The Female Reproductive System

The endocrine system | Main Anatomy Index | The urinary system

Last updated 30 March 2006

This page was contributed by David Boshell.

The Female Reproductive System

• The female reproductive system consists of internal organs, including the ovaries, oviducts, uterus and vagina; external genital structures, and the breasts.
• Menarche, the beginning of menstruation in females after puberty, occurs at about age 15.
• In the reproductive phase of female life, the menstrual cycle, taken between each menses (the start of bleeding), lasts about 28 days.
• Menopause occurs when this cycle eventually ceases, between the ages of 45 and 55.

The Ovary

• These are paired organs lying on either side of the uterus, next to the lateral pelvic wall.
• There are 2 main functions of the ovary:

1. Gametogenesis: the production of gametes. Oocytes are female gametes developing into mature ova; thus this process is known as oogenesis in the female.
2. The production of steroid hormones, including:

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  • Oestrogens, that promote growth and maturation of the sex organs and mammary glands giving mature female characteristics;
  • Progestogens, that prepare the sex organs (mainly the uterus) for pregnancy, and the mammary glands for lactation.

• These hormones have an important role in the menstrual cycle by preparing the uterus for implantation of a fertilised ovum.

Ovarian Structure

• The ovary consists of:

The Medulla of the Ovary

• The medulla, in the central portion
• It contains loose connective tissue, large blood vessels, lymphatics and nerves.

The Cortex of the Ovary

• The cortex, in the peripheral portion.
• It comprises the bulk of the tissue.

• It is lined by a layer of cuboidal germinal epithelium, over a dense connective tissue layer called the tunica albuginea.
• The primordial germ cells, however, are of extragonadal origin.
• The cortex contains ovarian follicles in richly cellular connective tissue stroma.
• Scattered smooth muscle fibres are present.

• At the hilum, the peritoneal fold of mesovarium is continuous with the medulla.

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Follicle development

Click here for a diagram of follicular development.

• The early stages of oogenesis occur during foetal development.
• All potential eggs are present at birth, arrested in their development at the first meiotic division.

• During the menstrual cycle, follicular maturation and ovulation continues, but only one oocyte will be released from the ovary.
• Most oocytes present at birth are lost through atresia: the death and resorption of immature oocytes.
• Thus, although there are some 400,000 follicles at birth, only 400 eggs will be ovulated in the reproductive life span.

Basic types of ovarian follicle include:

1. Primordial follicles, the earliest stage of development;
2. Growing follicles, including preantral (primary) and small antral (secondary) follicles.
3. Mature, pre-ovulatory (Graafian) follicles.

Primordial follicles

• These appear in the third month of foetal development.
• They are found in the stroma of the cortex just below the tunica albuginea.

• They are surrounded by a single layer of squamous follicular cells.
• The oocyte has a large, eccentric nucleus.

Preantral (Primary) Follicles

• This is the first stage of the growing follicle.
• The oocyte enlarges, and the surrounding follicular cells proliferate, becoming cuboidal.

• The zona pellucida, a deeply staining, acidophilic glycoprotein layer is deposited next to the follicular cells by microvilli from the oocyte.
• The follicular cells then stratify with an outer columnar layer, becoming known as granulosa cells.

• During this proliferation, surrounding stromal cells form the theca folliculi: a connective tissue sheath with 2 layers:

1. The theca interna, an inner, vascular layer of cuboidal secretory cells with luteinising hormone (LH) receptors, that, upon response to LH, synthesise and secrete androgens; the precursors of oestrogen;
2. The theca externa, an outer layer of connective tissue cells, smooth muscle cells and collagen fibres.

• A basal lamina separates the granulosa cells from the theca folliculi.
• In the process of atresia, this membrane thickens and hyalinises to become the glassy membrane.
• The follicle, about 0.2 mm wide, moves deeper into the cortical stroma.

• Factors required for oocyte and follicular growth include:

1. Follicle stimulating hormone (FSH);
2. Epidermal growth factor (EGF);
3. Insulin-like growth factor I (IGF-I);
4. Calcium ions (Ca²⁺).

Antral (Secondary) Follicles

• These follicles have a fluid filled, crescent shaped cavity, the antrum, containing liquor folliculi.
• The oocyte, positioned eccentrically, grows no further after reaching a diameter of about 125 micrometres.

• The granulosa cells surrounding the oocyte form the corona radiata.
• This is located in the cumulus oophorus, a mound of granulosa cells projecting into the antrum.

Pre-ovulatory (Graafian) Follicles

• The mature follicle has a diameter of about 10 mm.

