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| Hormones |
Hormones are small molecules that bind to specific target cells to modify the response of the cell, usually affecting gene expression. For example, estrogen is a small, hydrophobic molecule that binds to estrogen receptors. The estrogen-receptor complex then enters the nucleus and binds to specific DNA sequences in certain genes, and turns on or off transcription of those genes.
In females the ovaries secrete estrogens and progesterone, which are essential for the development of female genitalia during fetal development (Fig. 4). These hormones are also required for sexual development at puberty, and for pregnancy. The ovaries also produce a small amount of testosterone, although much less than testes in males produce.
In males the testes secrete the major androgen, testosterone. Synthesis of this hormone increases significantly at puberty, when it is responsible for adult sexual development. Androgens are also essential for the development of male genitalia during fetal development (Fig. 4). Some testosterone is converted to estrogen in males and is important for bone formation.
Animals in utero can be affected by hormones produced by nearby siblings of the opposite sex. The placement of an animal, such as a mouse in a litter, may have a long-term effect on physiology or behavior. Female mice that develop in the uterus between two males have shorter fertile periods than do females that develop between two females. Male mice prefer to mate with the females that develop in an all-female environment. Females who develop between two brothers in utero are more aggressive towards intruders than are females who develop with two sisters.
Hormones also affect mature adults. Males and females have receptors for estrogens, progesterone, and androgens in various tissues. Transsexuals (individuals who have a conflict between their biological sex and their perceived gender) must take hormones of the opposite sex if they choose to undergo a sex change. Males can develop breasts, decrease facial hair production, and change the texture of their skin and hair as a result of estrogen and progesterone therapy combined with anti-androgen drugs. Conversely, high levels of testosterone can have a masculizing effect on females. Interestingly, individual differences in natural hormone levels and hormone sensitivity mean that those undergoing a sex change require individualized hormone treatment programs.