4.3: Human Cells and Tissues - Biology

4.3: Human Cells and Tissues - Biology

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Dust Mop

This photo in Figure (PageIndex{1}) looks like a close-up of an old-fashioned dust mop, and the object it shows has a somewhat similar function. Can you guess what it is? The answer may surprise you. It is a scanning electron micrograph of human epithelial cells that line the bronchial passages. The floppy, dust-mop-like extensions are actually microscopic structures called cilia projecting from the outer surface of the epithelial cells. The function of the cilia is to trap dust, pathogens, and other particles in the air before it enters the lungs. The cilia also sway back and forth to sweep the trapped particles upward toward the throat, from which they can be expelled from the body.

Human Cells

Like the ciliated bronchial cells in the micrograph above, many other cells in the human body are very distinctive and well suited for special functions. To perform their special functions, cells may vary in a number of ways.

Variation in Human Cells

Some cells act as individual cells and are not attached to one another. Red blood cells are a good example. Their main function is to transport oxygen to other cells throughout the body, so they must be able to move freely through the circulatory system. Many other cells, in contrast, act together with other similar cells as part of the same tissue, so they are attached to one another and cannot move freely. For example, epithelial cells lining the respiratory tract are attached to each other to form a continuous surface that protects the respiratory system from particles and other hazards in the air.

Many cells can divide readily and form new cells. Skin cells are constantly dying and being shed from the body and replaced by new skin cells, and bone cells can divide to form new bone for growth or repair. Some other cells, in contrast, such as certain nerve cells, can divide and form new cells only under exceptional circumstances. That’s why nervous system injuries such as a severed spinal cord generally cannot heal by the production of new cells, resulting in a permanent loss of function.

Many human cells have the primary job of producing and secreting a particular substance, such as a hormone or an enzyme. For example, special cells in the pancreas produce and secrete the hormone insulin, which regulates the level of glucose in the blood. Some of the epithelial cells that line the bronchial passages produce mucus, a sticky substance that helps trap particles in the air before it passes into the lungs.

Different but Identical

All the different cell types within an individual human organism are genetically identical, so no matter how different the cells are, they all have the same genes. How can such different types of cells arise? The answer is the differential regulation of genes. Cells with the same genes can be very different because different genes are expressed depending on the cell type.

Examples of Human Cell Types

Many common types of human cells — such as bone cells and white blood cells — actually consist of several subtypes of cells. Each subtype, in turn, has a special structure and function. A closer look at these cell types will give you a better appreciation for the diversity of structures and functions of human cells.

Bone Cells

There are four main subtypes of bone cells, as shown in Figure (PageIndex{2}). Each type has a different form and function:

  1. Osteocytes are star-shaped bone cells that make up the majority of bone tissue. They are the most common cells in mature bone and can live as long as the organism itself. They also control the function of bone cells called osteoblasts and osteoclasts.
  2. Osteoblasts are cells with single nuclei that synthesize new bone. They function in organized groups of connected cells called osteons to form the organic and mineral matrix of bone.
  3. Osteogenic cells are undifferentiated stem cells that differentiate to form osteoblasts in the tissue that covers the outside of the bone.
  4. Osteoclasts are very large, multinucleated cells that are responsible for the breakdown of bones through resorption. The breakdown of bone is very important in bone health because it allows for bone remodeling.

White Blood Cells

White blood cells (also called leukocytes) are even more variable than bone cells. Five subtypes of white blood cells are shown in Figure (PageIndex{3}). All of them are immune system cells involved in defending the body, but each subtype has a different function. They also differ in the normal proportion of all leukocytes they make up.

  1. Monocytes make up about 5 percent of leukocytes. They are the biggest cells with extensions and a kidney-shaped nucleus. They engulf and destroy (phagocytize) pathogens in tissues.
  2. Eosinophils make up about 2 percent of leukocytes. They have and a bilobed nucleus. They attack larger parasites and set off allergic responses.
  3. Basophils make up less than 1 percent of leukocytes. Like eosinophils, these cells also have granules and a bilobed nucleus. They release proteins called histamines that are involved in inflammation.
  4. Lymphocytes make up about 30 percent of leukocytes. These are small cells with a large circular nucleus. They include B cells and T cells. B cells produce antibodies against non-self antigens, and T cells destroy virus-infected cells and cancer cells.
  5. Neutrophils are the most numerous white blood cells, making up about 62 percent of leukocytes. They have granules and a multilobed nucleus. They phagocytize single-celled bacteria and fungi in the blood.


Groups of connected cells form tissues. The cells in a tissue may all be the same type or they may be of multiple types. In either case, the cells in the tissue work together to carry out a specific function. There are four main types of human tissues: connective, epithelial, muscle, and nervous tissues.

Connective Tissue

The most diverse and abundant of all tissues, connective tissue holds cells together and supports the body. Connective tissue is made up of cells suspended in a non-cellular matrix. The matrix (also known as ground substance) is secreted by the connective tissue cells and determines the characteristics of the connective tissue. It is the consistency of the matrix that determines the function of the connective tissue. The matrix can be liquid, gel-like or solid, all depending on the type of connective tissue. For example, the extracellular matrix of bone is a rigid mineral framework. The extracellular matrix of blood is liquid plasma. Connective tissues such as bone and cartilage generally form the body's structure. There are many sub-types of the four major types of tissues in a human body, see the flow chart in Figure (PageIndex{5}).

Connective Tissue Proper

Fibroblast cells are responsible for synthesizing protein fibers for the matrix. Collagen fibers are strong, elastic fibers are flexible and reticular fibers form a supportive framework for organs and basement membranes. There are two subcategories of connective tissue proper.

Loose connective tissue

Thin and soft, this tissue contains many collagen and elastic fibers in a jell-like matrix. The cells in loose connective tissue are not close together. This tissue functions in binding the skin to underlying structures. There are three types of loose connective tissue.

  1. Areolar connective tissue is a common form of loose connective tissue. It is found in the skin and mucous membranes, where it binds the skin or membrane to underlying tissues such as muscles. It is also found around blood vessels and internal organs where it links and supports them.
  2. Adipose connective tissue is commonly known as fat. This tissue contains fat cells that are specialized for lipid storage. In addition to storing energy, this tissue also cushions and protects the organs.
  3. Reticular connective tissue is mostly composed of reticular protein fibers which make a skeleton, known as stroma, for the lymphatic and white blood cells. This type of tissue is found in the spleen and other lymphatic system structures.
Dense connective tissue proper

This tissue consists of three categories, dense regular connective tissue, dense irregular connective tissue, and elastic connective tissue. These tissues differ on the arrangement and composition of the fibrous elements of the extracellular matrix.

  1. Dense regular connective tissue has extracellular fibers that all run in the same direction and plane. Muscle tendons are a type of dense regular connective tissue.
  2. Dense irregular connective tissue contains collagen and elastic fibers which are found running in all different directions and planes. The dermis of the skin is composed of dense irregular connective tissue.
  3. Elastic connective tissue: Made up of freely branching elastic fibers with fibroblasts in the spaces between the fibers, this tissue allows the kind of stretch that is found in the walls of arteries.

