The differentiation and redifferentiation of the intestinal epithelium and its brush border membrane

This paper briefly reviews the progress of investigations during the past 30 years into the development of the intestinal epithelium, and surveys the directions in which research in this field is now advancing. Early studies showed that the differentiation of the epithelium is a discontinuous process, and also focused attention on the enzyme specialization of the brush border. The phases of differentiation were found to be under the control of the pituitary--adrenal system, with the outflow of glucocorticoids influencing both structural and biochemical aspects of enterocyte development in fetal and postnatal stages. More recent evidence has shown that thyroid hormones can also regulate epithelial differentiation, even in the absence of adrenocortical function. Organ culture is now becoming an important means of probing into the processes underlying overt differentiation, and methods for culturing pure populations of enterocytes are being perfected. Studies of developmental alterations in the constituents of the brush border membrane, and of synthesis and glycosylation of membrane constituents, are contributing to the development of an integrated account of the differentiation of this membrane, through which all nutrients must pass.