External genitals follow a pattern similar to internal organs, except that male and female genitalia all develop from the same tissue. Male and female organs that began from the same prenatal tissue are called homologous (HOE-mol-lig-gus). Until the 8th week, the undifferentiated tissue from which the genitalia will develop exists as a mound of skin, or tubercle, beneath the umbilical cord. In females, the external genitalia develop under the influence of female hormones produced by the placenta and by the mother and also the lack of influence from the Y chromosome. The genital tu
bercle develops into the clitoris, the labia minora, the vestibule, and the labia majora (see Figure 3.2).
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In males, by the 8th or 9th week the testes begin androgen secretion, which begins to stimulate the development of male genitalia. The genital tubercle elongates to form the penis, in which lies the urethra, culminating in an external opening called the urethral meatus. Part of the tubercle also fuses together to form the scrotum, where the testicles will ultimately rest when they descend.
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Figure 3.2
Development of the male and female external genitalia from the undifferentiated genital tubercle.
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Male development
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endocrine gland
Gland that secretes hormones into the blood.
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TABLE 3.2 The Sex Hormones
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A group of hormones that control male sexual development and include testosterone and androsterone. Androgens stimulate the development of male sex organs and secondary sex characteristics such as beard growth and a deepening voice. Testosterone also plays an important part (in both sexes) in stimulating sexual desire. The testes produce androgens in men (stimulated by luteinizing hormone), though a little is also produced by the adrenal glands. Women’s ovaries also produce a small amount of androgens, which helps stimulate sexual desire; too much production by the ovaries causes masculinization in women.
A group of hormones that control female sexual development and include estriol, estrone, and estradiol. Estrogen controls development of the female sex organs, the menstrual cycle, parts of pregnancy, and secondary sex characteristics such as breast development. The ovaries produce most of the estrogen in women, although the adrenal glands and the placenta also produce small amounts. Testes also produce a small amount of estrogen in men; if they produce too much, feminization may occur.
A female hormone secreted by the ovaries. Progesterone helps to prepare the lining of the uterus for the implantation of the fertilized ovum, to stimulate milk production in the breasts, and to maintain the placenta. Progesterone works in conjunction with estrogen to prepare the female reproductive system for pregnancy.
A hormone that affects the nervous system. It is produced in the hypothalamus of the brain and transported through the bloodstream to the pituitary gland. Gonadotropin means "gonad stimulating," and GnRH stimulates the pituitary to release hormones, such as follicle-stimulating hormone (FSH) and luteinizing hormone (LH), which themselves induce the ovaries and testes (as well as other glands) to secrete their hormones.
A hormone released by the pituitary gland when stimulated by GnRH, which stimulates the follicular development in females and the formation of sperm in the male.
A hormone released by the pituitary gland when stimulated by GnRH that stimulates ovulation and the release of other hormones, notably progesterone in the female and testosterone in the male. It also stimulates the cells in the testes to produce testosterone.
A pituitary hormone that stimulates milk production after childbirth and also production of progesterone.
A pituitary hormone that stimulates the ejection of milk from the breasts and causes increased contractions of the uterus during labor.
A hormone produced by the cells of the testes that signal the anterior pituitary to decrease FSH production if the sperm count gets too high.
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Gonadotropin-releasing hormone (GnRH)
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Follicle-stimulating hormone (FSH)
Luteinizing hormone (LH)
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after birth. Male and female brains have different tasks and so undergo different development. For example, female brains control menstruation and therefore must signal the release of hormones in a monthly cycle, whereas male brains signal release continuously. With the brain, as with sexual organs, the presence of androgens during the appropriate critical stage of development may be the factor that programs the central nervous system to develop male sexual behaviors.
