RESTORATION OF THE SUBEPIDERMAL TISSUES AND SCALE REGENERATION AFTER WOUNDING A CICHLID FISH, HEMICROMIS-BIMACULATUS

Experimental scale regeneration in fish is used as a means to study th e epidermal-dermal interactions controlling organogenesis. Scales and epidermis were removed from a large surface area (1 cm(2)) of the flan k in the cichlid Hemichromis bimaculatus, and scale regeneration and r estoration of the subepidermal tissues were studied using light and el ectron microscopy. In addition, cell proliferation in these regenerati ng dermal tissues was studied using tritiated thymidine incorporation. The original squamation pattern was entirely restored in the wound re gion, but a delay was observed in the central region compared to the p eripheral regions in which normal regeneration had occurred. This dela y was the consequence of the osmotic shock in wound regions that were exposed to the external environment for a long time because of the lat e covering of the wound by the healing epidermis. However, the osmotic shock was not as severe as expected because two means of protection o f the skin are involved: the scale-pocket lining (SPL) cells function as a barrier resisting the osmotic shock, and an exudate produced by t he wounded tissues inhibits water penetration. Scale regeneration was initiated in each scale pocket where the healing epidermis entered in contact with the SPL cells, and it proceeded centripetally from the ma rgins to the center of the wound, as did the healing of the epidermis. The shape and size of the regenerated scales reflected this centripet al re-epithelialization, suggesting that the first signal for scale re generation came from the epidermis. In the peripheral regions, which w ere rapidly protected from osmotic shock, the scales regenerated in a few days, whereas in the central regions, the dermis had to be restore d before scales were able to regenerate, approximately 14 days after s urgery. Healing of the dermis involved classical processes including m igration, and proliferation of fibroblast cells, followed by extracell ular matrix deposition (mainly collagen). The regenerated scales forme d in the central region of the wound differed from normal in that new scale-forming cells had to be recruited from the unwounded surrounding regions of the dermis, because of destruction of the SPL cell populat ions. J. Exp. Zool. 281:305-327, 1998. (C) 1998 Wiley-Liss, Inc.