Reproductive System Part II

Two key processes occur in the female reproductive system: oogenesis (egg development) and folliculogenesis (follicle development).

Before Birth: Oogonia divide to form millions of primary oocytes. These primary oocytes begin meiosis I but arrest in prophase I. They remain in this state for years.
At Birth: Only primary oocytes exist; no oogonia are present. The number of oocytes decreases from 6-7 million to about 2 million due to atresia (degeneration).
Puberty Onward:
- Meiosis I Completion: A primary oocyte completes meiosis I, resulting in a secondary oocyte and a polar body (which is discarded). This division is unequal, conserving resources in the secondary oocyte.
- Ovulation: The secondary oocyte is ovulated. Meiosis II starts but arrests in metaphase II.
- Fertilization Trigger: Meiosis II completes only if fertilization occurs. This results in a mature, haploid egg and another polar body.
- Zygote Formation: The haploid egg fuses with a haploid sperm to form a diploid zygote – the first cell of the new offspring.

Primordial Follicles: These are early-stage follicles containing primary oocytes, surrounded by simple squamous cells.
Primary Follicles: Form when primordial follicles are "picked". The squamous cells become cuboidal granulosa cells. The oocyte within remains a primary oocyte.
Secondary Follicles: Larger follicles with multiple layers of granulosa cells and a zona pellucida (clear fluid layer).
Tertiary Follicles (Antral Follicles): Characterized by a developed antrum (fluid-filled cavity).
Mature Follicles: The final stage of follicle development, typically around 14 days before ovulation. These follicles contain a secondary oocyte and are driven by FSH (follicle-stimulating hormone).

Cortex: The outer region of the ovary where follicles are located. All stages of follicle development can be observed simultaneously.
Medulla: The inner region with a rich blood supply to support the energy demands of the ovary. Primordial follicles are stationary, while primary follicles are moving towards ovulation. A dominant follicle outcompetes others and ovulates its oocyte along with the cumulus oophorus.
Corpus Luteum: After ovulation, the remaining follicle cells transform into the corpus luteum, which secretes progesterone to prepare the uterus for potential implantation.

Two main things happen in the female body: oogenesis (egg making) and folliculogenesis (follicle making).

Before Birth:
- Oogonia cells divide to create millions of primary oocytes (early egg cells).
- These oocytes start a process called meiosis I but then pause in prophase I. They stay like this for many years.
At Birth:
- Only primary oocytes are present; no oogonia. The number of oocytes drops from 6-7 million to about 2 million because many die off (atresia).
Puberty Onward:
- Meiosis I Completion: A primary oocyte finishes meiosis I, creating a secondary oocyte and a small polar body (which is discarded). This division is uneven to save resources for the secondary oocyte.
- Ovulation: The secondary oocyte is released from the ovary. Meiosis II begins but pauses in metaphase II.
- Fertilization Trigger: Meiosis II only finishes if a sperm fertilizes the egg. This results in a mature, haploid egg and another polar body.
- Zygote Formation: The haploid egg combines with a haploid sperm to form a diploid zygote – the first cell of a new baby.

Primordial Follicles: Early-stage follicles containing primary oocytes, surrounded by simple squamous cells.
Primary Follicles: Form when primordial follicles are “picked.” The squamous cells become cuboidal granulosa cells. The oocyte inside remains a primary oocyte.
Secondary Follicles: Larger follicles with multiple layers of granulosa cells and a zona pellucida (clear layer).
Tertiary Follicles (Antral Follicles): Follicles with a developed antrum (fluid-filled cavity).
Mature Follicles: The final stage, about 14 days before ovulation. These follicles contain a secondary oocyte and are driven by FSH (follicle-stimulating hormone).

Cortex: The outer part of the ovary where follicles are located. You can see all stages of follicle development here.
Medulla: The inner part with lots of blood vessels to provide energy to the ovary.
- Primordial follicles stay put, while primary follicles move towards ovulation.
- A dominant follicle wins out and releases its oocyte along with the cumulus oophorus.
Corpus Luteum: After ovulation, the remaining follicle cells become the corpus luteum, which releases progesterone to prepare the uterus for potential implantation.