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

肛门直肠管有两个起源;上部来自内胚层，下部来自外胚层。外胚层对耳道的作用过程尚不清楚。为了了解该区域的发育，从胚胎第13.0天（E13.0）到E18.5，每12小时制作一次小鼠胚胎的连续矢状切片。从这些切片中获得三维重建。在泄殖腔膜消失时（E13.5），内胚层衬里到达膜解体的部位。因此，整个管道内胚层起源。初级会阴的背侧端比其他外胚层上皮厚，在这个过渡区域，它从锐角变成钝角。当它伸直并形成会阴管后，次级会阴出现在尾部。在这些过程中，外括约肌出现在厚的外胚层的下间充质中，并起着形成外胚层肛管的拉绳的作用。

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.