Puberty

Puberty begins at ages 9 to 10 for most girls in the United States and Europe but significantly later in many countries. However, a 1997 study of more than 17,000 girls in the United States showed 3% of black girls and 1% of white girls beginning puberty by age 3, and 27% and 7%, respectively, by age 7. Puberty is triggered by the same hypothalamic and pituitary hormones in girls as it is in boys. Rising levels of gonadotropin-releasing hormone (GnRH) stimulate the anterior lobe of the pituitary to secrete follicle-stimulating hormone (FSH) and luteiniz-

Saladin: Anatomy & I 28. The Female I Text I I © The McGraw-Hill

Physiology: The Unity of Reproductive System Companies, 2003 Form and Function, Third Edition

Chapter 28 The Female Reproductive System 1059

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Chapter 28 The Female Reproductive System 1059

Female Puberty

Figure 28.11 Breast Cancer Screening and Treatment. (a) Nurse assisting a patient in mammography. (b) Mammogram of a breast with a tumor visible at the arrow (/eft), compared to the appearance of normal fibrous connective tissue of the breast (right). (c) Patient following mastectomy of the right breast. (d) The same patient following surgical breast reconstruction. What is the diagnostic benefit of compressing the breast for a mammogram?

Figure 28.11 Breast Cancer Screening and Treatment. (a) Nurse assisting a patient in mammography. (b) Mammogram of a breast with a tumor visible at the arrow (/eft), compared to the appearance of normal fibrous connective tissue of the breast (right). (c) Patient following mastectomy of the right breast. (d) The same patient following surgical breast reconstruction. What is the diagnostic benefit of compressing the breast for a mammogram?

ing hormone (LH). FSH, especially, stimulates development of the ovarian follicles, which, in turn, secrete estrogens, progesterone, inhibin, and a small amount of androgen. These hormone levels rise gradually from ages 8 to 12 and then more sharply in the early teens. The estrogens24 are feminizing hormones with widespread effects on the body. They include estradiol (the most abundant), estriol, and estrone. Most of the visible changes at puberty result from estradiol and androgens.

The earliest noticeable sign of puberty is thelarche25 (thee-LAR-kee), or breast development. Estrogen, progesterone, and prolactin initially induce the formation of lobules and ducts in the breast. Duct development is completed under the influence of glucocorticoids and growth hormone, while adipose and fibrous tissue enlarge the breast. Breast development is complete around age 20, but minor changes occur in each menstrual cycle and major changes in pregnancy.

25thel = breast, nipple + arche = beginning

Saladin: Anatomy & I 28. The Female I Text I I © The McGraw-Hill

Physiology: The Unity of Reproductive System Companies, 2003 Form and Function, Third Edition

1060 Part Five Reproduction and Development

Thelarche is soon followed by pubarche (pyu-BAR-kee), the growth of pubic and axillary hair, sebaceous glands, and axillary glands. Androgens from the ovaries and adrenal cortex stimulate pubarche as well as the libido. Women secrete about 0.5 mg of androgens per day, compared with 6 to 8 mg/day in men.

Next comes menarche26 (men-AR-kee), the first menstrual period. In Europe and America, the average age at menarche declined from age 16.5 in 1860 to age 12 in 1997, probably due mostly to improved nutrition. Menarche cannot occur until a girl has reached at least 17% body fat, so it is delayed until the mid- to late teens in some athletes and dancers. Adult menstruation generally ceases if a woman drops below 22% body fat. This is about the minimum needed to sustain pregnancy and lactation; thus the body reacts as if to prevent a futile pregnancy when it is too lean. Menarche does not necessarily signify fertility. A girl's first few menstrual cycles are typically anovulatory (no egg is ovulated). Most girls begin ovulating regularly about a year after they begin menstruating.

Estradiol stimulates many other changes of puberty. It causes the vaginal metaplasia described earlier. It stimulates growth of the ovaries and secondary sex organs. It stimulates growth hormone secretion and causes a rapid increase in height and widening of the pelvis. Estradiol is largely responsible for the feminine physique because it stimulates fat deposition in the mons pubis, labia majora, hips, thighs, buttocks, and breasts. It makes a girl's skin thicken, but the skin remains thinner, softer, and warmer than in males of corresponding age.

Progesterone27 acts primarily on the uterus, preparing it for possible pregnancy in the second half of each menstrual cycle and playing roles in pregnancy discussed later. Estrogens and progesterone also suppress FSH and LH secretion through negative feedback inhibition of the anterior pituitary. Inhibin selectively suppresses FSH secretion.

Thus we see many hormonal similarities in males and females from puberty onward. The sexes differ less in the hormones that are present than in the relative amounts of those hormones—high levels of androgens and low levels of estrogens in males and the opposite in females. Another difference is that these hormones are secreted more or less continually and simultaneously in males, whereas in females secretion is distinctly cyclic and the hormones are secreted in sequence. This will be very apparent as you read about the ovarian and menstrual cycles.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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