Uterine Cycle and Endometrium Structure

Uterine Structure

• The uterine structure consists of three layers: submucosal stratum (longitudinal), vascular stratum, and supravascular stratum (longitudinal).
• The myometrium is a layer of smooth muscle that surrounds the endometrium.
• The perimetrium is the outermost layer of the uterus.

Uterine Cycle

• The uterine cycle is a 28-day cycle that is influenced by estrogens and progesterone.
• The cycle consists of three phases: proliferative phase (days 5-14), secretory phase (days 15-28), and menstrual phase (days 1-4).
• During the menstrual phase, the functional layer of the endometrium is shed, leaving the basalis layer intact.
• The proliferative phase is driven by estrogen and progesterone, resulting in the regeneration of the endometrium.
• The secretory phase is dependent on progesterone, leading to the preparation of the uterus for implantation.

Endometrial Histology

• The endometrium consists of a simple columnar epithelium, lamina propria, and uterine glands.
• The uterine glands have a simple columnar epithelium, straight tubules, and narrow lumens.
• During the proliferative phase, the uterine glands are coiled and branched, with an accumulation of glycogen, and the lumens are dilated.

Menstruation

• When no fertilization occurs, the corpus luteum degenerates, leading to a drop in progesterone and estrogen levels.
• This results in the constriction of coiled arteries, ischemia, and necrosis of the functional layer.
• The rupture of arteries leads to hemorrhage, and the shedding of the functional layer, leaving the basalis layer intact.

Cervix and Vagina

• The cervix has a simple columnar epithelium and uterine glands.
• The vagina has a stratified squamous epithelium, lamina propria, and circular and longitudinal muscle layers.

Ovarian Cycle

• The ovarian cycle consists of three phases: follicular phase, ovulatory phase, and luteal phase.
• The follicular phase involves the development of primordial follicles to mature follicles.
• The ovulatory phase involves the release of the oocyte from the mature follicle and its capture by the oviducts.
• The luteal phase involves the formation of the corpus luteum from the residual follicular cells.

Ovary

• The ovary consists of the cortex, medulla, and tunica albuginea.
• The primordial follicle consists of a primary oocyte, follicular cells, and a basement membrane.
• The primary follicle develops into a secondary follicle, which consists of a primary oocyte, zona pellucida, granulosa cells, and a basement membrane.
• The secondary follicle further develops into a tertiary follicle, which consists of a secondary oocyte, zona pellucida, corona radiata, and a basement membrane.

Ovulation

• Ovulation is the hormone-stimulated process by which the oocyte is released from the ovary.
• Ovulation normally occurs around the 14th day of a typical 28-day cycle.
• In humans, usually only one oocyte is liberated during each cycle.

Corpus Luteum

• After ovulation, the portion of the follicle left behind forms the corpus luteum.
• The corpus luteum eventually degenerates into the corpus albicans, which is a scar of connective tissue.

Ovarian Structure

• The internal part of the ovary consists of loose connective tissue and blood vessels entering through the hilum from mesenteries.
• The tunica albuginea is a layer of connective tissue that covers the ovary.
• The cortex is the outer layer of the ovary, and the medulla is the inner layer.

Follicle Development

• Primordial follicles consist of a primary oocyte surrounded by follicular cells.
• As the oocyte grows, it starts secreting zona pellucida.
• The primary follicle develops into a secondary follicle, also known as a multilaminar primary follicle.
• The secondary follicle further develops into a tertiary follicle.
• The Graafian follicle is a mature follicle that reaches 10 mm in size.

Oogenesis

• Oogenesis begins in the female fetus, with primary oocytes arresting at prophase I in primordial follicles.
• At puberty, a population of primordial follicles begins to develop each month, typically producing one female gamete (a secondary oocyte).
• Oogonia are diploid cells that are the origin of oocytes, and they undergo mitotic divisions to produce primary oocytes.

Monthly Ovarian Cycle

• Up to approximately 20 primordial follicles mature into primary follicles every month.
• Some primary follicles mature into secondary follicles, while others undergo atresia.
• Typically, only one secondary follicle matures into a vesicular follicle.
• The secondary oocyte is a haploid cell that is arrested in the second meiotic metaphase.

Atresia

• Follicular atresia is a degenerative process in which follicular cells and oocytes die and are disposed of by phagocytic cells.
• Atresia can occur at any stage of follicular development, including nearly mature follicles.

Ovulation

• Ovulation is the hormone-stimulated process by which the oocyte is released from the ovary.
• Ovulation normally occurs midway through the menstrual cycle, around the 14th day of a typical 28-day cycle.
• In humans, usually only one oocyte is liberated during each cycle.

Corpus Luteum

• After ovulation, the portion of the follicle left behind forms the corpus luteum.
• The corpus luteum then degenerates into the corpus albicans, a scar of connective tissue.

Ovarian Cycle Phases

• The ovarian cycle consists of three phases: follicular, ovulatory, and luteal.
• The follicular phase involves the development of the primordial follicle into a mature follicle.
• The ovulatory phase involves the release of the oocyte from the mature follicle and capture by the oviducts.
• The luteal phase involves the residual follicular cells folding and becoming part of the corpus luteum.

Oviduct Structure

• The oviduct is lined by simple columnar epithelium with a lamina propria and tunica muscularis.
• The tunica serosa is the outermost layer of the oviduct.

Uterus Structure

• The uterus is lined by simple columnar epithelium with a lamina propria and underlying endometrium.
• The endometrium consists of functional and basal layers, with uterine glands in the functional layer.