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Ovarian Function in Reproduction: A Comprehensive Guide, Lecture notes of Anatomy

University of Southeastern Philippines (USEP)Anatomy

Ovaries are dynamic organs in which the type and amount of hormones, as well as the oocyte development, vary throughout the female cycle. The ovaries are innervated by autonomic nerves and receive an especially rich blood supply. They are anchored in place by ligaments (bands or sheets of tissue) connecting them to nearby organs.

Typology: Lecture notes

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FEMALE REPRODUCTIVE SYSTEM
consists of;
• paired ovaries, oviducts, uterus, vagina, external genitalia and the mammary glands.
evolved in primary functions;
• ovulation
• fertilization of an ovum by a sperm
• developing embryo and fetus
• birth and care of an infant
distinction of females to males are FEMALE SEXUAL CHARACTERISTICS:
• Primary Sexual Characteristics – includes internal structures of the reproductive system (ovaries,
female accessory ducts- oviducts, uterus and vagina) as well as the external genitalia.
• Secondary Sexual Characteristics - includes all external features (enlarged breast and the
characteristic distribution of fat in the torso) except external genitalia that distinguish adult
female from adult male.
OVARIES OR FEMALE GONADS
• Structure: oval-shaped or almond-shaped and have a lumpy surface
• Location: in the upper pelvic cavity, against the back of the pelvic wall on either side of the
uterus.
• Color: white or yellowish
• Size: Length is about 2.5-5 cm (1-2inches) and the width is about 1.5-3 cm (0.5-1 inch)
Two essential roles of the ovaries in reproduction;
• Production of the female gametes (oocytes or eggs)
• Secretion of several hormones (estrogens, progesterone and inhibin)
Ovaries are dynamic organs in which the type and amount of hormones, as well as the oocyte development
vary throughout the female cycle. The ovaries are innervated by autonomic nerves and receive especially
rich blood supply. They are anchored in place by ligaments (bands or sheet of tissue) connecting them to
nearby organs.
OVARIAN LIGAMENT – a thin, rope-like support that attaches the ovary to the uterus.
SUSPENSORY LIGAMENT – attaches the lateral surface of the ovary to the pelvic wall. It also carries the
blood vessels that supply nutrients and stimulatory hormones to the ovary.
BROAD LIGAMENT – a thin sheet of connective tissue that covers the ovaries, uterus, and oviducts
stabilizing their position and anchoring them to the walls and floor of the pelvic cavity. It also helps to
keep these delicate reproductive structures in proper alignment, which may be important in allowing the
egg to successfully pass from the ovary into the oviduct on its way to the uterus.
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FEMALE REPRODUCTIVE SYSTEM

consists of;

  • paired ovaries, oviducts, uterus, vagina, external genitalia and the mammary glands. evolved in primary functions;
  • ovulation
  • fertilization of an ovum by a sperm
  • developing embryo and fetus
  • birth and care of an infant distinction of females to males are FEMALE SEXUAL CHARACTERISTICS:
  • Primary Sexual Characteristics – includes internal structures of the reproductive system (ovaries, female accessory ducts- oviducts, uterus and vagina) as well as the external genitalia.
  • Secondary Sexual Characteristics - includes all external features (enlarged breast and the characteristic distribution of fat in the torso) except external genitalia that distinguish adult female from adult male. OVARIES OR FEMALE GONADS
  • Structure: oval-shaped or almond-shaped and have a lumpy surface
  • Location: in the upper pelvic cavity, against the back of the pelvic wall on either side of the uterus.
  • Color: white or yellowish
  • Size: Length is about 2.5-5 cm (1-2inches) and the width is about 1.5-3 cm (0.5-1 inch) Two essential roles of the ovaries in reproduction;
  • Production of the female gametes (oocytes or eggs)
  • Secretion of several hormones (estrogens, progesterone and inhibin) Ovaries are dynamic organs in which the type and amount of hormones, as well as the oocyte development vary throughout the female cycle. The ovaries are innervated by autonomic nerves and receive especially rich blood supply. They are anchored in place by ligaments (bands or sheet of tissue) connecting them to nearby organs. OVARIAN LIGAMENT – a thin, rope-like support that attaches the ovary to the uterus. SUSPENSORY LIGAMENT – attaches the lateral surface of the ovary to the pelvic wall. It also carries the blood vessels that supply nutrients and stimulatory hormones to the ovary. BROAD LIGAMENT – a thin sheet of connective tissue that covers the ovaries, uterus, and oviducts stabilizing their position and anchoring them to the walls and floor of the pelvic cavity. It also helps to keep these delicate reproductive structures in proper alignment, which may be important in allowing the egg to successfully pass from the ovary into the oviduct on its way to the uterus.

