[How sex dimorphism is established in the spinal nucleus of Onuf?]

Some perineal muscles are only present in males. They are innervated by motoneurons located in the ventral horn of the sacral spinal cord in the so-called Onuf's nucleus. This nucleus displays anatomical differences between male and female, constituting a sexual dimorphism. It has been experimentally demonstrated that androgens play a crucial role in the development of this dimorphism. Both proliferation and differentiation of motoneurons in this region are similar in male and female. However, in absence of androgens, motoneurons vanish by programmed cell death, the spinal nucleus after this phase contains a low number of neurons. On the contrary, in presence of androgens, motoneurons survive and the spinal nucleus contains a great number of cells. This hormonal effect is not vehiculated via direct action of androgens on motoneurons but by favoring the survival of striated muscle fibers of perineal muscles. These fibers secrete a trophic factor which acts on motoneurons. CNTF (Ciliary Neurotrophic Factor) is thought to play, at least partially, such a role. At last, after this stage of survival, androgens are involved in dendritic growth of these motoneurons. This effect is mediated by the transformation of androgens into estrogens by neuronal aromatase.