The migration of intestinal intervillous epithelial cells labeled in the fetus was followed in neonatal mice. At 17 days of gestation, a first group pregnant mice received three intraperitoneal injections of 3H-thymidine (150 μCi/injection) administered at 30 min intervals. Two mothers were sacrificed 3 hours after the first injection. Mice from different litters were also sacrificed on days 0, 2, 4, 8, 12, 14, and 16 after birth. A second group of pregnant mice was injected at 18½ days of gestation and offspring were sacrificed on days 6, 8, 10, 12, 14, and 16 after birth. Segments of duodenum and ileum were fixed in glutaraldehyde, postfixed in osmium tetroxide, dehydrated, and embedded in Epon. Sections were stained with aldehyde fuchsin and processed for radioautography. By following the leading front and trailing edge of labeled cells in the longest villi of the duodenum and ileum, we observed that 1) extrusion zones become active immediately after birth and 2) the longest villi do not elongate until 10 days after birth in the duodenum and 14 days in the ileum, that is, when all labeled epithelial cells originally present in the fetus have been extruded. Moreover, by measuring the distance between the internal limit of the inner circular layer of smooth muscle and the intervillous epithelium at 17 days of gestation (12.95 ± 1.18 μm) or the bottom of the crypts at day 3 (14.81 ± 0.91μm), we propose that crypts do not develop as downgrowth: rather the intervillous epithelium is reshaped and the crypt-villus junction moves upward, away from the muscularis externa.
In conclusion, by analyzing the growth pattern of the longest villi in neonatal mice, we have found that a steady state between cell production and cell extrusion is reached at birth in the duodenum and by day 4 in the ileum. This steady state will be lost however, after day 10, thus enabling lengthening of the villi.