OVARIAN MICROANATOMY

The external surface of each ovary is covered with a thin sheet of tissue, the surface epithelium it was once called as “ germinal epithelium ” because it was thought the female germ cells were derived from tissue. Now, it is known as that germ cells originated outside of the ovary during embryonic development. Tunica albuginea – this is underneath the surface epithelium that is a tough protective layer of connective tissue. This connective tissue framework is divided into more dense, outer ovarian cortex and a less dense central ovarian medulla. Ovarian cortex – contains the female germ cells. Each female germ cell or oocyte is enclosed in a tissue sac the, ovarian follicle (follicle in Latin is “little bag”). Between the oocyte and the follicular wall is a thin transparent membrane, the zona pellucida which is secreted by the oocyte. Ovarian medulla – contains large, spirally arranged blood vessels, lymphatic vessels, and the nerves. STAGES OF FOLLICULAR GROWTH Ovarian follicle is the basic functional unit of the ovary.

  1. Most follicles in the adult ovary are very small., nongrowing about 50 μm in diameter. These primordial follicles consist of an oocyte surrounded by a single layer approximately 15 squamous (flattened) granulosa cells , the membrane granulosa which is bounded by a basement membrane. Primordial follicles lie in the periphery of the ovarian cortex. Most of them never move beyond this stage of development, but few of these follicles initiate growth each day. The signal causing these follicles to enter the growth phase is unknown.
  2. Those that grow become primary follicles , which are about 100 μm in diameter. This initial growth of follicle is a consequence of a slight increase in the size of the oocyte as well as the growth of the granulosa layer. The granulosa is still a singe layer of cells in primary follicles but these cells are now cuboidal (cube- shaped) a sign that these cells are becoming more metabolically active in preparation for hormone secretion. The zona pellucida is a thin layer rich in glycoprotein , develops in the space between the oocyte and the granulosa cells.
  3. As the follicle continues to grow, granulosa cells undergo mitosis, resulting in multiple layers of granulosa cells around the oocyte, the follicle is now a secondary follicle with a membrana granulosa consisting of two to six cell layers. Fibroblast-like precursor cells from the surrounding stroma are recruited into a peripheral layer, the theca****. Blood vessels invade the thecal layer allowing the delivery of blood-borne nutrients and other materials to the follicle. In contrast, the granulosa layer is completely avascular. Growth of the follicle to this stage is a relatively slow process, lasting about four months.

FOLLICULAR ATRESIA

Each ovary contains many follicles which its number changes during a female’s life. The number of follicles peaks at 7million about midway through gestation and begins a decline in fetal life that continues until menopause. An infant girl has about 1million follicles in both of her ovaries, which this will be all she ever have in her entire life; no new follicles are formed after birth (although many recent evidences questions this long- held notion). In fact, over the next 50years or so, the size of her population will decline steadily. Puberty – about 200,00 will remain. Age of 35 – she will have fewer than 100,00 follicles. Menopause – her follicular supply will ne nearly depleted. Typically, a woman ovulates 1 oocyte per month over an approximately 40-year reproductive life span. This counts for the loss of 400-500 follicles. The vast majority of al follicles degenerate and die in the process known as atresia. This is a type of cell death called apoptosis or programmed cell death that plays a role in the selective elimination od many o the cells in the body. It is characterized by specific changes in the nucleus followed by cell fragmentation. The cellular remains of the granulosa cells and oocyte are removed by leukocytes (white blood cell scavengers), but atretic follicles leave tiny scars in the ovary. The death of a follicle can occur at any stage of follicular growth. Atresia begins as soon as oocytes begin to proliferate in the embryonic ovary and continues throughout a woman’s reproductive lifespan. Thus, atresia is the normal fate of ovarian follicles. OVARIAN STEROID HORMONE SYNTHESIS Ovarian follicles synthesize and secrete steroid hormones, all three major classes of reproductive steroid hormones (estrogens, progestins and androgens) are produced by the follicles. Cholesterol is the precursor for all steroid hormones secreted by the ovary. Each step of steroidogenesis (biosynthesis of steroids) is the enzymatic conversion of one steroid molecule to another. Most of these enzymes are members of cytochrome which add oxygen to the steroid ring structure. Steroidogenic enzymes are located within the mitochondria or smooth endoplasmic reticulum so the steroids must move to these organelles to be acted on by the enzymes. Steps:

1. Transport of cholesterol from outer to the inner mitochondrial membrane, a function provided by a protein called StAR (steroid acute regulatory protein).

