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First of all,I wanted to ask that whether epidermis is present in colon ? And if yes than how is muscle wall present with respect to epidermis. Can anyone give me a pic that has both the epidermis and muscle wall of colon.
Good question. The colon is lined by epithelial cells (as are all hollow organs). These particular epithelial cells are different from epidermal cells, which are another particular type of epithelial cells found in skin.
Epithelial cells are distinguished by the way they connect with each other using tight junctions to form a permeability layer between compartments.
In the colon, the purpose of the epithelial cells is to both to absorb water and to form a barrier between the luminal contents and the body. They form the layer facing the lumen. The muscular layers of the colon, both the circular and the longitudinal, are further out from the lumen. You can see them in this histology slide:
Epidermal thickness at different body sites: relationship to age, gender, pigmentation, blood content, skin type and smoking habits
Epidermal thickness and its relationship to age, gender, skin type, pigmentation, blood content, smoking habits and body site is important in dermatologic research and was investigated in this study. Biopsies from three different body sites of 71 human volunteers were obtained, and thickness of the stratum corneum and cellular epidermis was measured microscopically using a preparation technique preventing tissue damage. Multiple regressions analysis was used to evaluate the effect of the various factors independently of each other. Mean (SD) thickness of the stratum corneum was 18.3 (4.9) microm at the dorsal aspect of the forearm, 11.0 (2.2) microm at the shoulder and 14.9 (3.4) microm at the buttock. Corresponding values for the cellular epidermis were 56.6 (11.5) microm, 70.3 (13.6) microm and 81.5 (15.7) microm, respectively. Body site largely explains the variation in epidermal thickness, but also a significant individual variation was observed. Thickness of the stratum corneum correlated positively to pigmentation (p = 0.0008) and negatively to the number of years of smoking (p < 0.0001). Thickness of the cellular epidermis correlated positively to blood content (P = 0.028) and was greater in males than in females (P < 0.0001). Epidermal thickness was not correlated to age or skin type.
Organogenesis is the process by which the three germ tissue layers of the embryo, which are the ectoderm, endoderm, and mesoderm, develop into the internal organs of the organism. Organs form from the germ layers through the differentiation: the process by which a less-specialized cell becomes a more-specialized cell type. This must occur many times as a zygote becomes a fully-developed organism. During differentiation, the embryonic stem cells express specific sets of genes which will determine their ultimate cell type. For example, some cells in the ectoderm will express the genes specific to skin cells. As a result, these cells will differentiate into epidermal cells. Therefore, the process of differentiation is regulated by cellular signaling cascades.
In vertebrates, one of the primary steps during organogenesis is the formation of the neural system. The ectoderm forms epithelial cells and tissues, as well as neuronal tissues. During the formation of the neural system, special signaling molecules called growth factors signal some cells at the edge of the ectoderm to become epidermis cells. The remaining cells in the center form the neural plate. If the signaling by growth factors were disrupted, then the entire ectoderm would differentiate into neural tissue. The neural plate undergoes a series of cell movements where it rolls up and forms a tube called the neural tube. In further development, the neural tube will give rise to the brain and the spinal cord.
Figure (PageIndex<1>): Neural tube formation: The central region of the ectoderm forms the neural tube, which gives rise to the brain and the spinal cord.
The mesoderm that lies on either side of the vertebrate neural tube will develop into the various connective tissues of the animal body. A spatial pattern of gene expression reorganizes the mesoderm into groups of cells called somites, with spaces between them. The somites will further develop into the ribs, lungs, and segmental (spine) muscle. The mesoderm also forms a structure called the notochord, which is rod-shaped and forms the central axis of the animal body.
Figure (PageIndex<1>): Mesoderm: The mesoderm aids in the production of cardiac muscles, skeletal muscle, smooth muscle, tissues within the kidneys, and red blood cells.
