The relation between cell proliferation, differentiation and ultrastructural development in rat intestinal epithelium.

The ultrastructural development of the principal cells in rat small intestine was studied by morphometric analyses in relation to the exact cell position along crypt and villus. From the bottom to the tip of the crypt, a gradual increase occurred in absolute size of the total cell, the cytoplasm, the terminal web and of nearly all cell organelles. Also, the relative size of the cytoplasm, mitochondria, microvilli and endoplasmic reticulum increased during crypt cell differentiation. No sudden changes in ultrastructure were observed in the so-called "critical decision zone", normally located halfway up the crypt where the proliferative activity ceases. At the crypt-villous junction a 1.4-3 fold increase in cell size, cytoplasm, terminal web and of most organelles was noted. Expansion of the proliferative cell compartment over the total length of the crypt as occurs during recovery after a low X-irradiation dose (72 h after 400 R) does not affect the normal development of cellular ultrastructure. These findings are discussed in relation to biochemical and cell kinetic data.