Functions of the ovaries: * Produce viable oocyte * Produce female hormones
Important facts to know and understand about the ovary: * Ovaries contain all the eggs they will ever have before birth. * Ovarian follicles begin their development as primordial structures, which consist of an oocyte arrested at the diploid stage of the first meiotic division (in prophase I) surrounded by few flattened granulose cells. * Only the dominant follicle is selected because of its ability to convert androgens to oestrogens. The other follicles will not continue to grow and will undergo atresia.
An oocyte goes through 4 biological processes: * Oogenesis * Oogenesis is a mature oocyte. It is the process of producing female gametes, once the sex of the embryo is determined. The difference between oogenesis and spermatogenesis is that, in spermatogenesis meiosis begins at puberty to produce sperm in a sexually mature male, whereas meiosis occurs before birth in females. * Oogenesis is not a continuous process throughout the life of a female, unlike spermatogenesis in male. Oogenesis begins in utero when the primordial oocyte (known as Oogonia), enter meiosis but however Prevented from entering prophase I until ovulation. The Oogonia (primary oocyte) enter meiosis again before their release, then arrest in metaphase II. Meiosis will only be completed if the egg is fertilised.

Oocyte arrested at the diploid stage of the first meiotic division
Oocyte arrested at the diploid stage of the first meiotic division

Meiosis I
Meiosis I

Meiosis II, second arrest

Meiosis II, second arrest

Second polar bodies
Second polar bodies

* Folliculogenesis * The selected dominant follicle starts to develop in the following steps: * Primordial: This has the primary oocyte surrounded by a single squamous layer of granulosa cells. Membrane propria forms. * Primary follicles: Oocyte growth begins, the squamous granulosa cells become cuboidal. The zona pellucida forms. * Secondary follicles: Oocyte growth continues two layers of granulosa cells, theca layer form. Differentiation of theca and granulosa begins. * Tertiary follicles: Beginning of formation of antrum, theca cells differentiate to theca externa and granulosa cells differentiate to mural and cumulus. * Graafin follicle: Follicle reaches maximum size, ovulation occurs soon after.

* Fertilization * Once the mature ovum is ovulated it is ready to be fertilized, and once it’s in contact with a mature sperm, fertilization occurs. * Embryogenesis * Embryogenesis occurs after fertilization, it’s the development of blastocyst from a zygote.
Oocyte maturation: During oocyte maturation, the following steps occurs The nuclear (meiotic) includes * GVBD (Germinal Vesicle Break-Down) * Completion of first Meiotic division * Prophase I second meiotic division second arrest (in metaphase II)

The cytoplasmic oocyte maturation includes * Ca2+ Stores * mRNAs & Proteins synthesis * Cortical granules
Their function, * Breaks down nuclear lamins GVBD * Chromosome condensation * Spindle formation & stability
Their function, * Breaks down nuclear lamins GVBD * Chromosome condensation * Spindle formation & stability
The oocyte maturation is regulated by the Gonadotrophin-Induced LH, which includes the following aspects: * Meiosis-activating sterol (MAS) * Various Kinases E.g. MAPK cascade * Maturation promoting factor (MPF)

Oocyte activation: Is basically the series of processes that occur in the oocyte during fertilization. * Sperm entry causes calcium release into the oocyte * Cortical granule is released to block polyspermy * Degradation of proteins active in MII arrest e.g. CSF, MPF * Completion of meiosis * Formation of female pronucleus (PN) * Decondensation of sperm head formation of male PN
Control of Oogenesis & folliculogenesis * Both granulosa cells and thecal sells participate in estrogen production. The conversion of cholesterol into estrogen requires a number of sequential steps, the last of which is conversion of androgens into estrogens. Thecal cells readily produce androgens but have limited capacity to convert them into estrogens. * Granulosa cells, on the other hand, are readily able to convert androgens into estrogen but are unable to produce androgens in the first place. * LH hormones acts on the thecal cells to stimulate androgen production, whereas FSH acts on the granulosa cells to promote the conversion of thecal androgens (which diffuse into the granulosa cells from the thecal cells) into estrogen. * During follicular phase (the first half of the ovarian cycle), the ovarian follicle secretes estrogen under the influence of FSH, LH, and estrogen itself. In the second half of the ovarian cycle the follicular is transformed into the corpus luteum, which secretes progesterone as well estrogen during the leteal phase (the last half of the ovarian cycle). At this stage progesterone secretion causes negative feedback to the anterior pituitary to stop producing FSH to inhibit estrogen secretion.

Regulation of hormonal pattern: * Hypothalamus secrets GnRH * Anterior Pituitary secrets LH and FSH * stimulates follicle and/or CL (corpus luteum) growth and maintenance * Follicle produces Estradiol and Inhibin * estradiol has positive and negative feedback * inhibin inhibits FSH release * CL produces progesterone * progesterone has negative feedback