The endoderm consists, at first, of flattened cells, which subsequently become columnar. It forms the epithelial lining of the whole of the digestive tube (except part of the mouth and pharynx) and the terminal part of the rectum (which is lined by involutions of the ectoderm). It also forms the lining cells of all the glands which open into the digestive tube, including those of the liver and pancreas the epithelium of the auditory tube and tympanic cavity the trachea, bronchi, and air cells of the lungs the urinary bladder and part of the urethra and the follicle lining of the thyroid gland and thymus. Additionally, the endoderm forms internal organs including the stomach, the colon, the liver, the pancreas, the urinary bladder, the epithelial parts of trachea, the lungs, the pharynx, the thyroid, the parathyroid, and the intestines.
Anatomy of the Colon
The colon is not labeled very creatively—most of the labels for the colon correspond to their anatomical location and flow of stool. Your large intestine is broken down into six sections including the cecum, ascending colon, transverse colon, descending colon, sigmoid colon, and the rectum. The colon begins at the end of the small intestine, where it is called the cecum, and ends at the rectum. Cancers of the large intestine are casually referred to as colon, rectal or colorectal cancer.
The cecum is anatomically located in the lower right sight of your abdomen approximately where your appendix is attached. The cecum is the widest part of your entire colon and is approximately 5 centimeters long, or a third as long as a pen. Between 15 and 20 percent of all colon cancers occur in the cecum.
The ascending colon heads up vertically from the cecum to the transverse colon. The juncture between the cecum and transverse colon is called the right colic flexure, or the hepatic flexure for its proximity to your liver (hepatic system). Anatomically, the ascending colon is about 10 centimeters long and is seated on the right side of your abdomen.
The transverse colon connects your ascending and descending colon, traveling lengthwise across your abdomen. The transverse colon lies close to your stomach, liver, and gallbladder and is approximately 50 centimeters long.
The descending colon begins at the left colic flexure, also known as the splenic flexure for its proximity to the spleen. This portion of your colon lies in the left side of your abdomen, connecting your transverse colon to your sigmoid colon. The descending colon is approximately 10 centimeters long.
The sigmoid colon makes up the last 50 centimeters of the colon leading to the rectum and typically has an 'S' curve or shape to it. Roughly 20 to 25 percent of all colon cancers originate in the distal colon, which includes the descending and sigmoid colon
The rectum is the final portion of your large intestine leading to the anus. The digestive process has completely finished by the time stool reaches the rectum, where it waits to be passed as a bowel movement. About 25 to 30 percent of cancers originate in this 15-centimeter piece of the large intestine.
A tortuous colon is one that is longer than normal. In this relatively rare condition, in order for this longer tube to fit in your abdomen, the colon ends up with extra twists and turns.
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Anatomy of Earthworm (With Diagram) | Zoology
The body of Pheretima is nearly circular in cross-section and varies from 7 to 8 inches (18-19 cms) in length. The general colour of the body is brown but the dorsal surface is darker. A dark line extends from end to end in the mid-dorsal line.
The anterior end of the body is pointed and the posterior end is blunt. The animal is elon­gated and divided into a series of ring-like segments or metameres or somites, which are separated from one another by narrow trans­verse grooves.
Usually there are 120 true somites. A fleshy lobe, the prostcmium, projects over the mouth in front of the first seg­ment. The skin of segments, fourteenth, fifteenth and sixteenth, is swollen and pale in mature worms to form a saddle-shaped structure called clitellum or cingulum it secretes the material for produc­ing cocoons.
Every segment of the body, excepting the clitellar, and the first and last, bears numerous chitinous bristles called setae these are implanted on the body wall and are arranged in the form of rings. The setae serve as hold-fasts when the worm is moving over the ground or resting in its burrow.
The entire body is covered by a thin transparent cuticle which is secreted by the epidermis. The cuticle is porous and protects the body from injury.
A number of external openings are found in the body to carry on different functions:
(1) The mouth is a small crescentic aperture situated at the anterior end of the first segment just beneath the prostomium. It is meant for ingestion of food.
