A role of inductive factors in interstitial cell differentiation in hydra

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Abstract
The distribution of interstitial cells and I‐cell derivatives in hydra is asymmetrical. This asymmetry is expressed apico‐basally along the stalk and radially with respect to cell layer. An inductive factor concentrated from the tissues of hydra alters this expression of polarity (Lesh and Burnett, '66).To test the hypothesis that quantitative differences in the concentration of this inducer effect qualitative differences in I‐cell differentiation, regenerating annuli were exposed to tissue extracts containing varying levels of inductive activity. Two types of extracts were employed: (1) serial dilutions of an active extract; and (2) extracts from different species of hydra exhibiting decreasing levels of inductive capacity. Hydra pirardi annuli were incubated four hours in extract. After 24 hours annuli were fixed, sectioned, stained, and counts made of all cell types.Statistical analyses of these counts show that responding cell types demonstrate one or both of two responses: proliferation and/or differentiation. The initial response in normal annuli is a 1.9 ± fold increase in the proportion of I‐cells present. Annuli which undergo an increase in I‐cell complement before incubation develop alterations in the distribution of I‐cell derivatives. At concentrated inducer levels isorhiza cnidocytes (epidermis) and mucus cells (gastrodermis) differentiate. At lower concentrations stenotele cnidocytes and gland cells appear. The distribution of epithelio‐muscular cells and digestive cells (not I‐cell derivatives) is independent of inducer concentration.Also, the distribution of I‐cells and derivatives found after the experimental treatments coincides directly with the distribution of these cell types relative to the inductive regions of intact hydra.