Figure 1. Photograph and diagram illustrating the anatomy of a hydra. Reproduced with permission from Lenhoff HM and Lenhoff SG (1988) Trembley's polyps. Scientific American 256(4): 108–113. Photograph by Richard D. Campbell of the University of California at Irvine.

Figure 2. The developmental programme is never locked in hydra. (a) In adult polyps, active patterning processes are maintained through the coupling of differentiation and migration of cells located in the body column towards the extremities. (b) Vegetative reproduction through budding is allowed to occur only in a limited area of the gastric column. (c, d) Apical or basal regeneration is observed either after bisection of the animal (c), or on reaggregation after complete dissociation of the hydra into single cells (d). In (c) the time necessary to rebuild the amputated structure depends on the level of the bisection: in Hydra vulgaris, the apex is regenerated in about 2 days and basal region in 1 day after midgastric section. (e) Sexual development is required for survival in less temperate natural conditions. Te, testis; Ov, ovocyte.

Figure 3. Scheme depicting the various cellular steps observed before epimorphic regeneration is fully achieved in different model systems (time is not to scale). In urodeles, the two initial phases last about 2 weeks; in planarians, the blastema appears after 1 day; and in hydra, the apical-organizer activity is established within a few hours after cutting. Endodermal cells are involved in body regeneration but not in appendage regeneration. Reproduced from Galliot B (1997). Copyright © 1997 with permission from John Wiley & Sons, Inc.

Figure 4. (a) Level of apical-organizer activity deduced from the observed rate of secondary head induction in the host upon transplantation of the stump (according to MacWilliams, 1983). Two distinct components were characterized. The first was restricted to the stump region (no gradient), was detected even in absence of nerve cell differentiation, and decayed over 18 h. The second was measured as a gradient having its maxima in the stump, relying on differentiation of new nerve cells, and was still detectable after 48 h, by which time the new head had emerged. During the postcutting inhibition period, no induction of secondary head was observed. (b) Landmarks for molecular signalling during hydra regeneration. Posttranslational modifications are observed immediately after cutting while expression of head- and foot-specific genes requires several hours before reaching detectable levels in the stump. As observed for the postcutting inhibition in transplantation experiments, the time before induction of ‘early’ gene expression depends on the position of the section along the body column.