The Effects of Vitamin A and Citral on Epithelial Differentiation in vitro 2. The Chick Oesophageal and Corneal Epithelia and Epidermis

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Abstract
Differentiation of the oesophageal and corneal epithelia and epidermis of the chick embryo was studied in organ cultures in normal medium and in media containing added vitamin A and/or citral. In normal medium the oesophageal epithelium developed well for the first 6 days, but between 6 and 12 days many superficial layers of cells were sloughed and large mucous cells from the glands spread over the remaining epithelium. Vitamin A inhibited the normal stratification, and the oesophageal epithelium became pseudostratified and folded, with tall, columnar, ciliated cells and many small mucous glands. Citral inhibited the differentiation of ciliated and mucous cells and the epithelium became thicker than in normal medium. Vitamin A and citral together produced either a vitamin A or a citral effect, depending upon the relative concentrations of the two compounds. The corneal epithelium differentiated normally in the control cultures, but under the influence of additional vitamin A it remained thin and the superficial cells secreted mucus. In + citral medium the corneal epithelium became thicker than usual and the conjunctival epithelium keratinized. When both vitamin A and citral were added to the medium the corneal cultures showed a mild vitamin A effect. Skin cultures keratinized well in normal medium, but the epithelium remained thin and became mucus-secreting in response to high concentrations of vitamin A. Citral alone had little effect on epidermis, but it suppressed the vitamin A response almost completely when both compounds were added together. Citral produced changes resembling those of vitamin A deficiency in cultures of oesophagus and cornea, and it reduced the effects of added vitamin A. The results of these experiments therefore give further evidence of antagonism between vitamin A and citral. It is suggested that either deficiency of vitamin A or treatment with citral stimulates, whilst high concentrations of vitamin A inhibit, mitosis in the corneal epithelium. The results of these experiments indicate that each epithelium achieves its maximal synthesis of mucus at a characteristic concentration of vitamin A.