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Hydra immortality

Hydra immortality


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Recent research on immortality ha been on an organism named Hydra. I have heard about it being related to some FoxO gene, which has been found in several other organisms but how does telomerase affect the immortality of this amazing species?


The Immortal life of a Hydra

The Hydra, a small freshwater invertebrate, is an advantageous model organism for regenerative biologists. Named after the serpent from Greek mythology that grew two new heads for each one cut off, this tiny, jellyfish-like creature holds within its genomic code the key to biological immortality.

Hydra are unique because their stem cells exist in a continuous state of renewal. Kept safe and in isolation, these organisms show no signs of aging. Outside the lab, the only real threats they face are predators, weather extremes and disease.

Ever resilient, Hydra can survive dismemberment by regenerating lost sections of their bodies. Chop a Hydra into segments, and each segment will become a new Hydra. Blend one up, and you’re left with a soup of cells. If you ball up those cells using a centrifuge, they reorganize, eventually forming a new Hydra.

Assistant Professor Celina Juliano, Department of Molecular and Cellular Biology at the UC Davis College of Biological Sciences, and her colleagues are probing Hydra stem cells, hoping to find clues to the organism’s regenerative capabilities and longevity.

The eternal Embryo

Stem cells form the foundational building blocks of an organism’s body. During embryonic growth and development, stem cells differentiate, becoming various cells throughout the body. In many organisms, stem cells lose their ability to replace dysfunctional cells with age.

Hydra stem cells are different.

“If the animal has very powerful stem cells—because stem cells are the cells that give rise to all of your tissues—then you can regenerate very well,” said Juliano. “They sometimes call Hydra the eternal embryo.”

Continuous stem cell renewal is a major focus of Juliano’s research. However, this doesn’t fully explain Hydra longevity. To uncover this mystery, Juliano is interested in how Hydra maintains its genome despite undergoing a large number of cell divisions.

“The Hydra renews itself all the time, but just renewing yourself isn’t enough,” said Juliano. “The genome itself has to be faithfully propagated.”

Why don't Hydra age?

Juliano and her colleagues in the Juliano Lab are searching for the genetic elements that help prevent biological aging in Hydra. They’re particularly interested in transposons, which are also called “jumping genes.”

In organisms that age, such as humans, transposon expression is kept in check by a variety of genetic pathways during youth. As time plods on, aging cells lose control over transposon repression. Uncontrolled, these jumping genes bounce around the genome, replicating, proliferating and inserting themselves into the genome like unhinged saboteurs.

Some believe transposon expression plays a role in aging, but why the human body’s cells eventually lose control over this pathway is still a mystery.

Hydra cells never lose the ability to repress transposons.

“I wouldn’t say it’s the magic bullet of why Hydra doesn’t age,” said Juliano. “But this is one thing we’re interested in, how transposons are repressed in adult stem cells.”

A place for the North American Hydra Community

Though the Hydra is a miraculous animal, it is a much more popular model organism in Europe than in the United States and Australia.

“Funding is tight in the United States and grants are more likely to go to more established model organisms, thus making it difficult to work on Hydra,” said Juliano.

To vitalize and grow the North American Hydra community, Juliano and some colleagues organized Hydroidfest 2016, held at the UC Davis Bodega Marine Laboratory. Roughly 60 researchers from seven countries attended the meeting, which was funded by a grant from the National Science Foundation.

“It was small, but you had this feeling that it was the start of something special,” said Juliano. “We had people come to the meeting who weren’t Hydra biologists, and they were interested in starting projects in their labs.”

Along with Christine Schnitzler, an assistant professor of biology at the University of Florida, Juliano is organizing an expanded meeting, dubbed Cnidofest 2018: The Cnidarian Model Systems Meeting, from the phylum Cnidaria (which includes Hydra, corals, sea anemones and jellyfish). It will take place at the University of Florida’s Whitney Laboratory for Marine Biosciences in September 2018.

This article was first published by UC Davis College of Biological Sciences.


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HEXAPOLIS

Copyright: Daniel Stoupin

Posted By: Dattatreya Mandal January 9, 2018

Most of us would know about the Greek mythical beast Hydra, the serpentine water monster that could supposedly regenerate two heads in case one of its original ones were chopped off. However, the namesake real-life animal – the Hydra magnipapillata, or simply freshwater polyp, probably does one better than its mythical counterpart. That is because, unlike most multicellular organisms, the species hydra does not seem to show biological signs of cell deterioration with age.

In terms of its physical attribute, the hydra is an invertebrate that has cylindrical (tubular) anatomy with tentacles, and its entire body only grows to 10 mm. And in spite of such a ‘short stature’, the organism is pretty capable of preying on smaller aquatic animals. But its apparent signs of immortality come from the fascinating arrangement of stem cells that allows regenerative capacities. The latter scope is covered by the stem cells’ inherent ability to divide and morph into any cell type required by the body. Simply put, the hydra can revitalize itself with a fresh supply of cells.

In other words, the hydra, unlike other multicellular organisms, does not seem to abide by the biological phenomenon of senescence – loss of a cell’s power of division and growth thus causing deterioration and infertility by age. To that end, in a research conducted in 2015 (following an earlier research in 1998), the scientists created tiny artificial ‘paradises’ for around 2,256 living hydra specimens. This basically entailed the assigning of an entire Petri dish to each individual animal, complemented by fresh water supply thrice a week along with a nutritious and healthy diet of fresh brine shrimp.

