A histological and ultrastructural study of dedifferentiation and redifferentiation of digestive and gland cells in Hydra viridis

Abstract

The isolated gastrodermis of the green Hydra is capable of regenerating into a normal animal containing epidermis, mesoglea, and gastrodermis. Since the gastric region of the isolated gastrodermis contains only two cell types, namely, digestive cells and gland cells, our primary interest was to observe the cellular changes during dedifferentiation and redifferentiation into other cell types. Digestive cells at the periphery of the explant differentiate directly into epitheliomuscular cells, while some of the digestive cells in the interior of the mass are destroyed during the formation of the enteron. The peripheral gland cells undergo a complex cellular transformation. The granular endoplasmic reticulum is broken down; in some areas fragments of the endoplasmic reticulum are isolated in vacuoles associated with the Golgi apparatus and all secretory droplets are voided. The gland cell dedifferentiates into an interstitial cell, the cytoplasm of which is characteristically packed with free ribosomes and without endoplasmic reticulum. The interstitial cells subsequently divide and redifferentiate into cnidoblasts. The mechanism by which the endoplasmic reticulum of the gland cells is broken down is not known. Also, it is not known whether new ribosomes are synthesized during the redifferentiation of interstitial cells into cnidoblasts or that the ribosomes from the original gland cells are capable of the protein synthesis involved in nematocyst formation. The digestive cells and gland cells of the explant remain discretely located as they are in the normal animal. For this reason only peripheral gland and digestive cells undergo cellular transformation; while, except for the formation of the enteron, the cells in the interior of the mass become the future gastrodermis of the regenerate.