Meiosis

Meiosis occurs within the testes and ovary. This is a specialized process of cell division that produces the male gamete (spermatogenesis) and female gamete (oogenesis). There are notable differences between spermatogenesis and oogenesis, discussed below.

Primordial

Urogenital ridge Mesonephric Duct Paramesonephric Duct Indifferent Gonad

TDF No

Testosterone , . MIF feCOrS

Ovary

Primordial

Figure 11-1-1. Development Within Testes and Ovary

Meiosis consists of two cell divisions, meiosis I and meiosis II (Figure II-1-2).

Meiosis I

In meiosis I, the following events occur:

Synapsis—the pairing of 46 homologous chromosomes Crossing over— the exchange of segments of DNA

Disjunction—the separation of 46 homologous chromosomes without centromere splitting

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Figure 11-1-2. Meiosis

Meiosis II

In meiosis II:

• Synapsis does not occur.

• Crossing over does not occur.

• Disjunction occurs with centromere splitting.

Spermatogenesis

Primordial germ cells arrive in the indifferent gonad at week 4 and remain dormant until puberty.

When a boy reaches puberty, primordial germ cells differentiate into type A spermatogonia, which serve as stem cells throughout adult life.

Some type A spermatogonia differentiate into type B spermatogonia.

Type B spermatogonia enter meiosis I to form primary spermatocytes.

Primary spermatocytes form two secondary spermatocytes.

Secondary spermatocytes form two spermatids.

Spermatids undergo spermiogenesis, which is a series of morphological changes resulting in the mature sperm.

Oogenesis

Primordial germ cells arrive in the indifferent gonad at week 4 and differentiate into oogonia.

Oogonia enter meiosis I to form primary oocytes. All primary oocytes are formed by month 5 of fetal life and remain arrested in prophase (diplotene) of meiosis I until puberty.

No oogonia are present at birth.

When a girl reaches puberty, a primary oocyte completes meiosis I to form a secondary oocyte and polar body.

The secondary oocyte becomes arrested in metaphase of meiosis II and is ovulated.

At fertilization within the uterine tube, the secondary oocyte completes meiosis II to form a mature oocyte and polar body.

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Chapter Summary

The indifferent gonad begins development in a column of intermediate mesoderm called the urogenital ridge during week 4. Primordial germ cells arise in the wall of the yolk sac and migrate to the indifferent gonad. In the male, a testis develops from the indifferent gonad due to the presence of testis-determining factor (TDF), which is produced on the short arm of the Y chromosome. Testosterone secreted by the Leydig cells and mOllerian-inhibrting factor (MIF) secreted by the Sertoli cells also contribute to the development of the testis. In the female, an ovary develops in the absence of any factors.

Meiosis is a specialized type of cell division that produces the male and female gametes during spermatogenesis and oogenesis, respectively. Meiosis consists of two cell divisions: meiosis I and meiosis II. In meiosis I, the events include synapsis, exchange of DNA, and dysjundion, resulting in a reduction from 46 to 23 chromosomes. In meiosis II, (here is a reduction of DNA from 2n to 1 n.

Oogenesis begins in the female during the early weeks of development and by month 5 of fetal life all of the primary oocytes are formed and become arrested in prophase of meiosis I until puberty. After puberty, during each monthly menstrual cycle a secondary oocyte develops in the graafian follicle and is then arrested a second time in metaphase of meiosis II, which is then ovulated. Meiosis II is only completed if there is fertilization. In the male, spermatogenesis begins after puberty in the seminiferous tubules and moves through meiosis I and II without any arrested phases to produce spermatids. Spermatids undergo spermatogenesis to develop into the adult spermatozoa.

Clinical Correlate

Ectopic Tubal Pregnancy This is the most common form of ectopic pregnancy. It most usually occurs when the blastocyst implants within the ampulla of the uterine tube because of delayed transport. Risk factors include endometriosis, pelvic inflammatory disease (PID), tubular pelvic surgery, or exposure to diethylstilbestrol (DES). Clinical signs include abnormal or brisk uterine bleeding, sudden onset of abdominal pain that may be confused with appendicitis, last menses 60 days ago, positive human chorionic gonadotropin (hCG) test, and culdocentesis showing intraperitoneal blood.

Ectopic Abdominal Pregnancy

Ectopic abdominal pregnancy most commonly occurs in the rectouterine pouch (pouch of Douglas).

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