(2) The oval anal aperture for the exit of faeces lies at the posterior end in the last segment.
(3) The paired male gonopores are situated ventrally on the eighteenth segment, one on each side of the mid-ventral line. These are the openings of the sperm-ducts.
(4) The genital papillae are two pairs of cup-like depressions on the ventral surface of the seventeenth and nineteenth segments. The two papillae of a side, are placed one in front of and the other behind the corresponding male gonopore.
(5) The single female gonopore is a median aperture on the ventral side of the four­teenth segment it serves as an exit for the eggs.
(6) Four pairs of spermathecal apertures are situ­ated in the inter-segmental grooves between Vth and VIth, Vlth and VIIth, Vllth and VIIIth, and Vlllth and IXth somites. Each of these leads into a pouch called spermatheca which acts as the receptacle of sperms received from another worm at the time of mating.
(7) The body cavity or coelom communicates with the exterior through dorsal pores. These occur in the inter-segmental grooves along the mid-dorsal line. The first dorsal pore is in the groove between the XIIth and XIIIth segment, and there is one in each subsequent inter-segmental groove, excepting the last.
(8) The nephridiopores or the apertures of the excretory organs are minute openings found scattered all over the body, accepting the first three segments and the last.
Internal Anatomy of Earthworm:
If a worm is cut open from the anterior to the posterior end by an incision through the body wall in the mid-dorsal line, the internal structures may easily be studied. It is to be noted that the body of the earthworm is essentially a double tube. The body wall is the outer tube and the alimentary canal is the inner tube. The two tubes are separated by an extensive space, the body cavity or coelom.
External segmentation of the body corresponds to internal partition of the coelom into compart­ments by a number of septa which lie beneath the inter-segmental grooves. Thus, the body cavity is divided into 100 or more com­partments corresponding to the number of external somites.
Each compartment is lined internally by epithelium and is filled with a milky fluid which flows from compartment to compartment, because each septum is perforated by a large opening.
The alimentary canal remains suspended in the coelom and is held in position by the septa, which appear to arise from its wall. As the first septum lies between the IVth and Vth segments the coelom anterior to this region is a continuous cavity.
The milky coelomic fluid may spurt out through the dorsal pores when the worm is irritated. It contains large granular phago­cytes, small yellow cells, and a few circular nucleated cells of inter­mediate size. It keeps the skin of the worm moist, and helps in the excretion of waste products. The phagocytes devour bacteria which are injurious to the earthworm, and thus protect it.
Running along the dorsal surface of the alimentary canal is the dorsal blood vessels, beneath which is the supra-intestinal duct. There is a ventral blood vessel ventral to the gut and overlying the ventral nerve cord. Beneath the ventral nerve cord is a small sub-neural vessel.
The reproductive organs are suspended in the coelom and extend from the Vlth to the XlXth segments. The body wall and the gut wall are best seen in a cross-section passing through the middle of the worm (Fig. 71).
The body wall is composed of the following layers:
(i) The outermost layer is the thin and transparent cuticle which is a porous non-cellular membrane covering the epidermis. The epidermis secretes and replaces the cuticle from time to time.
(ii) The epidermis is composed of a single row of epithelial cells. Some of these are modified into gland cells secreting mucus which is poured out through the cuticle and cleanses the body surface. A few epidermal cells are modified as receptors and act as sense organs. Others are merely supporting cells. These are sacs in the epidermis for implantation of the setae, and minute muscle fibres for moving them.
(iii) Beneath the epidermis, there is the musculature of the body which consists of an outer circular layer and an inner longi­tudinal layer. The former produces a continuous sheath running round the body and the latter runs in parallel bundles along the length of the worm. Pigment cells and blood capillaries are scattered among the muscle fibres.
(iv) A thin layer of coelomic epithelium, consisting of a single row of flat cells, lies just beneath the longitudinal muscles. It forms the innermost lining of the body wall. The body wall is not only protective but it also carries on res­piration through its moist outer surface. The musculature along with the setae are responsible for locomotion.
