Normal and abnormal embryonic development of the anorectum in rats

BACKGROUND

The embryologic and pathologic aspect of anorectal malformation (ARM) remains poorly understood. There is no universally accepted theory to explain anorectal embryology and the abnormal development that produces ARM. The aim of this study was to observe the developmental processes of anorectum in rats and to explore the abnormal embryonic development that leads to ARM.

METHODS

Rat embryos with ARM were obtained by treating pregnant rats with administration of ethylenethiourea (ETU). Normal rat embryos and embryos with ARM from gestational days 12.5 to 20 were sectioned serially and sagittally and stained with H & E. The relevant structure including cloaca and urorectal septum (URS) were examined in a temporospatial sequence.

RESULTS

Characteristics of anorectum development in ARM rat embryos treated by ETU were as follows: (1) URS never fused with cloaca membrane. (2) Dorsal cloacal membrane was maldeveloped. (3) Cloacal configuration was abnormal. (4) Tail groove never appeared. All type of ARM was the rectourethal fistula and common cloaca in ETU-treated rat embryos and was discernible on gestation day 16.

CONCLUSIONS

Absence of the tail groove and maldevelopment of the dorsal cloacal membrane, cloacal configuration, and urorectal septum are likely to be responsible for the formation of ARM. Failure of fusion of the URS with cloacal membrane is the immediate reason for rectourethral fistula or common cloaca in ETU-treated rats.

Development of anorectal malformations using etretinate

PURPOSE

To investigate the pathogenesis of anorectal malformations (ARM), the authors studied cell proliferation and programmed cell death (apoptosis) patterns in murine embryos that develop ARM as a result of administering an overdose of etretinate, a long-acting vitamin A analogue (retinoid).

METHODS

Pregnant mice were fed 60 mg/kg of etretinate on the ninth gestational day (E9). Embryos were obtained between E9.5 and E13, and prepared for histological study. Cell proliferation was examined using proliferative cell-specific nuclear antigen (PCNA) expression. Apoptosis was identified by detecting in situ DNA fragmentation using the TdT-mediated dUTP-digoxigenin nick end-labeling (TUNEL) method.

RESULTS

Over 95% of etretinate-treated embryos had ARM including rectoprostatic urethral or rectocloacal fistula. In the histological study, ARM embryos showed defective cell proliferation in the cloacal membrane and excessive apoptosis in the dorsocaudal region on E11, which resulted in a lack of apoptosis in the anal orifice and a short tail on E12, respectively. Cells forming the urorectal septum showed the same pattern of cell proliferation and apoptosis both in ARM embryos and the controls. These results suggest that impairments of embryonal cellular dynamics in the cloacal membrane and dorsocaudal region induce some types of ARM.

Imperforate anus associated with a recto-bulbar-cutaneous fistula

This is the report of two newborn boys with imperforate anus associated with a long rectocutaneous fistula running deep into the scrotum and communicating, in its midportion, with the bulbar urethra. The findings are compared with those of a somewhat similar case in the literature, with some speculations as to the possible embryogenesis of the lesion.