Ovary: Cortex and Medulla

• A single ovarian follicle is considered the basic endocrine/reproductive unit of the ovary.
• It contains one germ cell surrounded by endocrine cells, organized in two layers separated by a basal membrane.
• The inner layer surrounding the oocyte contains granulosa cells.
• The outer layer consists of thecal cells.
• Both types of cells are responsible for producing ovarian hormones.
• The ovary's size is about 3x1.5x1 cm.
• The location of the ovary is the lateral pelvic wall.
• Components of the ovary consists of the cortex and medulla

Ovary: Cortex and Medulla

• The medulla or medullary region is in the ovary's central portion.
• The medulla or medullary region contains loose connective tissue, blood vessels, and nerves.
• The cortex or cortical region is located in the peripheral portion that surrounds the medulla.
• Follicles are embedded in a richly cellular connective tissue.
• The surface of the ovary is covered by tunica albuginea, made of connective tissue with one layer of cuboidal cells (aka germinal epithelium).

Ovarian Follicles

• Includes Primordial follicles, Growing follicles (primary, secondary/ antral follicles) and Mature or Graafian follicles
• Primordial follicles are found in the stroma of the cortex, just beneath the ovarian surface.
• A single layer of squamous follicular cells surrounds the oocyte.
• The primary follicle is the first stage of growing follicle
• Follicular cells become cuboidal.
• Zona pellucida forms in-between the oocyte and the adjacent follicular cells
• Follicular cells undergo stratification to form the granulosa layer of the primary follicle.

Ovaries: Gametogenesis and Steroidogenesis

• Ovaries produce gametes and steroid hormones.

• Two major groups of steroid hormones:

• Estrogens secreted by follicles promote growth and maturation of internal genitalia.

• Progesterone secreted by the corpus luteum prepares internal reproductive organs for pregnancy

Hormonal Regulation

• Gonadotropin-releasing hormone (GnRH): Secreted by the hypothalamus and regulates ovarian and uterine cycles.
• GnRH stimulates the release of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) from the anterior pituitary
• FSH: Initiates follicular growth and stimulates follicles to secrete estrogens.
• LH: Stimulates further development of ovarian follicles, ovarian follicles to secrete estrogens, and thecal cells to produce androgens (converted into estrogens).
• LH triggers ovulation and promotes the formation of the corpus luteum, which produces estrogens, progesterone, relaxin, and inhibin.

Hormones Secreted by Ovaries

• Estrogens secreted by ovarian follicles:
  • Promote development/maintenance of female reproductive structures and sex characteristics.
  • Inhibit release of GnRH, LH, and FSH.
• Progesterone:
  • Secreted mainly by the corpus luteum.
  • Works with estrogens to prepare/maintain endometrium for implantation and mammary glands for milk production.
  • Inhibits the secretion of GnRH and LH.
• Relaxin:
  • Produced by the corpus luteum.
  • Relaxes the uterus by inhibiting contraction of the myometrium.
  • At the end of pregnancy, it increases the flexibility of the pubic symphysis and dilates the uterine cervix.
• Inhibin:
  • Secreted by granulosa cells of growing follicles and by the corpus luteum.
  • Inhibits the secretion of FSH and LH.

Ovaries: Estrogens and Progestins

• Estrogens lead to:
  • Maturation of primordial germ cells
  • Provision of the hormonal timing for ovulation
  • Developing the tissues that will allow for implantation of the blastocyt
  • Establishment of an environment required for the maintenance of pregnancy
  • Provision of the hormonal influences for parturition and lactation
  • Anabolic effects on bone and cartilage
  • Vasodilation and heat dissipation
• Progestins:
  • Generally require the previous or concurrent presence of estrogens
  • Reduce the proliferative activity of the estrogens on the vaginal epithelium
  • Convert the uterine epithelium from proliferative to secretory, preparing it for implantation of the fertilized ovum
  • Enhance the development of the acinar portions of breast glands after estrogens have stimulated ductal development
  • Decrease peripheral blood flow; decrease heat loss

Human Chorionic Gonadotropin (hCG):

• Placental hormone structurally similar to LH.

• Supports the corpus luteum until the placenta produces enough progesterone.

  • At 6-8 weeks, the corpus luteum produces progesterone.
  • After 6-8 weeks, the placenta produces progesterone (30-40 times more).

• Placenta cannot synthesize cholesterol, relies on maternal supply.

• Estrogens include E1, E2, E3.

• The major hormone (E3) is synthesized by feto-placental function.

• Placental lactogen (PL): Chorionic somatomammotropin, placental growth hormone