  1. Cholesterol is then converted by an enzyme to pregnenolone , another precursor steroid. Steroidogenesis within the ovary can follow one of the two pathways.

In the Δ^5 pathway, which occurs predominantly in large tertiary follicles, pregnenolone is converted to 17 - hydroxypregnenolone , then to a weak androgen ( Dehydroepiandrosterone-DHEA ) and then to another weak androgen ( androstenedione ). A weak androgen is one that is not very potent in stimulating male tissue. In the Δ^4 pathway, pregnenolone is converted into progesterone , a progestin that not only serves as a precursor for other steroids, but enters the female’s blood and acts as a hormone on such target tissues as the uterus and mammary glands. Within the ovary the progesterone is then converted into 17 - hydroxypregnenolone , which is then changed to androstenedione. This pathway is predominant in the corpus luteum ( an endocrine gland formed from the follicle after it ovulates ). From here on both pathways can produce estrogens. Androstenedione is converted to testosterone (a potent androgen). Testosterone then can be converted to estradiol-17β ( estradiol ), the major estrogenic hormone secreted by ovarian follicles and the corpus luteum. NOTE: AROMATASE – the enzyme that converts testosterone to estradiol. ESTRONE – a weak estrogen that can be synthesized in both ovarian pathways. ESTRIOL – another weak estrogen that is mainly synthesize by the placenta.

depends on the coordinated biochemical activity of thecal and granulosa cells. This is known as the two-cell model (see figure above). To summarize:

  • Cholesterol precursor molecules are removed from the bloodstream by theca cells.
  • The metabolic machinery of these cells is specialized to synthesize androgens (primarily androstenedione) by the Δ^5 pathway.
  • Androgens produced by the theca cells diffuse to the granulosa.
  • Granulosa cells which cannot produce their own androgens take up this androstenedione and use it as a precursor for conversion to estrogens.
  • Granulosa and theca cells differ in their steroidogenic enzyme activity, and their cooperation is necessary for normal steroid production by the ovary. NOTE:
  • NOT ALL ANDROGENS PRODUCED BY THE FOLLICLE ARE CONVERTED TO ESTROGEN.
  • Low levels of androgens are present in a woman’s blood.
  • Weak androgens are predominant and most of these come from the adrenal glands.
  • Additional androgens including testosterone are secreted from growing follicles and from interstitial cells that originate from the thecal layer of an atretic follicle, which does not degenerate with the rest of the follicle.
  • NONSTEROIDAL HORMONES produced by the ovary includes INHIBIN, ACTIVIN, and FOLLISTATIN. HORMONAL CONTROL OF FOLLICULAR GROWTH AND STEROIDOGENESIS
  • Follicular growth and steroid hormone secretion are controlled by two pituitary gonadotropins: FSH (Follicle Stimulating Hormone) and the LH (Luteinizing Hormone).
  • The stimulus that induces primordial follicles to initiate growth is not known.
  • The signal probably comes from the ovary itself in the form of a local paracrine factor such as growth factor.
  • Low levels of FSH promote growth of primary and secondary follicles by stimulating the division of granulosa cells.
  • The rapid growth of the tertiary follicles is FSH dependent.
  • In large growing follicles, thecal and granulosa cells are LH and FSH dependents for steroid production.
  • As tertiary follicles grow, they release greater quantities of steroid hormones. In fact, the single dominant tertiary follicle produces most of the estrogen circulating in a woman’s bloodstream. However, just before the ovulation, hormone synthesis by ovary changes.
  • At this time, granulosa cells of the dominant follicle acquire LH receptors and LH causes the granulosa cells to synthesize a new set od steroidal enzymes. These cells not switch to Δ^4 pathway so they begin to secrete progesterone. This transformation is called luteinization.

