Sexual dimorphism in the vertebrate nervous system

Abstract

Many sex differences in the structure of vertebrate nervous systems have been discovered in the past twenty years.The recency of these discoveries is not due to any technological advance; indeed, many of the most interesting of these neural sexual dimorphisms can be detected with nineteenth century methods.Nor can there be much doubt that when the behavior of the sexes differs markedly, there must be sex differences in neural structure mediating those behavioral differences.If previous neuroanatomists assumed that sex differences in brain structure would be too subtle to be detected by current analytical tools, they were proved quite wrong-sex differences appear to be both frequent and prominent features of the nervous system in a wide variety of vertebrates (for surveys of neural sexual dimorphism, see Arnold and Gorski, 1984;Fishman and Breedlove, 1988;Kelley, 1988;De Vries, 1989).In this essay, I discuss progress in the understanding of how these neural sex differences arise, with the twin hopes of identifying those issues that remain unanswered or unaddressed and of convincing readers that the answers to these questions will shed light on the neurosciences beyond the field of sexual differentiation itself.Steroid hormones play a crucial role in the sexual differentiation of the vertebrate periphery.Whether an individual will develop as a male or female is determined by the parental contribution of sex chromosomes or, for some vertebrates, via environmental stimuli such as incubation temperature (Bull and Vogt, 1979).In either case, the direct consequence of this early, sex-determining event is that the initially indifferent gonads will develop as either testes or ovaries.For example, in male mammals this decision is executed by the testis determination factor (TDF) gene on the Y chromosome.If the TDF gene is expressed by the early, indifferent gonads, then testicular development is induced (Berta et al., 1990).In the absence of TDF function, the gonads develop as ovaries.From that point on, hormonal secretions of the gonad direct whether the rest of the organism, including the brain, develops in a male or female fashion.For mammals, testicular secretions provide active guidance for differentiation as a male, while ovarian secretions may assist but are not necessary for differentiation as a female (Wilson et al., 198 1).In other words, in the absence of gonadal secretions, the body, nervous system, and behavior of the animal will develop in a primarily female fashion.The testes, on the other hand, secrete at least two hormones crucial for male development: the peptide Mullerian regression hormone and the androgenic, steroid hormone testosterone (T).Mullerian regression hormone has not been shown to exert any influence on the CNS, but T