27.2 Pt.1

27.2 Anatomy and Physiology of the Ovarian Reproductive System

Learning Objectives

• By the end of this section, you will be able to:
    - Describe the structure and function of the organs of the ovarian reproductive system.
    - List the steps of oogenesis.
    - Describe the hormonal changes that occur during the ovarian and menstrual cycles.
    - Trace the path of an oocyte from ovary to fertilization.

Overview of the Female Reproductive System

• The female reproductive system:
    - Produces gametes (oocytes) and reproductive hormones.
    - Supports the developing fetus and facilitates childbirth.
    - Primarily located within the pelvic cavity, in contrast to the male reproductive system.

• Ovaries:
    - The female gonads, producing oocytes.

External Female Genitals: The Vulva

• Mons pubis:
    - A pad of fat over the pubic bone that becomes covered with pubic hair after puberty.

• Labia majora:
    - Larger outer folds of skin, hair-covered, starting posterior to the mons pubis.

• Labia minora:
    - Thinner inner folds of skin, pigmented, protecting the urethra and reproductive tract entrance.

• Clitoris (glans clitoris):
    - Encircled by the labia minora; analogous to the glans penis, it is sensitive and important for sexual sensation and orgasm.

• Hymen:
    - A thin membrane partially covering the vaginal entrance; cannot validate virginity.

• Bartholin’s glands:
    - Located near vulva, secretes mucus to keep the vestibular area moist.

Vagina

• Structure:
    - Muscular canal approximately 10 cm long serving as the reproductive tract entrance and exit for menses/childbirth.
    - Walls consist of fibrous adventitia, smooth muscle, and mucous membrane with transverse folds (rugae).

• Microbiome:
    - Predominantly contains Lactobacillus, which maintains acidic pH (below 4.5) to protect against pathogens.
    - Douching is discouraged as it disrupts healthy microbial balance.

Ovaries

• Structure:
    - Paired ovals, about 2 to 3 cm in length, similar to an almond.
    - Located in the pelvic cavity; supported by mesovarium and connected to the uterus by the ovarian ligament.

• Histology:
    - Outer layer: Ovarian surface epithelium.
    - Middle layer: Tunica albuginea (dense connective tissue).
    - Cortex: Contains ovarian stroma and developing oocytes.
    - Medulla: Contains blood vessels, lymph vessels, and nerves.

The Ovarian Cycle

• Definition:
    - A cyclical process (approximately every 28 days) including oogenesis and folliculogenesis.

Oogenesis

• Process Overview:
    - Begins with oogonia (formed during fetal development), undergoing mitosis.
    - Oogonia develop into primary oocytes, arrested in meiosis I until puberty.
    - Decline in primary oocyte count: from 1-2 million at birth to approximately 400,000 at puberty, eventually to zero by menopause.

• Ovulation:
    - Marks the transition into reproductive maturity. Occurs approximately once every 28 days after puberty.
    - Triggered by a surge of luteinizing hormone (LH), resuming meiosis of a primary oocyte into a secondary oocyte.
    - Results in unequal cell division, producing one viable secondary oocyte and a smaller polar body (first polar body).

Completion of Meiosis

• Meiosis II occurs only if the secondary oocyte is fertilized by a sperm, producing a haploid ovum that along with sperm becomes a diploid zygote.

• Cytoplasm from the oocyte provides nutrients to the developing zygote until implantation.

• Mitochondrial DNA from the ovum is inherited maternally; traces lineage back to a common female ancestor from approximately 200,000 years ago.

Folliculogenesis

• Definition:
    - Growth and development of ovarian follicles, typically leading to the ovulation of one dominant follicle.
    - Ovarian follicles are initially in a primordial state and undergo phases:
      - Primordial Follicles: Present at birth, inactive with a single layer of granulosa cells.
      - Primary Follicles: Mature from primordial follicles, transition to rounded granulosa cells.
      - Secondary Follicles: Develop from primary follicles, with added layers of connective tissue and theca cells.
      - Tertiary Follicles: Fully developed secondary follicles before ovulation.

• Approximately 99% of follicles undergo atresia during this process.

Hormonal Control of the Ovarian Cycle

• Hormonal Regulation:
    - Begins with GnRH produced by the hypothalamus, stimulating anterior pituitary to release FSH and LH.
    - FSH promotes follicle growth while LH stimulates estrogen production from granulosa and theca cells.

• Follicular Phase:
    - Tertiary follicles grow and secrete estrogen.
    - Estrogen levels rise, decreasing GnRH, FSH, and LH through negative feedback, leading to atresia of most follicles.
    - One dominant follicle survives, continuing to produce estrogen which, instead of negative feedback, triggers an LH surge.

• Ovulation Trigger:
    - The LH surge induces final maturation and ovulation of the dominant follicle, releasing oocyte into the peritoneal cavity.

Luteinization and Corpus Luteum Formation

• Following ovulation, cells in the ruptured follicle undergo luteinization, transforming into the corpus luteum, which secretes progesterone necessary for maintaining pregnancy.

• If fertilization does not occur, the corpus luteum degrades into the corpus albicans within 10 to 12 days, reinitiating FSH and LH.

Uterine Tubes (Fallopian Tubes)

• Structure:
    - Connect ovaries to uterus, consisting of three sections: isthmus, ampulla, and infundibulum with fimbriae.

• Function:
    - Transport of the oocyte from ovary to uterus.
    - Ciliated cells and smooth muscle contractions move the oocyte along the tube towards the uterus.

• Fertilization:
    - Typically occurs in the ampulla, where sperm meets the oocyte, often during its transit through the uterine tube.