OOCYTE MATURATION AND OVULATION

  • In human menstrual cycle, about 20 large tertiary follicles form in both ovaries before ovulation but, usually only one of these follicles in one ovary actually ovulates , the remaining follicles undergo atresia.
  • The ‘selected’ follicle is said to be the one who access most of the FSH to stimulate its growth, may be slightly larger, more vascular and have more granulosa cells and FSH receptors than its competitors.
  • The estrogen secretion of this secreted follicle increases and inhibits FSH secretion from the pituitary through negative feedback, thus the other tertiary follicles become deprived of FSH causing their degeneration.
  • Ovarian follicles contain gonadotropin-releasing hormone (GnRH) and that this substance inhibits follicular function.
  • OVULATION – extrusion od an oocyte from its follicle.
  • MEIOSIS - cell division found in germ cells.
  • The nucleus human cells contain 46 chromosomes (23 pairs of homologous chromosomes). This is called the diploid condition and symbolized by ‘2n’.
  • When germ cells first appear in the ovaries of female fetuses, they begin the process of meiosis that produces cells that have only 23 chromosomes , the haploid ‘n’ condition.
  • When a haploid ovum fuses with a haploid sperm, the diploid number of chromosomes is restored.
  • OOGENESIS – the process that results to the production of mature female gamete “sex cells”.
  • At birth, the ovaries of a female infant contain primary oocytes in the primordial follicles. These primary oocytes are arrested in the first meiotic division and this first meiotic arrest is maintained in the oocytes of the follicles of all stages except the most mature.
  • In menstrual cycle, only one follicle reaches the large Graafian stage every month.
  • Then, a surge of LH secretion from the pituitary gland occurs. LH will acts on follicle cells which triggers the resumption of the meiosis ( meiotic maturity ) in the oocyte. MEIOTIC DIVISION 1. The membrane of the oocyte nucleus ( germinal vesicle ) disintegrates, in a process called germinal vesicle breakdown. This is the reduction division of meiosis and produces two haploid daughter cells, still within the follicle. The reduction division is unequal and produces a large haploid secondary oocyte (about 190μm in diameter) and a tiny haploid first polar body. This polar body can divide again or remain single; in either case, it degenerates.
  1. The secondary oocyte then begins the second meiotic division, but this division is again arrested while the oocyte is still within the follicle; this is the second meiotic arrest. The second meiotic division is not completed (resulting in a haploid ootid and second polar body ) until after sperm penetration of the ovum, which occurs in the oviduct. Thus, the female germ cell is ovulated as a secondary oocyte (ovum, or egg) in the second meiotic arrest.

brake on meiotic resumption in the arrested oocyte is supported by the observation that oocytes removed from their surrounding follicles spontaneously reinitiate meiosis.

