Epidermal Tissue Homeostasis.

. It has previously been shown that in toad epidermis the cell birth rate ( K b ) exceeds the rate of cell loss through moulting ( K b ) and that the ‘surplus’ of cells seems to be removed in a controlled manner. Assuming that the epidermis is non‐expanding, a K b / K d ratio > 1 indicates that cell deletion additional to desquamation takes place. In normal toads this ratio is 2‐3. Following implantation of hydrocortisone pellets into intact toads (release rate, 18 μ/g toad/d), the K b / K d ratio, over a period of 14 d of hormone treatment, had increased to about 7, due mainly to an increased K b and to a lesser extent to a decreased K d . No change in the epidermal cell pool size had taken place. It was previously shown that, following removal of the pars distalis of the pituitary gland, the K b / K d ratio decreased with time, due to a decreasing K b and an increasing K d , eventually leading to a decreased epidermal cell pool size. In this paper it is shown that, in pars distalisectomized toads with hydrocortisone pellets implanted, the K b / K d ratio is restored to control levels by a restoration of the K b as well as the K d . the results differ from those of previous studies in which ACTH or adrenocortico‐steroids were administered discontinuously (by injection). Thus, by experimental manipulation, different K b / K d ratios can be obtained: low (< 1, pars distalis ablation), medium (2‐3, normal toads) and high (7, hydrocortisone implantation). the potentiality of this unique situation in analysing the important question of how the ‘surplus’ cells are deleted is discussed.