Front Matter

Why Testosterone is Essential for Engineering a Male Brain 203

volume than that of females (Figures 7.11 and 7.12). This correlates with quan­
titatively greater sexual intercourse behavior exhibited by males [110,111].
This difference in nuclear volume has been attributed to the presence of more
neurons in the dimorphic nucleus of the preoptic area of males than of females
[112]. Sex hormones determine neuron number in part by controlling the
occurrence of cell death during brain sexual differentiation in rodents [113].
In addition, gonadal steroids have direct effects on the size and the dendritic
growth of a particular part of the brain region, which also contribute to volume
differences in these regions.
In a study carried out by Reddy et al. [114] in sheep, a cluster of neurons was
found to exist bilaterally in the central portion of the medial preoptic area. This
structure is called the ovine sexually dimorphic nucleus (oSDN) because it is
approximately two times larger in heterosexual males and contains more neu­
rons than in rams that are homosexual or in ewes. The oSDN develops prior
to birth and is enlarged in females by exposure to exogenous testosterone
during a prenatal critical period that occurs after the external genitalia have
differentiated. Exposure of adult sheep to exogenous testosterone does not
affect oSDN volume, confirming that nuclear size in the male is pre‐engineered

Mullerian duct

Genital ridge

Mesonephros

Wolffian duct

AMH

Epididymis

Testis

Vas deferens

Ovary

Oviduct

Uterus

Seminal vesicle Vagina

Male Female

Figure 7.11 Differentiation of the male and female reproductive systems does not occur
until the fetal period of development. Source: Adapted from https://upload.wikimedia.org/
wikipedia/commons/thumb/e/e3/2915_Sexual_Differentation‐02.jpg/300px‐2915Sexual
Differentation‐02.jpg