  • An intracellular messenger molecule, cyclic AMP (cAMP) - meiosis-inhibiting signal.
  • A certain threshold level of cAMP in the oocyte cytoplasm is needed to maintain meiotic arrest. Thus, high cAMP may inhibit the progress of meiosis during the oocyte’s long period of stasis in meiosis I.
  • The major source of cAMP within the oocyte - is thought to be synthesized in granulosa cells and transported to the oocyte through the granulosa/oocyte gap junctions.
  • One of the consequences of the LH surge is the loss of these gap junctions, which would interrupt the flow of cAMP to the oocyte.
  • Other informational molecules involved in transmitting the LH signal from granulosa cells to the oocyte; epidermal growth factors (EGFs) , which may cause follicle maturation through paracrine action, and mitogen-activated protein kinase (MAPK) , an intracellular signal.
  • Maturation-promoting factor (MPF) - the actual events of meiotic maturation are controlled by this substance. This controls both mitosis and meiosis in cells of a wide range of organisms. This also causes the germinal vesicle (oocyte nuclear membrane) to break down and the chromosomes to divide. How does the oocyte escape from the follicle?
  • Just before ovulation, a small, pale (avascular) region , the stigma , appears on the follicular surface.
  • In this region, the surface epithelium and thecal layers of the follicle become thinner and dissociated , and the follicular wall exhibits a reduction in tensile (“breaking”) strength.
  • Also, the membrana granulosa degenerates in this region.
  • This thinning and weakening of the follicular wall at the stigma appear to be caused by estrogen-induced stimulation of the production of an enzyme ( collagenase ) from the connective tissue cells in this area.
  • Breakdown products of the destroyed connective tissue induce an inflammatory response , with migration of white blood cells and secretion of prostaglandins in this region.
  • Prostaglandins (PG) - facilitate ovulation by constricting blood vessels and reducing blood supply to the degenerating tissue.
  • Administration of inhibitors of prostaglandin synthesis is known to block ovulation.
  • Prostaglandins (PG) - human semen (ejaculate) contains this substance that causes contraction of the human uterus, this active substance in semen was soluble in fat and was thought to be secreted by the prostate gland into the semen.
  • Seminal vesicles not the prostate, are the major source of prostaglandins in semen, and prostaglandins are also produced in almost every tissue of the body.
  • Prostaglandins are a family of molecules derived from fatty acids. They all contain 20 carbon atoms. The most important kinds of prostaglandins are grouped into three categories : prostaglandins A, F, and E.
  • After the follicular wall has thinned, the pressure within the antral cavity causes the stigma to form a “cone” and then to tear.
  • Contractile – a smooth muscle-like cells are found in the follicular wall, but it is not clear what role they play in ovulation.
  • The oocyte, which has become detached from the membrana granulosa and is now floating freely with its cumulus oophorus in the follicular fluid, then oozes out with the escaping fluid through the tear in the follicular wall. Thus, ovulation has occurred. CORPUS LUTEUM
  • Once ovulation has occurred, the follicular wall remains as a collapsed sac. It then is called the corpus hemorrhagicum because a blood clot, derived from the torn blood vessels of the stigma, appears on its surface.
  • The luteinized granulosa cells (luteal cells) in this collapsed follicle begin to divide and invade the old antral cavity, thus forming the corpus luteum (Latin for “yellow body” ). It is yellow because of the presence of pigment in the luteal cells.
  • Cells of the corpus luteum secrete high levels of progesterone and moderate amounts of estradiol , thus, luteal cells follow the Δ^4 steroidogenic pathway. Secretion of both hormones from the corpus luteum requires LH.
  • The corpus luteum forms in the second half of the menstrual cycle , reaches a maximum diameter of 10 – 20mm , and then degenerates before menstruation.
  • The degenerated corpus luteum fills with connective tissue and is then called a corpus albicans (Latin for “ white body ”).
  • If pregnancy occurs , the corpus luteum does not die but instead survives to function in the first trimester of pregnancy.
  • FSH receptors are found only on granulosa cells , and they appear early in follicular development. This stimulates growth of the granulosa layer by mitosis and formation of the antrum. Thus, most follicular growth is FSH dependent.
  • LH receptors are present on early thecal cells and support growth and differentiation of the theca. Shortly before ovulation , LH receptors also appear on granulosa cells ; the action of LH on the granulosa leads to luteinization of the granulosa cells , final growth and maturation of the follicle , and ovulation.
  • The two gonadotropins regulate the two types of follicle cells (granulosa and theca) in their cooperative production of steroids. **OVARIAN DISORDERS
  1. OVARIAN CYSTS**
  • Cystic follicles are large fluid-filled sacs formed from unovulated follicles.
  • Luteinized cysts are solid masses filled with luteal cells. Polycystic ovary syndrome (PCOS), formerly called Stein–Leventhal syndrome - fairly common condition in which a woman’s ovaries contain many small cysts. This is an endocrine disorder in which circulating levels of androgens ( testosterone and androstenedione (a weak androgen) ) are
  • There is no good diagnosis for early stages of ovarian cancer because it is usually asymptomatic. The cancer is often advanced when it is diagnosed (typically about two years after its onset) and the epithelial cells may have become detached from the ovary and spread to the surface of other internal organs. Reduced risk of having ovarian cancer:
  • women who have had several pregnancies
  • who have nursed their infants
  • who have taken oral contraceptives Treatment
  • Surgical removal of all cancerous tissue in combination with chemotherapy.
  • Taxol - chemotherapeutic drugs used to shrink ovarian tumors. OVIDUCTS Are paired tubes extending from near each ovary to the top of the uterus and also called as fallopian tubes. Size : about 10 cm (4 in) long and has a diameter about the size of a drinking straw. Regions of Oviducts along its length:
  • oviductal infundibulum - nearest the ovary is a funnel-shaped portion.
  • ostium - the opening into the infundibulum, into which the ovulated egg enters.
  • fimbriae - the edges of the infundibulum have fingerlike projections.
  • When ovulation occurs, the ovum is captured by the infundibulum and enters the oviduct because of cilia on the oviductal lining that beat in a uterine direction. Proceeding along the tube toward the uterus, there is a wide oviductal ampulla , then a narrower oviductal isthmus , and finally the part of the oviduct that is embedded within the uterine wall, the intramural oviduct.
  • uterotubal junction - the point at which the oviduct empties into the uterine cavity. Each oviduct in cross section has three layers: (1) On the outside is a thin membrane, the oviductal serosa. (2) The middle layer consists of smooth muscle, the oviductal muscularis. Contraction of this muscle helps transport the ovum and, later, the early embryo toward the uterus. Near the time of ovulation, these contractions are 4–8s apart, whereas they are less frequent during other stages of the menstrual cycle. (3) The third layer, the internal lining of the oviduct , has many folds, with ciliated and nonciliated cells as well as mucous glands on the internal surface. Ciliary beating and mucous secretion play a role in ovum and embryo transport and in sperm movement up the oviduct.

