The Questions of Developmental Biology

Sex Determination and Behaviors
Organization/Activation Hypothesis
Does prenatal (or neonatal) exposure to particular steroid hormones impose permanent
sex-specific changes on the central nervous system? Such sex-specific neural changes have been
shown in regions of the brain that regulate "involuntary" sexual physiology. The cyclic secretion
of luteinizing hormone by the adult female rat pituitary, for example, is dependent on the lack of
testosterone during the first week of the animal's life. The luteinizing hormone secretion of
female rats can be made noncyclic by giving them testosterone 4 days after birth; conversely, the
luteinizing hormone secretion of males can be made cyclical by removing their testes within a
day of birth (Barraclough and Gorski 1962). It is thought that sex hormones may act during the
fetal or neonatal stage of a mammal's life to organize the nervous system in a sex-specific
manner, and that during adult life, the same hormones may have transitory, activational effects.
This idea is called the organization/activation hypothesis.
Interestingly, the hormone chiefly responsible for determining the male brain pattern is
estradiol, a type of estrogen.* Testosterone in fetal or neonatal blood can be converted into
estradiol by the enzyme P450 aromatase, and this conversion occurs in the hypothalamus and
limbic system two areas of the brain known to regulate hormone secretion and reproductive
behavior (Reddy et al. 1974; McEwen et al. 1977). Thus, testosterone exerts its effects on the
nervous system by being converted into estradiol. But the fetal environment is rich in estrogens
from the gonads and placenta. What stops these estrogens from masculinizing the nervous system
of a female fetus? Fetal estrogen (in both males and females) is bound by a-fetoprotein.
This protein is made in the fetal liver and becomes a major component of the fetal blood and
cerebrospinal fluid. It will bind and inactivate estrogen, but not testosterone.
Attempts to extend the organization/activation hypothesis to "voluntary" sexual behaviors
are more controversial because there is no truly sex-specific behavior that distinguishes the two
sexes of many mammals, and because hormonal treatment has multiple effects on the developing
mammal. For instance, injecting testosterone into a week-old female rat will increase pelvic
thrusting behavior and diminish lordosis a posture that stimulates mounting behavior in the
male when she reaches adulthood (Phoenix et al. 1959; Kandel et al. 1995). These behavioral
changes can be ascribed to testosterone-mediated changes in the central nervous system, but they
could also be due to hormonal effects on other tissues. Testosterone enables the growth of the
muscles that allow pelvic thrusting. And since testosterone causes females to grow larger and to
close their vaginal orifices, one cannot conclude that the lack of lordosis is due solely to
testosterone-mediated changes in the neural circuitry (Harris and Levine 1965; De Jonge et al.
1988; Moore 1990; Moore et al. 1992; Fausto-Sterling 1995).
In addition, the effects of sex steroids on the brain are very complicated, and the steroids
may be metabolized differently in different regions of the brain. Male mice lacking the
testosterone receptor still retain a male-specific preoptic morphology in the brain, and male mice
lacking the aromatase enzyme are capable of breeding (Breedlove 1992; Fisher et al. 1998).
These studies show that there is more to sex-specific morphology and behavior than steroid
hormones. Despite best-selling books that pretend to know the answers, we have much more to
learn regarding the relationship between development, steroids, and behavior. Moreover,
extrapolating from rats to humans is a very risky business, as no sex-specific behavior has yet
been identified in humans, and what is "masculine" in one culture may be considered "feminine"