The incredible results, according to Stephanie Pappas, writer for LiveScience, were as follows –

Over eight years, the researchers found no evidence of senescence in their coddled hydra. Death rates held constant at one per 167 hydras per year, no matter their age. (The “oldest” animals studied were clones of hydras that had been around for 41 years — though individuals were only studied for eight years, some were biologically older because they were genetic clones.) Likewise, fertility remained constant for 80 percent of the individual hydras over time. The other 20 percent fluctuated up and down, likely because of laboratory conditions.

Green hydra (Hydra viridissima). Copyright: Jan Hamrsky

Now it should be noted that this doesn’t mean that the hydra can’t be killed. In fact, the hydra can die as prey of bigger aquatic creature or even in the case of water contamination. But as we mentioned before, the hydra seems to nigh defeat the intrinsic multicellular process of senescence (or cell deterioration). As study researcher Daniel Martinez, a Pomona College biologist, said (back in 2015) –

I do believe that an individual hydra can live forever under the right circumstances. The chances of that happening are low because hydra are exposed to the normal dangers of the wild — predation, contamination, diseases. I started my original experiment wanting to prove that hydra could not have escaped aging. My own data has proven me wrong — twice.

Many, many hours of work went into this experiment. I’m hoping this work helps sparks another scientist to take a deeper look at immortality, perhaps in some other organism that helps bring more light to the mysteries of aging.

The study was originally published the journal Proceedings of the National Academy of Sciences, on Dec 7, 2015.


Hydra's immortality gene sheds light on human aging

The tiny freshwater polyp Hydra is a remarkable creature. It does not show any signs of aging and appears to be immortal. Researchers from Kiel University have examined this phenomenon and uncovered an important link to the aging process in humans that could lead to the development of advanced rejuvenation therapies.

How does the polyp Hydra do this? It accomplishes the feat of apparent immortality by reproducing through budding rather than mating. Each polyp contains stem cells capable of continuous proliferation. Without this endless supply of regenerating stem cells, the animals could not reproduce.

Geneticists at Kiel University, together with the University Medical Center Schleswig-Holstein, discovered that the same longevity gene that makes the hydra immortal may also explain why humans get older, and more infirm.

"Surprisingly, our search for the gene that causes Hydra to be immortal led us to the so-called FoxO gene," says Anna-Marei Böhm, PhD student and first author of the study.

All animals and humans have a FoxO gene. Until now, no one has been able to work out if FoxO plays a role in aging and why human stem cells become fewer and inactive with increasing age. The growing inactivity of stem cells as we age is critical. Because our stem cells lose the ability to proliferate and form new cells, aging tissue cannot regenerate any more. As a result, our muscles decline.

The Kiel researchers examined FoxO in several genetically modified polyps: Hydra with normal FoxO, with inactive FoxO and with enhanced FoxO. The scientists found that animals without FoxO possess significantly fewer stem cells.

“Our research group demonstrated for the first time that there is a direct link between the FoxO gene and aging“, says Thomas Bosch from the Zoological Institute of Kiel University, who led the Hydra study. “FoxO has been found to be particularly active in centenarians – people older than one hundred years – which is why we believe that FoxO plays a key role in aging – not only in Hydra but also in humans.”

The study has produced two conclusions. First, the FoxO gene plays a key role in the maintenance of stem cells and thus determines the life span of all animals. Secondly, the aging and longevity of organisms depends on two factors: the maintenance of stem cells and the maintenance of a functioning immune system.

The hypothesis can’t be verified yet on human beings as that would require genetic manipulation. Nonetheless, the research is a big step forward and more studies on the Hydra and the FoxO gene are planned which could lay the foundations for the development of advanced rejuvenation therapy for humans in the future.


Hydra: History, Habitat and Locomotion (With Diagram)

In this article we will discuss about Hydra:- 1. History of Hydra 2. Habit, Habitat and Culture of Hydra 3. Structure 4. Locomotion 5. Nutrition 6. Respiration, Excretion and Osmoregulation 7. Nervous System 8. Behaviour 9. Reproduction 10. Regeneration 11. Immortality 12. Symbiosis 13. Physiological Division of Labour.

  1. History of Hydra
  2. Habit, Habitat and Culture of Hydra
  3. Structure of Hydra
  4. Locomotion of Hydra
  5. Nutrition of Hydra
  6. Respiration, Excretion and Osmoregulation of Hydra
  7. Nervous System of Hydra
  8. Behaviour of Hydra
  9. Reproduction in Hydra
  10. Regeneration of Hydra
  11. Immortality of Hydra
  12. Symbiosis in Hydra
  13. Physiological Division of Labour in Hydra

1. History of Hydra:

Hydra, a small freshwater commonest polyp, is readily obtainable coelenterate. It serves a good example from the coelenterates to illustrate the fundamental characteristics of Metazoa. Leeuwenhoek (1702), first described it to the Royal Society of London but Trembley (1744) recognised its animal nature. Reaumur called it a polyp, while Linnaeus gave the name Hydra because of its special power of regeneration.

