Ovarian reserve

Histology

A clear description of cortex and medulla in the ovaries is not universally defined, but for functional understanding, cortex includes an area of developing and maturing germ cells, while the medulla is a composition of loose connective tissue. Further details of cortex include surface germinal epithelium, followed by loose mesenchymal cells interspersed between cortical sex cords. This close placement of germ cells and mesangium is meant to provide a well-nurtured and interactive environment for the timely maturation of ovarian follicles. One end of each ovary is designated as the hilum, which aligns the blood vessels and lymphatics to and from the ovaries and provides a surface attachment in the form of mesovarium to the broad ligament.

Germ cells

Ovaries are surrounded by a cuboidal germ cell epithelium. This is formed during embryogenesis by the differentiation of somatic cell lineage into primordial germ cells. Several growth factors and ligand interaction lead to the final settlement of these primordial cells on the genital ridge, after which they are referred to as oogonia . From there starts the development of primordial follicles with oogonia transforming into mitotic stage oocytes . These primordial oocytes then differentiate further into primary oocytes within the primary follicles ( Fig. 4.2 ). Finally, any progenitor cell’s capability to differentiate into primordial cells is stopped at the stage when oocytes enter into meiosis, and subsequently, further maturation is arrested. This landmark developmental step usually happens around the 8th week of gestation. In reproductive biology, this checkpoint has been conceptualized to mark the final oocyte pool in a human female for her lifetime. However, several experiments over the last decade have negated this concept. On the contrary, clear confirmation about ovarian stem cell reserve and its role in forming an essential component of ovarian reserve is still under development.

Endocrine function

After having laid down the essential cellular pillar for human development, it is important to understand its functional chemistry. Several intraovarian paracrine factors and genes take part in the recruitment of early primordial follicles for the developing cohort as detailed in Table 4.1 .

Stages from primary, secondary, and tertiary follicular growths are therefore follicular-stimulating hormone (FSH)-independent and occur over several menstrual cycles after puberty. Maturation of oocytes beyond the meiotic stage comes under the prepubertal FSH effect when each oocyte becomes surrounded by granulosa cell layer. This layer eventually converts into a selected Graafian follicle at the time of puberty when FSH levels are raised in a critical time frame. In each menstrual cycle, the estradiol levels reach their peak near mid-cycle resulting in the complete maturation of a dominant ovarian follicle to mark the process of ovulation . But it is only after the luteinizing hormone (LH) surge that ovulation occurs, releasing a mature ovum for fertilization.

The transition of the Graafian follicle to corpus luteum after ovulation, a process called luteinization , and its sustainability are dependent on the continuous supply of LH or its surrogate human chorionic gonadotropin. The key function of constant large production of progesterone from corpus luteum depends on low density lipoprotein (LDL)-cholesterol and functional mitochondrial steroidogenic acute regulatory (STAR) protein. If pregnancy does not take place, then the life of corpus luteum is around 14 days; subsequently, it converts into corpus albicans .