Apoptosis during regression of the tailgut and septation of the cloaca

Purpose: Apoptosis is involved in the embryonic morphogenesis of many organ s. The current study was undertaken to ascertain the role of apoptosis duri ng cloacal development in the rat. Methods: One hundred five rat embryos, ranging from gestational days 11 to 16, were sectioned serially in the transverse or sagittal planes and staine d with H&E. The cloaca, urorectal septum, rectum, urogenital sinus, Wolffia n ducts, and tailgut (TG) were examined consecutively in temporospatial seq uence. Results: The tailgut immediately distal to the hindgut starts to regress by apoptosis on day 12 of gestation in a craniocaudal direction and has regre ssed completely by day 13.5. A large number of apoptotic cells and debris c an be identified in the urorectal septum during cloacal septation, Vacuoles are formed by coalescence of apoptotic cells at the tip of urogenital sinu s from day 15 to 16, and, at the same time, sporadic apoptotic bodies in th e anal membrane contribute to its thinning. Conclusion: Results of the current study confirm that apoptosis occurs in a specific temporo-spatial sequence in the hindgut and cloaca and appears to be an important mechanism in TG regression, uro-rectal separation, urethra l opening, and rupture of the anal membrane. Copyright (C) 2000 by W.B. Sau nders Company.