Hormonal Influence on Sex Determination

Introduction: Steroid hormone action in the nervous system can be either activational or organizational. Hormones can activate target cells in specific social contexts, but these effects are transient because they depend on the presence of the hormone. In contrast, steroid hormones can organize the nervous system structure during development in permanent ways that persist beyond the period of developmental exposure to the hormone and establish a neural pattern for later activational responses in adulthood. Click for More Info Humans: It has been well established that in adolescence, the elevated levels of androgen and estrogen result in the appearance of secondary sex characteristics, such as facial hair in boys and breast development in girls. In addition, the two periods when the human brain is subject to steroid-dependent organization of neural circuits and behavior is around the time of birth and during adolescence. The onset of puberty is a brain event rather than a gonadal event. Based on both internal and external signals, the nervous system initiates gonadal maturation to complete the process of sperm and egg development as well as stimulate structural changes in regions of the adolescent brain. White matter volume increases through adolescence while gray matter first increases and then decreases to the adult volume in the early to mid 20's. Hippocampal and amygdalar volumes increase in a sex-dependent manner, with hippocampal enlargement occurring only in females and amygdala enlargement only in males. Gonadal hormones organize numerous neural circuits and behaviors during adolescence. Adolescence is clearly pivotal for behavioral development, especially in view of the potential for hormones to permanently affect the brain’s capacity for change. A particularly exciting and controversial discovery of a link between sexual behavior and brain structure concerns homosexuality in humans. Simon LeVay, then at the Salk Institute for Biological Studies in San Diego, reported that the size of a nucleus in the anterior hypothalamus of homosexual men more closely resembles the comparable structure in women than that in heterosexual men. Daughters born with a male co-twin had a reduced lifetime reproductive success compared to those born with a female co-twin, both because such daughters experienced a lower chance of marrying as well as producing fewer children. Nutritional and social factors did not affect the results, leading to the conclusion that exposure to testosterone from adjacent male fetuses in utero affected the reproductive success of the daughters. Click for More Info Other Animals: A key element of the organizational concept is the central role of sex steroid hormones. When cows were observed to give birth to twins of opposite sexes, the female twin was sterile and had masculine traits. Scientists have shown that androgenic hormones secreted by the male twin in the womb imbued the female twin with some male traits, and, in general, the gonads in embryos secrete the hormones that cause males to differ from females. In species that produce litters of many young from each pregnancy, the fetuses are arranged like peas in a pod inside the uterus. This grouping results in female and male fetuses residing next to one another in random order. Steroid hormones produced by one fetus's gonads influence the developing neural and secondary and accessory sex structures in an adjacent fetus, as shown by studies of rats, gerbils, and mice. The hormonal environment of the embryo influences the animal's adult sexual behavior by helping to organize the sexuality of the brain. In vertebrates steroid hormones act directly on specific neurons that are linked together in circuits. A nucleus of cells in the gerbil brain, present only in males, lies embedded in an area that helps to control copulatory behavior in male gerbils. Female gerbils injected with androgen early in life develop this "male" nucleus and take on some male behavioral characteristics. This nucleus provides a link between hormones, brain structure and sexual behavior.

Adapted and excerpted from: Pubertal hormones organize the adolescent brain and behavior, Male twins reduce fitness of female co-twins in humans Crews, David. 1994. Animal Sexuality. In Vista in Readings. Scientific American Magazine. (January): 108-114. Click above for a sample assignment question.