• Near this size, the stratum granulosum becomes thinner.

• Spaces between the granulosa cells widen, and the oocyte with its cumulus cells loosens from the others, preparing for ovulation.

• The thecal layers become more prominent, with lipid droplets appearing in the cytoplasm of theca interna cells.
• LH stimulates these cells to release androgens that are transported to the granulosa cells.
• Here, upon the response to FSH, they are converted to oestrogens.
• This stimulates the granulosa cells to proliferate, increasing the size of the follicle.

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Ovulation

• This is a hormone-mediated process resulting in the release of the secondary oocyte from the pre-ovulatory follicle.
• This is due to hormonal changes and enzymatic effects occurring in the middle of the menstrual cycle (day 14).

• A surge in the release of FSH and LH is induced in the pituitary about 24 hours before ovulation.
• This triggers resumption of the first meiotic division in the primary oocyte, producing two daughter cells:

1. The secondary oocyte, receiving half the chromatin and most of the cytoplasm;
2. The first polar body, which receives little cytoplasm and degenerates.

• The granulosa and theca cells then undergo luteinisation and produce progesterone.

• The second meiotic division begins immediately.
• The macula pellucida (stigma) is the site of rupture of the follicle, and the secondary oocyte with its granulosa cells is expelled, in the process of division.

• The oocyte, arrested at metaphase, is transported to the oviduct, where it remains viable for about 24 hours.
• If fertilisation occurs, the second meiotic division completes, producing:

1. A mature ovum with the maternal pronucleus containing a set of 23 chromosomes;
2. The second polar body, which degenerates.

• If fertilisation fails to occur, the oocyte then degenerates.

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The Corpus Luteum

• After ovulation, the follicle collapses, forming the corpus luteum.
• Connective tissue eventually invades the follicular lumen, and the granulosa and theca interna cells (luteal cells) become larger, filled with yellow lipid droplets.

• There are now 2 types of luteal cells:

1. Large, centrally located granulosa lutein cells, from the granulosa cells;
2. Smaller peripherally located theca lutein cells, from the theca interna.

• This structure becomes highly vascularised, secreting progesterone and oestrogens, preparing the endometrium of the uterus for implantation.

• If fertilisation occurs, the corpus luteum forms the corpus luteum of pregnancy.
• If fertilisation does not occur, the corpus luteum of menstruation remains active for 14 days before degenerating and forming a white scar called the corpus albicans.

The Ovarian Cycle

• This consists of 3 phases:

1. The follicular phase (days 1-14), where follicles develop under the influence of oestrogen;
2. Ovulation (day 14): rupture of the follicle;
3. The luteal phase, where the corpus luteum produces progesterone before degenerating.

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The Oviduct

• These are paired tubes, also known as Fallopian or uterine tubes.
• They extend bilaterally from the uterus to the ovaries.

• It carries the ova from the ovary to the uterus.

• There are 4 parts to the oviduct:

1. The funnel-shaped infundibulum opening into the peritoneal cavity next to the ovary with fringed extensions, called fimbriae, extending towards it;
2. The ampulla, the main part of the tube, which is the site of fertilisation;
3. The narrow isthmus, adjacent to the uterus;
4. The uterine or intramural part within the uterine wall;

The Wall of the Oviduct

• This is composed of 3 layers:

1. An external serosa or peritoneum, consisting of mesothelium and loose connective tissue;
2. The intermediate muscularis, consisting of thick, inner circular and thin, outer longitudinal muscle;
3. The inner mucosa, with thin longitudinal folds projecting into the lumen of the oviduct, numerous in the ampulla, but smaller in the isthmus.

• There is no submucosa.
• The simple, columnar epithelial mucosal lining has 2 types of cells:

1. Ciliated cells whose wave is directed toward the uterus;
2. Non-ciliated peg cells that secrete oviductal fluid, providing nutrients for the ovum.

Oviduct Transport

• At ovulation, the fimbriae are closely apposed to the ovarian surface where rupture will occur.
• As the egg is released, ciliated cells in the infundibulum sweep it towards the oviduct, to prevent it from passing into the peritoneal cavity.

• The egg is transported to the uterus for about 3 days by both peristaltic muscular activity and ciliary movement.

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The Uterus

• This is a hollow pear-shaped organ with a thick muscular wall.
• It is divided into:

1. The large upper body, with the fundus above the uterine tubes;
2. The lower, barrel-shaped cervix is separated from the body by the isthmus.

