On the nature of tissue interactions in embryonic skin

Abstract

s TUDIES of the interactions between epithelia and mesenchyme in developing embryonic organs have shown that mesenchyme has an important influence on epithelial growth and differentiation in, for example, salivary gland [5], pancreas [3], feather germs [ill, and preen gland [4]. In skin, McLaughlin [8, 91 and more recently (since this investigation began) Wessells [13] have found that the proximity of mesenchyme is necessary for the continued survival of epidermis, and for its normal differentiation. The mechanism of this dependence is less clear, but the participation of the intercellular material in such interactions has frequently been suggested (e.g. [6, 7, 91). In order fully to understand the relationship between tissues, it is necessary to know which components of one tissue contribute to the continued growth, differentiation, and functioning of the other. The work briefly reported here is an attempt to analyse the relationship between embryonic epidermis and dermis, in order to determine which mesenchymal constituents affect epidermal survival, differentiation and keratinization. The experiments were made on the scaly skin of the anterior tarsometatarsal region of the 12-day embryonic chick. After the skin had been treated with 0.04 per cent Versene (ethylene diamine tetra-acetic acid, di-sodium salt) for 20 min at 20°C the epidermis could be easily and cleanly peeled from the dermis. The dermis was subjected to various experimental treatments, and the two tissues were then recombined and cultured in vitro on cellulose acetate rafts [IO] on clots made of plasma and embryo extract [2]. The explants were fixed in acetic-Zenker for histological examination. The epidermis at 12 days consists of a layer of columnar basal cells and a single stratum of intermediate cells, above which is a two layered periderm. After separation and recombination with epidermal basal cells next to the dermis, both dermis and epidermis grew well, the periderm differentiated rapidly then degenerated, and layers of birefringent keratin were produced (Fig. 1). Isolated epidermis, however, did not survive under these conditions; the basal cells lost their columnar orientation within 3 hr, the epidermal sheet contracted and thickened, and the organization of cell layers was lost. Mitosis ceased after 18-24 hr, and most of the lower cells became necrotic, only a few cells showing traces of keratin, although the periderm underwent some differentiation (Fig. 2). No basement membrane formed. This behaviour occurred when isolated epidermis was placed on rafts, on millipore filters, or directly on the clot. Dermis was treated with trypsin to obtain a suspension of cells almost free of dermal intercellular material. When this suspension was placed on the basal surface of the epidermis, the orientation of the basal cells and the organization of cell layers were maintained, and a basement membrane stainable with the periodic acid-Schiff