Patterning of heads and feet during regeneration of Hydra oligactis aggregates

The de novo appearance of head organizing centers was monitored during regeneration of aggregates made from dissociated body column tissue of Hydra oligactis using the head-specific monoclonal antibody CP8 (L. C. Javois, R. D. Wood, and H. R. Bode, 1986, Dev. Biol. 117, 607-618). Newly differentiated CP8+ head organizing centers were present by 24 hr of regeneration and were restricted to one half of the aggregate. Depending on subsequent patterning events these CP8+ cells went on to develop into heads or body tentacles, or remained as CP8+ patches. A comparison of the number of initial CP8+ patches with the final number of CP8+ structures indicated that this number was fairly constant, suggesting that a limited number of head organizing centers were established early during regeneration. Examination of fully differentiated head structures revealed that they were segregated to one end of the aggregate with feet segregated away from the CP8+ structures. Increasing the surface area of the aggregates resulted in a more random initial spacing of CP8+ head organizing centers and ultimately a more random distribution of differentiated heads and feet. However, polarized polyps with one head and foot detached and the remaining structures were then segregated. The ability of existing pattern formation models to describe interactions between head- and foot-forming systems resulting in the observed spacing of heads and feet is discussed.