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Figure

The ovaries are located on each side against the lateral walls of the pelvic cavity. The right uterine tube is retracted to reveal the ovarian ligament.

more compact tissue and has a granular appearance due to tiny masses of cells called ovarian follicles.

A layer of cuboidal epithelial cells (germinal epithelium) covers the free surface of the ovary. Just beneath this epithelium is a layer of dense connective tissue called the tunica albuginea.

O What are the primary sex organs of the female? ^9 Describe the descent of the ovary. ^9 Describe the structure of an ovary.

Primordial Follicles

During prenatal development (before birth), small groups of cells in the outer region of the ovarian cortex form several million primordial follicles. Each of these structures consists of a single, large cell called a primary oocyte, which is closely surrounded by a layer of flattened epithelial cells called follicular cells.

Early in development, the primary oocytes begin to undergo meiosis, but the process soon halts and does not continue until puberty. Once the primordial follicles appear, no new ones form. Instead, the number of oocytes in the ovary steadily declines, as many of the oocytes degenerate. Of the several million oocytes formed originally, only a million or so remain at the time of birth, and perhaps 400,000 are present at puberty. Of these, probably fewer than 400 or 500 will be released from the ovary during the reproductive life of a female. Probably fewer than ten will go on to form a new individual!

A possible explanation for the increased incidence of chromosome defects in children of older mothers is that the eggs, having been present for several decades, had time to be extensively exposed to damaging agents, such as radiation, viruses, and toxins.

Oogenesis

Beginning at puberty, some primary oocytes are stimulated to continue meiosis. As in the case of sperm cells, the resulting cells have one-half as many chromosomes (23) in their nuclei as their parent cells. Egg formation is called oogenesis (o"o-jen'e-sis).

When a primary oocyte divides, the distribution of the cytoplasm is unequal. One of the resulting cells, called a secondary oocyte, is large, and the other, called the first polar body, is very small.

The large secondary oocyte represents a future egg cell (ovum) that can be fertilized by uniting with a sperm cell. If this happens, the oocyte divides unequally to produce a tiny second polar body and a large fertilized egg cell, or zygote (zi'got), that can divide and develop into an embryo (em'bree-o) (fig. 22.21). An embryo is the stage of prenatal development when the rudiments of all organs form. The polar bodies have no further function, and they soon degenerate.

Formation of polar bodies may appear wasteful, but it has an important biological function. It allows for production of an egg cell that has the massive amounts of cytoplasm and abundant organelles required to carry a zygote through the first few cell divisions, yet the right number of chromosomes.

99 Describe the major events of oogenesis. Q How does polar body formation benefit an egg?

An experimental procedure called polar body biopsy allows couples to select an egg that does not carry a disease-causing gene that the woman carries. First, oocytes with attached first polar bodies are removed from the woman and cultured in a laboratory dish. Then the polar bodies are screened with a DNA probe, which is a piece of genetic material that binds to a specific disease-causing gene and fluoresces or gives off radiation, which can be detected.

In polar body biopsy, bad news is really good news. Because of the laws of inheritance (discussed in chapter 24, p. 984), if the defective gene is in a polar body, it is not in the egg cell that it is physically attached to. Researchers can then fertilize the egg with sperm in the laboratory and implant it in the woman who donated it, with some confidence that the disorder carried in the family will not pass to this particular future child.

First polar body

Polar bodies degenerating

First polar body

Polar bodies degenerating

Ovarian cell

(46 chromosomes)

Mitosis

Ovarian cell

(46 chromosomes)

Mitosis

Primary oocyte (46 chromosomes)

First meiotic division

Secondary oocyte (23 chromosomes) Fertilization

Sperm cell (23 chromosomes)

Second meiotic division

Zygote

(46 chromosomes)

(b)

Figure 22.21

(a) During oogenesis, a single egg cell (secondary oocyte) results from the meiosis of a primary oocyte. If the egg cell is fertilized, it generates a second polar body and becomes a zygote. (b) Light micrograph of a secondary oocyte and a polar body (arrow) (700x).

