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 the 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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