Figure (PageIndex{10}): Cartilage is a connective tissue consisting of collagenous fibers embedded in a firm matrix of chondroitin sulfates. (a) Hyaline cartilage has chondrocytes in lacunae within a matrix. (b) Fibrocartilage has chondrocytes in lacunae within collage fibers in a matrix. (c) Elastic cartilage has chondrocytes in lacunae within elastic fibers in a matrix.


This connective tissue is relatively solid and is a non-vascularized tissue (does not have a blood supply). The matrix is produced by cells called chondroblasts. When these cells slow down, they reside is small spaces called lacunae. These mature cells in the lacunae are called chondrocytes. There are three types of cartilage: hyaline cartilage, elastic cartilage, and fibrocartilage.

  1. Hyaline cartilage is the most common type of cartilage, contains many collagen fibers and is found in many places including the nose, between the ribs and the sternum and in the rings of the trachea.
  2. Elastic cartilage has many elastic fibers in the matrix and supports the shape of the ears and forms part of the larynx.
  3. Fibrocartilage is tough and contains many collagen fibers and is responsible for cushioning the knee joint and for forming the disks between the vertebrae.


Bone is a hard, mineralized tissue found in the skeleton. The bone matrix contains many collagen fibers as well as inorganic mineral salts, calcium carbonate, and calcium phosphate, all features that make it a very rigid structure. Bone cells, called osteoblasts, secrete the osteoid substance that eventually hardens around the cells to form an ossified matrix. The osteon forms the basic unit of compact bone. Within the osteon, the osteocytes (mature bone cells) are located in lacunae. Because the bone matrix is very dense, the osteocytes get their nutrition from the central canal via tiny canals called canaliculi.


Blood is considered a type of fluid connective tissue because the matrix of blood is not solid. The fluid matrix is called plasma, and formed elements of this tissue include white blood cells, red blood cells, and platelets. Read more about the composition and function of blood in the cardiovascular system chapter.

Epithelial Tissue

Epithelial tissue is made up of cells that line inner and outer body surfaces, such as the skin and the inner surface of the digestive tract. Epithelial tissue that lines inner body surfaces and body openings is called mucous membrane. This type of epithelial tissue produces mucus, a slimy substance that coats mucous membranes and traps pathogens, particles, and debris. Epithelial tissue protects the body and its internal organs, secretes substances such as hormones in addition to mucus, and absorbs substances such as nutrients.

Epithelial Cell Classification

Most epithelial tissue is described with two names. The first name describes the number of cell layers present and the second describes the shape of the cells. One layer of epithelial cells is called simple and more than one layer of epithelial cells is called stratified. There are three basic shapes of epithelial cells, squamous, cuboidal, and columnar. Squamous cells are thin and flat; cuboidal cells have a shape of a cube; columnar cells have a shape of a pillar. For example, simple squamous epithelial tissue describes a single layer of cells that are flat and scale-like in shape.

Locations and Functions of Epithelial Tissues

These tissues are found at various locations in our body and they have many functions. Some locations and functions are listed below:

  • Simple squamous epithelium: This tissue is located in the sacs of the lungs and kidney where the exchange of nutrients and gas is essential.
  • Simple cuboidal epithelium: This tissue is located in the glands and their ducts and kidneys. The main function of this tissue is secretion.
  • Simple Columnar epithelium: This tissue lines the Gastrointestinal tract. The main function of this tissue is absorption and secretion.
  • Pseudostratified epithelium: This is a simple tissue with the appearance of stratification. This tissue is located in the respiratory tract. This tissue may contain cilia to move mucus.
  • Stratified squamous epithelium: This tissue is located where protection is needed such as skin.
  • Stratified cuboidal epithelium: This tissue is located in the sweat glands for protection.
  • Stratified columnar epithelium: This tissue is located in some sweat glands. The main function is to protect and secrete sweat components.
  • Transitional epithelium: This tissue lines bladder, urethra, and ureters. The tissue allows the urinary organs to expand and stretch.

Muscle Tissue

Muscle tissue is made up of cells that have the unique ability to contract or become shorter. There are three major types of muscle tissue, as pictured in Figure (PageIndex{14}): skeletal, smooth, and cardiac muscle tissues.

  1. Skeletal muscles are striated, or striped in appearance, because of their internal structure. Skeletal muscles are attached to bones, and when they pull on the bones, they enable the body to move. Skeletal muscles are under voluntary control.
  2. Smooth muscles are nonstriated muscles. They are found in the walls of blood vessels and in the reproductive, gastrointestinal, and respiratory tracts. Smooth muscles are not under voluntary control.
  3. Cardiac muscles are striated and found only in the heart. Their contractions cause the heart to beat. Cardiac muscles are not under voluntary control.

Nervous Tissue

Nervous tissue is made up of neurons and other types of cells generally called glial cells (Figure (PageIndex{15})). Neurons are composed of cell body and extensions. The cell body contains the nucleus and the extensions make connections with the other tissues and neurons. Neurons transmit electrical messages and the glial cells play supporting roles. Nervous tissue makes up the central nervous system (mainly the brain and spinal cord) and peripheral nervous system (the network of nerves that runs throughout the rest of the body).

Feature: My Human Body

If you are a blood donor, then you have donated tissue. Blood is a tissue that you can donate when you are alive. You may have indicated on your driver’s license application that you wish to be a tissue donor in the event of your death. Deceased people can donate many different tissues, including skin, bone, heart valves, and the corneas of the eyes. If you are not already registered as a tissue donor, the information below may help you decide if you would like to register.

Each year, approximately 30,000 people donate tissues, which supply tissues for up to 1 million tissue transplants. One tissue donor can enhance or save the life of more than 50 people! Unlike organs, which generally must be transplanted immediately after the donor dies, donated tissues can be processed and stored for a long time for later use. Donated tissues can be used to replace burned skin and damaged bone and to repair ligaments. Corneal tissues can be used for corneal transplants that restore sight in blind people. In fact, each year 48,000 patients have their sight restored with corneal transplants. Unfortunately, there are not enough tissues to go around, and the need for donated tissues keeps rising.


  1. Give an example of cells that function individually and move freely, and give an example of cells that act together and are attached to other cells of the same type.
  2. What are examples of cells that can readily divide and cells that can divide only under rare circumstances?
  3. Identify a type of cell that secretes an important substance and name the substance it secretes.
  4. Explain how different cell types come about when all the cells in an individual human being are genetically identical.
  5. Compare and contrast four subtypes of human bone cells.
  6. Identify three types of human white blood cells, and state their functions.
  7. Why are bone and blood both classified as connective tissues?
  8. Name another type of connective tissue, and describe its role in the human body.
  9. Based on the information in the table above of types of epithelial tissues, list four general functions of this type of tissue in the human body.
  10. Compare and contrast the three types of muscle tissues.
  11. Identify the four types of nervous tissues, where each type is found, and its basic function.
  12. Of the main types of human tissue, name two that can secrete hormones.
  13. Cells in a particular tissue:
    1. Are all of the same type
    2. Have different genes from cells in other tissues
    3. Work together to carry out a function
    4. Are always connected physically to each other
  14. Why are mucous membranes often located in regions that interface between the body and the outside world?
  15. Skin is a type of _____________ tissue.
  16. Body fat is a type of ____________ tissue.

