The Ovary is divided into the cortex or cortical region and the medulla.
- The cortex is in the periphery and surrounds the medulla. It contains ovarian follicles embedded in a richly cellular connective tissue stroma. Scattered smooth muscle fibers are present in the stroma around the follicles. The boundary between medulla and cortex is not distinct.
- The medulla is located in the centre. It contains loose connective tissue, large twisted blood vessels, lymphatic vessels, and nerves.
The ovarian surface is covered by a single layer of cuboidal, and in some parts squamous, cells. This layer is erroneously (and still) called ‘germinal epithelium’. Primordial germ cells are extra gonadal in origin. They migrate from the embryonic yolk sac into the cortex of the embryonic ovary. The dense connective tissue layer tunica albuginea lies between the germinal epithelium and the underlying cortex.
Ovarian follicles provide the micro-environment for the developing oocyte. At puberty, the initiation of the menstrual cycle (menarche) occurs. The follicles grow and mature, and a cyclic pattern of follicular maturation and ovulation is then established.
- Primordial Follicles are the earliest stage of follicular development. They first appear during the 3rd month of fetal development. These are in the mature ovary, located in the stroma, immediately below the tunica albuginea. Growing oocyte secretes sulphated acidic glycoproteins that are arranged as an extracellular coat called the zona pellucida. With mitosis, the single layer of cuboidal cells then becomes stratified to form the membrana granulosa (stratum granulosum). The follicle cells are now called the granulosa cells. Extensive gap junctions present between granulosa cells. There is no blood-follicle barrier at the basal layer of the granulosa cells. Movement of nutrients and small macromolecules can therefore occur from the blood to the follicular fluid, enabling normal development of the ovum and follicle. As the granulosa cells proliferate stromal cells immediately surrounding the follicle, forming a sheath of connective tissue cells called theca folliculi.
- Theca Folliculi are the stromal cells that form a connective tissue sheath around the proliferating granulosa cells (GC). External to the basal lamina. It further differentiates into the theca interna and theca externa.
- Theca Interna is the inner, highly vascularized layer of cuboidal secretory cells. They have ultrastructural features of steroid-producing cells. They contain a large number of luteinizing hormone receptors (LH). In response to LH stimulation, synthesize and secrete androgens that are precursors of estrogens. Also contains fibroblasts, collagen bundles, and a rich network of small vessels characteristic of endocrine organs.
- Theca Externa is the outer layer of connective tissue cells that contains mainly smooth muscle and collagen fibers. Boundaries between the thecal layers and surrounding stroma are not distinct. Basal lamina separates the vascular the vascular theca interna from the avascular granulosa layer.
- Secondary follicle is formed when the granulosa cell layer is 6-12 layers. Fluid filled cavities appear among the granulosa cells. Factors required for oocyte and follicular growth include Follicle Stimulating Hormone, Growth factors, and Calcium ions. Fluid-filled cavities coalesce to form an antrum. The follicle is now called a secondary or antral follicle. It is 10mm in diameter and the oocyte does not grow further due to secretion of the peptide oocyte maturation inhibitor from the granulosa cells.
- Later Secondary and mature follicles have a thickened elevation formed from granulosa cells called the cumulus oophorus, which projects into the antrum. The part that immediately surrounds the oocyte known as the corona radiata, which communicates with the oocyte via gap junctions. The mature (graafian) follicle has a diameter of 10 mm or more and causes a bulge on the ovarian surface. The stratum granulosum is thinner. Oocyte and cumulus cells are loosened from the granulosa cells. The corona radiata cells stay with the oocyte at ovulation.
The corpus luteum forms after ovulation when the collapsed follicle differentiates into this new function unit. It is formed from granulosa and theca interna cells. Bleeding from the capillaries in theca interna occurs into the follicular lumen, and it is then called the corpus hemorrhagicum (has a central clot). The follicular cavity is then invaded by connective tissue from the stroma. Granulosa cells and theca interna cells differentiate to form granulosa luteal cells and theca luteal cells, called luteinization. There are two types of luteal cells:
- Granulosa lutein cells are large (30μm), centrally located, and derived from granulosa cells. They secrete estrogen, progesterone, and inhibin.
- Theca lutein cells are smaller (15μm), peripherally located, and a deeper staining. They are derived from the theca internal layer and secrete androgens and progesterone.
The hormones prepare the uterus for implantation of the zygote (in the event of fertilization). After successful fertilization and implantation, the corpus luteum is maintain and continues to secrete progesterone and estrogen. The existence and function depends on the luteotropins human chorionic gonadotropin (hCG), LH, prolactin, and insulin (hCG is initially produced by the embryo and later by the placenta). Should fertilization and implantation not occur, the corpus luteum remains active only for 14 days (then called the corpus luteum of menstruation). The lack of hCG and luteotropins declines secretion of estrogen and progesteron. The corpus luteum degenerates and involutes 10 to 20 days after ovulation.The cells become loaded with lipid, decrease in size and undergo autolysis. A white scar is formed called the corpus albicans, which sinks deeper into the cortex and disappears over several months.