Comparative development study of the cloacal separation and formation of anal canal

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The anorectal canal has two origins ; the upper part is derived from endoderm and the lower part is derived from ectoderm. The process of ectodermal contribution to the canal is remains unclear. To understand the development of this area, serial sagittal sections of mouse embryos were made every 12 hrs from embryonic day 13.0 (E13.0) to E18.5. Three-dimensional reconstructions were obtained from these sections. At the disappearance of the cloacal membrane (E13.5), endodermal lining reached the site of disintegrated membrane. Thus the whole canal had endodermal origin. The dorsal end of the primary perineum was thicker than other ectodermal epithelium, and in this transitional region it changed from an acute to obtuse angle. After it straightened out and formed the canal, the secondary perineum appeared caudally. During these processes, the external sphincter appeared in the underlying mesenchyme of the thick ectoderm, and functioned as a drawstring to form the ectodermal anal canal.