The differentiation of nerve cells and nematocytes in Hydra attenuata has been investigated by labelling interstitial cell precursors with [3H]thymidine and following by autoradiography the appearance of labelled, newly differentiated cells. Nematocyte differentiation occurs only in the gastric region where labelled nematoblasts appear 12 h and labelled nematocytes 72-96 h after addition of [3H]thymidine. Labelled nerves appear in hypostome, gastric region, and basal disk about 18 h after addition of [3H]thymidine. The lag in the appearance of labelled cells includes cell division of the precursor as well as differentiation since nerves and nematocytes have 2n postmitotic nuclear DNA content.

A cell flow model is proposed for interstitial cells and their differentiated products. Stem cells occur as single interstitial cells or in pairs. Per cell generation about 60 % of the daughter cells of stem cell divisions remain stem cells and about 40 % differentiate nerves and nematocytes. Nerves differentiate directly from stem cells in about 1 day. Nematocyte differentiation requires 5-7 days including proliferation of a cluster of 4, 8, 16 or 32 interstitial cells and differentiation of a nematocyst capsule in each cell. The numbers of interstitial cells and nematoblasts predicted by the cell flow model from the rates of nerve differentiation (900 nerves/day/ hydra), nematocyte differentiation (1760 nematocyte nests/day/hydra) and stem cell proliferation (stem cell cycle = 24 h), agree with the numbers of these cells observed in hydra. The number of stem cells per hydra is 3000-6000 depending on assumptions about the time of determination. The ratio of nematocyte to nerve differentiation averaged over the whole hydra is 3:1. In the hypostome and basal disk interstitial cell differentiation occurs exclusively to nerve cells while in the gastric region the ratio of nematocyte to nerve differentiation is about 7:1.

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