Lamina propria macrophages involved in cell death (apoptosis) of enterocytes in the small intestine of rats.

In the rat small intestine, apoptotic enterocytes are exfoliated at the villus tip as a whole cell, in contrast to guinea pig enterocytes which are phagocytosed by macrophages in their cell body and shed off only in their apical cortex. While macrophages gather in the lamina propria of the intestinal villi in both species, their functions seem to differ. Unlike the guinea pig, lamina propria macrophages observed in the rat small intestine did not show morphological signs of phagocytosis, revealing few cellular elements in their phagosomes. At the "shoulder" of the villus, i.e., a certain distance proximal to the villus tip, subepithelial macrophages extended a thick process deep into the epithelium; their branched terminals penetrated the cytoplasm of enterocytes, resulting in the formation of excavated spaces in the cell body. Processes of macrophages frequently reached close to the brush border. At the shoulder of the villus, a few effete cells showed typical apoptotic signs and appeared to be pushed out into the lumen; still, the shedding of apoptotic enterocytes was recognized mainly at the very top of the villus, where no intraepithelial processes of macrophages could be seen. The present findings indicate that in the rat, lamina propria macrophages do not engulf aged enterocytes, but are involved in inducing their apoptosis.