GAMETOGENESIS

10 BEFORE WE ARE BORN    ESSENTIALS OF EMBRYOLOGY AND BIRTH DEFECTS

•

A basal layer, containing the blind ends of the uterine glands

The compact and spongy layers—the functional layer—disintegrate and are shed at menstruation and after parturition (childbirth). The basal layer has its own blood supply and is not cast off during menstruation.

Uterine Tubes

The uterine tubes, measuring 10 cm long and 1 cm in diameter, extend laterally from the horns of the uterus (see Fig. 2-2A). Each tube opens into a horn at its proxi- mal end and into the peritoneal cavity at its distal end.

The uterine tube is divided into the following parts:

infundibulum, ampulla, isthmus, and uterine part. The tubes carry oocytes from the ovaries and sperms to the fertilization site in the ampulla (see Fig. 2-2B). The uterine tube also conveys the dividing zygote to the uterine cavity.

Ovaries

The ovaries are almond-shaped reproductive glands that are located close to the lateral wall of the pelvis on each side of the uterus. The ovaries produce oocytes (see Fig.

2-5). When released from the ovary at ovulation, the secondary oocyte passes into one of the uterine tubes.

These tubes open into the uterus, which protects and nourishes the embryo and fetus until birth. The ovaries also produce estrogen and progesterone, the hormones responsible for the development of secondary sex charac- teristics and regulation of pregnancy.

Female External Sex Organs

The female external sex organs are known collectively as the vulva (Fig. 2-3). The labia majora, fatty external folds of skin, conceal the vaginal orifice, the opening of the vagina. Inside these labia are two smaller folds of mucous membrane, the labia minora. The clitoris, a small erectile

organ, is situated at the superior junction of these folds.

The vagina and urethra open into a cavity, the vestibule (cleft between the labia minora). The vaginal orifice varies with the condition of the hymen, a fold of mucous mem- brane that surrounds the orifice (see Fig. 2-3).

Male Reproductive Organs

The male reproductive organs (see Fig. 2-1B) include the penis, testes, epididymis, ductus deferens (vas deferens), prostate, seminal glands, bulbourethral glands, ejacula- tory ducts, and urethra. The oval testes (testicles) are located in the cavity of the scrotum. Each testis consists of many highly coiled seminiferous tubules that produce sperms. Immature sperms pass from the testis into a single, complexly coiled tube, the epididymis, where they are stored. From the epididymis, the ductus deferens carries the sperms to the ejaculatory duct. This duct descends into the pelvis, where it fuses with the ducts of the seminal glands to form the ejaculatory duct, which enters the urethra.

The urethra is a tube that leads from the urinary bladder through the penis to the outside of the body.

Within the penis, erectile tissue surrounds the urethra.

During sexual excitement, this tissue fills with blood, causing the penis to erect. Semen (penile ejaculate) con- sists of sperms mixed with seminal fluid produced by the seminal glands, bulbourethral glands, and prostate.

C H A P T E R 2    HuMAN REPRODuCTION 11

preceding cell (primary spermatocyte or primary oocyte).

This separation, or disjunction, of paired homologous chromosomes is the physical basis of segregation, or sepa- ration, of allelic genes during meiosis.

The second meiotic division follows the first division, without a normal interphase (i.e., without an intervening step of DNA replication). Each double chromatid chro- mosome divides, and each half, or chromatid, is ran- domly drawn to a different pole of the meiotic spindle;

thus, the haploid number of chromosomes (23) is retained.

Each daughter cell formed by meiosis has the reduced haploid number of chromosomes, with one representative of each chromosome pair (now a single chromatid chromosome).

Meiosis:

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Provides for constancy of the chromosome number from generation to generation by reducing the chro- mosome number from diploid to haploid, thereby pro- ducing haploid gametes.

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^Allows random assortment of maternal and paternal chromosomes between the gametes.

•

Relocates segments of maternal and paternal chromo- somes by crossing over of chromosome segments, which “shuffles” the genes and produces a recombina- tion of genetic material.

Spermatogenesis

Before puberty, primordial sperms (spermatogonia) remain dormant in the seminiferous tubules of the testes from the late fetal period. At puberty (after 12 years), they begin to increase in number. After several mitotic

cell divisions, the sperms grow and undergo gradual changes that transform them into primary spermato- cytes—the largest germ cells in the seminiferous tubules (see Fig. 2-5). Each primary spermatocyte subsequently undergoes a reduction division—the first meiotic divi- sion—to form two haploid secondary spermatocytes, which are approximately half the size of primary sper- matocytes (see Fig. 2-5). Subsequently, the secondary spermatocytes undergo a second meiotic division to form four haploid spermatids, which are approximately half the size of secondary spermatocytes. The spermatids are gradually transformed into four mature sperms during a process known as spermiogenesis (see Fig. 2-5).

During this metamorphosis (change in form), the nucleus condenses and the acrosome forms (see Fig.

2-4A). The acrosome contains enzymes that probably facilitate the sperm’s penetration of the zona pellucida (see Chapter 3, Fig. 3-1). When spermiogenesis is com- plete, sperms enter the lumina (cavities) of the seminifer- ous tubules (see Fig. 2-1B). The sperms then move to the epididymis, where they are stored and become func- tionally mature. Spermatogenesis requires approximately 2 months for completion. Maturation of sperms—

spermatogenesis—normally continues throughout the reproductive life of a male.

When ejaculated, the mature sperms are free- swimming, actively motile cells consisting of a head and a tail (see Fig. 2-4A). The neck of the sperm is the junc- tion between the head and tail. The head of the sperm, forming most of the bulk of the sperm, contains the nucleus. The anterior two thirds of the head are covered by the acrosome, a cap-like organelle containing enzymes that facilitate sperm penetration during fertilization. The Figure 2–4 Male and female gametes (germ cells). A, The parts of a human sperm (×1250).

