Dna Synthesis and Proliferation by Villous Epithelial Cells in Fetal Rats

Radioautographic studies with the labeled desoxyribonucleic acid (DNA) precursor, tritiated thymidine, have shown that cell proliferation in mammalian small intestinal epithelium normally is confined to the crypts (I). Following their formation in the crypts, most epithelial cells migrate onto the villi where they lose their capacity to proliferate as they mature into functional absorptive cells. Such restriction of cell proliferation to the crypts has been demonstrated in both adult rats (1, 2) and in young animals 7 days of age (3) and older (4). In fetal rats, formation of villi begins at the 17th day of gestation, and tall, well-developed villi have formed in the proximal intestine by the 19th day of gestation. In contrast, discrete, welldefined crypts do not develop until shortly after birth (5, 6, 7). In this report, we present evidence that in the duodenum of fetal rats, DNA synthesis and cell proliferation occur in epithelial cells along the entire length of the villi and that this unique pattern of cell renewal changes shortly before birth.