Oocyte

Maturing follicle

Oocyte

Maturing follicle

Primordial follicles

Light micrograph of the surface of a mammalian ovary (50x micrograph enlarged to 200x).

Primordial follicles ii.ii

Light micrograph of the surface of a mammalian ovary (50x micrograph enlarged to 200x).

JF Follicle Maturation

At puberty, the anterior pituitary gland secretes increased amounts of FSH, and the ovaries enlarge in response. At the same time, some of the primordial follicles mature (fig. 22.22). Within them, the oocytes enlarge, and the surrounding follicular cells divide mitoti-cally, giving rise to a stratified epithelium composed of granulosa cells. A layer of glycoprotein, called the zona pellucida (zo'-nah pel-lu'-sid-ah), gradually separates the oocyte from the granulosa cells; at this stage, the structure is called a primary follicle.

Meanwhile, the ovarian cells outside the follicle become organized into two layers. The inner vascular layer (theca interna) is largely composed of steroid-secreting cells, plus some loose connective tissue and blood ves sels. The outer fibrous layer (theca externa) consists of tightly packed connective tissue cells.

The follicular cells continue to proliferate, and when there are six to twelve layers of cells, irregular, fluid-filled spaces appear among them. These spaces soon join to form a single cavity (antrum), and the oocyte is pressed to one side of the follicle. At this stage, the follicle is about 0.2 millimeters in diameter and is called a secondary follicle.

Maturation of the follicle takes ten to fourteen days. The mature follicle (preovulatory, or Graafian, follicle) is about 10 millimeters or more in diameter, and its fluid-filled cavity bulges outward on the surface of the ovary, like a blister. The oocyte within the mature follicle is a large, spherical cell, surrounded by a thick zona pellucida, attached to a

Thecal cells

Granulosa cells

Thecal cells

Antrum

Corona radiata

Zona pellucida

Oocyte

Nucleus

Figure 22.23

(a) Structure of a mature (Graafian) follicle. (b) Light micrograph of a mature follicle (250x).

Nucleus

Antrum

Corona radiata

Zona pellucida

Oocyte

Follicular Bulge
(b)

Figure 22.23

(a) Structure of a mature (Graafian) follicle. (b) Light micrograph of a mature follicle (250x).

mantle of follicular cells (corona radiata). Processes from these follicular cells extend through the zona pellucida and supply nutrients to the oocyte (fig. 22.23).

Although as many as twenty primary follicles may begin maturing at any one time, one follicle (dominant follicle) usually outgrows the others. Typically, only the dominant follicle fully develops, and the other follicles degenerate (fig. 22.24).

Certain drugs used to treat female infertility, such as Clomid (clomiphene), may cause a woman to "super-ovulate." More than one follicle grows, more than one oocyte is released, and if all these oocytes are fertilized, the result is multiples. In 1997, an Iowa couple had septuplets after using a fertility drug.

Ovulation

As a follicle matures, its primary oocyte undergoes meio-sis I, giving rise to a secondary oocyte and a first polar body. A process called ovulation (o"vu-la'shun) releases these cells from the follicle.

Hormonal stimulation (LH) from the anterior pituitary gland triggers ovulation, causing the mature follicle to swell rapidly and its wall to weaken. Eventually the wall ruptures, and the follicular fluid, accompanied by the oocyte, oozes outward from the surface of the ovary. Figure 22.25 shows expulsion of a mammalian oocyte.

After ovulation, the oocyte and one or two layers of follicular cells surrounding it are usually propelled to the opening of a nearby uterine tube. If the oocyte is not fertilized within a short time, it degenerates. Figure 22.26 illustrates maturation of a follicle and the release of an oocyte.

O What changes occur in a follicle and its oocyte during maturation?

^9 What causes ovulation?

^9 What happens to an oocyte following ovulation?

FEMALE INTERNAL Accessory Organs

The internal accessory organs of the female reproductive system include a pair of uterine tubes, a uterus, and a vagina.

