Paper - The staged sequential development of the anus and rectum in human embryos and fetuses

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I have decided to take early retirement in September 2020. During the many years online I have received wonderful feedback from many readers, researchers and students interested in human embryology. I especially thank my research collaborators and contributors to the site. The good news is Embryology will remain online and I will continue my association with UNSW Australia. I look forward to updating and including the many exciting new discoveries in Embryology!

de Vries PA. and Friedland GW. The staged sequential development of the anus and rectum in human embryos and fetuses. (1974) J. Pediatr. Surg., 9(5): 755-69 PMID 4424274

The Staged Sequential Development of the Anus and Rectum in Human Embryos and Fetuses

By Pieter A. de Vries and Gerald W. Friedland


Human embryos do not normally have an “external cloaca”; they have at the most, a slight anal depression, but no proctodeum. The division of the cloaca into the anus and urogenital sinus is a process which involves spatially continuous mesoblastic and endodermal proliferations of the cloacal wall. This results in the establishment of a horseshoe-shaped septum, which constricts onto the cloacal membrane because of cellular growth.

The cloacal folds represent the heaped-up mesoblast around the cloacal orifice. Within the folds at the locus of the urorectal septum in front of the anus, the mesoblast is an inseparable part of the septal structure. In contradistinction, the separated anal tubercles fuse medially to separate the cloaca from the tailgut. These folds do not, however, contribute to the formation of an “external cloaca.” The apparent cephalocaudal and dorsoventral growth is explicable within the recognized parameters of axial cell migrations and proliferations.

Cite this page: Hill, M.A. (2020, September 26) Embryology Paper - The staged sequential development of the anus and rectum in human embryos and fetuses. Retrieved from

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