Apoptosis and differentiation in the crypt-villus unit of the rat small intestine

This investigation utilized immunocytochemical and fluorescent protocols to analyze the roles of cellular proliferation and apoptosis in the regulation of differentiation and senescence of the rat small intestinal mucosa. Specifically, the study localized apoptotic zones of the villus through the use of enzymatic tags; established the transition point between cell growth and differentiation, i.e. the point of no return where crypt cells differentiate into absorptive cells with barrier functions; and the role that plasmalemmal, cytoskeletal, junctional and extracellular matrix (ECM) elements may play in the regulation of differentiation and migration of epithelial cells from crypt to villus. Apoptosis was relegated to the villus tip forming a prominent 'apoptotic cuff' of cells. Close scrutiny of these cuffs reveals the presence of apoptotic cells adjacent to non-apoptotic (healthy) cells. Mid-villus epithelial cells were non-apoptotic and all cells in the crypt-villus unit expressed Bcl-2 activity. Intestinal lactase expression was prominent in post-mitotic cells along the villus, while cells in the crypt and base were negative for lactase activity. In contrast, all the cells of the crypt-villus unit were intensely reactive for F-actin. Close scrutiny of isolated cells and frozen sections indicates specific localization of actin in the microvillus region, apical cytoplasm, basolateral and lateral plasmalemma which was in close proximity to fibronectin in the basement lamina. Occludin positive junctional networks were prominent at villus tips, where senescent and apoptotic cells were also most prominent, suggesting that tight junctional integrity was essential to barrier, digestive and absorptive functions in all regions of the mucosa.