Actually Hydre was a nine-headed dragon serpentine of the Greek mythology. When one of its head was cut off, two new ones immediately appeared in its place. Thus, the name Hydra was given to this animal because of its special power to regenerate its lost part like Hydre.

Hydra is represented by several species in the different parts of the world. Some of the common species are H. vulgaris, which is orange-red coloured found in the freshwaters of America and Europe. H. fusca or H. oligactis which is now known as Pelmatohydra is the brown hydra reported chiefly from Punjab in India, North America and Europe.

H. viridis now known as Chlorohydra viridissima is the green hydra of America and Europe. Its green colour is due to the presence of a symbiotic green algae, Zoochlorellae in its endodermal cells. H. gangetica is found in the pond and other water reservoirs along the river Ganges.

2. Habit, Habitat and Culture of Hydra:

Hydra is found in freshwater ponds, pools, lakes, streams and ditches.

It usually remains attached to submerged vegetation or with any solid object. When it is undisturbed, its body remains extended with tentacles spread out and shows expansions and contraction without any apparent reason. It is carnivorous in habit and feeds on small insects, insect larvae and small crustaceans. It lives singly, i.e., solitary in habit. It reproduces sexually as well as asexually.

Collection and Culture of Hydra:

Hydra can be collected from freshwater lakes, ponds, etc., usually during winter months. If we collect ajar of water with Hydrilla plants and put it undisturbed for a day or two we may notice a number of Hydra attached either with the wall of glass jar or with the leaves of Hydrilla plant.

These may be examined under microscope by putting them on a glass slide with the help of dropper. For making its culture in laboratory, the same may be transferred in aquarium and a sufficient amount of food must be supplied daily. Its food generally consists of Daphnia readily available in stagnant water. By budding, their number increases very soon in the aquarium.

3. Structure of Hydra:

External Structure of Hydra:

Hydra is a polypoid coelenterate with a cylindrical body. It is easily visible to the naked eyes and when fully extended, it becomes elongated and slender. It measures from 2 to 20 mm in length. This variation in the length is due to its remarkable power of contraction and expansion.

Hydra appears tubular. It is sessile but its proximal or aboral end is drawn out into a slender stalk at the end of which is the basal disc or pedal disc for attachment to the substratum.

The pedal disc region of the body is provided with gland cells which secrete adhesive substance for attachment to the substratum and also a gas bubble fox floating. The free distal end or oral end of the body bears a conical elevation called hypostome.

The hypostome bears an aperture at its apex called mouth which opens into the gastro vascular cavity or enteron. The hypostome is encircled by a circlet of 6-10 tentacles (L., tentare = to feel). The tentacles are hollow their cavity is communicated to the gastro vascular cavity, slender, thread-like processes having nematocysts.

The tentacles can be greatly extended at the time of feeding or locomotion. At the proximal end of the body, it may bear lateral projections called buds in various stages of development. A well developed bud bears its own mouth, hypostome and tentacles.

When the fully formed buds are detached from the parent body, they give rise to new individuals. Gonads may also be present on its body. The testes occur near the oral end which are conical projections, while ovaries are situated towards the proximal end and these are oval projections.

Internal Structure of Hydra:

The internal structure of Hydra can be well explained with its longitudinal and transverse sections. However, the internal structure reveals the presence of body wall and a central cavity extended into the tentacles also called the coelenteron (Gr., koilos = hollow enteron = gut) or gastro vascular cavity or enteron.

4. Locomotion of Hydra:

Normally, a Hydra remains attached by the basal disc to some suitable object in the water. There it twists about and makes various movements of the tentacles and body in response to various stimuli and for the capture of food. All such movements are caused by the contraction or expansion of the contractile muscle fibres of the muscle processes of both epidermis and gastro dermis.

Actual locomotion is accomplished in several different ways which are as follows:

(i) Looping:

The most common, a type of walking (Fig. 31.15) similar to the looping of an inchworm or caterpillar. While standing erect, the body first extends and then bends and fixes the tentacles to the substratum by means of glutinant nematocysts. It then releases the attachment of the basal disc, reattaches the basal disc near the tentacles and again assuming an upright position by releasing its tentacles.

(ii) Somersaulting:

Somersaulting (Fig. 31.16) is like the looping. In this type of movement, Hydra extends its body and is bent to one side to place the tentacles on the substratum, the glutinant nematocysts help to fix the tentacles. The basal disc is freed from its attachment, and the animal stands on its tentacles, the body then contracts strongly till it appears like a small knob.

The body is then extended and bent to place the basal disc on the substratum, the tentacles loosen their hold and the animal regains an upright position. These movements are repeated and the Hydra moves from place to place. This is the normal method of locomotion.

(iii) Gliding:

Hydra can glide slowly along its attachment by alternate contraction and expansion of basal disc.

(iv) Cuttlefish-like movement:

The tentacles are fixed to the substratum and with the pedal disc up, Hydra moves over the substratum by pulling its tentacles along (Fig. 31.17).

(v) Floating:

Sometimes, Hydra can produce a bubble of gas secreted by some ectodermal cells of the basal disc which helps the animal to float on the surface of the water and is passively carried from one place to another by water current or wind below (Fig. 31.18).

