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Two differentiation pathways of the foot in two species of hydras hydra attenuata and hydra oligactis



Two differentiation pathways of the foot in two species of hydras hydra attenuata and hydra oligactis



Annales des Sciences Naturelles Zoologie et Biologie Animale 12(4): 159-171



In freshwater Hydras, the basal end consists of a foot with two histologically distinct parts, a peduncle and a basal disk which attaches the animal to a substrate. The epidermis of the pedals disk is composed chiefly of foot-specific epithelio-muscular cells; these cells secrete basal disk-specific droplets not found elsewhere in the epidermis. It is 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 tissues has world-wide reputation. The findings reported here indicate that this ability is not so universal, but varies mainly according to the species of Hydras. In Hydra attenuata regardless of the region which is amputated, the regeneration of the pedal part generally occurs. On the other hand, in Hydra oligactis, foot regeneration from the gastric column is generally absent. In this latter species, the restoration of the excised part requires the renewal of feeding. The morphogenetic events always involve the same sequence: at first, there is restoration of the budding activity that is at the beginning of peduncular growth, then at the end, secretory-disk differentiation. But foot formation 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 peduncular growth. This localization of the foot regenerative defect in H. oligactis strains is discussed. The H. oligactis strains are proposed as model for genetic analysis of cellular differentiation of the foot in Hydras.

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