Single cellular organisms maintain their population size by way of mitotic cellular division. Complex multicellular species like humans depend on a more sophisticated mechanism to achieve the same goal. The process of sexual reproduction is attained by several intermediate steps, namely gametogenesis (production of cells with half the complement of genetic material), copulation, fertilization (fusion of two haploid – having half the normal number of chromosomes – cells), gestation and parturition.
Gametogenesis in human females is a hormonally regulated process carried out in the ovaries. These reproductive organs (analogous to the testis in males) are uniquely designed to execute its modus operandi – from the production of viable gametes to the expulsion of such for fertilization. The embryology, gross anatomy, neurovascular supply and some clinically significant points relating to the ovaries will be outlined in this article.
The female gonads (ovaries) are derivatives of the paramesonephric gonadal ridge. They become distinct ovaries between the 6th and 7th gestational weeks, when epiblastic primordial germ cells begin their invasion of the genital ridges. While the genital ridges are being populated with primordial germ cells, its epithelial lining grows into the mesenchymal layer that is deep to it. They continue to grow and form primitive sex cords which subsequently form irregular clusters of cells. These cords will later degenerate and become the vascularized stroma of the ovary or medullary cords.
The medullary cords also degenerate and are replaced by cortical cords in the 3rd gestational month. The cortical cords eventually form clusters of cells that progress into primordial follicles. It should be noted that the absence of the Y chromosome is integral to the development of the ovaries. The Y chromosome carries the SRY (sex-determining region on Y) gene that is responsible for the production of the testis-determining factor protein, which promotes male gonadal differentiation.
The ovaries are a bilateral pair of flattened, egg-shaped, intraperitoneal discs that reside just within the true pelvis. They are longer than they are wide and altogether smaller than their male homologue, the testes. The organs have superior and inferior poles, as well as anterior, posterior, medial and lateral surfaces.
The female gonads are surrounded by fibrous connective tissue known as the tunica albuginea, which lies deep to a superficial sheath of cuboidal epithelium. Deep to the capsule is a cortical layer in which ovarian follicles reside. A third layer, beneath the cortex is the vascular medulla.
Neonatal females possess approximately one million primordial follicles. About 96% of these follicles degenerate by the time the individual reaches puberty. The normal ovaries of peripubescent females will have several primordial follicles developing during a menstrual cycle. Typically, only one follicle achieves full maturity to release a viable ovum (process of ovulation).
The ovaries are situated in an inferomedial manner when the subject is in the anatomical position. The superior (tubal) pole is covered by the fimbriae (finger-like projections) of the fallopian (uterine) tube. The superior pole is attached to the suspensory ligament of the ovary, which not only anchors the ovary to the posterior pelvic wall, but also acts as a conduit for the ovarian vessels. Additionally, the vesicular appendix (hydatid of Morgagni) is related to the superior pole. For completion, each vesicular appendix is connected proximally to the epoöphoron, which is a network of tubules residing in the mesosalpinx (superior aspect of the broad ligament of the uterus).
Its inferior pole is directed towards the body of the uterus and has a connection there by way of the ligament of the ovary (different from the suspensory ligament of the ovary). The ligament of the ovary progresses as the round ligament (both of which are derived from the gubernaculum), which secures each ovary to the cornu of the uterus. Furthermore, each round ligament also attaches to the inner part of the labia majora.
The mesovarium, which is a double layer of peritoneum, secures the ovary at its anterior surface to the posterior part of the broad ligament. With the exception of this part of the ovary, the rest of the organ is covered by a sheath of cuboidal epithelium and not peritoneum. The posterior surface of the ovary is exposed to the peritoneal cavity. Its medial border is related to the uterosacral fold as well as the body of the uterus. Finally, the lateral surface of the ovary is related to the angle of the internal and external iliac vessels, the ureteric fold and the obturator nerve.
The ovarian arteries are direct branches from the anterolateral surfaces of the abdominal aorta. They arise bilaterally, just inferior to the renal arteries and continue on an inferolateral course. The arteries cross the ureters anteriorly, on the surface of the psoas muscle group. It then enters the suspensory ligament of the ovary to access the organ at its superior pole. Along its course across the ovary, the ovarian artery gives several tubal branches to the fallopian tube before anastomosing with the ovarian branch of the uterine artery (branch of internal iliac artery).
On either side, the ovarian veins begin as a pampiform plexus in the mesovarium before they coalesce to give two left and right ovarian veins. They also travel through the suspensory ligament of the ovary towards their destination. Along their path, the pair of veins on either side fuses to form a single left and right ovarian vein. Each vein continues superomedially, also crossing their respective ureter. The left ovary drains into the renal vein, and the right ovary drains into the inferior vena cava.
Preganglionic fibers of the lesser splanchnic nerves (T10 and T11) synapse at aortic ganglia at the root of the ovarian arteries. The postganglionic nerve fibers then travel with the ovarian artery to provide vasoconstrictive innervation.
Postganglionic fibers from the inferior hypogastric plexus, which receive fibers from the preganglionic pelvic splanchnic nerves, provide parasympathetic innervation to the ovaries. These fibers travel along the uterine arteries and are vasodilatory in nature. It is important to note that the denervated ovaries are still capable of ovulating since the process is hormonally regulated.
There are three pathways for lymphatic flow from the ovaries:
- superiorly, to the para-aortic lymph nodes adjacent to the ovarian artery
- inferiorly, to the medial group of superficial inguinal nodes through the inguinal canal alongside the round ligament
- and horizontally to the opposite ovary across the uterine fundus.
Pain arising from the ovaries is perceived at the periumbilical region. This can be mistaken for appendicular pain since painful sensation from the vermiform appendix is also perceived by the patient in this area. The presence of adnexal (fallopian tube, ovaries and adjoining structures) tenderness and absence of migratory pain from the umbilicus to the lower right quadrant should aid in differentiating between appendicitis and ovarian pathologies. Additionally, ovarian pain can also be referred to the medial aspect of the thigh by route of the adjacent obturator nerve.
Benign cystic lesions are the most frequently encountered pathologies of the ovaries. They can either originate from ruptured or whole Graafian follicles (cystic follicles) or as luteal cysts (normal cyst lined by luteinized granulosa cells that subsequently rupture). Cysts may be singular or multiple in occurrence, and they may also be functional (unlike neoplastic ovarian growths).
Polycystic ovarian syndrome (PCOS) is an intricate disorder of the ovaries in which there are high levels of androgens, a menstrual cycle without the release of an ovum, abnormal menstrual cycles, and reduced fertility.