(vi) Climbing:

Hydra can climb by attaching its tentacles to some distant objects and then releasing the basal disc and by contracting the tentacles the body is drawn up to a new position (Fig. 31.19).

(vii) Swimming:

By freeing itself from the substratum and with the help of wave-like movements of the tentacles, Hydra swims in water.

5. Nutrition of Hydra:

(i) Food and its Ingestion:

The food consists of small crustaceans like Cyclops, small annelids and insect larvae, thus, it is carnivorous. On touching a tentacle by the prey, the stenoteles penetrate it and inject a poisonous toxin to paralyze it, the volvents coil around the bristles to hold the food.

The tentacle holding the captured animal contracts and bends over the mouth, the other tentacles also bend and help to transfer the food into the mouth where it is engulfed by movements of the mouth and hypostome peristaltic contractions of the body wall force it into the enteron.

Hydra will normally swallow only living prey. It has been shown that it will engulf only those animals which contain a chemical called glutathione which is present in tissue fluids of most animals, and it is released when the body is punctured by stenoteles this shows that glutathione is necessary to evoke the feeding reaction.

(ii) Digestion of Hydra:

The mucous gland cells of the hypostome cover the engulfed food with mucus, then enzymatic gland cells produce a proteolytic enzyme like trypsin which partly digests the proteins into polypeptides in an alkaline medium in the enteron, this digestion is extracellular.

Some endoderm cells form pseudopodia and engulf the smaller partly digested particles into food vacuoles. The contents of the food vacuoles are first acidic, then they become alkaline, the remaining digestion is completed in the vacuoles, and it is called intracellular digestion.

Thus, Hydra combines the intracellular digestion of Protozoa and extracellular digestion of higher animals. Some endoderm cells after taking in food into food vacuoles separate from the body wall and wander about in the enteron to the parts where digested food is needed.

Digested food is assimilated to endoderm cells and transferred to ectoderm or into the enteron from where it is distributed to all parts thus, the enteron cavity serves a double function of digestion and circulation. Hydra can digest proteins, fats and some carbohydrates, but it does not digest starch. Some digested food forms oil globules which are stored in the ectoderm.

(iii) Egestion:

The indigestible materials, like exoskeleton of crustaceans, are ejected through the mouth on contraction of the body. The mouth, thus, functions as anus also.

6. Respiration, Excretion and Osmoregulation of Hydra:

There are no special organs for respiration and excretion. Gaseous exchange occurs through the general body surface. Nitrogenous wastes are largely in the form of ammonia, which also diffuses through the general body surface. It is also thought that the gastro dermis of basal disc is said to accumulate some excretory matter, which may be discharged through a pore.

Osmoregulation of Hydra:

The water that continuously enters into the body cells by endosmosis is finally collected into the gastro vascular cavity and from here expelled out through mouth due to a wave of muscle contraction passing from the basal disc region to the hypostomal region.

7. Nervous System of Hydra:

There are many nerve cells, each with two to four branching nerve fibres. The nerve fibres are primitive because they do not form axons or dendrites, moreover the nerve fibres form actual contacts with fibres of other nerve cells. Recent studies have shown that there are no synapses, thus, they form a continuous nerve net (Fig. 31.21).

In Hydra, there are two nerve nets, one in connection with the ectoderm which is more highly developed, and the other near the endoderm, the two nerve nets lie in and on either side of the mesogloea. But the ectodermal nerve net is more strongly developed and is particularly concentrated around the mouth and basal disc regions.

The two nerve nets are joined to each other and to the sensory cells of both ectoderm and endoderm, they are also joined to the epitheliomuscular cell. The fibres of both nerve nets are continuous and there are no synapses. Sensory cells are receptors for touch, light and chemicals, and stimuli pass from them through the nerve nets to muscle processes which act as effectors.

This is a diffused nervous system which works as a receptor → conductor → effector system. The nerve cells form conducting chains between receptors and effectors. The messages radiate in all directions from the point of stimulation but there is no coordination because the messages do not evoke responses equally in all the effectors.

8. Behaviour of Hydra:

The movements of Hydra connected with feeding are automatic, they are governed by the external environment. It responds to contact, if a tentacle is touched then the other tentacles and even the body may contract this shows that there is a transmission of the stimulus, the stimulation is conducted in all directions by the nerve nets.

The response is greatest near the point of stimulation and it gets progressively less in more distant regions because each nerve net offers some resistance to the passage of impulses, this resistance occurs at the numerous nerve cells. Hydras are found more towards the top of a pond than at great depth, thus, they can obtain more oxygen.

If Hydra is attached near the bottom, then the body is held upright, but at average depth it is horizontal with the hypostome lower than the foot. It also hangs with the head down by its foot from the surface of water with the aid of a gas bubble. It can alter the shape of the body becoming long and slender or small and contracted like a barrel.

Behaviour of Hydra depends on its physiological state, the response of a well-fed Hydra to stimuli is slow and sluggish, but a hungry Hydra will respond vigorously to the same stimuli.

However, Hydra responds to various stimuli in the following way:

(i) Light:

Hydra shows positive response to mild light but avoids or shows negative response to both strong light or very less light. Actually, it becomes restless and moves in a number of directions in darkness.

