TISSUE KINETICS, ION-TRANSPORT, AND RECRUITMENT OF MITOCHONDRIA-RICH CELLS IN THE SKIN OF THE TOAD (BUFO-BUFO) IN RESPONSE TO EXPOSURE TO DISTILLED WATER

Mitochondria-rich cells (MRC) of the amphibian epidermis are responsib le for active chloride uptake at low external salinity, and new MRCs a re recruited in response to exposure to distilled (deionized) water. T he time-course of this recruitment, the tissue kinetics and ion transp ort have been studied in toads (Bufo bufo) immediately before, and aft er 2,7, and 14 days exposure to distilled water. General epidermal str ucture was not affected. However, the numbers of MRCs per mm(2) (D-MRC ) increased throughout the experiment as revealed by staining of epide rmal sheets with AgNO3 (Ag) or methylene blue (MB). Part of the increa sed D-MRC was accounted for by an increase in MRC subpopulation(s) tha t stained neither with Ag nor MB. The cell birth rate (K-b) decreased and cell loss by moulting (K-d) increased without any significant chan ge in epidermal cell pool size, indicating a reduced apoptotic rate. T he increase in D-MRC was accompanied by a 3-fold increase in Cl- curre nt (I-Cl). At day-2 there was a transient reduction in the I-Cl per MR C. H+ secretion was progressively reduced during prolonged exposure to distilled water. Thus, at day-2 MRCs appeared incompletely differenti ated as indicated by decreased I-Cl and H+ flux per MRC, and by the in creased proportion of MRCs unstained by Ag or MB. Full Cl- (but not H) transport capacity, was restored at day-7. We conclude that increase d D-MRC following exposure to low external Cl-, rather than being due to an increased K-b, is the combined effect of a decreased apoptotic r ate and an increased rate of differentiation, where 'morphological dif ferentiation' precedes 'functional differentiation'.