Quantitative analysis of motor neurons of the levator ani muscle in fetal rats with spina bifida occulta

Background

With the combination of microsurgery and microinjection techniques, we investigated the development of motor neurons in the spinal cord of fetal rats with spina bifida occulta by injecting the retrograde trace FG into the levator ani muscle.

Methods

The fetal rats were divided into 3 groups. On the day 9 of gestation, 6 mature Wistar rats (weighing 250-300 g) in the control group (group 1) were subcutaneously injected with 0.5 mL of normal saline at their hind limbs at 9:00 am and 4:00 pm. At these 2 time points, 15 rats in the treatment group (group 2 and group 3) were subcutaneously injected with 20% sodium valproate solution (400 mg/kg of body weight) at their hind limbs, too. On the day 20 of gestation, pregnant rats were anesthetized with 10% chloral hydrate (300 mg/kg of body weight) intraperitoneally, and then fetal microsurgery and microinjection were performed to expose the levator ani muscle, whereas 5% FG was administered with microinjector. Twenty-four hours later, transcardial perfusion of 4% paraformaldehyde in phosphate-buffered saline (PBS) was given to the operated fetus. After the spine sample was stained with Alcian blue GX, the image of stained spine was measured using a computer system for the distance of the 2 cartilaginous ends of the vertebra arch. Then, the lumbosacral spinal cord was cryopreserved in 20% sucrose in PBS for a later serial transverse cryosection after 24 hours. The FG-labeled motor neurons were visualized with a wide-band ultraviolet-fluorescent filter, and the number of the FG-labeled motor neurons was recorded. Nine fetal rats survived in group 1. Eighteen fetal rats survived in the treatment group, including 7 (with no malformation) of 18 fetuses in group 2 and 11 fetuses with spina bifida occulta in group 3.

Results

The FG-labeled motor neurons in the ventral horn of normal spinal cord clustered at the dorsolateral and dorsomedial corner of the ventral horn. The FG-labeled motor neurons in the ventral horn of deformed spinal cord were less than that of normal spinal cord, and the motor neurons were scattered around the space between the dorsomedial and dorsolateral corners. The number of FG-labeled motor neurons was 244 ± 41 in group 3, 426 ± 36 in group 1, and 397 ± 20 in group 2. The data were stastistically significant if P < .05.

Conclusion

The motor neurons that innervate the levator ani muscle in fetal rats with spina bifida occulta are fewer than the normal fetal rats, and they are arranged in abnormal distribution.