Prefertilization Events

I. GAMETES (oocytes and spermatozoa), descendants of primordial germ cells, are produced in the adult by either oogenesis or spermatogenesis, processes that involve meio-sis. Primordial germ cells originate in the wall of the yolk sac of the embryo and migrate into the gonad region.

II. MEIOSIS (Figure 1-1), which occurs only during the production of gametes, consists of two cell divisions (meiosis I and meiosis II) and results in the formation of four gametes containing 23 chromosomes and IN amount of DNA (23,IN). Meiosis:

A. Reduces the number of chromosomes within the gametes to ensure that the human species number of chromosomes (46) can be maintained from one generation to another

B. Redistributes maternal and paternal chromosomes to ensure genetic variability

C. Promotes the exchange of small amounts of maternal and paternal DNA via crossover during meiosis I

III. FEMALE GAMETOGENESIS (OOGENESIS; Table 1-1)

A. Primordial germ cells (46,2N) arrive in the ovary at week 4 of embryonic development and differentiate into oogonia (46,2N).

B. Oogonia enter meiosis 1 and undergo DNA replication to form primary oocytes (46,4N). All primary oocytes are formed by the fifth month of fetal life and remain dormant in prophase (diplotene) of meiosis I until puberty.

C. During a woman's ovarian cycle, a primary oocyte completes meiosis I to form a secondary oocyte (23,2N) and a first polar body, which probably degenerates.

D. The secondary oocyte enters meiosis II, and ovulation occurs when the chromosomes align at metaphase. The secondary oocyte remains arrested in metaphase of meiosis II until fertilization occurs.

E. At fertilization, the secondary oocyte completes meiosis II to form a mature oocyte (23,IN) and a second polar body.

IV. HORMONAL CONTROL OF THE FEMALE REPRODUCTIVE CYCLE (Figure 12)

A. The hypothalamus secretes gonadotropin releasing hormone (GNRH).

B. In response to GNRH, the adenohypophysis secretes the gonadotropins follicle-stimulating hormone (FSH) and luteinizing hormone (LH).

Oogonia or

Type B spermatogonia (46, 2N)

Meiosis I

Primary gametocyte (46, 4N)

Secondary gametocyte (23, 2N)

Gamete (23,1N)

Oogonia or

Type B spermatogonia (46, 2N)

Meiosis I

Primary gametocyte (46, 4N)

Secondary gametocyte (23, 2N)

Type Dark Spermatogonia

Alignment and disjunction Centromeres do not split

Meiosis II

Alignment and disjunction Centromeres split

Figure 1-1. Schematic representation of meiosis I and meiosis II, emphasizing the chromosomal changes and amount of DNA that occur during either oogenesis or spermatogenesis. Note that only one pair of homologous chromosomes is shown (white = maternal origin; black = paternal origin). Synapsis is the process of pairing of homologous chromosomes. The point at which the DNA molecule crosses over is called the chiasma, which is where the exchange of small amounts of maternal and paternal DNA occur. Note that synapsis and crossing over occur only during meiosis I. (Adapted from Dudek RW, Fix JD: BRS Embryology, 2nd ed. Baltimore, Williams & Wilkins, 1998, p 4.)

Alignment and disjunction Centromeres do not split

Meiosis II

Alignment and disjunction Centromeres split

Figure 1-1. Schematic representation of meiosis I and meiosis II, emphasizing the chromosomal changes and amount of DNA that occur during either oogenesis or spermatogenesis. Note that only one pair of homologous chromosomes is shown (white = maternal origin; black = paternal origin). Synapsis is the process of pairing of homologous chromosomes. The point at which the DNA molecule crosses over is called the chiasma, which is where the exchange of small amounts of maternal and paternal DNA occur. Note that synapsis and crossing over occur only during meiosis I. (Adapted from Dudek RW, Fix JD: BRS Embryology, 2nd ed. Baltimore, Williams & Wilkins, 1998, p 4.)

Table 1-1

Number of Chromosomes and Amount of DN A Contained in Cells During the Stage of Gametogenesis

No. of Chromosomes,

Cell Type Amount of DNA

Primordial germ cell, oogonia, spermatogonia (type A and B), zygote, 46, 2N

blastomeres, all normal somatic cells

Primary oocyte, primary spermatocyte 46, 4N

Secondary oocyte, secondary spermatocyte 23, 2N

Oocyte (ovum), spermatid, sperm 23, IN

C. FSH stimulates the development of a secondary follicle to a graafian follicle within the ovary.

D. Granulosa cells of the secondary and graafian follicle secrete estrogen.

E. Estrogen stimulates the endometrium of the uterus to enter the proliferative phase.

F. LH stimulates ovulation.

G. Following ovulation, granulosa lutein cells of the corpus luteum secrete progesterone.

H- Progesterone stimulates the endometrium of the uterus to enter the secretory phase.

V. MALE GAMETOGENESIS (SPERMATOGENESIS; see Table 1^1) is classically di-

vided into three phases: spermatocytogenesis, meiosis, and spermiogenesis.

A. Spermatocytogenesis

1. Primordial germ cells (46,2N) arrive in the testes at week 4 of embryonic development and remain dormant until puberty. At puberty, primordial germ cells differentiate into type A spermatogonia (46,2N).

2. Type A spermatogonia undergo mitosis to provide a continuous supply of stem cells throughout the reproductive life of the male (called spermatocytogenesis). Some type A spermatogonia differentiate into type B spermatogonia (46,2N).

