2 DIFFERENTIATION PATHWAYS OF THE FOOT IN 2 SPECIES OF HYDRAS - HYDRA-ATTENUATA AND HYDRA-OLIGACTIS

In freswater Hydras, the basal end consists of a foot with two histolo gically distinct parts, a peduncle and a basal disk which attaches the animal to a substrate. The epidermis of the pedal disk is composed ch iefly of foot-specific epithelio-muscular cells; these cells secrete b asal disk-specific droplets not found elsewhere in the epidermis. It i s generally assumed that peduncle epitheliomuscular cells are able to undergo functional changes and differentiate into foot secretory cells . In the same way, the high regenerative capacity of gastric column ti ssues has world-wide reputation. The findings reported here indicate t hat this ability is not so universal, but varies mainly according to t he species of Hydras. In Hydra attenuata, regardless of the region whi ch is amputated, the regeneration of the pedal part generally occurs. On the other hand, in Hydra oligactis, foot regeneration from the gast ric column is generally absent. In this latter species, the restoratio n of the excised part requires the renewal of feeding. The morphogenet ic events always involve the same sequence: at first, there is restora tion of the budding activity that is at the beginning of peduncular gr owth, then at the end, secretory-disk differentiation. But foot format ion rates can vary very much with the different geographic origins of the strain. One strain H. oligactis St-Nizier, shows a particular foot defect and the basal disk is formed after a particularly long growth period. It is not the case in the H. oligactis Tete-d'Or strain where the more rapid pedal-disk formation cannot be related to the peduncula r growth. The localization of the foot regenerative defect in H. oliga ctis strains is discussed. The H. oligactis strains are proposed as mo del for genetic analysis of cellular differentiation of the foot in Hy dras.