Hormonal and meta-hormonal determinants of sexual dimorphism

The role of hormones in the determination of sexual characteristics has been known for several decades. It has been shown, for example, that several products, including sex steroids, may influence the body development pattern, metabolic pathways, fat and muscle distribution and vocal cord anatomy, thus producing an overall outcome consistent with a masculine or feminine phenotypic pattern. These qualities are usually described as secondary sexual traits, so as to be distinguished from primary sex traits, usually referring to the gonads and external genitalia. However, it must be noted that hormonal regulation may not explain the full range of the sexual phenotype, since the central nervous system retains a significant role in the establishment of sexual identity, thus giving rise to a higher sex determination stage exclusively described in humans, namely behavioral or psychological sex. Recently, it has been suggested that differences among the sexes are not limited to brain function but they may also refer to anatomical differences and different biochemical profiles, including a distinct pattern of AR and ER distribution. This new aspect of sexual dimorphism suggests a whole system of meta-hormonal regulation, recently described as the sexual brain model. The role of local androgen and/or estrogen concentrations in the initial establishment of brain sexual dimorphism is still under evaluation, since the first results are relatively inconclusive and no direct cause and effect relationship has been proven so far. On the other hand, sex hormones have recently been found to participate in processes well beyond their initially suggested spectrum of action. For instance, ER interacts with EGFR in a number of ways, affecting development of a number of epithelial structures. Estrogen receptors have also been detected in a number of non-classic targets of steroids, such as the brain and the lungs. This observation may imply that sexual dimorphism goes a lot deeper than previously estimated, affecting virtually every organic system, suggesting, in essence, the existence of two different functional models for the whole human body, formulated and conserved throughout the evolutionary progress.