NOTE: Hormones play a role in oviduct function.

  • Estrogens cause secretion of mucus in the oviducts; they also cause the oviductal cilia to beat faster and the smooth muscle to contract more frequently.
  • Progesterone reduces both mucous secretion and muscular contraction. Salpingitis - inflammation of the oviducts can lead to blockage by scar tissue and subsequent infertility. This term comes from the Greek word salpinx , which means “tube”. Tubal blockage can be repaired in some cases by microsurgery. Salpingectomy means surgical removal of the oviducts. Tubal sterilization (surgical cutting, tying off, or blocking the oviducts) can be used as a contraceptive measure. UTERUS (WOMB) Is a single, inverted pear-shaped organ situated in the pelvic cavity above the urinary bladder and in front of the rectum. SIZE:
  • nulliparous women (those who have never given birth to a child) - about 7.5cm (3 in) long, 5 cm (2 in) wide, and 1.75 cm (1in) thick.
  • multiparous women (those who have previously borne children), the uterus is larger and its shape is more variable. Uterine ligaments – the bands or cords of tissue that supports the uterus.
  • A pair of broad ligaments attach the uterus to the pelvic wall on each side.
  • Paired uterosacral ligaments attach the lower end of the uterus to the sacrum (“tail bone”).
  • The lateral cervical ligaments connect the cervix and vagina to the pelvic wall.
  • The paired, cord-like round ligaments attach on one end to the uterus near the entrance of the oviducts and on the other end to the lower pelvic wall. NOTE: These ligaments, in addition to carrying blood vessels and nerves to the uterus, also serve to support the uterus and other pelvic organs in their normal position. Regions of Uterus in frontal section:
  • Uterine fundus - the dome-shaped region above the points of entrance of the oviducts.
  • Uterine corpus or “body” - is the tapering central portion, which ends at an external constriction, the uterine isthmus.
  • Uterine cervix - a narrow region that is adjacent to the vagina. The wall of the uterine fundus and corpus has three layers of tissue:
  • The external surface of the uterus is covered by a thin membrane , the perimetrium. This outer coat , or serosa , is continuous with the layer covering the oviducts and other peritoneal organs.

The wall of the vagina has folds that allow it to stretch during coitus or childbirth; it is normally collapsed. The vaginal canal leads from the vulva (external genitalia) to the external cervical os. The opening of the external cervical os into the vagina is circumscribed by a recess called the fornix, which allows support for a diaphragm contraceptive. Layers of vaginal wall:

