Current Topics in Developmental Biology, Vol. 11 by A.A. Moscona, Alberto Monroy (Eds.)

By A.A. Moscona, Alberto Monroy (Eds.)

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The figure shows an animal with two buds. (b) Schematic drawing of a cross section through the wall of the gastric column, showing the two cell layers with various cell types. umn are interstitial cells which look undifferentiated and occur singly as well as in nests. Further, one finds-also in nests-large numbers of nematoblasts which are precursors of the nematocytes. The latter are produced in the gastric column, but most nematocytes migrate to the tentacles that catch and kill the prey. There are different types of nematocytes with different spatial distributions in the animal.

163-183 32 ALFRED GIERER Two general types of concepts on polarity may be distinguished. One is to assume that the tissue contains asymmetric cells aligned parallel to each other, so that in any section the distal (“arrowhead”) terminal is different from the proximal one. This, in turn, could give rise to activation of head formation a t the “arrowhead” terminal. In a n alternative general scheme, one assumes that there is a graded distribution of some cell type or substance in the gastric column, and that there are mechanisms deciding, within a cut section, which part is relatively highest in concentration, leading to head formation in the portion which was closest to the original head.

SOMEPROPERTIES OF HYDRA Hydra is a few millimeters long and has an asymmetrical polar organization with tentacles, hypostome, and the gastric column including the budding zone, peduncle, and basal disk (Fig. l a). It is made of two cell layers, ectoderm and endoderm, with a mesoglea in between (Fig. lb). Further, there are mucous cells in the hypostome, gland cells in the endoderm of the gastric column, and secreting basal disc cells in the foot. Embedded mostly in the ectodermal layer of the gastric col‘c This definition of prepatterns, which is somewhat more general as well as more abstract than that in part of the literature, will be used in this article.

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