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Chapter 11 • Female Reproductive System

Ovarian Follicles

An ovarian follicle consists ofan oocyte surrounded by one or more layers offol­ licular cells, the granulosa cells. In utero, each ovary initially contains 3 million primordial germ cells. Many undergo atresia as the number of follicles in a nor­ mal young adult woman is estimated to be 400,000. A typical woman willovulate only around 450 ova during her reproductive years. Allother follicles (with their oocytes) willfail to mature and willundergo atresia.

Before birth, primordial germ cells differentiate into oogonia that proliferate by mitotic division until they number in the millions. They all enter prophase ofthe first meiotic division in utero and become arrested (they are now designated as primordial follicles). The primordial follicles consist of a primary oocyte sur­ rounded by a single layer of squamous follicular cells, which are joined to one another by desmosomes.

Around the time of sexual maturity, the primordial follicles undergo further growth to become primary follicles in which the oocyte is surrounded by 2 or more layers of cuboidal cells. In each menstrual cycle after puberty, several primary follicles enter a phase of rapid growth. The oocyte enlarges and the surrounding follicular cells (now called granulosa cells) proliferate. Gap junc­ tions form between the granulosa cells. A thick layer of glycoprotein called the zona pellucida is secreted (probably by both the oocyte and granulosa cells) in the space between the oocyte and granulosa cells. Cellular processes of the granulosa cells and microvilli of the oocyte penetrate the zona pellucida and make contact with one another via gap junctions. Around this time the stroma surrounding the follicle differentiates into a cellular layer called the theca fol­ liculi. These cells are separated from the granulosa cells by a thick basement membrane. As development proceeds, 2 zones are apparent in the theca: the theca interna (richly vascularized) and the theca externa (mostly connective tissue). Cells of the theca interna synthesize androgenic steroids that diffuse into the follicle and are converted to estradiol by the granulosa cells.

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Figure 1-1 1-4. Ovary

Small primordial follicles are at top and 3 primary follicles (arrows) with cuboidal granulosa cells and a thin zona pellucida are below.

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Chapter 11 • Female Reproductive System

Ovulation

Ovulation occurs approximately mid-cycle andis stimulated by a surge ofluteiniz­ ing hormone secretedbythe anteriorpituitary. Ovulationconsists ofruptureofthe mature follicle andliberation ofthe secondaryoocyte (ovum)thatwillbe caughtby the infundibulum, the dilated distal end ofthe oviduct. The ovum remains viable for a maximum of24 hours. Fertilization most commonly occurs in the ampulla of the oviduct. Ifnot fertilized, the ovum undergoes autolysis in the oviduct.

Corpus Luteum

After ovulation, the wall ofthe follicle collapses and becomes extensively infold­ ed, forming a temporary endocrine gland called the corpus luteum. During this process the blood vessels and stromal cells invade the previously avascular layer of granulosa cells and the granulosa cells and those of the theca interna hyper­ trophy and form lutein cells (granulosa lutein cells and theca lutein cells). The granulosa lutein cells now secrete progesterone and estrogen and the thecalutein cells secrete androstenedione and progesterone. Progesterone prevents the devel­ opment ofnew follicles, thereby preventing ovulation.

In the absence ofpregnancy the corpus luteum lasts only 10-14 days. The lutein cells undergo apoptosis and are phagocytized by invading macrophages. The site ofthe corpus luteum is subsequently occupied by a scar of dense connective tis­ sue, the corpus albicans.

When pregnancy does occur, human chorionic gonadotropin produced by the placenta willstimulate the corpus luteum for about 6 months and then decline. It continues to secrete progesterone until the end ofpregnancy. The corpus luteum ofpregnancy is large, sometimes reaching 5 cm in diameter.

OVIDUCTS

The oviduct (Fallopian tube) is a muscular tube of about 12 cm in length. One end extends laterally into the wall ofthe uterus and the other end opens into the peritoneal cavitynext to the ovary. The oviduct receives the ovum from the ovary, provides an appropriate environment for its fertilization, and transports it to the uterus. The infundibulum opens into the peritoneal cavity to receive the ovum. Finger-like projections (fimbriae) extend from the end of the tube and envelop the ovulation site to direct the ovum to the tube.

Adjacent to the infundibulum is the ampulla, where fertilization usually takes place. A slender portion ofthe oviduct called the isthmus is next to the ampulla. The intramural segment penetrates the wall ofthe uterus.

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The myometrium is composed of bundles of smooth-muscle fibers separated by connective tissue. During pregnancy, the myometrium goes through a period of growth as a result of hyperplasia and hypertrophy. The endometrium consists of epithelium and lamina propria containing simple tubular glands that occasionally branch in their deeper portions. The epithelial cells are a mixture of ciliated and secretory simple columnar cells.

The endometrial layer can be divided into 2 zones. The functionalis is the part that is sloughed offat menstruation and replaced during each menstrual cycle, and the basalisis the portion retained after menstruation that subsequently proliferates and provides a new epithelium and lamina propria. The bases ofthe uterine glands, which lie deep in the basalis, are the source ofthe stem cells that divide and migrate to form the new epithelial lining.

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Figure 1-11 -7. Uterine wall with endometrium

Simple tubular glands to the right of arrow and myometrium to the left of arrow

VAGINA

The wall of the vagina has no glands and consists of 3 layers: the mucosa, a muscular layer, and an adventitia. The mucus found in the vagina comes from the glands of the uterine cervix. The epithelium of the mucosa is stratified squamous. This thick layer of cells contains glycogen granules and may contain some keratohyalin. The muscular layer of the vagina is composed of longitudi­ nal bundles of smooth muscle.

Section I • Histology and Cell Biology

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Figure 1-11-8. Vaginal epithelium with vacuolated stratified squamous epithelial cells that contain glycogen, which is removed during histological processing

MAMMARY GLANDS

The mammary glands are not part ofthe reproductive tract but are important ac­ cessory glands. The mammary gland is a tubuloalveolar gland consisting of 15-20 lobes drained by an equal number of lactiferous ducts that open at the tip of the nipple. The lobes are separated by connective tissue and varying amounts of adipose tissue. The histological structure varies according to sex, age, and physi­ ologic status.

At birth the gland consists only of short branching lactiferous ducts with no as­ sociated alveoli. As puberty approaches in the female, the ducts elongate and branch under the influence of ovarian hormones, and small spherical masses of epithelial cells appear at the ends ofthe branches. These cells are capable ofform­ ing functional acini in response to hormonal stimulation. Between the epithelial cells and the basal lamina are myoepithelial cells.

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Figure 1-11-9. Breast tissue containing modified mammary gland tissue (arrow) surrounded by dense regular connective tissue (arrowhead)

The mammary glands enlarge significantly during pregnancy as a result of pro­ liferation ofalveoli at the ends ofthe terminal ducts. Alveoli are spherical collec­ tions of epithelial cells that become the active milk-secreting structures during lactation. The milk accumulates in the lumen ofthe alveoli and in the lactiferous ducts. Lymphocytes and plasma cells are located in the connective tissue sur­ rounding the alveoli. The plasma cell population increases significantly at the end ofpregnancy and is responsible for the secretion oflgA that confers passive im­ munity on the newborn.

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