(ii) Temperature:

Hydra prefers mild temperature which suits best for its life activities, say from 20 to 25° C. Any increase or decrease from these levels in temperature is avoided by Hydra.

(iii) Electricity:

Hydra reacts to weak constant electric currents by bending towards the anode and then contracting the entire body. If attached by the basal disc, the oral end bends towards the anode but if fixed by tentacles, the basal disc bends towards the anode side.

(iv) Chemicals:

Hydra always shows negative response to injurious chemicals but exhibits positive response to food.

9. Reproduction in Hydra:

(i) Asexual Reproduction of Hydra:

Hydra reproduces asexually by budding. In fact this is the usual means of reproduction during the warmer months of the year.

A bud (Fig. 31.22) develops as a simple evagination of the body wall. The ectoderm cells increase in number at one point to form a protuberance below which the endoderm cells acquire reserve food, then both ectoderm and endoderm are pushed out to form a bud which contains a diverticulum of the enteron.

The bud arises at the junction of the stalk and stomach, and several buds may be formed at the same time. At the distal end, the bud grows tentacles one by one and a mouth is formed. The attachment of the bud to the mother Hydra constricts to separate the bud, but endoderm cells at the base unite before this, after constriction ectoderm grows over the foot to cover the endoderm.

The bud grows into a new Hydra which migrates towards the surface of water for dispersal, but it finally gets fixed by its basal disc so that it becomes a solitary individual. Budding occurs during the warmer months when food is plentiful.

(ii) Sexual Reproduction of Hydra:

In Hydra, it starts with the development of temporary structures called gonads during autumn months. Actually, sexual reproduction occurs during the un-favourable conditions like excessive high and low temperatures of the water in which Hydra lives or also due to an increase in the amount of free carbon dioxide in the surrounding water.

Generally, the gonads develop due to the repeated proliferation of the interstitial cells of the epidermis which form bulging’s on the body wall. The bulging’s of gonads differ from the bulging’s of buds as the mesogloea and gastro dermis do not enter into the gonads.

Mostly, the species of Hydra are dioecious, i.e., sexes separate the individuals bear either male or female gonad, e.g., H. oligactis. But some species are monoecious or hermaphrodite also, i.e., both male and female gonads are found on the body of same individual, e.g., H. viridissima. Usually, the testes develop towards the distal part of the body, while ovaries develop towards the proximal part of the body.

In H. oligactis, where sexes are separate, male and female can be marked easily. The males are smaller and bear 1-8 conical testes having a teat-like structure over them. The females are comparatively longer and bear 1-2 oval ovaries.

Testes and Sperma­togenesis:

The interstitial cells of epidermis multiply rapidly to increase in number and finally push out the other cells of epidermis to form swellings on the outer body surface of Hydra. Thus, the structure formed is called testis. The testes are rounded spherical structures in dioecious forms, while they are blunt conical structures in monoecious forms. Now, the interstitial cells start behaving like sperm mother cell or spermatogonia.

These divide to form secondary spermatogonia which develop into spermatocytes. The spermatocytes undergo two maturation divisions, one being reduction division to form spermatids. The spermatids then differentiate to form spermatozoa.

Each spermatozoa being haploid carries 15 chromosomes in H. oligactis and possesses a cylindrical head containing nucleus, a middle piece and a vibrating long tail. Due to the pressure of spermatozoa in testis, its wall ruptures to release spermatozoa in the surrounding water.

The ovary also develops in the same way as testis from the interstitial cells of the epidermis.

The interstitial cells behave like oogonia. Now, one of the oogonia, usually that which is centrally located, becomes larger and amoeboid called oocyte. The other oogonia are used up as nourishment and for forming yolk. The oocyte undergoes two maturation divisions, one of them being reduction division to form a large yolk-laden ovum and two polar bodies.

The ovum being haploid contains 15 chromosomes in H. oligactis. The ovum is a large yolk-laden mass occupying most of the space inside the ovary. The ovum remains surrounded by epidermal cells in the beginning but when it matures the epidermal cells break up and withdraw.

Thus, the ova becomes naked on all sides except where it is attached to the body of. Hydra by an epidermal cup. Each ovary produces a succession of ova but usually one at each time, sometimes there are found two in H. viridissima or more in H. dioecia. The ovum remains attached with the parent body and secretes a protective gelatinous sheath around it.

Cross-fertilisation occurs as a rule in the different species of Hydra. To avoid self-fertilisation, even in monoecious species, the testes mature first, i.e., protandrous condition exists. However, fertilisation takes place when mature spermatozoa released from testes approach randomly to the naked ovum surrounded in gelatinous sheath.

Many sperms may penetrate the gelatinous sheath to reach the ovum but only one of them reaches to the ovum and fuses with it completely to form the zygote which becomes diploid with 30 chromosomes. The process of fertilisation takes place effectively only when the sperm reaches the ovum within its viable condition that usually remains for two hours from its being exposed to naked otherwise it perishes.

The development of zygote starts soon after fertilisation when it still remains attached to the parent body. The zygote undergoes total equal cleavage, i.e., holoblastic to form a hollow ball of cells. The cells are called blastomeres which soon get arranged to form a single layered embryo with a central cavity called blastocoel. The embryo is now known as blastula.

