Reproductive System Notes

Stages of Follicular Development

• Follicular development occurs in the cortex of the ovary.
• Typically, only one dominant follicle reaches full maturity each cycle.

1. Primordial Follicle

• Most basic type, present at birth.
• Contains:
  • Primary oocyte (stuck in meiosis I).
  • A single layer of squamous (flat) follicular cells.
• These follicles remain dormant until puberty.

2. Primary Follicle

• Begins developing in response to FSH at puberty.
• Contains:
  • Primary oocyte.
  • Cuboidal granulosa cells (instead of flat).
• Granulosa cells start secreting estrogen.
• A glycoprotein layer called the zona pellucida forms around the oocyte.

3. Secondary Follicle

• Grows larger with more granulosa cells.
• An outer layer called the theca cells (from ovarian connective tissue) develops.
• Theca cells + granulosa cells work together to produce more estrogen.
• Fluid-filled spaces start to appear between granulosa cells.

4. Antral (Tertiary) Follicle

• Characterized by a large fluid-filled space called the antrum.
• The antrum pushes the oocyte off to one side and surrounds it with a cumulus oophorus (a cluster of supporting granulosa cells).
• The oocyte is now fully surrounded by:
  • Corona radiata (inner granulosa cells).
  • Zona pellucida (glycoprotein barrier).
• Only one antral follicle becomes dominant and continues to ovulation.

5. Mature (Graafian or Vesicular) Follicle

• This is the fully mature follicle that will release the oocyte.
• Has:
  • A large antrum.
  • A secondary oocyte that has completed meiosis I and entered meiosis II (arrested in metaphase-II).
• LH surge triggers ovulation leading to the follicle rupturing, and the secondary oocyte being released.

Hormonal Roles in These Stages:

Follicular Phase (Days 1–13 of the Ovarian Cycle)

• Purpose: To mature a follicle that will release an oocyte during ovulation.

Step-by-Step Process:

1. GnRH is Released (Hypothalamus)

• Around day 1 of the cycle, GnRH (gonadotropin-releasing hormone) is secreted in pulses.
• It stimulates the anterior pituitary gland.

2. FSH and LH are Secreted (Pituitary Gland)

• FSH (Follicle-Stimulating Hormone) rises, which stimulates the growth of several ovarian follicles (fluid-filled sacs containing immature oocytes).
• LH (Luteinizing Hormone) is also secreted in low levels during this time.

3. Follicles Begin to Develop

• A few follicles begin to mature, but only one becomes dominant (called the Graafian follicle).
• The developing follicles start producing estrogen as they grow.

4. Estrogen Levels Increase

• As the dominant follicle grows, estrogen levels rise significantly (mostly from granulosa cells).
• Estrogen has two main effects:
  • Stimulates thickening of the endometrium (uterine lining) - the proliferative phase of the uterine cycle.
  • Provides negative feedback to the hypothalamus and pituitary to suppress FSH (so no more follicles mature).

5. LH Surge Triggered

• Around Day 13, very high estrogen levels switch to positive feedback on the hypothalamus and pituitary.
• This causes a surge in LH (and a smaller surge in FSH).

Outcome:

1. The LH surge triggers ovulation on Day 14.
2. The mature follicle ruptures, releasing the secondary oocyte into the uterine tube.
3. Marks the end of the follicular phase and start of the luteal phase.

Hormone Summary During Follicular Phase:

What is Ovulation?

The release of the secondary oocyte from the mature (Graafian) follicle in the ovary.

After Ovulation:

The leftover follicle (now empty) becomes the corpus luteum, which secretes progesterone (and some estrogen) to maintain the endometrial lining in case of implantation.

What Are Ovarian Follicles?

Ovarian follicles are fluid-filled sacs in the ovaries that each contain an immature oocyte (egg) and surrounding support cells. They support the maturation of the oocyte and secrete hormones (mostly estrogen) to prepare the body for potential pregnancy.

Ovarian, Uterine Phase and Hormones

Hormones in Luteal Phase:

Luteal Phase (Days 15–28)

What is the Luteal Phase?

The post-ovulation phase where the ruptured follicle becomes the corpus luteum.

Step-by-Step Luteal Phase:

1. The empty follicle becomes the corpus luteum under LH influence.
2. The corpus luteum secretes large amounts of progesterone, and some estrogen.
3. Progesterone prepares the endometrium for potential implantation:
   • Thickens the lining
   • Increases glandular secretions
   • Reduces uterine contractions

If Fertilization Does Not Occur:

• Corpus luteum degenerates after about 10 days and becomes a scar called the corpus albicans.
• Progesterone and estrogen levels drop sharply.
• Endometrium sheds, beginning the menstrual phase of the uterine cycle (Day 1 of the next cycle).
• Loss of negative feedback causes GnRH, FSH, and LH levels to rise again, starting a new cycle.

If Fertilization Does Occur:

• The implanting embryo releases hCG (human chorionic gonadotropin).
• hCG keeps the corpus luteum alive, which continues progesterone production.
• Progesterone maintains the uterine lining until the placenta takes over hormone production around week 10 of pregnancy.

Hormones in Luteal Phase:

Ovarian & Uterine Cycle Link

UTERINE (MENSTRUAL) CYCLE

FERTILIZATION & EARLY PREGNANCY

• Fertilization occurs in the ampulla of the uterine tube (within 12–24 hours of ovulation).
• Zygote → Morula → Blastocyst
• Implantation: Blastocyst embeds in endometrium (~Day 6–7)
• hCG (human chorionic gonadotropin): Secreted by the implanted embryo, maintains the corpus luteum.
• Placenta takes over hormone production by ~Week 10.

PUBERTY & SEXUAL MATURATION

• Males: Increased testosterone leads to sperm production, a deeper voice, muscle growth, and body hair.
• Females: Increased estrogen/progesterone leads to breast development, menstrual cycles, and fat distribution.

SEXUAL DIFFERENTIATION

• SRY gene (Y chromosome) triggers testes development in embryos.
• Without SRY, ovaries develop by default.
• Hormones (especially testosterone) shape external genitalia.

KEY VOCABULARY

• Gonads – Primary sex organs (testes in males, ovaries in females).
• Gametes – Sex cells (sperm and oocytes).
• Fertilization – Fusion of sperm and egg to form a zygote.
• Puberty – Developmental period when reproductive organs mature.
• Secondary sex characteristics – Traits not directly related to reproduction (voice change, body hair, breast development).

MALE REPRODUCTIVE SYSTEM

Anatomy & Functions

Hormonal Regulation in Males

• Hypothalamus releases GnRH (gonadotropin-releasing hormone).
• Anterior Pituitary:
  • Releases FSH: Stimulates Sertoli cells to support sperm development.
  • Releases LH: Stimulates Leydig cells to produce testosterone.
• Testosterone:
  • Promotes spermatogenesis.
  • Maintains secondary sex characteristics.
  • Provides negative feedback to the hypothalamus and pituitary.

FEMALE REPRODUCTIVE SYSTEM

Anatomy & Functions

Hormonal Regulation in Females

• Hypothalamus releases GnRH.
• Anterior Pituitary:
  • FSH: Stimulates follicle growth.
  • LH: Triggers ovulation; supports corpus luteum.
• Ovaries:
  • Produce estrogen (stimulates endometrial growth, secondary sex traits).
  • Produce progesterone (maintains endometrium).

OVARIAN CYCLE (about 28 days)