Uterine Tubes

The uterine tubes (fallopian tubes, or oviducts) are suspended by portions of the broad ligament and open near the ovaries. Each tube, which is about 10 centimeters long and 0.7 centimeters in diameter, passes medially to the uterus, penetrates its wall, and opens into the uterine cavity.

Blood Vessels The Uterus

Blood vessel Degenerating follicle Mature Primordial Germinal follicle follicles epithelium

Figure 22.24

Light micrograph of a mammalian (monkey) ovary (30x). If ovulation does not occur, the follicle degenerates.

Blood vessel Degenerating follicle Mature Primordial Germinal follicle follicles epithelium

Figure 22.24

Light micrograph of a mammalian (monkey) ovary (30x). If ovulation does not occur, the follicle degenerates.

Uterine tube

Uterine tube

Oocyte Ovary

Figure 22.25

Light micrograph of a follicle during ovulation (75x).

Oocyte Ovary

Figure 22.25

Light micrograph of a follicle during ovulation (75x).

Near each ovary, a uterine tube expands to form a funnel-shaped infundibulum (in"fun-dib'u-lum), which partially encircles the ovary medially. On its margin, the infundibulum bears a number of irregular, branched extensions called fimbriae (fim'bre) (fig. 22.27). Although the infundibulum generally does not touch the ovary, one of the larger extensions (ovarian fimbria) connects directly to the ovary.

The wall of a uterine tube consists of an inner mu-cosal layer, a middle muscular layer, and an outer cover ing of peritoneum. The mucosal layer is drawn into many longitudinal folds and is lined with simple columnar epithelial cells, some of which are ciliated (fig. 22.28). The epithelium secretes mucus, and the cilia beat toward the uterus. These actions help draw the egg cell and expelled follicular fluid into the infundibulum following ovulation. Ciliary action and peristaltic contractions of the tube's muscular layer aid transport of the egg down the uterine tube.

Uterus

The uterus receives the embryo that develops from an egg cell that has been fertilized in the uterine tube, and sustains its development. It is a hollow, muscular organ, shaped somewhat like an inverted pear.

The broad ligament, which also attaches to the ovaries and uterine tubes, extends from the lateral walls of the uterus to the pelvic walls and floor, creating a drape across the top of the pelvic cavity (see fig. 22.27). A flattened band of tissue within the broad ligament, called the round ligament, connects the upper end of the uterus to the anterior pelvic wall (see figs. 22.20 and 22.27).

The size of the uterus changes greatly during pregnancy. In its nonpregnant, adult state, it is about 7 centimeters long, 5 centimeters wide (at its broadest point), and 2.5 centimeters in diameter. The uterus is located medially within the anterior portion of the pelvic cavity, superior to the vagina, and is usually bent forward over the urinary bladder.

Uterine tube

Corpus albicans

Corpus luteum

Primordial follicle

Uterine tube

Corpus albicans

Corpus luteum

Primordial follicle

Primary

Primary And Secondary Follicles Eggs

Secondary oocyte

Zona pellucida

Follicular cells

Corona radiata

Primary oocyte

Follicular fluid

First polar body

Figure 22.26

As a follicle matures, the egg cell enlarges and becomes surrounded by follicular cells and fluid. Eventually, the mature follicle ruptures, releasing the egg cell.

Secondary oocyte

Zona pellucida

Primary

Follicular cells

Corona radiata

Primary oocyte

Follicular fluid

First polar body

Figure 22.26

As a follicle matures, the egg cell enlarges and becomes surrounded by follicular cells and fluid. Eventually, the mature follicle ruptures, releasing the egg cell.

Blood Vessels Figures

Figure

The funnel-shaped infundibulum of the uterine tube partially encircles the ovary.

Figure

(a) Light micrograph of a uterine tube (250x micrograph enlarged to 800x). (b) Scanning electron micrograph of ciliated cells that line the uterine tube (4,000x).

Connective tissue layer

Basement membrane

Nucleus Cytoplasm Cilia

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Responses

  • CALUM
    What does it mean for diameter of the follicle to be 22.26 before ovulation?
    4 months ago

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