Size, shape and orientation of cells in budding hydra and regulation of regeneration in cell aggregates.

Changes in the number, shape, volume, orientation and vacuolization of cells involved in the budding of hydra were measured in histological sections. Before evagination, a group of about 800 epithelial cells are visibly recruited for the bud to be produced and this number increases to about 5,000 within a day. Thereafter, bud size increases mainly by proliferation of the cells within the bud. Upon recruitment for budding, the epithelial cells assume a columnar shape, with a smaller contact area facing the mesoglea, accompanied by a decrease in volume which is mostly accounted for by devacuolization. In later stages, cells progressively resume the form typical for non-budding areas of hydra. Evagination proceeds without reorientation of epithelio-muscular fibers, whereas elongation of the bud is accompanied by fiber reorientation.The process of sorting out and regeneration in aggregates of previously dissociated hydra cells was followed using various ratios of endodermal to ectodermal epithelial cells. From different initial compositions, the ratio in the regenerate rapidly approaches 1∶1, the ratio found in normal hydra tissue.The experimental findings are discussed in the context of theoretical notions on pattern formation, evagination, elongation and stability of layered structures.