David, Charles N. and Gierer, A.
CELL CYCLE KINETICS AND DEVELOPMENT OF HYDRA ATTENUATA. III. NERVE AND NEMATOCYTE DIFFERENTIATION.
In: Journal of Cell Science, Vol. 16: pp. 359-375
The differentiation of nerve cells and nematocytes in Hydra attenuata has been investigated
by labelling interstitial cell precursors with PHJthymidine 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 2H 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 2JI 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/hydia) 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