HYDRA REGENERATION FROM RECOMBINED ECTODERMAL AND ENDODERMAL TISSUE .2. DIFFERENTIAL STABILITY IN THE ECTODERMAL AND ENDODERMAL EPITHELIAL ORGANIZATION

Hydra tissue consists of the ectodermal and the endodermal layers, Whe n the two layers were separated by procaine treatment and then recombi ned, the ectodermal epithelial. cells spread as a single cell layer ov er the endoderm as in epiboly in vertebrate embryogenesis, and the res ultant spherical structure subsequently regenerated into a complete hy dra. In this study, light and electron microscopy were used to examine the structural changes which took place in the cells and tissue durin g this epibolic ectodermal spreading process, Within a fen hours after tissue recombination, the endoderm underwent dramatic changes; it los t its epithelial sheet organization, and turned into a mass of irregul arly shaped cells without the apical-basal cell polarity initially pre sent, In contrast, the ectoderm maintained its basic epithelial sheet organization as it spread over the endoderm, Later, the endodermal epi thelial cells reorganized themselves into a single-layered epithelial sheet underneath the spreading ectodermal layer, The resultant spheric al structure consisted of a single layer of ectodermal epithelial cell s outside, a single layer of endodermal epithelial cells inside, and a n empty cavity in the center as in normal hydra tissue, This structure regenerated into hydra in the following days, These and other observa tions demonstrate that the two-layered epithelial sheet organization i s highly dynamic, and that its stability is maintained by strong inter actions between the two layers in normal hydra, It is suggested that t his dynamic nature of the hydra tissue, particularly the high plastici ty of the endodermal epithelial sheet organization, may be an importan t element for the high regenerative capacity of this organism.