  • Tunica mucosa - The layer next to the lumen (interior space) of the vagina. This layer includes the epithelial lining of the vagina, which consists of many layers of squamous (flattened) cells. This layer expands under the influence of estrogen and undergoes hormone induced changes in thickness and glycogen content during the menstrual cycle.
  • Tunica muscularis - the middle layer of the vaginal wall, that contains numerous bundles of smooth muscles that are embedded in connective tissue. A sphincter of skeletal muscles at the vaginal opening is under voluntary control.
  • Tunica adventitia – this is a thin layer surrounding the tunica muscularis, this elastic connective tissue supports nerve bundles, most of which control blood flow and smooth muscle contraction of the vaginal tissue. FEMALE EXTERNAL GENITALIA include the mons pubis, labia majora, labia minora, vaginal introitus, hymen, and clitoris (Figure 2.12). These organs, collectively called the vulva.
  • The mons pubis is a cushion of fatty tissue, covered by skin (has many touch receptors and a few pressure receptors) and pubic hair (forms the shape of an inverted pyramid – 25% of women this hair extends in a line up to the navel), that lies over the pubic symphysis.
  • The labia majora (“major lips”) are fleshy folds of tissue that extend down from the mons pubis and surround the vaginal and urethral orifices This contain fat, and the pigmented skin has some pubic hair, sweat and oil glands, and fewer touch and pressure receptors than the mons pubis and are homologous to the male scrotum, they are derived embryologically from the same tissue.
  • The labia minora (“minor lips”) are paired folds of smooth tissue underlying the labia majora, light pink to brownish black in color, these tissues cover the vaginal and urethral openings, but upon sexual arousal they become more open. The hairless skin of the labia minora has oil glands (but no sweat glands) and a few touch and pressure receptors. In older women or in women who have low estrogen levels, the skin of the labia minora becomes thinner and loses surface moisture.
  • The vestibule is the cavity between the labia minora occupied by the opening of the vagina, the vaginal introitus. In women who have not previously had coitus, the introitus often is covered partially by a membrane of connective tissue known as the hymen. However, the presence or absence of a hymen is not a reliable indicator of virginity or sexual experience. In rare cases, a wall of tissue completely blocks the introitus, a condition called imperforate hymen. The

condition is present in about 1 out of 2000 young women. Because an imperforate hymen can block menstrual flow, surgery is required to alleviate the problem.

  • Urethral orifice the anterior to the vaginal introitus, this is where urine passes. Below and to either side of the urethral orifice are openings of two small ducts leading to the paired lesser vestibular glands (Skene’s glands). These glands are homologous to the male prostate glands and secrete a small amount of fluid. At each side of the introitus are openings of another pair of glands, the greater vestibular glands (Bartholin’s glands). These glands secrete mucus and are homologous to the bulbourethral glands of the male. Sometimes, Bartholin’s glands can form a cyst or abscess as the result of infection.
  • The glans clitoris lies at the anterior junction of the two labia minora, above the urethral orifice and at the lower border of the pubic bone. Its average length is about 1–1.5cm (0.5 in) and it is about 0.5 cm in diameter. The clitoral shaft , like the shaft of the penis contains a pair of corpora cavernosa , spongy cylinders of tissue that fill with blood and cause the clitoris to erect slightly during sexual arousal. At the base of the glans clitoris, the corpus cavernosa tissue branches and each “leg” or crus extends under the surface of the labia minora. Another spongy cylinder present in the penis, the corpus spongiosum , is not found in the clitoris; this tissue in the female is represented by the labia minora. The clitoral glans is partially covered by the clitoral prepuce, which is homologous to a similar structure covering the glans of the penis. The clitoris is rich in sensory receptors. Mammary Glands (breasts or mammae) Are paired skin glands (evolved from sweat glands) positioned over ribs two through six on the chest. Function: to secrete milk (modified sweat) during breast-feeding, and they also can serve as a stimulus for sexual arousal in both males and females. In humans, usually only a single pair persists, but in some individuals more than one pair are present, a condition called polythelia. The male mammary glands are usually quiescent, but they are capable of growing and even secreting milk if properly stimulated by certain hormones. Such development of the breasts in males is called gynecomastia. Functional Anatomy: Each human female breast is covered by skin and contains a variable amount of fat and glandular tissue (divided into 15 to 20 lobes separated by fat and ligamentous tissue). Breast size does not affect the ability to secrete milk. suspensory ligaments of Cooper - provides support for the breast, but it tends to be less effective in older women. Each mammary lobe is composed of several lobules , which are grape-like clusters of mammary alveoli. The alveolus is the functional unit of the mammary gland. It is a hollow sphere of milk-secreting cells. Each alveolus receives an extensive blood supply, which provides raw materials for milk synthesis and transports the hormones that control alveolar growth and function.