Explore More

Each person’s body is completely unique, which means that everyone reacts differently to drugs and other medical treatments. In the TED talk below, tissue engineer Nina Tandon talks about a possible solution to this problem: making artificial tissues that are engineered to be the same as the patient’s and then using the tissues to test the effectiveness of specific drugs or other treatments.

The Glutamate Dehydrogenase Pathway and Its Roles in Cell and Tissue Biology in Health and Disease

Glutamate dehydrogenase (GDH) is a hexameric enzyme that catalyzes the reversible conversion of glutamate to α-ketoglutarate and ammonia while reducing NAD(P)⁺ to NAD(P)H. It is found in all living organisms serving both catabolic and anabolic reactions. In mammalian tissues, oxidative deamination of glutamate via GDH generates α-ketoglutarate, which is metabolized by the Krebs cycle, leading to the synthesis of ATP. In addition, the GDH pathway is linked to diverse cellular processes, including ammonia metabolism, acid-base equilibrium, redox homeostasis (via formation of fumarate), lipid biosynthesis (via oxidative generation of citrate), and lactate production. While most mammals possess a single GDH1 protein (hGDH1 in the human) that is highly expressed in the liver, humans and other primates have acquired, via duplication, an hGDH2 isoenzyme with distinct functional properties and tissue expression profile. The novel hGDH2 underwent rapid evolutionary adaptation, acquiring unique properties that enable enhanced enzyme function under conditions inhibitory to its ancestor hGDH1. These are thought to provide a biological advantage to humans with hGDH2 evolution occurring concomitantly with human brain development. hGDH2 is co-expressed with hGDH1 in human brain, kidney, testis and steroidogenic organs, but not in the liver. In human cerebral cortex, hGDH1 and hGDH2 are expressed in astrocytes, the cells responsible for removing and metabolizing transmitter glutamate, and for supplying neurons with glutamine and lactate. In human testis, hGDH2 (but not hGDH1) is densely expressed in the Sertoli cells, known to provide the spermatids with lactate and other nutrients. In steroid producing cells, hGDH1/2 is thought to generate reducing equivalents (NADPH) in the mitochondria for the biosynthesis of steroidal hormones. Lastly, up-regulation of hGDH1/2 expression occurs in cancer, permitting neoplastic cells to utilize glutamine/glutamate for their growth. In addition, deregulation of hGDH1/2 is implicated in the pathogenesis of several human disorders.

Keywords: GDH GDH deregulation and diseases expression glioma hGDH1 hGDH2 human tissues regulation structure.

Conflict of interest statement

The authors declare no conflict of interest.


The glutamate dehydrogenase (GDH) pathway…

The glutamate dehydrogenase (GDH) pathway and the Krebs cycle function. As shown here,…

Structural model of hGDH1. Shown…

Structural model of hGDH1. Shown is a cartoon diagram of the apo form…

hGDH1 and hGDH2 expression in…

hGDH1 and hGDH2 expression in glial cells. Punctate immunoreactivity for hGDH1 ( A…

Localization of hGDH2-positive “puncta” in…

Localization of hGDH2-positive “puncta” in the periphery and in the perinuclear cytoplasmic area…

Association of hGDH2 with the…

Association of hGDH2 with the nuclear membrane in small cortical neurons. A delicate…

4.3 Connective Tissue Supports and Protects

As may be obvious from its name, one of the major functions of connective tissue is to connect tissues and organs. Unlike epithelial tissue, which is composed of cells closely packed with little or no extracellular space in between, connective tissue cells are dispersed in a matrix . The matrix usually includes a large amount of extracellular material produced by the connective tissue cells that are embedded within it. The matrix plays a major role in the functioning of this tissue. The major component of the matrix is a ground substance often crisscrossed by protein fibers. This ground substance is usually a fluid, but it can also be mineralized and solid, as in bones. Connective tissues come in a vast variety of forms, yet they typically have in common three characteristic components: cells, large amounts of amorphous ground substance, and protein fibers. The amount and structure of each component correlates with the function of the tissue, from the rigid ground substance in bones supporting the body to the inclusion of specialized cells for example, a phagocytic cell that engulfs pathogens and also rids tissue of cellular debris.

Functions of Connective Tissues

Connective tissues perform many functions in the body, but most importantly, they support and connect other tissues from the connective tissue sheath that surrounds muscle cells, to the tendons that attach muscles to bones, and to the skeleton that supports the positions of the body. Protection is another major function of connective tissue, in the form of fibrous capsules and bones that protect delicate organs and, of course, the skeletal system. Specialized cells in connective tissue defend the body from microorganisms that enter the body. Transport of fluid, nutrients, waste, and chemical messengers is ensured by specialized fluid connective tissues, such as blood and lymph. Adipose cells store surplus energy in the form of fat and contribute to the thermal insulation of the body.

Embryonic Connective Tissue

All connective tissues derive from the mesodermal layer of the embryo (see Figure 4.3). The first connective tissue to develop in the embryo is mesenchyme , the stem cell line from which all connective tissues are later derived. Clusters of mesenchymal cells are scattered throughout adult tissue and supply the cells needed for replacement and repair after a connective tissue injury. A second type of embryonic connective tissue forms in the umbilical cord, called mucous connective tissue or Wharton’s jelly. This tissue is no longer present after birth, leaving only scattered mesenchymal cells throughout the body.

Classification of Connective Tissues

The three broad categories of connective tissue are classified according to the characteristics of their ground substance and the types of fibers found within the matrix (Table 4.1). Connective tissue proper includes loose connective tissue and dense connective tissue . Both tissues have a variety of cell types and protein fibers suspended in a viscous ground substance. Dense connective tissue is reinforced by bundles of fibers that provide tensile strength, elasticity, and protection. In loose connective tissue, the fibers are loosely organized, leaving large spaces in between. Supportive connective tissue —bone and cartilage—provide structure and strength to the body and protect soft tissues. A few distinct cell types and densely packed fibers in a matrix characterize these tissues. In bone, the matrix is rigid and described as calcified because of the deposited calcium salts. In fluid connective tissue , in other words, lymph and blood, various specialized cells circulate in a watery fluid containing salts, nutrients, and dissolved proteins.

  • Areolar
  • Adipose
  • Reticular
  • Hyaline
  • Fibrocartilage
  • Elastic
  • Regular elastic
  • Irregular elastic
  • Compact bone
  • Cancellous bone

Connective Tissue Proper

Fibroblasts are present in all connective tissue proper (Figure 4.12). Fibrocytes, adipocytes, and mesenchymal cells are fixed cells, which means they remain within the connective tissue. Other cells move in and out of the connective tissue in response to chemical signals. Macrophages, mast cells, lymphocytes, plasma cells, and phagocytic cells are found in connective tissue proper but are actually part of the immune system protecting the body.

