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Development, Vol 121, Issue 5 1273-1282, Copyright © 1995 by Company of Biologists
JOURNAL ARTICLES |
U. Technau and T. W. Holstein
Hydra's head is a compound structure with a hypostome at the apical extreme and a circle of tentacles more basally. During head regeneration, it is thought (P. M. Bode, T. A. Awad, O. Koizumi, Y. Nakashima, C. J. P. Grimmelikhuijzen and H. R. Bode (1988) Development 102, 223-235; R. Weinziger, L. M. Salgado, C. N. David and T. C. G. Bosch (1994) Development 120, 2511-2517) that the conditions for tentacle formation are fulfilled before those for hypostome formation. Using a new hypostome-specific marker, we have found that the order of hypostome and tentacle formation is variable. In regenerating basal tissue, the hypostome marker is expressed before tentacles appear but in apical tissue, the tentacles appear first. This observation appears inconsistent with current views but can be explained by a hierarchical model (H. Meinhardt (1993) Developmental Biology 157, 321-333) in which tentacles require an inductive influence of the hypostome. In basal regenerates, the hypostome forms first and then induces tentacles. In apical regenerates, inductive factor remains from the amputated hypostome, and tentacle may form before the new hypostome. We have also observed that the mode of expression of the tentacle marker differs in basal and apical tissue. In basal tissue, the marker first appears in the definitive tentacle zone; in apical tissue, the marker first appears in the position of the presumptive hypostome and is then displaced to its final position, as described by previous workers. This observation is also expected according to the above-cited model.
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