B. Meiosis

1. Type B spermatogonia enter meiosis I and undergo DNA replication to form pri^ mary spermatocytes (46,4N).

2. Primary spermatocytes complete meiosis I to form two secondary spermatocytes (23,2N).

3. Secondary spermatocytes complete meiosis II to form four spermatids (23, IN).

C. Spermiogenesis

1. Spermatids undergo a postmeiotic series of morphologic changes (called spermiogenesis) to form sperm (23,IN). These changes include formation of the acro-some; condensation of the nucleus; and formation of the head, neck, and tail. The total time for sperm formation is approximately 64 days.

2. Newly ejaculated sperm are incapable of fertilization until they undergo capacita-tion, which occurs in the female reproductive tract and involves the unmasking of sperm glycosyltransferases and removal of proteins that coat the surface of the sperm.

Figure 1-2. Hormonal control of the female reproductive cycle. The various patterns of hormone secretion from the hypothalamus, adenohypophysis, and ovary are shown. These hormones prepare the endometrium of the uterus for implantation of a conceptus. The menstrual cycle of the uterus consists of five phases. (1) The menstrual phase (days 1-4) is characterized by the necrosis and shedding of the functional layer of the endometrium. (2) The proliferative phase (days 4-15) is characterized by the regeneration of the functional layer of the en-dometrium and a low basal body temperature (97.5°F). (3) The ovulatory phase (days 14-16) is characterized by ovulation of a secondary oocyte and coincides with the surge in luteinizing hormone. (4) The secretory phase (days 15-25) is characterized by secretory activity of the endometrial glands and an elevated basal body temperature (over 98°F); implantation of a conceptus occurs in this phase. (5) The premenstrual phase (days 25-28) is characterized by ischemia due to reduced blood flow to the endometrium. E = estrogen; FSH = follicle-stimu-lating hormone; LH = luteinizing hormone; GNRH = gonadotropin releasing hormone; P = progesterone. (From Dudek RW: High-Yield Histology, 2nd ed. Philadelphia, Lippincott Williams & Wilkins, 2000, p 170.)

VI. CLINICAL CORRELATIONS

A. Offspring of older women

1. Prolonged dormancy of primary oocytes may he the reason for the high incidence of chromosomal abnormalities in the offspring of older women. All primary oocytes are formed by the fifth month of fetal life; thus, a female infant is born with her entire supply of gametes. Primary oocytes remain dormant until ovulation. Oocytes that ovulated late in the woman's reproductive life may have been dormant for as long as 40 years.

Achondroplasia Bone Histology

GnRH

Secretory

Ovulation

Proliferative

Menstrual

Premenstrual

Adenohypophysis

Ovary

Uterus

Hypothalamus

2. The incidence of trisomy 21 (Down syndrome) increases with the advanced age of the mother.

B. Offspring of older men. An increased incidence of achondroplasia (a congenital skeletal anomaly characterized by retarded bone growth) is associated with advanced paternal age.

C. Male fertility depends on the number and motility of sperm. Fertile males produce from 20 to more than 100 million sperm/ml of semen. Sterile males produce less than 10 million sperm/ml of semen. Normally up to 10% of sperm in an ejaculate may be grossly deformed (two heads or two tails), but these sperm probably do not fertilize an oocyte owing to their lack of motility.

D. Hormonal contraception

1. Oral contraceptives a. Combination pills contain a combination of estrogen and progesterone.

(1) They are taken for 21 days and then discontinued for 7 days.

(2) The primary mechanism of action is the inhibition of GNRH, FSH, and LH secretion, which prevents ovulation.

b. Progesterone-only pills contain only progesterone.

(1) They are taken continuously.

(2) The primary mechanism of action is not known; however, progesterone-only pills cause thickening of cervical mucus, which makes is hostile to sperm migration, and thinning of the endometrium, which causes it to be unprepared for conceptus implantation.

2. Medroxyprogesterone acetate (Depo-Provera) is a progesterone-only product that offers a long-acting alternative to oral contraceptives. It can be injected intramuscularly and will prevent ovulation for 2—3 months.

3. Levonorgestrel (Norplant) is a progesterone-only product that offers an even longer-acting alternative to oral contraceptives. The capsules containing levonorgestrel can be implanted subdermally and will prevent ovulation for 1-5 years.

4. Luteinizing hormone releasing hormone (LHRH) analogues. Chronic treatment with an LHRH analogue (e.g., buserelin) results paradoxically in a downregula-tion of FSH and LH secretion, thereby preventing ovulation.

5. Postcoital contraception ("morning-after pill") can be used after unprotected intercourse.

a. Dosage. Diethylstilbestrol (DES; 25 mg) is taken twice a day for 5 days; or, two combination pills are taken up to 72 hours after intercourse followed by two pills 12 hours later b. Mechanism of action. The high doses of steroids disrupt the endometrium so that implantation may not occur.

C- Precaution. Because of the potential teratogenic effects of steroids, a therapeutic abortion is recommended if postcoital contraception fails and pregnancy occurs.

E. Anovulation is the absence of ovulation in some women owing to inadequate secretion of FSH and LH. This condition is often treated with clomiphene citrate. By competing with estrogen for binding sites in the adenohypophysis, clomiphene citrate suppresses the normal negative feedback loop of estrogen on the adenohypophysis. This in turn stimulates FSH and LH secretion and induces ovulation.

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