Cell Types

The most abundant cell in connective tissue proper is the fibroblast . Polysaccharides and proteins secreted by fibroblasts combine with extra-cellular fluids to produce a viscous ground substance that, with embedded fibrous proteins, forms the extra-cellular matrix. As you might expect, a fibrocyte , a less active form of fibroblast, is the second most common cell type in connective tissue proper.

Adipocytes are cells that store lipids as droplets that fill most of the cytoplasm. There are two basic types of adipocytes: white and brown. The brown adipocytes store lipids as many droplets, and have high metabolic activity. In contrast, white fat adipocytes store lipids as a single large drop and are metabolically less active. Their effectiveness at storing large amounts of fat is witnessed in obese individuals. The number and type of adipocytes depends on the tissue and location, and vary among individuals in the population.

The mesenchymal cell is a multipotent adult stem cell. These cells can differentiate into any type of connective tissue cells needed for repair and healing of damaged tissue.

The macrophage cell is a large cell derived from a monocyte, a type of blood cell, which enters the connective tissue matrix from the blood vessels. The macrophage cells are an essential component of the immune system, which is the body’s defense against potential pathogens and degraded host cells. When stimulated, macrophages release cytokines, small proteins that act as chemical messengers. Cytokines recruit other cells of the immune system to infected sites and stimulate their activities. Roaming, or free, macrophages move rapidly by amoeboid movement, engulfing infectious agents and cellular debris. In contrast, fixed macrophages are permanent residents of their tissues.

The mast cell, found in connective tissue proper, has many cytoplasmic granules. These granules contain the chemical signals histamine and heparin. When irritated or damaged, mast cells release histamine, an inflammatory mediator, which causes vasodilation and increased blood flow at a site of injury or infection, along with itching, swelling, and redness you recognize as an allergic response. Like blood cells, mast cells are derived from hematopoietic stem cells and are part of the immune system.

Connective Tissue Fibers and Ground Substance

Three main types of fibers are secreted by fibroblasts: collagen fibers, elastic fibers, and reticular fibers. Collagen fiber is made from fibrous protein subunits linked together to form a long and straight fiber. Collagen fibers, while flexible, have great tensile strength, resist stretching, and give ligaments and tendons their characteristic resilience and strength. These fibers hold connective tissues together, even during the movement of the body.

Elastic fiber contains the protein elastin along with lesser amounts of other proteins and glycoproteins. The main property of elastin is that after being stretched or compressed, it will return to its original shape. Elastic fibers are prominent in elastic tissues found in skin and the elastic ligaments of the vertebral column.

Reticular fiber is also formed from the same protein subunits as collagen fibers however, these fibers remain narrow and are arrayed in a branching network. They are found throughout the body, but are most abundant in the reticular tissue of soft organs, such as liver and spleen, where they anchor and provide structural support to the parenchyma (the functional cells, blood vessels, and nerves of the organ).

All of these fiber types are embedded in ground substance. Secreted by fibroblasts, ground substance is made of polysaccharides, specifically hyaluronic acid, and proteins. These combine to form a proteoglycan with a protein core and polysaccharide branches. The proteoglycan attracts and traps available moisture forming the clear, viscous, colorless matrix you now know as ground substance.

Loose Connective Tissue

Loose connective tissue is found between many organs where it acts both to absorb shock and bind tissues together. It allows water, salts, and various nutrients to diffuse through to adjacent or imbedded cells and tissues.

Adipose tissue consists mostly of fat storage cells, with little extracellular matrix (Figure 4.13). A large number of capillaries allow rapid storage and mobilization of lipid molecules. White adipose tissue is most abundant. It can appear yellow and owes its color to carotene and related pigments from plant food. White fat contributes mostly to lipid storage and can serve as insulation from cold temperatures and mechanical injuries. White adipose tissue can be found protecting the kidneys and cushioning the back of the eye. Brown adipose tissue is more common in infants, hence the term “baby fat.” In adults, there is a reduced amount of brown fat and it is found mainly in the neck and clavicular regions of the body. The many mitochondria in the cytoplasm of brown adipose tissue help explain its efficiency at metabolizing stored fat. Brown adipose tissue is thermogenic, meaning that as it breaks down fats, it releases metabolic heat, rather than producing adenosine triphosphate (ATP), a key molecule used in metabolism.

Areolar tissue shows little specialization. It contains all the cell types and fibers previously described and is distributed in a random, web-like fashion. It fills the spaces between muscle fibers, surrounds blood and lymph vessels, and supports organs in the abdominal cavity. Areolar tissue underlies most epithelia and represents the connective tissue component of epithelial membranes, which are described further in a later section.

Reticular tissue is a mesh-like, supportive framework for soft organs such as lymphatic tissue, the spleen, and the liver (Figure 4.14). Reticular cells produce the reticular fibers that form the network onto which other cells attach. It derives its name from the Latin reticulus, which means “little net.”

Dense Connective Tissue

Dense connective tissue contains more collagen fibers than does loose connective tissue. As a consequence, it displays greater resistance to stretching. There are two major categories of dense connective tissue: regular and irregular. Dense regular connective tissue fibers are parallel to each other, enhancing tensile strength and resistance to stretching in the direction of the fiber orientations. Ligaments and tendons are made of dense regular connective tissue, but in ligaments not all fibers are parallel. Dense regular elastic tissue contains elastin fibers in addition to collagen fibers, which allows the ligament to return to its original length after stretching. The ligaments in the vocal folds and between the vertebrae in the vertebral column are elastic.

In dense irregular connective tissue, the direction of fibers is random. This arrangement gives the tissue greater strength in all directions and less strength in one particular direction. In some tissues, fibers crisscross and form a mesh. In other tissues, stretching in several directions is achieved by alternating layers where fibers run in the same orientation in each layer, and it is the layers themselves that are stacked at an angle. The dermis of the skin is an example of dense irregular connective tissue rich in collagen fibers. Dense irregular elastic tissues give arterial walls the strength and the ability to regain original shape after stretching (Figure 4.15).

Disorders of the.

Connective Tissue: Tendinitis

Your opponent stands ready as you prepare to hit the serve, but you are confident that you will smash the ball past your opponent. As you toss the ball high in the air, a burning pain shoots across your wrist and you drop the tennis racket. That dull ache in the wrist that you ignored through the summer is now an unbearable pain. The game is over for now.

After examining your swollen wrist, the doctor in the emergency room announces that you have developed wrist tendinitis. She recommends icing the tender area, taking non-steroidal anti-inflammatory medication to ease the pain and to reduce swelling, and complete rest for a few weeks. She interrupts your protests that you cannot stop playing. She issues a stern warning about the risk of aggravating the condition and the possibility of surgery. She consoles you by mentioning that well known tennis players such as Venus and Serena Williams and Rafael Nadal have also suffered from tendinitis related injuries.

What is tendinitis and how did it happen? Tendinitis is the inflammation of a tendon, the thick band of fibrous connective tissue that attaches a muscle to a bone. The condition causes pain and tenderness in the area around a joint. On rare occasions, a sudden serious injury will cause tendinitis. Most often, the condition results from repetitive motions over time that strain the tendons needed to perform the tasks.