The cells of blastula divide rapidly and some of them delaminate into the blastocoel to completely obliterate it.

Now the embryo is called gastrula which has an outer layer of cells, forming ectoderm and an inner core of cells, forming endoderm. The solid gastrula is neither ciliated nor free swimming because it is still attached to the parent body. This type of gastrula is characteristically called stereo gastrula which represents the planula stage of Hydra.

The outer ectodermal layer of the embryo soon secretes some secretion which hardens to form a protective covering round it called theca. The theca is two-layered being formed of an inner thin membrane and outer thick and chitinous layer. The theca may be smooth as in H. oligactis or spiny, it may be oval or round.

At this stage, the embryo gets detached from the parent body, settles at the bottom and remains dormant till the advent of favourable environmental conditions. After the approach of favourable conditions, the embryo again becomes active and development starts. The endodermal cells get arranged into a layer beneath the ectodermal layer and, thus, a new cavity called coelenteron or gastro vascular cavity appears.

A layer of mesogloea develops between ectoderm and endoderm. These two germ layers, i.e., ectoderm and endoderm give rise to their different derivatives, a circlet of tentacles develop, hypostome and mouth is formed. Thus, a young Hydra is formed.

With the above developments, the embryo increases in size and the theca ruptures to release young Hydra resembling a polyp. It soon elongates and gets fixed by its aboral end and grows into an adult.

10. Regeneration of Hydra:

Regeneration may be defined as the ability of certain animals to restore the lost or worn out parts of their bodies. Hydra has the considerable power of regeneration. Trembley (1744 or 1745) first of all demonstrated that an individual Hydra can be cut into several pieces, and each will regenerate the lost parts, developing a whole new individual.

The parts usually retain their original polarity, with oral ends developing tentacles and aboral ends, basal discs.

Parts of two different individuals, often of different species, may be brought together and grafted together in various arrangements. The germ layers, however, will not mix. The epidermis will only fuse with epidermis and gastro dermis with gastro dermis.

Trembley (1744 or 1745) also demonstrated that if the head end of Hydra is split into two and the parts are slightly separated it results into a Y-shaped Hydra or two-headed individual having two mouths and two sets of tentacles. Each head may be again split in a similar manner. In this way Trembley succeeded in producing a seven-headed Hydra.

Occasionally, even in nature, a Hydra becomes turned inside out. In the laboratory this can be accomplished mechanically or by overdoses of glutathione. Trembley (1744) thought that under these conditions the epidermis becomes gastrodermis and gastrodermis becomes epidermis.

More modern studies, however, demonstrate conclusively that this does not occur. Rather, the Hydra usually turns itself right side again, but if it does not, the layers switch location by migration of cells through the mesogloea.

11. Immortality of Hydra:

Brien (1955) and others have found that a Hydra is at least potentially immortal. There is a growth zone just below the base of tentacles in which interstitial cells give rise to new body cells of all types.

As these new cells are formed, other cells are pushed toward the end of the tentacles or the basal disc where the old cells are shed. In this manner all of the cells are renewed once in about 45 days. So far as is known, this process of cell replacement continues indefinitely. If the interstitial cells of growth zone are destroyed by X-rays, the Hydra lives only a few days.

12. Symbiosis in Hydra:

Symbiosis (Gr., sym = together + bios = living) is an association of two different species of individuals in which both the partners are benefited.

The degree of association in a symbiotic relationship varies from rather loose associations in which the two partners benefit relatively little from each other, to a very intimate association in which the two partners may be regarded as a single organism.The green Hydra, Chlorohydra viridissima exhibits a very good example of symbiosis.

The gastro dermal cells of. C.viridissima are harboured by a large number of unicellular green alga Chlorella referred as Zoochlorella in this case (or Zooxanthelld).

The algae are passed from one generation of Hydra to the next through the eggs. It seems impossible to deprive a Chlorohydra of its Zoochlorella and, thus, it is evident that they are mutually benefited the alga gets shelter and protection and also at the same time obtains carbon dioxide from hydra’s respiration and nitrogenous compounds from its excretory wastes, in return the Hydra obtains oxygen and carbohydrates from alga due to its photosynthetic activity.

In this association, one individual in which other harbours are called host and symbiont respectively.

13. Physiological Division of Labour in Hydra:

H.M. Edward, a French scientist advocated that even the primitive multicellular animals exhibit physiological division of labour like those of higher Metazoa and human society. We know that in human society different set of people like washer man, cobbler, blacksmith, carpenter, potter, farmer, teacher, doctor, engineer, etc., perform different functions for the society they are specialised to do their jobs efficiently.

Likewise, for proper functioning of a multicellular body the different life activities are performed by different cells present in its body. Certain cells become specialised for one function, others for different functions unlike to that of a unicellular body in which all life activities are performed by the single cell.

In lower Metazoa, similar cells performing similar functions form tissue, while in higher Metazoa, similar tissues together constitute an organ and similar organs performing similar functions form systems. All these are specialised to do their jobs efficiently. This is called physiological division of labour where different cell types are specialised structurally and physiologically to perform different functions.

This phenomenon is well illustrated by coelenterates. Hydra, however, exhibits it but still at a primitive level.

