Foliculogenesis | Desarrollo folicular y Ovogénesis: ¿Cómo se forman los ovulos?

Folliculogenesis: Understanding the Female Reproductive Cycle

Introduction to Folliculogenesis

• Folliculogenesis is a crucial process in the female reproductive cycle, influencing ovum production and hormone relationships. This video explores its significance.

Stages of Follicular Development

• The development of ovarian follicles progresses from primordial follicles to mature Graafian follicles, involving size increase and maturation stages.

• Not all ovarian follicles complete their maturation; many undergo atresia (degeneration) before reaching maturity. Only those that fully develop become Graafian follicles, which release the ovum during ovulation.

Phases of Folliculogenesis

• The initial stage is the primordial follicle, consisting of an oocyte surrounded by a single layer of pregranulosa cells formed during fetal development around the third month of gestation.

• Approximately 5 million primordial follicles are formed by the fifth month of gestation; however, many degenerate through atresia, resulting in about 1 to 2 million at birth and only around 500,000 at puberty. This loss continues until menopause.

Transitioning Through Follicular Stages

• Primordial follicles can be stimulated to develop into primary follicles characterized by cuboidal granulosa cells surrounding an enlarging oocyte.

• In secondary follicles, multiple layers of granulosa cells form around the oocyte, along with the creation of a zona pellucida composed of glycoproteins that encase it.

Preantral and Antral Follicles

• Preantral follicle stage sees continued growth in granulosa cells and formation of internal and external theca layers surrounding the oocyte; this stage becomes dependent on FSH (follicle-stimulating hormone).

• Antral or tertiary follicles feature a fluid-filled cavity called antrum; their development relies on gonadotropins (FSH and LH), marking a critical phase as menstrual cycles begin post-puberty with recruitment for further development each cycle.

Selection Process During Menstrual Cycles

• As FSH levels rise during each menstrual cycle, several follicles are recruited for maturation; however, most will not reach full maturity due to declining FSH levels leading to selection dominance where only one follicle becomes dominant (Graafian follicle).

Ovulation Process

• The Graafian follicle is fully developed and releases its contained ovum during ovulation; as it grows larger, the oocyte shifts laterally within it while remaining surrounded by granulosa cells forming cumulus oophorus structures.

Post-Ovulation Changes

• After ovulation occurs around day 14 due to peak gonadotropin levels, remaining follicular cells transform into luteal bodies producing estrogen and progesterone essential for preparing endometrial lining for potential embryo implantation if pregnancy occurs or degenerating if not.

Hormonal Regulation in Folliculogenesis

Hormonal Regulation in Folliculogenesis and Oogenesis

The Role of Androgens and Estrogens

• Androgens can be converted into estrogens through the action of follicle-stimulating hormone (FSH), which is crucial for controlling the menstrual cycle.

• The increase in estrogen levels from the dominant follicle is essential for inducing the LH peak necessary for ovulation.

Parallel Processes: Folliculogenesis and Oogenesis

• Oogenesis, or the development of oocytes, must occur simultaneously with folliculogenesis; primordial follicles contain primary oocytes arrested in prophase of meiosis I during fetal development.

• Primary oocytes resume maturation at puberty, but this requires continued development of primordial follicles into Graafian follicles.

Triggering Ovulation

• Luteinizing hormone (LH) triggers the resumption of meiosis in primary oocytes just before ovulation, resulting in secondary oocytes and a polar body.

• The expansion of cumulus cells may reduce inhibitory effects on meiosis exerted by surrounding cells on the oocyte.

Final Stages of Meiosis

• Secondary oocytes are arrested in metaphase II until fertilization occurs; if fertilization happens, it results in a fertilized egg (zygote) and another polar body.