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Web Topic 6.3: Sexual Dimorphism Can Arise as an Indirect Effect of Hormonal Levels

[Referenced on textbook p. 171]

In the textbook, we describe direct effects of androgens on neural development. Androgens can also affect neural development indirectly, however. Here’s an example.

Marc Breedlove and his colleagues (then at the University of California, Berkeley) studied the development of the rat’s equivalent of Onuf’s nucleus—called the spinal nucleus of thebulbocavernosus (or SNB) because its motor neurons innervate the bulbocavernosus muscle (the muscle we are calling the bulbospongiosus muscle; see Chapter 4), as well as other muscles of the rat’s genital area (Breedlove et al., 1999).

To understand Breedlove’s research, you have to know that during fetal development, an excess of motor neurons is generated throughout the spinal cord. Cell death subsequently eliminates those motor neurons that fail to establish connections with muscles. This pruning process is mediated by trophic factors secreted by muscles: if the axonal endings of the motor neurons fail to pick up these trophic factors, they die.

Breedlove found that during early fetal development, both male and female rats possess a bulbocavernosus muscle, but in female fetuses the muscle subsequently dies (see Figure 1). That’s because the muscle requires the presence of androgens, which are not present at sufficient levels in female fetuses. Deprived of their target muscle and the trophic factors it produces, the motor neurons in the female rat’s SNB also die. Thus the sexual dimorphism in the rat’s SNB is an indirect consequence of the sexual dimorphism in the genital musculature. (See Web Activity 6.4.)

Figure 1  Development of sexual dimorphism of the rat spinal nucleus of the bulbocavernosus.

The situation in humans is somewhat different because women do have a bulbospongiosus muscle—it forms a sling around the clitoral shaft and the vaginal introitus (see Chapter 3). Nevertheless, the total amount of genital musculature innervated by Onuf’s nucleus is less in women than in men. Therefore, fewer motor neurons survive in the Onuf’s nucleus of women.

References

Breedlove, S. M., Cooke, B. M., & Jordan, C. L. (1999). The orthodox view of brain sexual differentiation. Brain, Behavior and Evolution 54: 8–14.

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