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PART 5 Reproduction and Development

Puberty in Females

The first episode of menstrual bleeding, called menarche (me-nar′k ē ), occurs during puberty, between ages 11 and 16. Atthis time, the vagina, uterus, uterine tubes, and external genitaliabegin to enlarge as well. Adipose tissue is deposited in the breastsand around the hips, causing them to enlarge and assume adultform. The ducts of the breasts develop, pubic and axillary hairgrows, and the voice changes, although more subtly than in males.The development of sex drive is also associated with puberty.The changes associated with puberty are primarily the resultof elevated rates of estrogen and progesterone secretion by theovaries. Before puberty, estrogen and progesterone are secreted invery small amounts, and LH and FSH levels remain very low dueto a lack of GnRH released from the hypothalamus. At puberty,not only are GnRH, LH, and FSH secreted in greater quantitiesthan before puberty but also the adult cyclic pattern of FSH andLH secretion is established.

ASSESS YOUR PROGRESS

65. Define menarche. Describe other physical changes thatoccur during female puberty.

66. What changes occur in LH, FSH, estrogen, andprogesterone secretion during puberty?

So that it is easier to discuss the processes occurring in differ-ent components of the female reproductive system, the menstrualcycle is divided into the ovarian cycle and the uterine cycle. The ovarian cycle describes the changes that are associated specifi-cally with the ovaries, and the uterine cycle describes the changesthat are associated specifically with the uterus.

Ovarian Cycle

The term ovarian cycle refers to the regular events that occur inthe ovaries of sexually mature, nonpregnant women during themenstrual cycle. This cycle can be divided into the follicular phase,which occurs before ovulation, and the luteal phase, which occursafter ovulation. The hypothalamus and anterior pituitary releasehormones that control the events of the ovarian cycle. FSH from theanterior pituitary is primarily responsible for initiating the develop-ment of primary follicles, and as many as 25 follicles begin tomature during each ovarian cycle. However, normally only 1 isovulated. The follicles that start to develop in response to FSH maynot ovulate during the same ovarian cycle in which they begin tomature, but they may ovulate one or two cycles later. The remainingfollicles degenerate. Larger, more mature follicles appear to secreteestrogen and other substances that have an inhibitory effect on other,less mature follicles.Early in the ovarian cycle, the release of GnRH from the hypo-thalamus increases, as does the sensitivity of the anterior pituitaryto GnRH. These changes stimulate the anterior pituitary to produceand release small amounts of FSH and LH. FSH and LH stimulatefollicular growth and maturation. They also cause an increase inestrogen secretion by the developing follicles. FSH exerts its maineffect on the granulosa cells, whereas LH exerts its initial effect onthe theca interna cells and later on the granulosa cells.LH stimulates the theca interna cells to produce androgens,which diffuse from these cells to the granulosa cells. FSH stimulatesthe granulosa cells to convert androgens to estrogen. In addition,FSH gradually increases LH receptors in the granulosa cells. Estro-gen produced by the granulosa cells increases LH receptors in thetheca interna cells. Consequently, theca interna cells and granulosacells cooperate to produce estrogen. Estrogen, in turn, increasesreceptors for LH in both theca interna cells and granulosa cells.After LH receptors in the granulosa cells have increased, LHstimulates the granulosa cells to produce progesterone, which diffusesfrom the granulosa cells to the theca interna cells, where it is convertedto androgens. These androgens are also converted to estrogen bythe granulosa cells. Thus, the production of androgens by the thecainterna cells increases, resulting in a gradual increase in estrogensecretion by granulosa cells throughout the follicular phase, eventhough only a small increase in LH secretion occurs. FSH levelsactually decrease during the follicular phase because developingfollicles produce inhibin, which has a negative-feedback effect onFSH secretion.The gradual increase in estrogen levels, especially late in thefollicular phase, begins to have a positive-feedback effect on LHand FSH release from the anterior pituitary. Consequently, as theestrogen level in the blood increases, it stimulates greater LH and FSHsecretion. The sustained increase in estrogen is necessary for thispositive-feedback effect. In response, LH and FSH secretion

Menstrual Cycle

The term menstrual (men′stroo- ă l) cycle technically refers to thecyclic changes in sexually mature, nonpregnant females thatbegin with menses. Typically, the menstrual cycle is about28 days long, although it can be as short as 18 days in somewomen and as long as 40 days in others (figure 28.18). Menses (men′s ē z) is a period of mild hemorrhage that occurs approxi-mately once each month, during which the functional layer of theendometrium is sloughed and expelled from the uterus. Men-struation (men-stroo- ā ′sh ŭ n) is the discharge of the blood andother elements of the endometrium. Although the term menstrualcycle refers specifically to changes in the uterus, the term is oftenused to refer to all the cyclic events in the female reproductivesystem, including alterations in hormone secretion and changes inthe ovaries (table 28.2).The first day of menses is considered day 1 of the menstrualcycle, and menses typically lasts 4–5 days. Ovulation occurs onabout day 14 of a 28-day menstrual cycle; however, the timingof ovulation varies from individual to individual and even withina single individual from one menstrual cycle to the next. Thetime between ovulation (day 14) and the next menses is typically14 days. The time between the first day of menses and the day ofovulation is more variable than the time between ovulation andthe next menses. The time between the ending of menses andovulation is called the proliferative phase, because of the rapidproliferation of the uterine mucosa, or the follicular phase, because of the rapid development of ovarian follicles. The periodafter ovulation and before the next menses is called the secre-tory phase, because of the maturation of and secretion by uter-ine glands, or the luteal phase, because of the existence of thecorpus luteum.