Sexual dimorphism in the number and size of SNB motoneurons: delayed development during normal ontogeny

The spinal nucleus of the bulbocavernosus (SNB) is a sexually dimorphic pool of motoneurons that innervates the perineal musculature. In the Mongolian gerbil, the SNB lies dorsolateral to the central canal within the lumbosacral spinal cord. Previously, no information was available on the normal development of the sexual dimorphism of this structure in the Mongolian gerbil, although evidence exists for a peripubertal development of the SNB in the gerbil. At each age from postnatal day 1 (PND1) through PND15 and at PND25, male and female gerbils were aldehyde perfused. Spinal cords were gelatin-embedded, cryoprotected, frozen and sectioned coronally through the lumbosacral transition zone and stained for Nissl substance with thionin. Examination by light microscopy revealed that the number of visible male and female SNB motoneurons significantly increased from PND1 through PND25. The size of the motoneurons also significantly increased in both sexes, however, until PND15 male gerbil SNB showed two significantly different sized populations of motoneurons. These data suggest the development of the SNB in the Mongolian gerbil is delayed, compared to the rat, and may continue well beyond the perinatal time period.