Persons whose jobs and hobbies involve performing the same movements over and over again are often at the greatest risk of tendinitis. You hear of tennis and golfer’s elbow, jumper's knee, and swimmer’s shoulder. In all cases, overuse of the joint causes a microtrauma that initiates the inflammatory response. Tendinitis is routinely diagnosed through a clinical examination. In case of severe pain, X-rays can be examined to rule out the possibility of a bone injury. Severe cases of tendinitis can even tear loose a tendon. Surgical repair of a tendon is painful. Connective tissue in the tendon does not have abundant blood supply and heals slowly.

While older adults are at risk for tendinitis because the elasticity of tendon tissue decreases with age, active people of all ages can develop tendinitis. Young athletes, dancers, and computer operators anyone who performs the same movements constantly is at risk for tendinitis. Although repetitive motions are unavoidable in many activities and may lead to tendinitis, precautions can be taken that can lessen the probability of developing tendinitis. For active individuals, stretches before exercising and cross training or changing exercises are recommended. For the passionate athlete, it may be time to take some lessons to improve technique. All of the preventive measures aim to increase the strength of the tendon and decrease the stress put on it. With proper rest and managed care, you will be back on the court to hit that slice-spin serve over the net.

Interactive Link

Watch this animation to learn more about tendonitis, a painful condition caused by swollen or injured tendons.

Supportive Connective Tissues

Two major forms of supportive connective tissue, cartilage and bone, allow the body to maintain its posture and protect internal organs.


The distinctive appearance of cartilage is due to polysaccharides called chondroitin sulfates, which bind with ground substance proteins to form proteoglycans. Embedded within the cartilage matrix are chondrocytes , or cartilage cells, and the space they occupy are called lacunae (singular = lacuna). A layer of dense irregular connective tissue, the perichondrium, encapsulates the cartilage. Cartilaginous tissue is avascular, thus all nutrients need to diffuse through the matrix to reach the chondrocytes. This is a factor contributing to the very slow healing of cartilaginous tissues.

The three main types of cartilage tissue are hyaline cartilage, fibrocartilage, and elastic cartilage (Figure 4.16). Hyaline cartilage , the most common type of cartilage in the body, consists of short and dispersed collagen fibers and contains large amounts of proteoglycans. Under the microscope, tissue samples appear clear. The surface of hyaline cartilage is smooth. Both strong and flexible, it is found in the rib cage and nose and covers bones where they meet to form moveable joints. It makes up a template of the embryonic skeleton before bone formation. A plate of hyaline cartilage at the ends of bone allows continued growth until adulthood. Fibrocartilage is tough because it has thick bundles of collagen fibers dispersed through its matrix. Menisci in the knee joint and the intervertebral discs are examples of fibrocartilage. Elastic cartilage contains elastic fibers as well as collagen and proteoglycans. This tissue gives rigid support as well as elasticity. Tug gently at your ear lobes, and notice that the lobes return to their initial shape. The external ear contains elastic cartilage.

Bone is the hardest connective tissue. It provides protection to internal organs and supports the body. Bone’s rigid extracellular matrix contains mostly collagen fibers embedded in a mineralized ground substance containing hydroxyapatite, a form of calcium phosphate. Both components of the matrix, organic and inorganic, contribute to the unusual properties of bone. Without collagen, bones would be brittle and shatter easily. Without mineral crystals, bones would flex and provide little support. Osteocytes, bone cells like chondrocytes, are located within lacunae. The histology of transverse tissue from long bone shows a typical arrangement of osteocytes in concentric circles around a central canal. Bone is a highly vascularized tissue. Unlike cartilage, bone tissue can recover from injuries in a relatively short time.

Cancellous bone looks like a sponge under the microscope and contains empty spaces between trabeculae, or arches of bone proper. It is lighter than compact bone and found in the interior of some bones and at the end of long bones. Compact bone is solid and has greater structural strength.

Fluid Connective Tissue

Blood and lymph are fluid connective tissues. Cells circulate in a liquid extracellular matrix. The formed elements circulating in blood are all derived from hematopoietic stem cells located in bone marrow (Figure 4.17). Erythrocytes, red blood cells, transport oxygen and some carbon dioxide. Leukocytes, white blood cells, are responsible for defending against potentially harmful microorganisms or molecules. Platelets are cell fragments involved in blood clotting. Some white blood cells have the ability to cross the endothelial layer that lines blood vessels and enter adjacent tissues. Nutrients, salts, and wastes are dissolved in the liquid matrix and transported through the body.

Lymph contains a liquid matrix and white blood cells. Lymphatic capillaries are extremely permeable, allowing larger molecules and excess fluid from interstitial spaces to enter the lymphatic vessels. Lymph drains into blood vessels, delivering molecules to the blood that could not otherwise directly enter the bloodstream. In this way, specialized lymphatic capillaries transport absorbed fats away from the intestine and deliver these molecules to the blood.

Interactive Link

View the University of Michigan Webscope to explore the tissue sample in greater detail.


In performing their protective role, fixatives denature proteins by coagulation, by forming additive compounds, or by a combination of coagulation and additive processes. A compound that adds chemically to macromolecules stabilizes structure most effectively if it is able to combine with parts of two different macromolecules, an effect known as cross-linking. Fixation of tissue is done for several reasons. One reason is to kill the tissue so that postmortem decay (autolysis and putrefaction) is prevented. [1] Fixation preserves biological material (tissue or cells) as close to its natural state as possible in the process of preparing tissue for examination. To achieve this, several conditions usually must be met.

First, a fixative usually acts to disable intrinsic biomolecules—particularly proteolytic enzymes—which otherwise digest or damage the sample.

Second, a fixative typically protects a sample from extrinsic damage. Fixatives are toxic to most common microorganisms (bacteria in particular) that might exist in a tissue sample or which might otherwise colonize the fixed tissue. In addition, many fixatives chemically alter the fixed material to make it less palatable (either indigestible or toxic) to opportunistic microorganisms.

Finally, fixatives often alter the cells or tissues on a molecular level to increase their mechanical strength or stability. This increased strength and rigidity can help preserve the morphology (shape and structure) of the sample as it is processed for further analysis.

Even the most careful fixation does alter the sample and introduce artifacts that can interfere with interpretation of cellular ultrastructure. A prominent example is the bacterial mesosome, which was thought to be an organelle in gram-positive bacteria in the 1970s, but was later shown by new techniques developed for electron microscopy to be simply an artifact of chemical fixation. [2] [3] Standardization of fixation and other tissue processing procedures takes this introduction of artifacts into account, by establishing what procedures introduce which kinds of artifacts. Researchers who know what types of artifacts to expect with each tissue type and processing technique can accurately interpret sections with artifacts, or choose techniques that minimize artifacts in areas of interest.

