Overview of the Female Reproductive System

• To produce ova via oogenesis
• To receive spermatozoa
• To provide an environment for fertilization
• To provide an environment for fetal development
• To expel the developed fetus
• To provide nutrition to the newborn

Ovary

• An outer cortex , which is occupied by the developing follicles and contains several hundred thousand ova.
• An inner medulla , which contains prominent blood vessels, nerves, and scattered clusters of interstitial cells.

Ovarian Follicles

• Oogonia are small, diploid germ cells that migrate to the ovarian cortex and multiply by mitosis. Some of these cells develop the potential to become mature female gametes and become primordial follicles. This occurs during the first few months of gestation.
• Primordial follicles consist of a primary oocyte surrounded by a layer of squamous follicular cells . The oocytes in these follicles undergo arrest before the completion of the first stage of meiosis, and therefore have double the genetic material of the oogonia (4N). This arrest continues until a female reaches sexual maturity.
• After puberty, a few primordial follicles develop into primary follicles with each ovarian cycle. Like its primordial follicle precursor, the early primary follicle is encapsulated by a single layer of cuboidal epithelial cells. With continued development, this lining becomes stratified epithelium composed of cuboidal granulosa cells surrounded by an outer layer of squamous theca cells . The primary oocyte grows and becomes encased in an acellular layer of proteoglycans and glycoproteins known as the zona pellucida . This layer separates the developing oocyte from the surrounding granulosa cells.
• As the follicle continues to develop, the oocyte is displaced from its center by the accumulation of the liquor folliculi in the developing follicular antrum . The formation of the antrum represents the transition to a secondary follicle . A group of cells known as the cumulus oophorus develops between the oocyte and the zona granulosa . The zona granulosa produces estrogen, whereas the theca interna produces androstenedione that is converted to estrogen by the enzyme aromatase in the granulosa cells. The outer theca externa has no known function.
• With continuing hormonal stimulation, the first meiotic division is completed and a Graafian follicle develops. In this process, the primary oocyte becomes a secondary oocyte (2N) and the first polar body is formed. In this follicle, the oocyte is surrounded by the zona pellucida and a group of cells known as the corona radiata , which is derived from the cumulus oophorus. The follicle has a large fluid-filled antrum and is enveloped by follicular cells comprising the zona granulosa, as well as internal and external theca cells.

Corpus Luteum

• If fertilization does not take place, luteinizing hormone from the anterior pituitary ceases and the corpus luteum becomes the corpus albicans , a fibrous tissue mass with macrophage infiltration.
• If fertilization and implantation do take place, hCG from the placenta rescues the corpus luteum, which becomes the corpus luteum of pregnancy . It continues to produce estrogen and progesterone as the placenta prepares to take on this role.

Genital Tract

• The oviducts or fallopian tubes transport the ovum from the ovary to the uterus and serve as the site of fertilization. They have a muscular wall with a folded mucosa composed of a ciliated columnar epithelium. The epithelium contains ciliated cells that move the ovum along and non-ciliated secretory cells that nourish and protect it. The oviducts are divided into four parts: infundibulum , ampulla , isthmus , and pars interstitialis . A thick epithelium and thin muscle layer characterize the former two regions; the latter two regions have more muscle and a thinner epithelium.
• The uterus is the site of implantation. Its structure changes considerably based upon the stage of the menstrual cycle. Uterine structure throughout the menstrual cycle is covered in detail in the next section.
• The cervix is the junction between the uterus and vagina. It protects the uterus and upper genital tract from bacterial invasion. The external os of the cervix represents an important transitional area between mucous-secreting endocervical cells on the uterine side to a stratified squamous epithelium on the vaginal side. Underlying the epithelium is a great deal of collagenous tissue containing numerous leukocytes, which protect against bacterial infiltration.
• The vagina is the site of entry for the spermatozoa and exit for the fetus. The stratified squamous epithelium is notable for its production of glycogen, which is broken down into lactic acid as a defense mechanism against pathogens. The lamina propria of the vagina contains many elastic fibers and few glands. While mucous secretion does not come from the vagina itself, the lamina propria contains a vascular network that produces a transudate that serves as an additional lubricant. The vagina has inner circular and outer longitudinal layers of smooth muscle, and an adventitia that merges with that of the bladder and rectum.

The Uterus

• The mucosa of the uterus is known as the endometrium. It is a columnar epithelium that contains numerous tubular glands. Some of the epithelial cells are ciliated while the rest contain microvilli. It has two component sub-layers. The stratum functionalis , which changes over the course of the cycle, and the stratum basalis , which remains relatively constant. The endometrium is highly vascularized, with a blood supply consisting of straight arteries that form a plexus in the stratum basalis and spiral arteries that extend to the surface of the stratum functionalis.
• The muscularis layer of the uterus is the myometrium . It is a thick smooth muscle layer that expands during pregnancy and serves to protect and expel the fetus. The middle layer of the myometrium, the stratum vasculare , contains many large blood vessels.
• The serosal layer of the uterus is the perimetrium . It is composed primarily of connective tissue.

The Uterine Cycle

• The proliferative phase begins at the end of the menstrual flow and is maintained by the production of estrogen by the developing follicles. In this phase, the endometrium is densely populated by fibroblasts and the uterine glands are small (10 micron) invaginations. From the day 8 to 14, the uterine glands elongate as the entire endometrium doubles or triples in thickness. The final endometrial thickness may be as much as five millimeters. During the late stages of this phase, blood vessels from the stratum vasculare invade the endometrium.
• The secretory (luteal) phase begins at ovulation and is governed by progesterone production from the corpus luteum. The glands become closely packed, coiled, and dilated (> 50 microns). The columnar epithelium discharges a carbohydrate-rich secretion that nourishes the ovum. Coiled arteries extend toward the epithelium and anastamose to enhance the blood supply of the endometrium.
• The menstrual phase of the cycle is initiated if fertilization does not occur. The corpus luteum fails because it is not rescued by the hCG from the placenta. The lack of progesterone results in vasoconstriction, ischemia, and the sloughing of tissue.

Placenta

• Facilitates the physiologic exchange between maternal and fetal circulations.
• Serves as a selective filter for nutrients and maternal macromolecules like IgG.
• Provides a site for gas exchange.
• Serves as an endocrine organ secreting a number of hormones, which include hCG and progesterone.

• There are two fetal cell types that contribute to the placenta, both of which are derived from the trophoblast. The cytotrophoblast layer contains mononuclear cuboidal cells that play a supportive role. The squamous syncytiotrophoblast layer is formed by fused cytotrophoblasts; it is therefore multinucleate and contains no internal cell membrane boundaries. This layer develops finger-like projections into the endometrium and secretes hCG, estrogen, and progesterone.
• The syncytiotrophoblast layer forms the chorionic villi , which contain fetal blood vessels embedded in mesencyhmal tissue and extend into the lacunae of the endometrium, where they are bathed in maternal blood. This blood is found within the lacunae of the decidua basalis , which is derived from the stratum functionalis of the endometrium.
• At first, there are five layers separating fetal and maternal blood: the syncytiotrophoblast layer, its basement membrane, cytotrophoblast cells, the endothelial cell basement membrane, and the endothelial cells lining the fetal capillaries. Over time, the cytotrophoblast layer regresses and the basement membrane of the syncytiotrophoblast may fuse with that of the fetal blood vessels. At this point there are merely three layers separating the maternal and fetal blood: the syncytiotrophoblast layer, the fused basement membrane, and the endothelial cells of the fetal capillaries.

Umbilical Cord