Development of the human tail bud and splanchnic mesenchyme.

The purpose of this paper was to shed some light on anorectal development from a viewpoint of the tail bud and splanchnic mesenchyme for better understanding of the morphogenesis of the human anorectum. Human embryos ranging from Carnegie stage 11 to 23 (CS 11 to 23) were adopted in this study. Seventeen embryos preserved at the Congenital Anomaly Research Center of Kyoto University Graduate School of Medicine were histologically examined. The cloaca, extending caudally to the hindgut, was dramatically enlarged, particularly both its dorsal portion and membrane, that is, the cloacal membrane resulting from the development of the tailgut derived from the tail bud. The splanchnic mesenchyme surrounding the hindgut was spread out in the direction of the urorectal septum ventrally, suggesting that it participated in the formation of the septum. No fusion of the urorectal septum and the cloacal membrane was found. The splanchnic mesenchyme proliferated and developed into smooth muscle (circular and longitudinal) layers from cranial to caudal along the hindgut. The tail bud seems to cause both the adequate dilation of the dorsal cloaca and the elongation of the cloacal membrane; its dorsal portion in particular will be necessary for normal anorectal development. The splanchnic mesenchyme developed and descended toward the pectinate line and formed the internal sphincter muscle at the terminal bowel.