Fixation is usually the first stage in a multistep process to prepare a sample of biological material for microscopy or other analysis. Therefore, the choice of fixative and fixation protocol may depend on the additional processing steps and final analyses that are planned. For example, immunohistochemistry uses antibodies that bind to a specific protein target. Prolonged fixation can chemically mask these targets and prevent antibody binding. In these cases, a 'quick fix' method using cold formalin for around 24 hours is typically used. Methanol (100%) can also be used for quick fixation, and that time can vary depending on the biological material. For example, MDA-MB 231 human breast cancer cells can be fixed for only 3 minutes with cold methanol (-20 °C). For enzyme localization studies, the tissues should either be pre-fixed lightly only, or post-fixed after the enzyme activity product has formed.

There are generally three types of fixation processes depending on the initial specimen:

Heat fixation: After a smear has dried at room temperature, the slide is gripped by tongs or a clothespin and passed through the flame of a Bunsen burner several times to heat-kill and adhere the organism to the slide. Routinely used with bacteria and archaea. Heat fixation generally preserves overall morphology but not internal structures. Heat denatures the proteolytic enzyme and prevents autolysis. Heat fixation cannot be used in the capsular stain method as heat fixation will shrink or destroy the capsule (glycocalyx) and cannot be seen in stains.

Immersion: The sample of tissue is immersed in fixative solution of volume at a minimum of 20 times greater than the volume of the tissue to be fixed. The fixative must diffuse through the tissue to fix, so tissue size and density, as well as type of fixative must be considered. This is a common technique for cellular applications. Using a larger sample means it takes longer for the fixative to reach the deeper tissue.

Perfusion: Fixation via blood flow. The fixative is injected into the heart with the injection volume matching cardiac output. The fixative spreads through the entire body, and the tissue doesn't die until it is fixed. This has the advantage of preserving perfect morphology, but the disadvantages are that the subject dies and the cost of the volume of fixative needed for larger organisms is high.

In both immersion and perfusion fixation processes, chemical fixatives are used to preserve structures in a state (both chemically and structurally) as close to living tissue as possible. This requires a chemical fixative.

Crosslinking fixatives – aldehydes Edit

Crosslinking fixatives act by creating covalent chemical bonds between proteins in tissue. This anchors soluble proteins to the cytoskeleton, and lends additional rigidity to the tissue. Preservation of transient or fine cytoskeletal structure such as contractions during embryonic differentiation waves is best achieved by a pretreatment using microwaves before the addition of a cross linking fixative. [4] [5]

The most commonly used fixative in histology is formaldehyde. It is usually used as a 10% neutral buffered formalin (NBF), that is approx. 3.7%–4.0% formaldehyde in phosphate buffer, pH 7. Since formaldehyde is a gas at room temperature, formalin – formaldehyde gas dissolved in water (

37% w/v) – is used when making the former fixative. Formaldehyde fixes tissue by cross-linking the proteins, primarily the residues of the basic amino acid lysine. Its effects are reversible by excess water and it avoids formalin pigmentation. Paraformaldehyde is also commonly used and will depolymerise back to formalin when heated, also making it an effective fixative. Other benefits to paraformaldehyde include long term storage and good tissue penetration. It is particularly good for immunohistochemistry techniques. The formaldehyde vapor can also be used as a fixative for cell smears.

Another popular aldehyde for fixation is glutaraldehyde. It operates similarly to formaldehyde, causing the deformation of proteins' α-helices. However glutaraldehyde is a larger molecule than formaldehyde, and so permeates membranes more slowly. Consequently, glutaraldehyde fixation on thicker tissue samples can be difficult this can be troubleshot by reducing the size of the tissue sample. One of the advantages of glutaraldehyde fixation is that it may offer a more rigid or tightly linked fixed product—its greater length and two aldehyde groups allow it to 'bridge' and link more distant pairs of protein molecules. It causes rapid and irreversible changes, is well suited for electron microscopy, works well at 4 °C, and gives the best overall cytoplasmic and nuclear detail. It is, however, not ideal for immunohistochemistry staining.

Some fixation protocols call for a combination of formaldehyde and glutaraldehyde so that their respective strengths complement one another.

These crosslinking fixatives, especially formaldehyde, tend to preserve the secondary structure of proteins and may also preserve most tertiary structure.

Precipitating fixatives – alcohols Edit

Precipitating (or denaturing) fixatives act by reducing the solubility of protein molecules and often by disrupting the hydrophobic interactions that give many proteins their tertiary structure. The precipitation and aggregation of proteins is a very different process from the crosslinking that occurs with aldehyde fixatives.

The most common precipitating fixatives are ethanol and methanol. They are commonly used to fix frozen sections and smears. Acetone is also used and has been shown to produce better histological preservation than frozen sections when employed in the Acetone Methylbenzoate Xylene (AMEX) technique.

Protein-denaturing methanol, ethanol and acetone are rarely used alone for fixing blocks unless studying nucleic acids.

Acetic acid is a denaturant that is sometimes used in combination with the other precipitating fixatives, such as Davidson's AFA. [6] The alcohols, by themselves, are known to cause considerable shrinkage and hardening of tissue during fixation while acetic acid alone is associated with tissue swelling combining the two may result in better preservation of tissue morphology.

Oxidizing agents Edit

The oxidizing fixatives can react with the side chains of proteins and other biomolecules, allowing the formation of crosslinks that stabilize tissue structure. However they cause extensive denaturation despite preserving fine cell structure and are used mainly as secondary fixatives.

Osmium tetroxide is often used as a secondary fixative when samples are prepared for electron microscopy. (It is not used for light microscopy as it penetrates thick sections of tissue very poorly.)

Potassium dichromate, chromic acid, and potassium permanganate all find use in certain specific histological preparations.

Mercurials Edit

Mercurials such as B-5 and Zenker's fixative have an unknown mechanism that increases staining brightness and give excellent nuclear detail. Despite being fast, mercurials penetrate poorly and produce tissue shrinkage. Their best application is for fixation of hematopoietic and reticuloendothelial tissues. Also note that since they contain mercury, care must be taken with disposal.

Picrates Edit

Picrates penetrate tissue well to react with histones and basic proteins to form crystalline picrates with amino acids and precipitate all proteins. It is a good fixative for connective tissue, preserves glycogen well, and extracts lipids to give superior results to formaldehyde in immunostaining of biogenic and polypeptide hormones However, it causes a loss of basophils unless the specimen is thoroughly washed following fixation.

HOPE fixative Edit

Hepes-glutamic acid buffer-mediated organic solvent protection effect (HOPE) gives formalin-like morphology, excellent preservation of protein antigens for immunohistochemistry and enzyme histochemistry, good RNA and DNA yields and absence of crosslinking proteins.

3D Bioprinting of Living Tissues

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Progress in drug testing and regenerative medicine could greatly benefit from laboratory-engineered human tissues built of a variety of cell types with precise 3D architecture. But production of greater than millimeter sized human tissues has been limited by a lack of methods for building tissues with embedded life-sustaining vascular networks.