The head, composed mostly of the nucleus, is partly covered by the acrosome, an organelle containing enzymes. B, A sperm drawn to approximately the same scale as the oocyte. C, The human secondary oocyte (×200) is surrounded by the zona pellucida and corona radiata.

Acrosome Follicular cells of

corona radiata

Cytoplasm Nucleus

Zonapellucida Middle piece

of tail

Principal piece of tail

End piece of tail Head

A B C

Neck Nucleus covered by acrosome

First polar body

12 BEFORE WE ARE BORN    ESSENTIALS OF EMBRYOLOGY AND BIRTH DEFECTS

SPERMATOGENESIS

Testis

Spermatogonium 46,XY

Primary spermatocyte 46,XY

Secondary spermatocytes

Spermatids

Normal sperms SPERMIOGENESIS 23,X

23,X 23,X 23,Y 23,Y

23,X 23,X 23,Y 23,Y

23,Y

NORMAL GAMETOGENESIS

OOGENESIS

Ovary

Primary oocyte 46,XX in primary follicle

Primary oocyte 46,XX in growing follicle Follicular cells

Zona pellucida

Antrum

Sperm

Second polar body Corona radiata

Fertilized oocyte

First polar body Primary oocyte 46,XX in larger follicle

Secondary oocyte 23,X in mature follicle Second

meiotic division First meiotic division

Second meiotic division completed

First meiotic division completed

Figure 2–5 Normal gametogenesis: conversion of germ cells into gametes. The illustrations compare spermatogenesis and oogenesis. Oogonia are not shown in this figure because they differentiate into primary oocytes before birth. The chromosome complement of the germ cells is shown at each stage. The number designates the total number of chromosomes, including sex chromosome(s) (shown after the comma). Note: (1) After the two meiotic divisions, the diploid number of chromosomes, 46, is reduced to the haploid number, 23; (2) four sperms form from one primary spermatocyte, whereas only one secondary oocyte results from matura- tion of a primary oocyte; (3) the cytoplasm is conserved during oogenesis to form one large cell, the oocyte.

C H A P T E R 2    HuMAN REPRODuCTION 13

Figure 2–6 Diagrammatic representation of meiosis. Two chromosome pairs are shown.

A to D, Stages of prophase of the first meiotic division. The homologous chromosomes approach each other and pair; each member of the pair consists of two chromatids. Observe the single crossover in one pair of chromosomes, resulting in the interchange of chromatid segments. E, Metaphase. The two members of each pair become oriented on the meiotic spindle. F, Anaphase. G, Telophase. The chromosomes migrate to opposite poles. H, Distribu- tion of parental chromosome pairs at the end of the first meiotic division. I to K, Second meiotic division, which is similar to mitosis, except that the cells are haploid.

A B

Chromosome

Centromere

Meiotic spindle

S phase (DNA synthesis)

Single chromatid chromosome Double chromatid chromosome

C D

E

H

I

J

K

F G

14 BEFORE WE ARE BORN    ESSENTIALS OF EMBRYOLOGY AND BIRTH DEFECTS

tail provides the motility of the sperm, assisting with its transport to the site of fertilization in the ampulla of the uterine tube. The tail of the sperm consists of three parts:

middle piece, principal piece, and end piece. The middle piece contains the energy-producing mitochondria, which fuel the lashing movements of the tail. Hox genes influ- ence microtube dynamics at the molecular level in shaping the head of the sperm and in the formation of the tail.

Oogenesis

Oogenesis refers to the sequence of events by which oogonia (primordial oocytes) are transformed into primary oocytes. The maturation process begins during the fetal period; however, is not completed until after puberty—16 years. During early fetal life, oogonia pro- liferate by mitosis and enlarge to form primary oocytes (see Fig. 2-5). At birth, all primary oocytes have com- pleted prophase (first stage of mitosis) of the first meiotic division (see Fig. 2-6). The oocytes remain in prophase until puberty. Shortly before ovulation, a primary oocyte completes the first meiotic division. Unlike the corre- sponding stage of spermatogenesis, the division of cyto- plasm is unequal (see Fig. 2-5). The secondary oocyte receives almost all the cytoplasm, whereas the first polar body receives very little, causing it to degenerate after a short time. At ovulation (release of oocyte), the nucleus of the secondary oocyte begins the second meiotic divi- sion, but progresses only to metaphase.

If the secondary oocyte is fertilized by a sperm, the second meiotic division is completed and a second polar body is formed (see Fig. 2-5). The secondary oocyte released at ovulation is surrounded by a covering of amorphous material—the zona pellucida—and a layer of follicular cells—the corona radiata—(see Fig. 2-4C).

The secondary oocyte is large, being just visible to the unaided eye.

Up to 2 million primary oocytes are usually present in the ovaries of a neonate. Most of these oocytes regress during childhood so that, by puberty, no more than 40,000 remain. Of these, only approximately 400 oocytes mature into secondary oocytes and are expelled at ovula- tion (see Fig. 2-5).

Comparison of Male and Female Gametes

Compared with sperms, the oocytes are massive, are immotile, and have an abundance of cytoplasm (see Fig 2-4B and C). In terms of sex chromosome constitution, there are two kinds of sperms (see Fig. 2-5): 22 autosomes plus either an X sex chromosome (i.e., 23,X) or a Y sex chromosome (i.e., 23,Y). There is only one kind of sec- ondary oocyte: 22 autosomes plus an X sex chromosome (i.e., 23,X). The difference in sex chromosome comple- ment forms the basis of primary sex determination.