Morphallactic Regeneration in Hydra.pptx

Regeneration of hydra - DEVELOPMENTAL BIOLOGY CSIR-NET JRF LIFESCIENCE

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HYDRA MORPHOLLACTIC REGENRATION TOPIC OF DEVELOPMENT BIOLOGY FOR CSIR NET JUNE 2022

Morphallaxis. Here, regeneration occurs through the repatterning of existing tissues, and there is little new growth. Such regeneration is seen in Hydra (a cnidarian) .
The head activation gradient
Grafting experiments
The head inhibition gradient
The basal disc activation and inhibition gradients.


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In this video we discussed about regeneration and types of Regeneration of hydra (morphallaxis).

1. Morphallactic Regeneration in Hydra Presented by :- Lovyansh lifescience

2. Morphallaxis Morphallaxis . Here, regeneration occurs through the repatterning of existing tissues, and there is little new growth. Such regeneration is seen in Hydra (a cnidarian).

3. Hydra Hydra' is a genus of freshwater cnidarians. Most hydras are tiny-about 0.5 c m long. A hydra has a tubular body, with a "head" at its distal end and a "foot" at its proximal end. The "foot," or basal disc, enables the animal to stick to rocks or the undersides of pond plants. The "head" consists of a conical hypostome region (containing the mouth) and a ring of tentacles (which catch food) beneath it. Hydra, like all cnidarians, has only ectoderm and endoderm; these animals lack a true mesoderm. Hydras can reproduce sexually, but they do so only under adverse conditions (such as crowding or cold weather). They usually multiply asexually, by budding off a new individual The buds form about two-thirds of the way down the animal's body axis.

4. The head activation gradient Every portion of the hydra body column along the apical-basal axis is potentially able to form both a head and a foot. However, the polarity of the hydra is coordinated by a series of morphogenetic gradients that permit the head to form only at one place and the basal disc to form only at another

5. The hypostome as an "organizer Ethel Browne noted that the hypostome acted as an organizer" of the hydra. This notion has been confirmed by Broun and Bode (2002), who demonstrated that (1) when transplanted, the hypostome can induce host tissue to form a second body axis, (2) the hypostome produces both the head activation and head inhibition signals, (3) the hypostome is the only "self-differentiating" region of the hydra, and (4) the head inhibition signal is actually a signal to inhibit the formation of new organizing centers.

6. G rafting experiments Evidence for such gradients was first obtained from grafting experiments begun by Ethel Browne in the early 1900s. When hypostome tissue from one hydra is transplanted into the middle of another hydra, the transplanted tissue forms a new apical-basal axis, with the hypostome extending outward . When a basal disc is grafted to the middle of a host hydra, a new axis also forms, but with the opposite polarity, extending a basal disc When tissues from both ends are transplanted simultaneously into the middle of a host, no new axis is formed, These experiments have been i nterpreted to indicate the existence of a head activation gradient (highest at the hypostome ) and a foot activation gradient (highest at the basal disc). The head activation gradient can be measured by implanting rings of tissue from various levels of a donor hydra into a particular region of the host trunk The higher the level of head activator in the donor tissue, the greater the percentage of implants that will induce the formation of new heads. The head activation factor is concentrated in the head and decreases linearly toward the basal elise . Three peptides have been associated with this head activation gradient. Two of them, Heady and Head Activator, are critical for head formation and the initiation of the bud. The other, Hym301, regulates the number of tentacles formed

7. The head inhibition gradient Rand and colleagues showed that the nonmal regeneration 0f the hypostome is inhibited when an intact hypostome is grafted adjacent to the amputation site. Moreover, if a graft of subhypostomal tissue (from the region just below the hypostome , where there is a relatively high concentration of head activator) is placed in the same region of a host hydra, no secondary axis forms . The host head appears to make an inhibitor that prevents the grafted tissue from forming a head and secondary axis. However, if one grafts subhypostomal tissue to a decapitated host hydra, a second axis does form A gradient of this inhibitor appears to extend from the head down the body column, and can be measured by grafting subhypostomal tissue into various regions along the trunks of host hydras. This tissue will not produce a head when implanted into the apical area of an intact host hydra, but it will form a head if placed lower on the host. Thus, there is a gradient of head inhibitor as well as head activator

8. The basal disc activation and inhibition gradients Certain properties of the basal disc suggest that it is the source of both a foot inhibition and a foot activation gradient). The inhibition gradients for the head and the foot may be important in determining where and when a bud can form. In young adult hydras, the gradients of head and foot inhibitors appear to block bud formation. However, as the hydra grows, the sources of these la bile substances grow farther apart, creating a region of tissue about two-thirds down the trunk where levels of both inhibitors are minimal. This region is where the bud forms Certain mutants of Hydra have defects in their ability to form buds, and these defects can be explained by alterations of the inhibition gradients. The L4 mutant of Hydra magnipapillata , for instance, forms buds very slowly, and only after reaching a size about twice as long as wild-type individuals. The amount of head inhibitor in these mutants was found to be much greater than in wild-type Hydra.

9. the specification of cells as they migrate from the basal region through the body column may be mediated by a gradient of tyrosine kinase. The product of the shinguard gene is a tyrosine kinase that extends in a gradient from the ectoderm just above the basal disc through the lower region of the trunk. Buds appear to form where this gradient fade; The shinguard gene appears to be activated through the product of the manacle gene, a putative transcription factor that is expressed earlier in the basal disc ectoderm The inhibition and activation gradients also inform the hydra "which end is up" and speCify positional values along the apical-basal axis. When the head is removed, the head inhibitor no longer is made, causing the head activator to induce a new head. The region with the most head activator (i.e., those cells directly beneath the amputation site) will form the head. Once the head is made, it generates the head inhibitor, and thus equilibrium is restored.

10. If a hydra's body column is cut into several pieces, each piece will regen era te a head at its original apical end and a foot at its original basal end. No cell ctivision is required for this to happen, and the result is a small hydra. Since each cell retains its plasticity, each piece can re-form a smaller organism; this is the basis of morphallaxis .