Multidisciplinary research at the Wyss Institute has led to the development of a multi-material 3D bioprinting method that generates vascularized tissues composed of living human cells that are nearly ten-fold thicker than previously engineered tissues and that can sustain their architecture and function for upwards of six weeks. The method uses a customizable, printed silicone mold to house and plumb the printed tissue on a chip. Inside this mold, a grid of larger vascular channels containing living endothelial cells in silicone ink is printed, into which a self-supporting ink containing living mesenchymal stem cells (MSCs) is layered in a separate print job. After printing, a liquid composed of fibroblasts and extracellular matrix is used to fill open regions within the construct, adding a connective tissue component that cross-links and further stabilizes the entire structure.

Confocal microscopy image showing a cross-section of a 3D-printed, 1-centimeter-thick vascularized tissue construct showing stem cell differentiation towards development of bone cells, following one month of active perfusion of fluids, nutrients, and cell growth factors. The structure was fabricated using a novel 3D bioprinting strategy invented by Jennifer Lewis and her team at the Wyss Institute and Harvard SEAS. Credit: Lewis Lab, Wyss Institute at Harvard University

The resulting soft tissue structure can be immediately perfused with nutrients as well as growth and differentiation factors via a single inlet and outlet on opposite ends of the chip that connect to the vascular channel to ensure survival and maturation of the cells. In a proof-of-principle study, one centimeter thick bioprinted tissue constructs containing human bone marrow MSCs surrounded by connective tissue and supported by an artificial endothelium-lined vasculature, allowed the circulation of bone growth factors and, subsequently, the induction of bone development.

This innovative bioprinting approach can be modified to create various vascularized 3D tissues for regenerative medicine and drug testing endeavors. The Wyss team is also investigating the use of 3D bioprinting to fabricate new versions of the Institute’s organs on chips devices, which makes their manufacturing process more automated and enables development of increasingly complex microphysiological devices. This effort has resulted in the first entirely 3D-printed organ on a chip – a heart on a chip – with integrated soft strain sensors.

3.1.4 How the body is coordinated

Students should have knowledge and understanding of the following content.

The human body has automatic control systems, which may involve nervous responses or chemical responses coordinated by hormones.

Reflex actions are automatic and rapid.

Examples include the response of the pupil in the eyes to bright light, and the knee jerk reaction.

Knowledge of the reflex arc is not required.

Suggested activity for TDA Compare the speed of the catching reflex of two people.

Hormones are secreted by glands and are transported to their target organs by the bloodstream.

Knowledge of the names of specific hormones is not required.

Several hormones are involved in the menstrual cycle of a woman, including some that are involved in promoting the release of an egg.

Students should be familiar with a diagram of the menstrual cycle.

The uses of hormones in controlling fertility include:

  • giving oral contraceptives that contain hormones to inhibit eggs from maturing
  • giving &lsquofertility drugs&rsquo to stimulate eggs to mature.

The names of the hormones involved and the mechanism by which they work are not required.

Aging changes in organs, tissues, and cells

All vital organs begin to lose some function as you age during adulthood. Aging changes occur in all of the body's cells, tissues, and organs, and these changes affect the functioning of all body systems.

Living tissue is made up of cells. There are many different types of cells, but all have the same basic structure. Tissues are layers of similar cells that perform a specific function. The different kinds of tissues group together to form organs.

There are four basic types of tissue:

Connective tissue supports other tissues and binds them together. This includes bone, blood, and lymph tissues, as well as the tissues that give support and structure to the skin and internal organs.

Epithelial tissue provides a covering for superficial and deeper body layers. The skin and the linings of the passages inside the body, such as the gastrointestinal system, are made of epithelial tissue.

Muscle tissue includes three types of tissue:

  • Striated muscles, such as those that move the skeleton (also called voluntary muscle)
  • Smooth muscles (also called involuntary muscle), such as the muscles contained in the stomach and other internal organs
  • Cardiac muscle, which makes up most of the heart wall (also an involuntary muscle)

Nerve tissue is made up of nerve cells (neurons) and is used to carry messages to and from various parts of the body. The brain, spinal cord, and peripheral nerves are made of nerve tissue.

Cells are the basic building blocks of tissues. All cells experience changes with aging. They become larger and are less able to divide and multiply. Among other changes, there is an increase in pigments and fatty substances inside the cell (lipids). Many cells lose their ability to function, or they begin to function abnormally.

As aging continues, waste products build up in tissue. A fatty brown pigment called lipofuscin collects in many tissues, as do other fatty substances.

Connective tissue changes, becoming more stiff. This makes the organs, blood vessels, and airways more rigid. Cell membranes change, so many tissues have more trouble getting oxygen and nutrients, and removing carbon dioxide and other wastes.

Many tissues lose mass. This process is called atrophy. Some tissues become lumpy (nodular) or more rigid.

Because of cell and tissue changes, your organs also change as you age. Aging organs slowly lose function. Most people do not notice this loss immediately, because you rarely need to use your organs to their fullest ability.

Organs have a reserve ability to function beyond the usual needs. For example, the heart of a 20-year-old is capable of pumping about 10 times the amount of blood that is actually needed to keep the body alive. After age 30, an average of 1% of this reserve is lost each year.

The biggest changes in organ reserve occur in the heart, lungs, and kidneys. The amount of reserve lost varies between people and between different organs in a single person.

These changes appear slowly and over a long period. When an organ is worked harder than usual, it may not be able to increase function. Sudden heart failure or other problems can develop when the body is worked harder than usual. Things that produce an extra workload (body stressors) include the following:

  • Illness
  • Medicines
  • Significant life changes
  • Sudden increased physical demands on the body, such as a change in activity or exposure to a higher altitude

Loss of reserve also makes it harder to restore balance (equilibrium) in the body. Drugs are removed from the body by the kidneys and liver at a slower rate. Lower doses of medicines may be needed, and side effects become more common. Recovery from illnesses is seldom 100%, leading to more and more disability.

Side effects of medicine can mimic the symptoms of many diseases, so it is easy to mistake a drug reaction for an illness. Some medicines have entirely different side effects in the elderly than in younger people.

No one knows how and why people change as they get older. Some theories claim that aging is caused by injuries from ultraviolet light over time, wear and tear on the body, or byproducts of metabolism. Other theories view aging as a predetermined process controlled by genes.

No single process can explain all the changes of aging. Aging is a complex process that varies as to how it affects different people and even different organs. Most gerontologists (people who study aging) feel that aging is due to the interaction of many lifelong influences. These influences include heredity, environment, culture, diet, exercise and leisure, past illnesses, and many other factors.

Unlike the changes of adolescence, which are predictable to within a few years, each person ages at a unique rate. Some systems begin aging as early as age 30. Other aging processes are not common until much later in life.

Although some changes always occur with aging, they occur at different rates and to different extents. There is no way to predict exactly how you will age.


