Reproductive System (Female)

Female Reproductive System
     The female reproductive system is designed to produce the female gametes (ova), transport the developing conceptus (uterine tubes), support the development of the embryo (uterus) and provide a receptacle to receive sperm from the male (vagina).
     The ovaries are flattened oval organs roughly the shape of almonds. They are intraperitoneal and their position in the pelvic cavity is stabilized by a mesentery called the mesovarium that comes off the posterior surface of a mesentary called the broad ligament. In addition, the lateral end of the ovary is held against the wall of the pelvic cavity by the suspensory ligament, by which the ovarian artery and vein reaches the ovary, and the medial end of the ovary is attached to the uterus by the ovarian ligament.
     The peritoneum covers the ovaries as a simple cuboidal epithelium called the germinal epithelium. The thick connective tissue layer underneath the germinal epithelium is the tunica albuginea.
     The interior of the ovaries is divided into the superficial cortex and the deeper medulla.
Oogenesis and Ovarian Cycle
     The production of female gametes is a process called oogenesis. Oogenesis occurs as part of the monthly ovarian cycle. The female stem cells, called oogonia (sing. oogonium), complete their mitotic divisions before birth and by birth have already begun the process of meiosis as primary oocytes. However, the first reduction division of meiosis freezes during prophase and roughly 2 million primary oocytes, frozen in prophase, are present at birth. From birth to puberty, primary oocytes degenerate until at puberty only 400,000 primary oocytes remain. This process by which primary oocytes disappear is called atresia (adj. atretic).
     The primary oocytes are surrounded by simple squamous epithelium in a structure called a primordial follicle. The primordial follicles are found in the outer edge of the cortex in clusters known as egg nests. At puberty, rising levels of follicle stimulating hormone (FSH) begins the ovarian cycle by which a select number of primordial follicles begin further development.
     Complete development of a follicle is divided into the following stages:
  Primary follicle
     A primordial follicle becomes a primary follicle when the squamous follicular cells divide and become cuboidal. At the same time the primary oocyte becomes bigger. When there are two or more layers of cells surrounding the primary oocyte they are called granulosa cells. As the primary follicle gets bigger, a fluid filled space containing macromolecules, called the zona pellucida, appears between the primary oocyte and granulosa cells.
     Growth of the follicle is also associated with development of the cells immediately surrounding the follicle called thecal cells. Some thecal cells along with granulosa cells secrete female sex hormones called estrogens, of which estradiol is the most important.
  Secondary follicle
     A few of the primary follicles continue to grow while most degenerate through atresia. In the follicles that remain, the granulosa cells secrete a fluid called follicular fluid. This fluid coalesces into a fluid-filled cavity called the antrum. With the appearance of an antrum the follicle is called a secondary follicle.
  Mature Graafian follicle
     Usually only one follicle remains midway into the ovarian cycle. This follicle enlarges partly as the result of further accumulation of fluid into the antrum. The primary oocyte projects into antrum in a mound of granulosa cells called the cumulus oophorus. The follicle is now large enough to span the width of the cortex and creates a conspicuous bulge on the surface of the ovary. The follicle is now called a mature Graafian follicle.
     At about 14 days, or midway into the ovarian cycle, a sudden rise in luteinizing hormone (LH) released by the pituitary causes ovulation. About 3 hours before ovulation the primary oocyte resumes the first division of meiosis. The division results in a secondary oocyte that receives all the cytoplasm and a polar body that contains only the genetic material and not much else. The polar body is essentially discarded.
    As a result of the upsurge of LH, the cumulus oophorus detaches from the follicular wall, the fluid pressure within the follicle increases and the follicular wall weakens. The follicular wall finally ruptures and the secondary oocyte is extruded. Granulosa cells remain attached to the zona pellucida of the secondary oocyte and form the corona radiata.
  Corpus luteum
     The ruptured follicle collapses and the granulosa cells and internal thecal cells transform into steriod-hormone-producing cells. Though some estrogens continue to be synthesized by these cells, these cells now synthesize progestins of which progesterone is the most important. Progesterone promotes the secretory phase of the uterus. The accumulation of a yellow pigment in these cells is the reason this structure is called the corpus luteum (yellow body).
  Corpus albicans
     If pregnancy does not occur the corpus luteum begins to degenerate after 12 days. Fibroblasts invade the deteriorating structure and form pale scar tissue that is called a corpus albicans (white body).
Uterine Tube
     The uterine tubes are lined by an epithelium that has both ciliated and nonciliated simple columnar epithelial cells. The nonciliated cells secrete lipids and glycogen that provide nourishment for spermatozoa and the developing conceptus and the ciliated cells create currents that move material toward the uterus. The developing pre-embryo is also moved toward the uterus by peristaltic waves of the muscular layer of the uterine tubes.