We have noted that the ectoderm of Hydra is protective, muscular and sensory,its nematocysts are used for defence and for obtaining food. The ectoderm of basal disc is glandular which helps in fixing the Hydra with the substratum its central part can produce gas bubble which helps in floating. The endoderm is digestive, vascular, muscular and also secretory.

The interstitial cells form gonads and replace both ectodermal and endodermal cells. The enteron carries on digestion and circulation. The mouth serves for ingestion of food and egestion of wastes.

The tentacles are used for obtaining food and for locomotion. All this division of labour is possible because Hydra is beginning to show a differentiation of its parts. Thus, it can be said that the physiological division of labour is correlated with a morphological differentiation of structure.


The Undying Hydra: A Freshwater Mini-Monster That Defies Aging

Could this tiny creature, named after a mythical multiheaded monster, hold the secret to eternal youth? Related to jellyfish and anemones, the hydra has an almost otherworldly ability to heal itself and stave off aging.

Additional Resources

    at UC Davis studies regenerative biology, stem cells and the immortal hydra
  • As part of the Vale Lab at UCSF, Taylor Skokan studies the hydra's ability to reorganize their cells and reassemble their body plan from a disorganized aggregation of cells. studies hydra as a model system to investigate questions in developmental and evolutionary biology

TRANSCRIPT

Everything that lives, must one day die, right?

Well, maybe not everything.

It&rsquos named after a many-headed monster from Greek mythology.

Chop off one head and two grow in its place.

That myth isn&rsquot too far from the real hydra&rsquos death-defying abilities.

They&rsquore cousins to jellyfish &mdash see the resemblance?

But hydra live in freshwater.

Its body is a hollow column with walls only two cells thick.

Its head is just a bunch of tentacles surrounding a mouth, no eyes or brain.

The whole thing is about as long as a grain of rice.

The hungry hydra stretches out its tentacles to snag swimming prey.

Microscopic harpoons hook the water flea, injecting it with paralyzing neurotoxins.

The hydra uses those nutrients to make more hydra.

It&rsquos a clone sprouting right from the hydra&rsquos side.

Hydra can reproduce sexually too, but most often, they just clone themselves.

Hydra are constantly regenerating their own bodies too, replacing all of their cells every 20 days.

They can do that because roughly half of the cells in their bodies are stem cells, which can develop into all the different types of specialized cells you need to build, or rebuild a body.

Stem cells only make up a tiny percentage of our bodies

And our stem cells degrade over time &mdash that&rsquos why we age.

But a hydra can make near-perfect copies of its stem cells &hellip basically forever.

It&rsquos called non-senescence &mdash biological immortality.

Having all those stem cells allows hydra to recover from all kinds of . damage.

As long as the chopped-off chunk has some stem cells, it's ready to regrow.

In a couple days, the severed head has a new body.

And the foot? It&rsquos got a brand new head!

Soon, they&rsquoll both be as good as new &hellip except for maybe some lingering trust issues.

To test the limits of their ability to recover, researchers came up with an experiment.

This hydra is genetically engineered so that under ultraviolet light its exterior cells glow purple and interior ones glow green.

Scientists basically blend up a bunch of these hydra.

Leaving a heap of mixed-up cells.

Right away, the cells start reorganizing.

Those purple outer layer cells migrate out.

The cells from the inside squirm back into the center.

The hydra dumps cells from the interior to restore its hollow shape.

The stem cells start dividing &mdash and differentiating &mdash to rebuild the rest of the animal.

Soon a head starts to form, and sometimes a few little heads compete to be the new top.

After a few tumultuous days what was once a mixed-up pile of cells takes on a familiar shape.

Researchers hope to harness hydra&rsquos ability to regenerate to someday slow human aging, or even regrow damaged organs.

Maybe this tiny monster will one day show us the way to the mythical fountain of youth.

Hey Deep Peeps &mdash our PBS friends here on YouTube are celebrating Earth Day with a ton of special episodes.

Head over to Above the Noise, where host Myles Bess asks . can we make room for wildlife in our cities?


Mapping cells in the 'immortal' regenerating hydra

The tiny hydra, a freshwater invertebrate related to jellyfish and corals, has an amazing ability to renew its cells and regenerate damaged tissue. Cut a hydra in half, and it will regenerate its body and nervous system in a couple of days. Researchers at the University of California, Davis have now traced the fate of hydra's cells, revealing how three lines of stem cells become nerves, muscles or other tissues.

Celina Juliano, assistant professor in the UC Davis Department of Molecular and Cellular Biology, project scientist Stefan Siebert and colleagues including Jeff Farrell, a postdoctoral researcher at Harvard University, sequenced the RNA transcripts of 25,000 single hydra cells to follow the genetic trajectory of nearly all differentiated cell types.

"The beauty of single-cell sequencing and why this is such a big deal for developmental biologists is that we can actually capture the genes that are expressed as cells differentiate from stem cells into their different cell types," Juliano said.

The study gives developmental biologists a high-resolution map of the three stem cell developmental lineages in hydra. The data set will help researchers understand regulatory gene networks in place early in evolution that are shared among many animals, including humans, Siebert said. Understanding how the hydra regenerates its entire nervous system, for example, could help us better understand neurodegenerative diseases in humans.

