The Importance of Mesenchymal Factors in the Differentiation of Chick Epidermis

Abstract

ABSTRACT Experiments were undertaken to investigate the degree to which the underlying mesenchyme influences the differentiation of an epithelium. Isolated epidermis of the 5-day chick embryo was explanted in vitro in contact with three different types of mesenchyme from chick embryos of the same age. These mesenchymes were derived from the gizzard, proventriculus, and heart. In a control series epidermis was re-implanted on limb mesenchyme. In each combination the epidermis behaved in a different and characteristic way. The cellular processes by which an isolated epithelium becomes established on mesenchyme in vitro are described. In the control series, epidermis re-implanted on limb mesenchyme keratinized normally, but in contact with cartilage it degenerated. On gizzard mesenchyme epidermis is prevented from keratinizing and is induced to secrete mucus and sometimes to become ciliated. On proventriculus mesenchyme epidermis is initially prevented from keratinizing and secretes mucus, but after about 7 days it reverts to its normal differentiation and keratinizes. Expiants of heart mesenchyme become subdivided into two regions: a central myoblastic area surrounded by a zone of fibroblastic outgrowth. On the central region of myoblasts the epidermis is prevented from keratinizing and spreads to a single-layered squamous epithelium. On heart fibroblasts, however, the epidermis keratinizes more densely than in the controls. The f our types of mesenchyme differ in the nature of the intercellular material that they contain, and in the amount of epithelial spreading they permit. The various effects of the mesenchyme are probably exerted specifically by the fibroblasts, which arrange themselves in contact with the epithelium in most types of culture. These epithelio-mesenchymal interactions are discussed in relation to the initiation and maintenance of embryonic and adult differentiation. It is suggested that the intercellular material of the mesenchyme may participate in the mechanisms underlying these reactions.