The contraction of the circular muscles elongates, whereas contraction of the longi­tudinal muscles shortens the length of the body. The two kinds of muscles are brought into play alternately. The setae are used for fixation on the substratum. The skin contains the receptor organs and therefore serves as a sensory membrane.
The got wall is composed of the following layers:
(I) An outer serous coat formed by a layer of tall narrow cells. They are seen to be loaded with minute yellowish granules, specially in the intestinal region of the gut, and are therefore called chloragogen cells or yellow cells. They collect waste products, and when fully loaded, drop down into the coelom and are discharged through the dorsal pores, thus helping in excretion.
(II) Beneath the serous coat are two layers of muscles—the outer one being longitudinal and the inner one circular in disposition. The muscula­ture is poorly developed and consists of non-striped fibres. In the region of the oesophagus and the gizzard the musculature is well-formed.
(Ill) The internal lining of the gut is a single layer of epithelial cells some of which are glandular and others are absorptive. The glandular cells secrete digestive juice.
Sensory receptors are classified into five categories: mechanoreceptors, thermoreceptors, proprioceptors, pain receptors, and chemoreceptors. These categories are based on the nature of stimuli each receptor class transduces. What is commonly referred to as “touch” involves more than one kind of stimulus and more than one kind of receptor. Mechanoreceptors in the skin are described as encapsulated (that is, surrounded by a capsule) or unencapsulated (a group that includes free nerve endings). A free nerve ending, as its name implies, is an unencapsulated dendrite of a sensory neuron. Free nerve endings are the most common nerve endings in skin, and they extend into the middle of the epidermis. Free nerve endings are sensitive to painful stimuli, to hot and cold, and to light touch. They are slow to adjust to a stimulus and so are less sensitive to abrupt changes in stimulation.
Figure 2. Four of the primary mechanoreceptors in human skin are shown. Merkel’s disks, which are unencapsulated, respond to light touch. Meissner’s corpuscles, Ruffini endings, Pacinian corpuscles, and Krause end bulbs are all encapsulated. Meissner’s corpuscles respond to touch and low-frequency vibration. Ruffini endings detect stretch, deformation within joints, and warmth. Pacinian corpuscles detect transient pressure and high-frequency vibration. Krause end bulbs detect cold.
There are three classes of mechanoreceptors: tactile, proprioceptors, and baroreceptors. Mechanoreceptors sense stimuli due to physical deformation of their plasma membranes. They contain mechanically gated ion channels whose gates open or close in response to pressure, touch, stretching, and sound.” There are four primary tactile mechanoreceptors in human skin: Merkel’s disks, Meissner’s corpuscles, Ruffini endings, and Pacinian corpuscle two are located toward the surface of the skin and two are located deeper. A fifth type of mechanoreceptor, Krause end bulbs, are found only in specialized regions. Merkel’s disks (shown in Figure 2) are found in the upper layers of skin near the base of the epidermis, both in skin that has hair and on glabrous skin, that is, the hairless skin found on the palms and fingers, the soles of the feet, and the lips of humans and other primates. Merkel’s disks are densely distributed in the fingertips and lips. They are slow-adapting, unencapsulated nerve endings, and they respond to light touch. Light touch, also known as discriminative touch, is a light pressure that allows the location of a stimulus to be pinpointed. The receptive fields of Merkel’s disks are small with well-defined borders. That makes them finely sensitive to edges and they come into use in tasks such as typing on a keyboard.
Which of the following statements about mechanoreceptors is false?
- Pacini corpuscles are found in both glabrous and hairy skin.
- Merkel’s disks are abundant on the fingertips and lips.
- Ruffini endings are encapsulated mechanoreceptors.
- Meissner’s corpuscles extend into the lower dermis.
Muscle spindles are stretch receptors that detect the amount of stretch, or lengthening of muscles. Related to these are Golgi tendon organs, which are tension receptors that detect the force of muscle contraction. Proprioceptive and kinesthetic signals come from limbs. Unconscious proprioceptive signals run from the spinal cord to the cerebellum, the brain region that coordinates muscle contraction, rather than to the thalamus, like most other sensory information.
