Division H Of Meiosis

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figure 13-3 Meiotic processses in germ cells in women as related to their age and sexual development. Modified from Odell, W. D. (1979). The reproductive system in women. In "Endocrinology" (L. J. DeGroot, G. F. Cahill, L. Martini, D. H. Nelson, W. D. Odell, J. T. Potts, Jr., E. Steinberger, and A. I. Winegard, eds.), Vol. 3, p. 1385. Grune & Stratton, New York.

Ovulation of a single ovum each month requires only about a total of 400 oogonia over the 30-40 year period of reproductive fertility. At the time of menopause the number of viable oogonia in the ovary is virtually zero. These events are summarized in Figure 13-3. It is interesting that the ovum that is ultimately fertilized is the product of an intense selection procedure; there is a ratio of atresia to selection of ~1000:1. Figure 13-4 presents a schematic diagram of a single oocyte. During follicular development just prior to ovulation, the primary oocyte undergoes a meiotic or reductive division so that the number of chromosomes is reduced by one-half (see Figure 13-3). This yields the secondary oocyte, which contains 22 autosomes, one X sex chromosome, and the bulk of the cell cytosol; in addition, a polar body or polycyte is formed that also has 22 autosomes, one sex chromosome, and only very little of the cell cytosol.

Annulate Lamellae

figure 13-4 Schematic diagram of a mature primary oocyte. Abbreviations: G, multiple Golgi complexes; CA, compound aggregates; AL, annulate lamellae; V, vesicles; FI, wavy filaments; NI, nucleolus. Reproduced with permission from Lentz, T. L. (1971). "Cell Fine Structure," p. 269. Saunders, Philadelphia, PA.

figure 13-4 Schematic diagram of a mature primary oocyte. Abbreviations: G, multiple Golgi complexes; CA, compound aggregates; AL, annulate lamellae; V, vesicles; FI, wavy filaments; NI, nucleolus. Reproduced with permission from Lentz, T. L. (1971). "Cell Fine Structure," p. 269. Saunders, Philadelphia, PA.

C. Corpus Luteum

At the site of ovulation where the mature follicle ruptures and releases the ovum, the cells comprising the follicle, under the actions of LH, enlarge and differentiate into lutein cells; as a consequence a yellowish substance (lutein) accumulates in the cytoplasm. The lutein cells are derived by maturation from both the theca interna and follicular cells. These cells are the principal sites of production of progesterone and estrogen after ovulation. Subsequently, capillaries grow into these cells, giving rise to the corpus luteum, which is a typical endocrine organ. In the human, the corpus luteum may range in size from 1.5 to 4 cm.

If the released oocyte is not fertilized within 1-2 days, then the corpus luteum will continue to increase in size for 10-12 days. This is then followed by virtual total regression of the gland to produce a small white ovarian scar known as the corpus albicans and concomitant cessation of progesterone and estrogen secretion (see Figure 13-3).

If the released oocyte is fertilized, the corpus luteum continues to grow and function for the first 3 months of pregnancy. Then it slowly regresses, leaving a white scar on the ovary. The progesterone from the corpus luteum is essential for maintenance of the first 2 months of pregnancy; after this time the production of progesterone by the placenta is adequate for the continued maintenance of pregnancy.

D. Fallopian Tubes

The fallopian tubes are also variously referred to as uterine tubes or oviducts. Each tube is 11-12 cm long and extends from the trumpet-shaped end, which is in contact with the ovary, down to the proximal end, which penetrates the uterine wall (see Figure 13-1). During the ovulatory process, the many small fingerlike projections or fimbriae of the ciliated cells on the inner surface of the fallopian tubes actively massage or undulate to aid in the translocation of the released mature oocyte down to the uterus. During the tubular passage of the oocyte, if it is not fertilized, it will eventually disintegrate and disappear; the biochemical-endocrine basis of these changes is not yet known.

If the oocyte becomes fertilized, it promptly completes its mitotic division and becomes a zygote. Usually the zygote will reach the uterus in 4-5 days, where it will implant and continue to develop during pregnancy. If the zygote fails to reach the uterus, the woman is said to be subject to an ectopic pregnancy.

E. Uterus

In the nonpregnant adult woman the uterus is a muscular, thick-walled, pear-shaped organ 5 cm wide X 7.5 cm long X 2.5 cm thick. The cavity of the uterus communicates below with the vagina and above with the fallopian tubes. The uterine wall comprises three chief layers: (1) the outer or perimetrium; (2) the middle or myometrium; and (3) the inner or endometrium.

