ABSTRACT
The epithelial cells of the small intestine are highly differentiated cells expressing a variety of specific functions. As the immature absorptive cells (the principal cells of the intestinal epithelium), goblet cells, endocrine cells, Paneth cells and other minor cell types migrate out of the crypts towards the tip of the villus, they differentiate progressively. Differentiated enterocytes are characterized by the presence of microvilli at their apical surface firmly anchored to and sustained by a cytoskeleton. The small intestinal cells are polarized and their plasma membrane consists of an apical-and a basolateral domain accompanied by tight junctions between adjoining cells, forming a relatively impermeable barrier from the lumen to the systemic environment. The apical surface of enterocytes has its own characteristic set of proteins and this specialized surface, called the brush-border, contains many enzymes including disaccharidases, aminopeptidases and transport enzymes of glucose and amino acids (Hauri et al., 1985). These proteins are found exclusively in the apical domain of the enterocyte.
