Vital marking of single cells in developing tissues: India ink injection to trace tissue movements in hydra.

A new vital marking procedure was devised, studied, and applied to the phenomenon of hydra column tissue movements. The method involves injecting colloidal carbon (India ink) into the ectodermal epithelium of hydra. The carbon is taken up specifically by epithelial cells and packed into lysosome residual bodies. Each marked cell is then identifiable by its aggregated carbon particle, which appears as a black dot; this is passed to a daughter cell during cell division. Using this method to stain vitally the ectoderm of the green hydra, Chlorohydra viridissima, whose endodermal cells were marked by symbiotic algae, the relative movements of ectoderm and endoderm along the body column were analysed. Ectoderm and endoderm move in different directions or at different rates in the top quarter of the gastric column, but move together in the lower part of the column. This indicates that some epithelial cell movements involve cells migrating relative to the mesoglea; other movements involve translocation of the entire body wall, including ectoderm, mesoglea and endoderm.