[Quantitative analysis of motoneurons innervating the muscle levator ani in rats with anorectal malformation].

OBJECTIVE

To quantify the motorneurons innervating the muscle levator ani (rat equivalent of puborectalis in human) by retrograde tract-tracing in a rat model of anorectal malformation.

METHODS

Anorectal malformation is induced in fetuses by gavage feeding of ethylenethiourea (ETU; 125 mg/kg body weigh) to pregnant outbreed Wistar rats at day 11 of gestation. At E20, fetal surgery was performed to expose the muscle levator ani and fluorogold (FG) was injected. Fetuses were collected 24 hours later, fixed by paraformaldehyde infusion cardially. The lumbosacral spinal cord was removed for cryosection. Serial longitudinal sections (60 micro m) encompassing the whole width of lumbosacral spinal cord were studied with fluorescent microscopy. The number of FG-labelled motoneurons were scored and compared between fetuses with or without malformation in the ETU-injection group, and normal controls.

RESULTS

In the ETU-injection group, 55.6% (95/171) of fetuses displayed anorectal malformation, 56.7% of fetuses displayed anomalies of neural tube. FG was injected to the muscle levator ani of 8 fetuses without defects of anorectum or neural tube, 5 fetuses with imperforate anus only, 10 fetuses with anomalies of neural tube, and 15 fetuses with combined imperforate anus and anomalies of neural tube. Fetuses (n = 9) from non-injected with ETU group were injected with FG as normal controls. The total number of FG-labelled motoneurons in the fetuses without defects of anorectum and neural tube, with imperforate anus, with anomalies of neural tube, with combined imperforate anus and anomalies of neural tube, and normal controls were 109 +/- 38, 55 +/- 26, 48 +/- 30, 54 +/- 29 and 135 +/- 29 respectively. FG-labelled motoneurons in the fetuses with imperforate anus, anomalies of neural tube, and combined imperforate anus and anomalies of neural tube are significantly fewer than that in fetuses without defects of anorectum or neural tube (P < 0.05), and in normal controls (P < 0.005). The number of FG-labelled motoneurons did not differ between the fetuses without defects of anorectum or neural tube and normal controls (P > 0.05).

CONCLUSION

Fewer motoneurons that innervates the puborectalis may contribute to the poor postoperative anorectal function in patients with anorectal malformation.