Explainer| The Menstrual cycle

The female reproductive system is made up of the gonads, duct system and accessory glands. Produces garments and transports them for fertilisation, the process of birth and nurturing. When the oocyte is matured it is expelled from the ovaries and then travels down the fallopian tube. Eggs pass through the fallopian tube to the uterus where they are fertilised. They burrow into the walls of the uterus and perhaps turn into the embryo if it is fertilised.

The distal part of the uterus is the cervix and represents boundaries between the uterus and the Vergina. There are also a number of the ligament that anchor the uterus the main important ones are the ligaments that anchor the ovaries- suspensory ligament anchor the ovaries medially. Mesovarium connects the ovaries to the posterior surface of the broad ligaments.

Uterine ligaments, broad ligaments anchor the uterus to the wall of the pelvic cavity and keeps the uterus tilted towards the bladder. Uterosacral, anchor the cervix of the uterus to the sacrum.

Ovaries are almond shaped, have an inner medulla that has a rich vascular bed in the rich connective tissue, and the outer cortex which has the ovarian follicles that contain the oocyte. There is no sharp distinction between the cortex and the medulla. The surface of the ovaries is covered with a continues germinal epithelium, under this epithelium is a layer of a poorly defined layer of dense connective tissue called the tunica albuginea.

There are 3 types of follicles, primordial, growing and tertiary, until puberty, the only follicles that are seen are the primordial follicles. The oocyte is in resting phase of prophase 1. As the primary oocyte enlarges after puberty the follicular cells become cuboidal cells and at this stage, the structure is called a primary oocyte. Which is surrounded by and an intracellular material called the zona pellucida.

As it continues to grow the single follicular layers of the oocyte undergoes mitosis and becomes multilayered or stratified, this is called the granulosa layer. The connective tissue layer that surrounds the cells also differentiates into a poorly defined layer. The theca interna surrounds the granulosa and the externa that surrounds the interna, this is important following ovulation.

As the follicles keep on growing mainly as results of the number of granulosa cells, small fluid-filled cavities start to form among the granulosa cells to the fluid-filled antrum. Secondary follicles are marked by the presence of a fluid-filled antrum.

Granulosa cells form a hill, at one region of the follicular wall, called the cumulus opharus, which contains the oocyte and from the time the oocyte stops growing. Growth continues until the follicle is about 1cm in diameter due to an increase in the fluid this is mature follicles or the Graafian follicle. The layer of the follicular cells that are closest to the oocyte become elongated forms the corona radiator. At ovulation, the follicles raptures and the oocyte is expelled into the peritoneal cavity.

Only one follicle develops into the Graafian follicle the rest will go through follicular atresia (which is the disintegration of the oocyte) also disintegration of the granulosa cells and vascularization of the granulosa cells happens. And eventually, the Atretic Follicle is replaced by connective tissue about 99% follicles degenerate.

Ovulation

Is not just an increase in follicular fluid pressure, at approximately 24-30 hours there is an increase in luteinizing hormone as well FSH. Also, stigma or macula pellucida forms on the surface of the follicles and fluid begin to ooze from it. Theca externa begins to release proteolytic enzymes, for example, Collagenases that begins to weaken the walls of the theca externa.

At that time prostaglandins are released new blood vessels begin into the follicle. At the same, the prostaglandins cause these blood vessels to dilate so that more blood enters the follicles. This swelling combined with the disintegration of the follicular wall causes the wall to rapture.

Following the rapture, the remnants of the follicles begin to form a structure called corpus luteum, the granulosa layer fold and a central blood clot is formed. This is followed by granulosa cells increasing in size and differentiation to form the granulosa lutein cells, which produce progesterone.

Theca cells increase in size to become theca lutein cells which produce oestrogens.

Capillaries grow into the area and the corpus lutein is formed, the corpus lutein is known as the vascular endocrine gland. If fertilisation happens it sustains the growing fetus until the placenta is formed. If not it disintegrates and forms the Corpus albicans.

From day 1-4 there is the shedding of the functionalis  of the endometrial lining and some primordial follicles become primary follicles.

5-14, the follicles keep on developing into the eggs, which releases oestrgens that cause the proliferation of the cells in the endometrium and replaces the functionalis layer. This is called the oestrogenic phase.

Day 14 is when ovulation happens causing the remnants of the follicles to luteinize causing the corpus luteum. 14-25 the corpus luteum secretes progesterone and estrogens. Progesterone causes the glands of the endometrium to become secretory glands.

They become a vascularised layer of the superficial endometrium. Causes the thickening of the endometrium and helps with storage of glycogen. These changes are minimal in the week after ovulation, this stage is the luteal phase.

25-28, corpus luteum regresses to become the corpus albicans which is a scar of CT and the progesterone levels fall. If pregnancy occurs the corpus luteum will be marinated until implantation.

Reference

Hodges, J. K. (1998). Endocrinology of the ovarian cycle and pregnancy in the Asian (Elephas maximus) and African (Loxodonta africana) elephant. Animal reproduction science53(1-4), 3-18.

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