The endometrium is a mucous membrane composed of two layers: the thin, basal layer and the outer, functional layer. The functional layer changes dramatically under hormonal influence during the menstrual cycle and is almost completely lost during the process of menstruation. In the secretory phase of the menstrual cycle (14-28 days), the cells are quite tall and columnar with numerous microvilli.

The volume of the uterine cavity changes in capacity from 2-5 ml before pregnancy to 5000-7000 ml at the term of pregnancy; the increase in uterine mass is from 60 g at puberty to 1000 g at the end of pregnancy, which is a 16-fold increase. The primary stimulus for the growth of the myometrium during pregnancy is estrogen produced by the placenta.

TABLE 13-1 Hormones Related to Female Development, Reproduction, and Lactation

Hormone

Site of production

Principal target tissue

Principal biological function

Steroid hormones 17/3-Estradiol Estrone

Dehydroepiandrosterone sulfate (DHEA) Estriol (from DHEA sulfate) Progesterone

Peptide hormones (nonpregnant state) Gonadotropin-releasing hormone (GnRH)

Follicle-stimulating hormone (FSH)

Luteinizing hormone (LH)

Inhibin

Activin

Peptide hormones (pregnant/lactating state)0

Prolactin (of the pregnant / lactating states)

Human chorionic gonadotropin

Human placental lactogen (HPL) [also known as human chorionic somatomammotropin (HCS)] Relaxin Oxytocin

Other Prostaglandins

Ovary and follicle Placenta Fetal adrenal Placenta Corpus luteum

Hypothalamus

Adenohypophysis

Adenohypophysis

Follicle granulosa cell

Follicle granulosa cell

Adenohypophysis

Trophoblast and placenta

Trophoblast and placenta

Ovary

Neurohypophysis

Fetus

Uterine endometrium

Uterine endometrium, mammary gland

Adenohypophysis

Ovarian granulosa and thecal cells

Corpus luteum

Hypothalamus-pituitary

Hypothalamus

Mammary tissue

Maternal corpus luteum Maternal tissue

Uterine cervix Uterus and mammary tissue

Uterus

Cell proliferation

Prepare for implantation of the blastocyte and development of mammary alveolar system

Stimulation of release of FSH and LH

Maturation of ovarian follicle and stimulation of estrogen production Stimulate progesterone production Participate in feedback inhibition of FSH secretion Increase secretion of FSH

Stimulate milk production

Stimulate progesterone production Produce peripheral insulin resistance in the mother

Softening of uterine cervix Milk release

' All of the peptide hormones of the nonpregnant state are also functional during pregnancy and lactation.

F. Vagina

The vagina is a 9-cm membranous tube that extends from the lower portion or cervix of the uterus downward and forward to the external opening in the vestibule (see Figure 13-2). The wall of the vagina is composed of fibroelastic tissue and smooth muscle, which is lined with a mucous membrane formed of squamous epithelial cells. These cells are quite responsive to estrogen, and their physical appearance under the light microscope changes corresponding to the phase of the ovulatory cycle.

The Papanicolaou test, or Pap smear, which consists of a light microscopic examination of cells obtained painlessly from the vagina and cervix of the uterus, is an important diagnostic test that is of considerable clinical importance in the early diagnosis of cancer of the uterus. The foreign cancerous cells can easily be distinguished from the regular uniform appearance of the epithelial cells.

A description of the anatomical organization of the mammary glands and the placenta is presented in Chapter 14.

III. CHEMISTRY, BIOCHEMISTRY, AND BIOLOGICAL RESPONSES

Table 13-1 tabulates the 13-15 hormones pertinent to female development, reproduction, and lactation.

A. Steroid Hormones

1. Structural and Metabolic Relationships

The two most important steroid hormones of the adult female are estradiol and progesterone. In addition, estrone, estriol, and dehydroepiandrosterone play important roles in pregnancy. The structures of these compounds are presented in Figure 13-5.

As reviewed in Chapter 2, the naturally occurring estrogens are typically 18-carbon steroids that have an aromatic A ring with a phenolic hydroxyl. The ovarian follicular cell in the nonpregnant female is the cellular site of production of estradiol. Also, substantial quantities of estrone may be secreted by the ovarian follicular cell, as well as smaller quantities of 17a-estradiol, 16a-estriol (commonly referred to as estriol), and 6a-hydroxy-17/3-estradiol. Figure 2-23 reviews the pathways of biosynthesis of the principal estrogens in the nonpregnant female

In the pregnant female the principal estrogen is estriol; it has a biological activity approximately equivalent to that of 17/3-estradiol. Estriol is synthesized in the placenta from the precursor dehydroepiandrosterone

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