Sexual dimorphism in the liver.

That the liver in oviparous females supplies the major part of the egg yolk proteins requires a marked degree of sexual dimorphism of this organ. In addition to vitellogenin, several minor components (e.g. vitamin binding proteins) are supplied by the liver to the oocyte in oviparous animals and to the developing embryo in viviparous females. Other metabolic adjustments to maintain reproductive competency of the female (e.g. increased lipid synthesis, detoxification of the waste products of the developing embryo, and reproductively sensible steroid metabolism) are some of the physiological bases for the differences between the female and male liver. Sex-differences in several other hepatic proteins, enzymes, and hormone receptors have also been established. alpha 2mu Globulin, Bond's protein, and carbonic anhydrase are clear examples of the sex specificity of rat liver. Differential expression of the genes for the male- and female-specific proteins in the liver is brought about by the androgenic and estrogenic hormones. The hepatic receptors for these hormones also show a marked degree of sexual dimorphism. During development and aging, these receptors seem to appear when the need for these hormones is most critical. The timely appearance of the hepatic estrogen and androgen receptor and the facilitated action of these hormones are mediated through "pre- and neonatal imprinting" by the sex hormones, especially androgen. Exploration of the physiological and molecular basis of this "imprinting" mechanism remains an exciting area of contemporary endocrinology.