The role of cell death in the development of a sexually dimorphic neuromuscular system
Hormonally regulated cell death is thought to underlie many sex differences in the nervous system, but little is known about the molecular mechanisms involved. This question was investigated in a sexually dimorphic neuromuscular system in mice. Motoneurons in the spinal nucleus of the bulbocavernosus (SNB) innervate striated perineal muscles including the bulbocavernosus (BC) and levator ani (LA). In rats, SNB cell number and BC/LA muscle size are similar in males and females prenatally, but the postnatal persistence of this neuromuscular system is androgen dependent. Androgens reduce cell death of SNB motoneurons during a perinatal critical period and, as a result, males have more SNB motoneurons than do females in adulthood. The Bcl-2 family of proteins, which includes anti-apoptotic (e.g., Bcl-2) and pro-apoptotic (e.g., Bax, Bak) family members, regulates cell death in neurons and other tissues. I found that Bax is essential for the normal sex difference in SNB motoneuron number in adult mice. Experiments to characterize the time course of developmental cell death in the SNB of mice proved inconclusive based on counts of retrogradely-labeled SNB motoneurons and pyknotic cells in the SNB region, but suggest that developmental cell death may overlap with a period of late secondary SNB migration. Further, SNB motoneuron location differs between perinatal male and female mice.
It's controversial whether the sex difference in perineal muscle size is due to sexually dimorphic cell death or to androgen dependent growth, especially of the LA. We find that Bax deletion slightly increased LA fiber number in adult females, but did not eliminate the gross sex difference in perineal muscle size. Modest effects of Bax deletion on the perineal muscles in adulthood may result from functional redundancy between Bax and Bak. In confirmation, I found that Bax/Bak DKO females have a more than 10-fold increase in LA fiber number over that in wild type females. In addition, wild type females have a higher density of TUNEL-positive cells than do males in both the BC and the LA during perinatal life. Thus, cell death makes an important contribution to the sexually dimorphic development of the BC and the LA.