  • Cells shrink. If enough cells decrease in size, the entire organ atrophies. This is often a normal aging change and can occur in any tissue. It is most common in skeletal muscle, the heart, the brain, and the sex organs (such as the breasts and ovaries). Bones become thinner and more likely to break with minor trauma.
  • The cause of atrophy is unknown, but may include reduced use, decreased workload, decreased blood supply or nutrition to the cells, and reduced stimulation by nerves or hormones.
  • Cells enlarge. This is caused by an increase of proteins in the cell membrane and cell structures, not an increase in the cell's fluid.
  • When some cells atrophy, others may hypertrophy to make up for the loss of cell mass.
  • The number of cells increases. There is an increased rate of cell division.
  • Hyperplasia usually occurs to compensate for a loss of cells. It allows some organs and tissues to regenerate, including the skin, lining of the intestines, liver, and bone marrow. The liver is especially good at regeneration. It can replace up to 70% of its structure within 2 weeks after an injury.
  • Tissues that have limited ability to regenerate include bone, cartilage, and smooth muscle (such as the muscles around the intestines). Tissues that rarely or never regenerate include the nerves, skeletal muscle, heart muscle, and the lens of the eye. When injured, these tissues are replaced with scar tissue.
  • The size, shape, or organization of mature cells becomes abnormal. This is also called atypical hyperplasia.
  • Dysplasia is fairly common in the cells of the cervix and the lining of the respiratory tract.
  • The formation of tumors, either cancerous (malignant) or noncancerous (benign).
  • Neoplastic cells often reproduce quickly. They may have unusual shapes and abnormal function.

As you grow older, you will have changes throughout your body, including changes in:

Cell Structure Summary Sheet

  • Organelles are structures that carry out different functions within a cell.
  • Organelles in a cell are analogous to the organs in a body.
  • Organelles are suspended in a water-based fluid called cytosol.
  • The nucleus stores the genetic information (chromosomes) of eukaryotic cells.
  • The nucleus is roughly spherical and is surrounded by two membranes.
  • The nucleus is the 'brain' of a cell.
  • Mitochondria are the power houses of a cell.
  • Mitochondria convert biomolecules (i.e. fats and sugars) into energy.
  • By-products of energy production in mitochondria may damage DNA and cause mutations.
  • Ribosomes are made up of two large complexes comprised of RNA and protein.
  • Ribosomes are located in the cytosol. Their function is to read RNA and produce proteins in a process known as translation.
  • The cytoskeleton is an intricate network of proteins that criss-cross the cytoplasm of a cell.
  • Actin and tubulin are the proteins used to build main fibers of the cytoskeleton (microfilaments and microtubules, respectively).
  • The cytoskeleton serves several key functions:
    • Provides structure to cells and a place to anchor organelles
    • Cell motility
    • Control of cell division during mitosis

    To divide, the cell goes through a process called the cell cycle. There are four main stages or phases.

    • Gap 1 or G1 phase, where the cell grows in size, and checks that everything is OK for it to divide.
    • Synthesis or the S phase, where the cell copies its DNA.
    • Gap 2 or G2 phase, where the cells check that all its DNA has been correctly copied.
    • Mitosis or M phase, where the cell finally divides in two.

    The diagram below shows mitosis or M phase.

    7. Concluding Remarks

    Fibroblast growth factors (FGFs) that signal through FGF receptors (FGFRs) regulate a wide range of biological functions, including cell proliferation, survival, migration, and differentiation. Among the signal pathways, RAS/MAP kinase is known to be predominant in the case of FGFs. While the biological functions of FGFs are largely implicated in many types of cells in vitro through this signaling pathway, the maintenance of stability and half-life in vivo should be considered. Biomaterial-based systems, including delivery carriers of FGFs and scaffolds of stem cells regulated by the FGFs functions, have recently been potentially developed and shown to have many good results in vivo. Future clinical applications of FGFs in the regeneration of tissues, including skin, muscle, tendon/ligament, bone, tooth, and nerve tissues will be realized when their biological functions are maximized by the appropriate use of biomaterials and stem cells.

    Table 3

    Tissue applications of fibroblast growth factors.

    Target tissueSubfamily of FGFMaterials/carriersIn vivo/in vitroAnimal/cellFunctions/effectsRef
    SkinFGF2Gelatin microsphereIn vivoGuinea pigWound healing[86]
    FGF2Chitosan hydrogelIn vivoMouseWound healing[87]
    VesselsFGF2Gelatin hydrogelIn vivoMouseVascularization[88]
    FGF2Heparinized collagenIn vitroEndothelial cellCell growth[68]
    FGF2Heparinized PLGA scaffoldIn vivo/In vitroMouseVascularization[88]
    FGF2PLGA microsphere-alginate porous scaffoldIn vivoRatCapillary penetration, vascularization[89]
    MuscleFGF2PLGA nanoparticleIn vivo/In vitroMouseArteriogenesis[90]
    AdiposeFGF2MatrigelIn vivoMouseAdipogenesis[91]
    FGF2Matrigel-gelatin microspheresIn vivoMouseAdipogenesis[92]
    FGF2Gelatin microsphere-collagen scaffoldIn vivoMouse/rabbitAdipose regeneration[93, 94]
    Tendon/ LigamentFGF2Gelatin-PLA scaffoldIn vivoRabbitACL and bone regeneration[95]
    FGF2Silk/PLGA scaffoldIn vitroBMSCsProliferation, differentiation[96]
    CartilageFGF2PGA scaffoldIn vitroChondrocyteDedifferentiation[97]
    FGF2Collagen spongeIn vivoMouseCartilage regeneration[98]
    FGF2Gelatin microsphere-polymer scaffoldIn vivo/In vitroMouseChondrogenesis, vascularization[99]
    FGF2Collagen-PGLA-PLCL scaffoldIn vivoChondrocyteTracheal regeneration[100]
    FGF2Collagen-PLLA scaffoldIn vitroChondrocyteProliferation[101]
    BoneFGF1Hydroxyapatite-fibrin scaffoldIn vivoRatOsteogenic markers, bone regeneration[81]
    FGF2Hyaluronate scaffoldIn vitroBMSCsOsteogenic markers, mineralization[102]
    FGF2Gelatin hydrogelIn vivoRabbitMineralization, bone regeneration[103]
    FGF2Hydroxyapatite porous granulesIn vitroMC3T3-E1Cell proliferation, osteoblast differentiation[104]
    FGF2Collagen-bioactive glassIn vivoRatBone regeneration[105]
    FGF2Ti based metals-matrigelIn vivo/In vitroRatBone regeneration[106]
    FGF2Hydroxyapatite/collagen scaffoldIn vivoRabbitBone regeneration, cartilage regeneration[107]
    FGF2Ti implant-melatoninIn vivoRatOsseointegration[108]
    DentalFGF2Gelatin microsphereIn vivoDogPeriodontal regeneration[109]
    FGF2Tricalcium phosphateIn vivoDogAlveolar tissue regeneration[110]
    NerveFGF1pHEMA-MMAIn vivoRatAxonal regeneration[111]
    FGF2Polyamide nanofiber scaffoldIn vitroAstrocyteNeurite outgrowth[5]
    FGF2Porous PLA scaffoldIn vivoRatCell migration, angiogenesis[112]
    FGF2Polyethylene glycolIn vivoRatSpinal cord injury repair[113]
    FGF2Polymer tube channelIn vivo/In vitroRatPeripheral nerve regeneration[90]
    FGF2Gelatin hydrogelIn vivoGuinea pigFacial nerve functions[114]

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