The Effects of Vitamin A and Citra ] on Epithelial Differentiation in vitro 2 . The Chick Oesophageal and Corneal

T H E effects of vitamin A and citral on the differentiation of chick tracheal epithelium in vitro were described in a previous paper (Aydelotte, 1963a). High concentrations of vitamin A inhibited the development of tracheal mucous cells but the epithelium became well ciliated. Citral in low concentrations favoured the differentiation of mucous cells but few ciliated cells developed; in higher concentrations of citral the tracheal epithelium became stratified and occasionally keratinized. The changes produced by citral resembled those in the tracheal epithelium of vitamin A deficient chicks (Aydelotte, 19636) and when vitamin A and citral were both added to the culture medium, the combined effect was intermediate between those given by the two compounds separately. These results, therefore, supported the suggestion put forward by Leach & Lloyd (1956) that citral inhibits vitamin A. The investigation of the effects of vitamin A and citral in vitro has been extended to the oesophageal and corneal epithelia and epidermis of the chick embryo. These epithelia are stratified, and therefore differ from the tracheal epithelium, and, moreover, they are known to be influenced by the environmental concentration of vitamin A. In vitamin A deficient chicks, the oesophageal mucous glands atrophy as they are gradually replaced by keratinizing epithelium (Seifried, 1930; Aydelotte, 19636). The chick corneal epithelium, normally smooth, stratified and squamous, becomes slightly thicker and rough, whilst the adjacent conjunctival epithelium changes from a thin, secretory membrane to a thick, keratinized layer (Beach, 1923; Aydelotte, 19636). Keratinized epidermis, on the other hand, is relatively little affected by deficiency