Anatomy and Physiology of the Ovarian Reproductive System

Core Concepts

The ovaries: structure and location

The ovaries are paired, almond-shaped organs in the upper pelvic cavity, one on each side of the uterus. Each is anchored by the ovarian ligament (to the uterus), the suspensory ligament (pelvic wall), and the mesovarium (a peritoneal fold). The outer cortex holds the follicles — sacs containing oocytes at various stages — and the inner medulla holds blood vessels. Unlike the testis, the ovary stays inside the body cavity and needs no temperature regulation.

Oogenesis: making the female gamete

Oogenesis differs fundamentally from spermatogenesis: all the oogonia (stem cells) a female will ever have are created before birth and arrest in prophase I as primary oocytes (about 1–2 million at birth, falling to roughly 300,000–400,000 by puberty — commonly taught reference figures; verify against current texts). No new oocytes are added after birth.

Each cycle, FSH recruits a cohort of follicles; usually one becomes the dominant follicle. Near ovulation the primary oocyte completes meiosis I, producing a secondary oocyte (which keeps almost all the cytoplasm) and a tiny first polar body. The secondary oocyte arrests in metaphase II — meiosis II is completed only if a sperm fertilizes it. Only about 400–500 oocytes ever ovulate in a lifetime, each a single large cell whose cytoplasm nourishes the early embryo.

The ovarian cycle and its hormones

The ovarian cycle has two phases split by ovulation (commonly taught as day 14 of a 28-day cycle; real cycles vary widely).

• Follicular phase (days 1–13): FSH stimulates follicle growth while granulosa and theca cells produce estrogen, which rises steadily and rebuilds the endometrium (proliferative phase).
• Ovulation (about day 14): Rising estrogen triggers positive feedback — an LH surge — rupturing the dominant follicle and releasing the secondary oocyte into the tube.
• Luteal phase (days 15–28): The ruptured follicle becomes the corpus luteum, secreting progesterone (and some estrogen), which converts the endometrium into a glandular, secretory lining ready for implantation. With no pregnancy, the corpus luteum degenerates into the scar-like corpus albicans around day 24–26; falling progesterone triggers menstruation and the cycle restarts. With pregnancy, hCG from the embryo keeps the corpus luteum alive through the first trimester.

Uterine tubes, uterus, and vagina

The uterine tubes extend from the uterus toward the ovaries; finger-like fimbriae sweep the ovulated oocyte into the tube, and cilia plus smooth muscle move it along slowly — which is why fertilization normally occurs in the ampulla (the widened middle section). The uterus is a pear-shaped muscular organ with three layers: outer perimetrium (serosa), middle myometrium (muscle contracting in labor and menstruation), and inner endometrium (the lining that builds and sheds monthly). The vagina is a distensible muscular tube from the uterus to the outside; its acidic environment (commonly taught as maintained by lactobacilli) resists infection.

External genitalia and mammary glands

The external genitalia (the vulva) include the mons pubis, labia majora and minora, the clitoris (erectile tissue homologous to the penis), and the vestibule with the vaginal and urethral openings. The mammary glands are modified sweat glands over the pectoral muscles; each has 15–25 lobes whose lactiferous ducts converge at the nipple. Estrogen and progesterone drive growth at puberty; milk production is triggered after birth by prolactin, covered later in the book.

Common Confusions