Brain Imprinting During Gonadal Development

The development of sex differences in human beings and other mammals is primarily triggered by the presence of X and Y sex chromosomes. Exposure to gonadal hormones at various stages of development contributes to sex-specific behaviors. Notably, various organs, including the brain, liver, and immune system, exhibit sex differences under the influence of genetic factors and hormonal milieu. Sexual differentiation of the human brain is primarily determined by surge of sex steroids during three critical periods of development namely, embryogenesis, the perinatal period, and puberty, where sexually dimorphic peaks of gonadal hormones are found. Gonadal hormones act on the fetal brain, producing either an activation effect, resulting in a transient modification, or an organization effect, leading to a permanent modification of neuronal function. Testosterone exposure during the critical periods of sexual development induces changes in the sexually dimorphic nuclei of the hypothalamus, contributing to male gender identity. In females, genetic factors play a more significant role, and hormonal influence on the brain is comparatively less pronounced. Recent research has highlighted the role of transcription factors, signaling molecules, and the immune system in the sexual differentiation of the brain. Understanding the concept of sexual differentiation in both brain structure and function is crucial for making informed decisions regarding sex assignment and the clinical management of infants with ambiguous genitalia.