     The uterine tube can be divided into four regions:
  1. Infundibulum
     The infundibulum is the funnel-like, open end of the uterine tubes. The edge of the infundibulum has numerous finger-like projections called fimbriae. The cells lining the inside surfaces of the fimbriae and infundibulum have cilia that ensure that the ovulated secondary oocyte enters the tube and is propelled toward the uterus.
  2. Ampulla
     The ampulla is the expanded intermediate region of the uterine tube.
  3. Isthmus
     The ampulla narrows near the uterus to form a short segment called the isthmus.
  4. Intramural part
     This is the final segment of the tube within the wall of the uterus.
     The uterus is a pear-shaped organ that provides support for the developing embryo and fetus. The uterus has a muscular wall whose contractions assist in the expulsion of the fetus during birth. In most women the uterus bends over the urinary bladder in a position known as anteflexion. However, in some women the uterus bends back toward the sacrum in a position known as retroflexion.
     The largest region of the uterus is called the body. The rounded portion of the uterus superior to the attachment of the uterine tubes is the fundus. Inferiorly, the body ends at a constriction called the isthmus. The cylindrical portion of the uterus below the isthmus is the cervix.
     The inferior end of the cervix protrudes into the proximal end of the vagina. The passageway within the cervix is the cervical canal which opens in the vagina at the external orifice and opens into the uterine cavity at the internal orifice.
    The uterus is intraperitoneal and its position is stabilized by a number of ligaments:
  1. Broad ligament
     The peritoneum on the surface of the uterus extends laterally from the sides of the uterus as a mesentery that attaches to the interior walls of the pelvic cavity. This sheet of mesentery is called the broad ligament.
  2. Uterosacral ligaments
     The uterosacral ligaments are folds of fascia that extend from the lateral surfaces of the uterus to the sacrum.
  3. Round ligaments
     These ligaments extend anteriorly from the lateral surfaces of the uterus near the attachment of the uterine tubes through the inguinal canal and end in the connective tissue of the labia majora.
  4. Cardinal ligaments
     These ligaments extend from the base of the uterus and vagina to the lateral walls of the pelvis.
  Uterine Wall
     The uterine wall can be divided into three layers:
  1. Endometrium
     The mucosa, or innermost lining of the uterus, is called the endometrium. The endometrium contains numerous glands and blood vessels that provide physiological support for the conceptus.
  2. Myometrium
     This is the muscular wall of the uterus. It contains layers of smooth muscle that contract to provide the force that assists in moving the fetus from the uterus into the vagina.
  3. Perimetrium
     The peritoneum of the pelvic cavity is present as a serosa on the fundus and anterior and posterior surfaces of the uterus. This serosa is called the peritoneum.
  Histology of the Uterus
     The uterine and ovarian arteries provide blood to the uterus. Within the myometrium arcuate arteries encircle the endometrium. The endometrium is supplied by radial arteries that branch from the arcuate arteries. Two types of arteries then supply blood to two zones of the endometrium:
  Functional zone
     The functional zone is the innermost zone of the endometrium. This zone contains most of the uterine glands and it is supplied by spiral arteries.
  Basilar zone
     This zone is adjacent to the myometrium. It contains the terminal ends of the uterine glands and is not sloughed off with the menses. It is supplied by the straight arteries.
  Uterine Cycle
     The uterine cycle is coordinated with the ovarian cycle as it is influenced by the same hormonal cycle. The uterine cycle can be divided into three phases:
  1. Menses
     The uterine cycle begins with the menses. The menses is the period during which menstruation occurs. Menstruation involves the degeneration and detachment of the functional zone of the uterus. The dead tissue is sloughed and along with some blood exits the uterus through the cervix and vagina.
  2. Proliferative Phase
     After the menses, and under the influence of estrogens secreted by the developing follicles of the ovaries, the functional zone of the endometrium is completely restored. By the end of this phase, which occurs during ovulation, the functional zone is several millimeters thick and highly vascularized. Uterine glands secreting a mucus rich in glycogen extend the full thickness of the endometrium to the basilar zone.
  3. Secretory Phase
     The secretory phase begins at ovulation and continues as long as the corpus luteum remains intact. The secretion of progestins by the corpus luteum stimulates enlargement and enhanced secretion of the endometrial glands and the elongation and further development of the spiral arteries.
     The vagina is an elastic, muscular tube that extends from the cervix to the vestibule of the external genitalia. The recess that surrounds the part of the cervix that protrudes into the vagina is called the fornix. The boundary between  the vaginal and the vestibule is indicated by an elastic, epithelial fold, the hymen.
External Genitalia (aka Vulva, Pudendum)
     The vestibule is the central space that leads into the vagina.