An investigation of the proctodeal region with respect to development and breakdown of the cloacal membrane in Xenopus laevis

Abstract

Anorectal malformations are common congenital anomalies which disable the newborn. In order to gain insight into the development and causes of these anomalies, the normal development of the hindgut requires elucidation at the structural and molecular level. During normal embryonic development of the alimentary canal, the proctodeum develops at the posterior end of the embryo and together with the stomodeum, is the only region where the ectoderm and endoderm are directly juxtaposed. The juxtaposed ectoderm and endoderm go on to form the cloacal membrane, which eventually perforates to allow continuity to the exterior. This study investigated morphological features and mechanisms involved in proctodeal development and cloacal membrane dissolution in Xenopus laevis embryos. Embryos were collected, staged and processed for various techniques. Analysis using time course, confocal, scanning electron microscopy and histology revealed the presence of a proctodeal depression by stage 21. This is earlier in development than the initial appearance of the stomodeum. Prominent morphological features found in the proctodeal region included cilia and vacuolated cells. Immunolocalization of the basement membrane between the proctodeal ectoderm and hindgut endoderm, and apoptosis detection proved inconclusive, but the results suggest that they may play a role in hindgut development. BMP-4 expression, often associated with apoptosis and thought to be involved in anorectal malformations, decreased in the proctodeal region as the embryos developed. The combination of morphological and molecular evidence as found in Xenopus laevis provides a greater understanding of hindgut development. These results may stimulate further research into anorectal malformations in an attempt to provide a more comprehensive understanding of the mechanisms involved in the occurrence of these anomalies.