Development and loss of early projections in a sexually dimorphic rat spinal nucleus.

The spinal nucleus of the bulbocavernosus (SNB) contains 3-4 times more motoneurons in adult male rats compared to females. This large dimorphism in motoneuron number is produced perinatally by an androgen-regulated cell death. To determine if the early projections of the SNB to its target musculature may be involved in the creation of this sexual dimorphism, and how these projections might interact with androgens, HRP tracing techniques were used to retrogradely label SNB motoneurons during prenatal and postnatal development in males, females, and masculinized females. HRP labeling revealed that the prenatal formation of early projections of the SNB in males and females is comparable. SNB motoneuron number increases through the day before birth in all groups, and during this increase, labeled cells can be seen outside the SNB, which we hypothesize are in the process of migrating into the SNB from the lateral motoneuron column. Postnatally, SNB motoneuron number declines, especially in females, and by postnatal day 10 the sexual dimorphism in cell number and projections has been established. These results indicate that although masculine androgen levels are critical in determining SNB motoneuron survival, they are not necessary for initial axon outgrowth of SNB motoneurons. However, androgens may be involved in the regulation of SNB motoneuron migration and the stabilization of the peripheral projections of the SNB. Both male and female SNB motoneuron axons are present at their target muscles during the time in which sex differences in motoneuron number develop, suggesting that the interaction of SNB motoneurons with their targets could be involved in the dimorphic regulation of cell survival.