Spreading, proliferation, and differentiation of the epidermis after wounding a cichlid fish, Hemichromis bimaculatus

A large superficial wound has been experimentally provoked in the cichlid f ish Hemichromis bimaculatus us to study the interactions between the epider mal cells and the substrate on which they spread, on the one hand, and the restoration of the subepidermal tissues and the epithelial-mesenchymal inte ractions preceding scale regeneration, on the other hand. The re-epithelialization process, e.g., migration, spreading, differentiati on, and proliferation of the epidermal cells, has been followed step by ste p, using Light, scanning and transmission electron microscopy, and tritiate d thymidine incorporation, until complete reorganization of the healing epi dermis. Wound healing is fast (500 mu m/hr) and proceeds centripetally from the wou nd margins. The epidermal cells spread on EI wound surface which is compose d of two different matrices: the remains of basement membrane materials cov ering the scale-pockets, and collagen fibrils of cut dermal strips. Even th ough both matrices favour cell spreading and attachment, migrating cells sh ow a different behaviour. The re-epithelialization of the wound follows an orderly sequence similar to amphibian and mammalian wound healing, i.e., a "leap frog" mechanism of cell locomotion involving three epidermal layers. The basal layer cells, which spread on the substrate, and the superficial l ayer cells which protect the epidermis, differentiate first. Whatever the t ype of substrate over which the epithelium spreads (basement membrane mater ial or collagen fibrils), the epidermal basal layer cells differentiate as soon as they become attached. The incorporation of tritiated thymidine has revealed that there is no proliferation in the healing epidermis until afte r complete closure of the wound, but that the rapid re-epithelialization of the large surface requires the recruitment of epidermal cells at the wound margins. The present study offers new data on the dynamics of re-epithelialisation a nd on the resistance of cichlid skin to such wounds. It is also clearly sho wn that the epidermal basal layer cells differentiate rapidly, a step which is interpreted as the first stage of epithelial-mesenchymal interactions t hat will lead to scale regeneration. Anat Rec 254:435-451, 1999. (C) 1999 W iley-Liss, Inc.