The rectum and anal canal is the most distal part of the gastrointestinal tract, and it is known to have developed and formed in the embryonic period. Congenital anomalies in these regions known as the imperforate anus or anorectal malformation
are
not
uncommon disease with various clinical presentations, and their anatomical structures closely related with developmental stages during the embryonic period. Their morphologic classification depends on the basis of embryologic development, and it
is
also
important to understand the normal anatomy in adult.
The classical knowledges about the embryology of gastrointestinal tract which included the rectum and anal canal was well summarized in the textbook of embryology which was written by Keibel and Mall in 1912. Many studies about the anal canal and
rectum
were reported therafter, but confusions exist due to lack of absolute standard criteria.
To solve such problems, developmental stage was contrived and developed with a long history by many authors and it is now commonly used in grouping of the embryologic development. In is de Vries and Friedland in 1974 who first studied about the
rectum
and anal canal in human embryos using developmental stage.
The author analyzed the development of the rectum and anal canal of 31 cases of human embryos which were classified with Carnegie stage, and observed its changes of morphology by using three-dimensional computerized reconstruction and microscopic
observations.
@ES The results were as follows:
@EN 1) The cloaca was in primitive state until state 12, and it began to differentiate as the mesonephric duct opened to the cloaca between stage 13~14.
2) The urorectal septum appeared in stage 15 and it grew rapidly in stage 16 and fused with the cloacal membrane in stage 19.
3) The urorectal septum grew along the plane between the mesodermal tissues around the bilateral urorectal cleavago line.
4) The external cloaca was not present in human embryos.
5) The anal membrane ruptured between stage 20~23 and it does not seem to be associate with developmental stage.
6) Layers of the wall of the rectum and anal canal could be differentiate each other and most of structures were formed within its wall except anal column, anal sinus and transverse rectal folds at the end of the embryonic period.
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