Barorecptors detect pressure changes in an organ. They are found in the walls of the carotid artery and the aorta where they monitor blood pressure, and in the lungs where they detect the degree of lung expansion. Stretch receptors are found at various sites in the digestive and urinary systems.
In addition to these two types of deeper receptors, there are also rapidly adapting hair receptors, which are found on nerve endings that wrap around the base of hair follicles. There are a few types of hair receptors that detect slow and rapid hair movement, and they differ in their sensitivity to movement. Some hair receptors also detect skin deflection, and certain rapidly adapting hair receptors allow detection of stimuli that have not yet touched the skin.
Histology of the Small Intestine
The small intestine wall has four layers: the outermost serosa, muscularis, submucosa, and innermost mucosa.
Describe the histology of the small intestine
- The outermost layer of the intestine, the serosa, is a smooth membrane consisting of a thin layer of cells that secrete serous fluid, and a thin layer of connective tissue.
- The muscularis is a region of muscle adjacent to the submucosa membrane. It is responsible for gut movement (also called peristalsis ). It usually has two distinct layers of smooth muscle: circular and longitudinal.
- The submucosa is the layer of dense irregular connective tissue or loose connective tissue that supports the mucosa it also joins the mucosa to the bulk of underlying smooth muscle.
- The mucosa is the innermost tissue layer of the small intestines and is a mucous membrane that secretes digestive enzymes and hormones. The intestinal villi are part of the mucosa.
- The three sections of the small intestine look similar to each other at a microscopic level, but there are some important differences. The jejunum and ileum do not have Brunner’s glands in the submucosa, while the ileum has Peyer’s patches in the mucosa, but the duodenum and jejunum do not.
- Brunner’s glands: Compound, tubular, submucosal glands found in that portion of the duodenum that is above the hepatopancreatic sphincter (sphincter of Oddi).
- Peyer’s patches: Patches of lymphoid tissue or lymphoid nodules on the walls of the ileum in the small intestine.
- intestinal wall: The wall of the small intestine is composed of four layers, from the outside to the inside: serosa, muscularis, submucosa, and mucosa.
The Small Intestine’s Layers
Section of duodenum: This image shows the layers of the duodenum: the serosa, muscularis, submucosa, and mucosa.
The small intestine has four tissue layers:
- The serosa is the outermost layer of the intestine. The serosa is a smooth membrane consisting of a thin layer of cells that secrete serous fluid, and a thin layer of connective tissue. Serous fluid is a lubricating fluid that reduces friction from the movement of the muscularis.
- The muscularis is a region of muscle adjacent to the submucosa membrane. It is responsible for gut movement, or peristalsis. It usually has two distinct layers of smooth muscle: circular and longitudinal.
- The submucosa is the layer of dense, irregular connective tissue or loose connective tissue that supports the mucosa, as well as joins the mucosa to the bulk of underlying smooth muscle.
- The mucosa is the innermost tissue layer of the small intestines, and is a mucous membrane that secretes digestive enzymes and hormones. The intestinal villi are part of the mucosa.
The three sections of the small intestine look similar to each other at a microscopic level, but there are some important differences. The jejunum and ileum do not have Brunner’s glands in the submucosa, while the ileum has Peyer’s patches in the mucosa, but the duodenum and jejunum do not.
Brunner’s glands (or duodenal glands) are compound tubular submucosal glands found in the duodenum. The main function of these glands is to produce a mucus-rich, alkaline secretion (containing bicarbonate) in order to neutralize the acidic content of chyme that is introduced into the duodenum from the stomach, and to provide an alkaline condition for optimal intestinal enzyme activity, thus enabling absorption to take place and lubricate the intestinal walls.
Peyer’s patches are organized lymph nodules. They are aggregations of lymphoid tissue that are found in the lowest portion of the small intestine, which differentiate the ileum from the duodenum and jejunum.
