Development 102, 223-235 (1988) Printed in Great Britain © The Company of Biologists Limited 1988 223 Development of the two-part pattern during regeneration of the head in hydra P. M. BODE 1 , T. A. AWAD 1 , O. KOIZUMI 2 , Y. NAKASHIMA 2 , C. J. P. GRIMMELIKHUIJZEN 3 and H. R. BODE 1 'Developmental Biology Center, Department of Developmental and Cell Biology, University of California at Irvine, Irvine, CA 92717, USA 2 Physiological Laboratory, Department of Science, Fukuoka Women's University, Fukuoka, Japan ^Zoological Institute, University of Heidelberg, Heidelberg, FRG Summary The head of a hydra is composed of two parts, a domed hypostome with a mouth at the top and a ring of tentacles below. When animals are decapitated a new head regenerates. During the process of regeneration the apical tip passes through a transient stage in which it exhibits tentacle-like characteristics before becom- ing a hypostome. This was determined from markers which appeared before morphogenesis took place. The first was a monoclonal antibody, TS-19, that specifically binds to the ectodermal epithelial cells of the tentacles. The second was an antiserum against the peptide Arg-Phe-amide (RFamide), which in the head of hydra is specific to the sensory cells of the hypostomal apex and the ganglion cells of the lower hypostome and tentacles. The TS-19 expression and the ganglion cells with RFamide-like immunoreac- tivity (RLI) arose first at the apex and spread radially. Once the tentacles began evaginating in a ring, both the TS-19 antigen and RLI+ ganglion cells gradually disappeared from the presumptive hypostome area and RLI+ sensory cells appeared at the apex. By tracking tissue movements during morphogenesis it became clear that the apical cap, in which these changes took place, did not undergo tissue turnover. The implications of this tentacle-like stage for pattern- ing the two-part head are discussed. Key words: hydra, regeneration, pattern formation, two- part pattern, head, monoclonal antibody. Introduction The basic structure of hydra is a hollow cylinder composed of two tissue monolayers, an ectoderm and an endoderm separated by a thin extracellular matrix, the mesoglea. At the base of the cylinder is a holdfast, which is simply a disc of ectodermal cells secreting adhesive substances. The head, however, is a more complicated two-part structure. The upper half is a smooth dome or cone, the hypostome, that opens to serve as a mouth. The lower half is the tentacle zone from which six or seven tentacles protrude. When the head is removed from an animal a new head regenerates. Much has already been learned about the patterning process, such as the factors affecting its location, as well as information on its properties and kinetics as it develops (see Bode & Bode, 1984£> for review). However, most analyses yielded information only on the presumptive head as a unit. An important question concerning this process is how the two parts of the head dome arise. Recent observations on tentacle regeneration in adult bud- ding animals and the most common abnormalities that result have suggested a possible sequence of events (Bode & Bode, 1987). The apical end heals over rapidly after decapi- tation. After one day of regeneration the first one or two tentacles are visible in some animals. These occasionally emerge from a small area at the apex. During the next two days new tentacles appear in more basal positions, forming a ring around the dome. Some of the early tentacles may remain above the ring, well within the hypostome area where the mouth would normally open. Occasionally regener- ation is incomplete and only a single tentacle forms at the apical end.