Table 1. Cell Death versus Cell Birth.

Sex differences in the size of multiple brain regions have been attributed to sex differences in the rate of cell death, with steroid hormones either stimulating or repressing apoptotic processes. Recent evidence indicates that precise cellular mechanisms mediated cell death are specific for each sub-nuclei, and additional mechanisms likely remain to be discovered. In the rat brain, estrogens are a dominant mediator of sex differences in cell death, with the exception of the SNB of the spinal cord which is regulated by androgens. In contrast to these reproductively relevant nuclei, the hippocampus and amygdala are characterized by sex differences in cell birth, with more new neurons born in male hippocampus compared to female but more new astrocytes born in the female amygdala compared to the developing male.

Brain Region	Cell fate	Sex difference	Mechanism	References
SNB	death	M>F	CNTF, androgen	34, 35, 36
SDN-POA	death	M>F	unknown	32
AVPV	death	F>M	TNFα, caspase	74, 75
BNST	death	M>F	epigenetics	71
Hippocampus	birth	M>F	unknown	77, 78
Amygdala	birth	F>M	endocannabiniods	89

These findings suggest that both cell death and cell birth are important contributors to sex differences in the brain and highlight how much we still have to learn regarding mechanisms that establish the cellular profiles of the male and female brain. SNB – spinal nucleus bulbocavernosus, SDN-POA – sexually dimorphic nucleus of the preoptic area, AVPV = anteroventral periventricular nucleus, BNST = bed nucleus of the stria terminalis, M = male, F = female.