Oogenesis

• The female reproductive system consists of paired ovaries, paired uterine tubes, uterus, vagina, accessory glands, and external genitalia.
• Increased vascularity and glandular secretions prepare the endometrium for potential embryo implantation.
• Thickening of the endometrium due to increased edema (fluid accumulation) and the secretion of glycogen by the endometrial glands.
• Each ovary is attached to the posterior surface of the broad ligament by the mesovarium.
• The ovary is composed of cortex, ovarian follicles, medulla, connective tissues and blood vessels.
• The cortex of the ovary is the outer layer and houses the ovarian follicles, which are the basic functional units responsible for oogenesis.
• Oogenesis occurs within the ovarian follicles located in the cortex.
• Each ovarian follicle consists of an oocyte surrounded by layers of specialized cells.
• The primary function of these follicles is to nurture and support the development of the oocyte, allowing it to mature through various stages until it is released during ovulation.
• The cortex is where the majority of the oocyte development and follicular maturation take place.
• The medulla of the ovary is the inner region, situated beneath the cortex, and contains a higher density of blood vessels, lymphatics, nerves, and connective tissue.
• Oogenesis involves the transformation of primordial germ cells into mature oocytes (egg cells).
• Primordial germ cells form in the embryo before birth and migrate to the developing ovaries.
• During fetal development, the oogonia undergo mitotic divisions, forming primary oocytes.
• These primary oocytes are arrested in prophase I of meiosis, remaining in this stage until puberty.
• At puberty, with the onset of the menstrual cycle, some of the primordial follicles containing primary oocytes start to mature in response to hormonal signals.
• Each month, one or a few follicles start to grow, and an oocyte within a follicle continues through the meiotic process.
• Just before ovulation, the primary oocyte completes the first meiotic division (meiosis I) and forms a secondary oocyte and a smaller polar body.
• If the secondary oocyte is fertilized by a sperm cell, it completes meiosis II, resulting in the formation of a mature ovum (egg cell) and another polar body.
• The process of oogenesis continues cyclically, with the release of an oocyte during each menstrual cycle until menopause.
• Primordial germ cells are the precursors to oogonia and are formed during early embryonic development within the yolk sac endoderm.
• These primordial germ cells undergo mitotic divisions and transform into oogonia, which migrate from their original location in the yolk sac endoderm to the genital ridges within the developing embryo.
• Throughout the menstrual cycle, the functional layer thickens in response to hormonal changes, preparing for potential embryo implantation.
• LH stimulates the Corpus Luteum to produce and secrete progesterone and estrogen.
• The surge of LH triggers various events: completion of the first meiotic division in the primary oocyte, forming the secondary oocyte, rupture of the mature Graafian follicle, leading to the release of the secondary oocyte (ovulation), and formation of the Corpus Luteum from the remnants of the follicle after ovulation.
• High levels of progesterone exert negative feedback on the pituitary gland, inhibiting further secretion of LH.
• The basal layer serves a crucial function in regenerating the functional layer after menstruation, ensuring the endometrial lining is restored for the next menstrual cycle.
• The endometrium consists of tubular glands that extend into the endometrial tissue, involved in secreting substances essential for embryo implantation and nourishment.
• The functional layer of the endometrium contains spiral arteries, which supply blood to the endometrial tissue and are crucial for the menstrual cycle and pregnancy.
• During the proliferative (follicular) phase, the endometrium regenerates and rebuilds, primarily the functionalis layer.
• In the absence of fertilization and pregnancy, the Corpus Luteum eventually degenerates due to decreasing LH levels.
• The menstrual phase marks the onset of menstruation, typically lasting for about 3-7 days.
• High levels of estrogen exert negative feedback on the release of follicle-stimulating hormone (FSH) during the menstrual cycle.
• Estrogen plays a crucial role in promoting the growth and maturation of secondary follicles to mature Graafian follicles.
• The luteal (secretory) phase follows ovulation and extends from around Day 15 to Day 28 of the menstrual cycle.
• The basal layer of the endometrium, situated beneath the functional layer and adjacent to the myometrium, the muscular layer of the uterus, does not undergo desquamation (shedding) during menstruation.
• When estrogen levels reach a certain threshold, it induces a positive feedback loop, causing a surge in luteinizing hormone (LH) secretion.
• If no pregnancy occurs, the functional layer is shed during menstruation.
• Progesterone plays a vital role in preparing and maintaining the endometrial lining of the uterus for potential embryo implantation.
• The basal layer acts as a reservoir or reserve layer that remains intact even after menstruation.