Because the lumen of the gastrointestinal tract is exposed to the external environment, much of it is populated with potentially pathogenic microorganisms. Peyer’s patches function as the immune surveillance system of the intestinal lumen and facilitate the generation of the immune response within the mucosa.
Micrograph of the small intestine: A low-magnification micrograph of small intestinal mucosa that shows villi.
Intestinal villi (singular: villus) are tiny, finger-like projections that protrude from the epithelial lining of the mucosa. Each villus is approximately 0.5–1.6 mm in length and has many microvilli (singular: microvillus), each of which are much smaller than a single villus.
Villi increase the internal surface area of the intestinal walls. This increased surface area allows for more intestinal wall area to be available for absorption. An increased absorptive area is useful because digested nutrients (including sugars and amino acids) pass into the villi, which is semi-permeable, through diffusion, which is effective only at short distances.
In other words, the increased surface area (in contact with the fluid in the lumen) decreases the average distance traveled by the nutrient molecules, so the effectiveness of diffusion increases.
The villi are connected to blood vessels that carry the nutrients away in the circulating blood.
The epidermis acts as a barrier that protects the body from ultraviolet (UV) radiation, harmful chemicals, and pathogens such as bacteria, viruses, and fungi.
Historically, it was thought that the function of the epidermis was to regulate fluid and protect the body from mechanical injury. In recent years, we've come to understand that it is a complex system that plays a key role in how the immune system communicates and target defense.
Within the epidermis are several distinct layers, consisting of (from bottom to top):
- Stratum basale, also known as the basal cell layer, is the innermost layer of the epidermis. This layer contains column-shaped basal cells that are constantly dividing and being pushed toward the surface. The stratum basale is also home to melanocytes that produce melanin (the pigment responsible for skin color). When exposed to the sunlight, melanocytes produce more melanin to better protect the skin from UV exposure. Abnormalities in the development of these cells can lead to melanoma, the most deadly type of skin cancer.
- Stratum spinosum also referred to as the squamous cell layer, is the thickest layer of the epidermis located just above the basal layer. These are composed of basal cells that have matured into squamous cells, known as keratinocytes. Keratinocytes are responsible for producing keratin, a protective protein that makes up skin, nails, and hair. The squamous layer is also home to Langerhans cells which attach themselves to foreign substances as they infiltrate the skin. It is also responsible for synthesizing cytokines, a type of protein that helps regulate the immune response.
- Stratum granulosum is made up of keratinocytes that have moved up from the squamous layer. As these cells move closer toward the skin's surface, they begin to flatten and stick together, eventually drying and dying out.
- Stratum corneum is the outermost layer of the epidermis. It consists of 10 to 30 layers of dead keratinocytes that are constantly being shed. Shedding of these cells slows significantly with age. The complete cell turnover, from basal cell to stratum corneum, takes around four to six weeks for young adults and about a month and a half for older adults.
- Stratum lucidum only exists on the palms of the hands and soles of the feet. It consists of four layers rather than the typical four.
The innermost layer of the skin is the hypodermis or subcutis. Composed of fat and loose connective tissue, this layer of the skin insulates the body and cushions and protects internal organs and bones from injury. The hypodermis also connects the skin to underlying tissues through collagen, elastin, and reticular fibers that extend from the dermis.
A major component of the hypodermis is a type of specialized connective tissue called adipose tissue that stores excess energy as fat. Adipose tissue consists primarily of cells called adipocytes that are capable of storing fat droplets. Adipocytes swell when fat is being stored and shrink when fat is being used. The storage of fat helps insulate the body and the burning of fat helps generate heat. Areas of the body in which the hypodermis is thick include the buttocks, palms, and soles of the feet.
Other components of the hypodermis include blood vessels, lymph vessels, nerves, hair follicles, and white blood cells known as mast cells. Mast cells protect the body against pathogens, heal wounds, and aid in blood vessel formation.
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