The Role of Estrogen in Sexual Differentiation

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The Role of Estrogen in Sexual Differentiation

Most, if not all, species with two sexes exhibit sexually dimorphic behavior and physical characteristics. These dimorphisms can be attributed to differences in the brain, such as size or function of structure, and these brain structures can be affected by the hormones circulated throughout the organism. It has been held that the sexual dimorphisms rely only on the presence or absence of androgen, namely, testosterone, during the critical period of development for an organism; however, new research suggests that the presence of estrogen, specifically estradiol, has an active role in sexual differentiation.

Several sexual dimorphic structures in the brain have been observed in laboratory experiments. The corpus callosum in male rats is much larger than that in female rats, and this size difference is uncorrelated with total brain weight. These findings led many to investigate the relationship between human male and female corpus callosa. A paper published by de Lacoste-Utamsing and Holloway stated that the splenium of the callosum is larger in women than in men, but their finding has since been challenged by several reports stating that there exists no sexual dimorphism. Analysis done from 1982-1994 reveals a small difference of corpus callosum size in favor of males, but it is hypothesized that age, handedness, overall brain size and weight, and incorrect statistics were not taken into account. (3)

There has also been controversy in the research involving the brain region INAH-3 in humans. The heterosexual male INAH-3 is larger than that of heterosexual females; the INAH-3 in homosexual males is on the average smaller than that of heterosexual males and approximately the same size of heterosexual females. The general population has attempted to use this fact as an explanation of the biological basis of homosexuality, though the differences in structure may not be causally related to the sexual orientation of the man. Because we can only observe behaviors when doing experiments with lab animals, the data cannot firmly establish a basis for sexual orientation.

The traditional view on sexual differentiation is that organizational effects from hormones which occur during neonatal development are the master plan for the organisms sex and corresponding behaviors and characteristics. Exposure to androgen, namely, testosterone, would result in a male organism, while exposure to neither androgen nor estrogen would result in the default sex: female.

Characteristics resulting from organizational effects include formation of genitalia and traits such as aggression.
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