Regenerates from three lines of stem cells

Hydra continuously renew their cells from three different stem cell populations. The researchers analyzed sets of messenger RNA molecules, called transcriptomes, from individual hydra cells and grouped the cells based on their expressed genes. They could then build a decision tree showing how each lineage of stem cells gives rise to different cell types and tissues. For example, the interstitial stem cell lineage produces nerve cells, gland cells and the stinging cells in the animal's tentacles.

"By building a decision tree for the interstitial lineage, we unexpectedly found evidence that the neuron and gland cell differentiation pathway share a common cell state," said Juliano. "Thus, interstitial stem cells appear to pass through a cell state that has both gland and neuron potential before making a final decision."

The single cell molecular map also allowed Juliano and colleagues to identify genes that may control these decision-making processes, which will be the focus of future studies.

Researchers are especially interested in hydra's ability to regenerate its nervous system, which could give insights into treating trauma or degenerative disease in humans.

"All organisms share the same injury response pathway but in some organisms like hydra, it leads to regeneration," said coauthor and graduate student Abby Primack. "In other organisms, like humans, once our brain is injured, we have difficulty recovering because the brain lacks the kind of regenerative abilities we see in hydra."


Meet the animal that refuses to die

By Joanna Rothkopf
Published September 26, 2014 6:07PM (EDT)

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Everything dies, right? Wrong, actually. According to NPR reporters Robert Krulwich and Adam Cole, an organism called the hydra has achieved biological immortality in a lab setting.

The extremely charming video below tells the story of a scientist named Daniel Martinez who decided, quite simply, to get some hydra and watch them until they died. This was in the 1990s--they haven't shown any signs of senescence (or biological mortality) yet.

This is weird, explains Krulwich, because in biology, there is a correlation between the age one has babies and the age that one dies. If you're a fly, for instance, you have your babies after a few weeks, and die after a few months. If you're an elephant, you wait 13 years to have babies and then die when you're 40 or 50. Hydra reproduce after only a few days, but so far have shown no signs of coming close to dying, even though they technically should have after a number of weeks.

Watch the video below to find out why.

Joanna Rothkopf

MORE FROM Joanna RothkopfFOLLOW @joannarothkopf


Mapping the cells of immortality in the hydra

Just a few millimeters long, the hydra has the ability to completely regenerate damaged body parts including its nervous system, making it practically immortal. UC Davis researchers are discovering how the hydra achieves this and what the implications might be for human medicine. (Stefan Siebert/UC Davis).

The tiny hydra, a freshwater invertebrate related to jellyfish and corals, has an amazing ability to renew its cells and regenerate damaged tissue. Cut a hydra in half, and it will regenerate its body and nervous system in a couple of days.

Researchers at the University of California, Davis, have now traced the fate of the hydra’s cells, revealing how three lines of stem cells become nerves, muscles or other tissues.

Celina Juliano, assistant professor in the UC Davis Department of Molecular and Cellular Biology, project scientist Stefan Siebert and colleagues including Jeff Farrell, a postdoctoral researcher at Harvard University, sequenced the RNA transcripts of 25,000 single hydra cells to follow the genetic trajectory of nearly all differentiated cell types.

“The beauty of single-cell sequencing and why this is such a big deal for developmental biologists is that we can actually capture the genes that are expressed as cells differentiate from stem cells into their different cell types,” Juliano said.

The study gives developmental biologists a high-resolution map of the three stem cell developmental lineages in the hydra. The dataset will help researchers understand regulatory gene networks in place early in evolution that are shared among many animals, including humans, Siebert said. Understanding how the hydra regenerates its entire nervous system, for example, could help us better understand neurodegenerative diseases in humans.

Regenerates from three lines of stem cells

Hydras continuously renew their cells from three different stem cell populations. The researchers analyzed sets of messenger RNA molecules, called transcriptomes, from individual hydra cells and grouped the cells based on their expressed genes. They could then build a decision tree showing how each lineage of stem cells gives rise to different cell types and tissues. For example, the interstitial stem cell lineage produces nerve cells, gland cells and the stinging cells in the animal’s tentacles.

“By building a decision tree for the interstitial lineage, we unexpectedly found evidence that the neuron and gland cell differentiation pathway share a common cell state,” said Juliano. “Thus, interstitial stem cells appear to pass through a cell state that has both gland and neuron potential before making a final decision.”

The single-cell molecular map also allowed Juliano and colleagues to identify genes that may control these decision-making processes, which will be the focus of future studies.

Researchers are especially interested in the hydra’s ability to regenerate its nervous system, which could give insights into treating trauma or degenerative disease in humans.

“All organisms share the same injury response pathway but in some organisms like hydra, it leads to regeneration,” said co-author and graduate student Abby Primack. “In other organisms, like humans, once our brain is injured, we have difficulty recovering because the brain lacks the kind of regenerative abilities we see in hydra.”

Additional co-authors include Jack Cazet and Yashodara Abeykoon at UC Davis and Christine Schnitzler, University of Florida. The work was partly supported by grants from the National Institutes of Health and DARPA.


Watch the video: Ύδρα ή αλλιώς Hydra (May 2022).