Abstract
Sex differences in the nervous system come in many forms. Although a majority of sexually dimorphic characteristics in the brain have been described in older animals, mechanisms that determine sexually differentiated brain characteristics often operate during critical perinatal periods. Both genetic and hormonal factors likely contribute to physiological mechanisms in development to generate the ontogeny of sexual dimorphisms in brain. Relevant mechanisms may include neurogenesis, cell migration, cell differentiation, cell death, axon guidance and synaptogenesis. On a molecular level, there are several ways to categorize factors that drive brain development. These range from the actions of transcription factors in cell nuclei that regulate the expression of genes that control cell development and differentiation, to effector molecules that directly contribute to signalling from one cell to another. In addition, several peptides or proteins in these and other categories might be referred to as 'biomarkers' of sexual differentiation with undetermined functions in development or adulthood. Although a majority of sex differences are revealed as a direct consequence of hormone actions, some may only be revealed after genetic or environmental disruption. Sex differences in cell positions in the developing hypothalamus, and steroid hormone influences on cell movements in vitro, suggest that cell migration may be one target for early molecular actions that impact brain development and sexual differentiation.