17.          REGENERATION

Regeneration is the reactivation of development in post-embryonic life which is responsible for restore missing tissues and is known as regeneration.

Regeneration can be observed as we go back to the embryonic state, the term is appropriate in many ways.

Regeneration by two methods:

Epimorphosis: dedifferentiation of adult structure takes place to form a relatively undifferentiated mass of cells and then redifferentiation takes place to form the new structure. Example - planarian flatworm regeneration, regenerating amphibian limbs.

Morphallaxis: Repatterning of existing tissues cause the regeneration, and there is little new growth takes place. Example -  Hydra (a cnidarian).

17.1.      Epimorphosis

Epimorphic regeneration in Salamander takes place by cell dedifferentiation and dedifferentiation. This cause the formation of the blastema. It is an aggregation of fairly dedifferentiated cells which is derived from the originally differentiated tissue, those cells undergo proliferation and redifferentiation to form the new limb parts. Bone, dermis, and cartilage just beneath the site of amputation contribute to the regeneration blastema.

After amputation of the limb of salamander, the outer epidermal cells cause the formation of the stump. Now, this epidermis is called a wound epidermis. Wound epidermis thickens differentiate into the apical epidermal cap (AEC) (homologous to apical ectodermal ridge). Beneath the wound epidermis, extracellular matrices tissues are present. These tissues are degraded by proteases within the next 4 days and dedifferentiation of the individual cell takes place. Thus cartilage cells, fibroblasts, bone cells, and myocytes all lose their differentiated state and downregulation of differentiation governing gene myf 4 and myf 5, takes place along with expression of msxl that are connected with the proliferation of embryonic limb’s progress zone mesenchyme also this mesenchyme causes regeneration of blastema, show continuous proliferation and which will ultimately redifferentiate to the new structures of the limb. The growth of the regeneration blastema depends on the presence of both the apical ectodermal cap and nerves. The apical epidermal cap secretes Fgf 8 and stimulates the growth of the blastema but also require the presence of a nerve, which releases factors necessary for the proliferation of the blastema cells and the best nerve mitogen is newt anterior gradient protein (nAG). Within 5 days of amputation, the induction of newt anterior gradient protein (nAG) takes place in the Schwann cells that surround the neurons because of the expression of nAG very low in normal limb. When nAG gene is introduced by electroporation into denervated limbs than it causes regeneration of limbs.

17.2.      Morphallaxis

Hydra belongs to genus cnidarians. It has a tubular body, head or hypostome present at its distal end and a foot or basal disc present at its proximal end. The foot or basal disc enables the animal to stick to rocks. Conical hypostome region containing the mouth along with a ring of tentacles, which is responsible for catching the food present beneath it.

17.2.1.   The head activation gradient

Hydra has a columnar body along the apical-basal axis formation of both the head and the foot. A series of morphogenetic gradients define the polarity of the hydra that permits the formation of the head only at one place and the basal disc formation only at another. If transplantation of hypostome tissue of one hydra takes place into the middle of another hydra, then the formation of a new apical-basal axis takes place along with the hypostome extending outward. Same case with the basal disc, but here the opposite polarity and extending a basal disc. If transplantation of tissues from both ends occurs simultaneously into the middle of a host then no new axis is formed or if the form has little polarity. These experiments have been deduced the presence of a head activation gradient, which is highest at the hypostome along with foot activation gradient which is highest at the basal disc.

17.2.2.   The hypostome as an "organizer"

Hypostome of the hydra act as an "organizer" Because hypostome cause induction of formation of secondary axis if transplanted in host, hypostome cause the production of both head activation along with head inhibition signals, the hypostome act as "self-differentiating" region, the head inhibition  signal  cause the inhibition of  the formation of new organizing centers.

Three genes are important for hypostome function in hydra. First Wnt, crucial for the hypostome region during the bud elongation and Wnt present at apical region and inhibit GSK3 as a result stabilization of β catenin in the cell nucleus takes place.

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