• The uterine wall is composed of a mucosa, the endometrium; a muscularis, the myometrium; and an external serosa of visceral peritoneum called the perimetrium.
• There is no submucosa.

• The myometrium and endometrium undergo cyclic changes each month, constituting the menstrual cycle.

The Myometrium

• This is the thickest layer of the uterine wall.
• It consists of 3 indistinct layers of muscle:

1. A middle muscle layer, the stratum vasculare, containing large blood vessels, with spiralling, interlacing smooth muscle fibres;
2. Outer and inner layers with smooth muscle fibres running down the long axis of the uterus.

• In pregnancy, these muscle fibres hypertrophy and divide, and there is an increase in connective tissue.
• There are elastic fibres in the outer layer.

The Endometrium

• This layer proliferates, then degenerates during the menstrual cycle.
• It has 2 layers:

1. The thick stratum functionale. It is lined with simple columnar epithelium that invaginates into the endometrial stroma to form simple, tubular uterine glands. This layer is sloughed off at menstruation;
2. The basal stratum basale, retained during menstruation, serves as a source for regeneration of the stratum functionale.

Endometrial Vasculature

• Arcuate arteries in the myometrium, from the uterine artery, branch to radial arteries entering the basal layer of the endometrium.
• These branch into straight arteries supplying this layer, and spiral arteries supplying capillaries in the stratum functionale.

• This capillary bed contains dilated segments called lacunae.
• The vasculature in this layer proliferates under the influence of oestrogen, and degenerates under the influence of progesterone.

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The Menstrual Cycle

Click here for a diagram of the menstral cycle.

• This 28 day cycle has 3 continuous phases:

1. Menstrual phase (days 1-5), when the corpus luteum degenerates and ovarian hormone production declines. In the stratum functionale, arteries constrict and rupture, surface epithelium is disrupted, constituting vaginal discharge;
2. Proliferative phase (days 5-14), influenced by oestrogen from the ovaries, occurring with follicular maturation. Cells and spiral arteries of the stratum basale proliferate rapidly.
3. Secretory phase (days 15-28). This stage begins with ovulation, with the endometrium swelling under the influence of progesterone from the corpus luteum.

The Cervix

• The cervix has 2 constricted openings:

1. The internal os at the uterine end
2. The external os at the vaginal end. This is the site of transition between vaginal stratified squamous epithelium in the porto vaginalis, and the mucous secreting simple columnar epithelium of the cervical canal.

• The cervical mucosa differs from the rest of the uterine endometrium. It:

1. Lacks spiral arteries;
2. Has little change in thickness over the menstrual cycle, and is not sloughed in the menstrual phase;
3. Have large, branched, mucous secreting cervical glands (Nabothian glands). Normally in the menstrual cycle, the mucous inhibits sperm migration, but during midcycle, a lot less viscous mucous is produced, providing a more favourable environment for the passage of sperm and fertilisation.

• The cervical epithelial cells are constantly exfoliated into the vagina.
• These cells can be prepared on a Papanicolaou cervical smear for screening of lesions related to cervical cancer.

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The Vagina

• This is a fibromuscular tube extending from the cervix to the vestibule. The vaginal wall consists of:

1. The vaginal mucosa internally, lined by non-keratinised stratified squamous epithelium, with many rugae. Connective tissue papillae from the lamina propria project into this layer. There are no glands here, as the vagina is lubricated by mucous produced by the cervical glands.
2. The intermediate vaginal muscularis, with inner circular smooth muscle and outer longitudinal fibres.
3. The outer vaginal adventitia, with an inner layer of dense fibroelastic tissue, and an outer layer of loose connective tissue.

The Mammary Glands (Breasts)

• These are modified apocrine sweat glands that develop during puberty.

• It is composed of lobes of branched (lobulated) tubuloalveolar glands in subcutaneous tissue, radiating from the nipple.
• Each lobule drains into ducts that carry milk to lactiferous sinuses beneath the areola, where it pools before being secreted through the nipple via lactiferous ducts.
• The intralobular connective tissue is loose.
• The dense interlobular connective tissue contains adipose tissue.

• The areola becomes pigmented during puberty, increasing after pregnancy.
• It contains sebaceous glands, sweat glands, and modified mammary glands (of Montgomery).

• Underlying smooth muscle fibres are arranged circumferentially and radially, and longitudinally along the lactiferous ducts.
• This facilitates lactation from, and erection of the nipple.

• The nourishing, yellow pre-milk released temporarily after childbirth, colostrum, contains many antibodies that provide the newborn with some passive immunity.

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