11.2 Oogenesis and Ovarian Cycle

Oogenesis and the ovarian cycle are crucial processes in female reproduction. These intricate mechanisms involve the formation of egg cells and their maturation within ovarian follicles. Understanding these processes is key to grasping how the female reproductive system prepares for potential pregnancy.

The ovarian cycle is tightly regulated by hormones from the hypothalamus, pituitary, and ovaries. This hormonal dance orchestrates follicle development, ovulation, and the preparation of the uterus for possible implantation. The cycle's phases and their timing are essential for successful reproduction.

Oogenesis Process and Stages

Fetal Development and Early Stages

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• Oogenesis forms female gametes (ova) in ovaries throughout reproductive life
• Primordial germ cells migrate to developing ovaries during fetal development
• Oogonia differentiate from primordial germ cells through mitotic divisions
• Primary oocytes form from oogonia and enter prophase I of meiosis
  • Arrest at diplotene stage of prophase I until puberty (dictyate stage)
• Approximately 1-2 million primary oocytes present at birth

Puberty and Maturation

• Select primary oocytes resume meiosis I at puberty
• Meiosis I produces secondary oocyte and first polar body
  • Unequal cytokinesis results in large secondary oocyte and small polar body
• Secondary oocyte begins meiosis II but arrests at metaphase II
  • Remains arrested until fertilization occurs
• If fertilization occurs, meiosis II completes
  • Results in mature ovum and second polar body
• One functional ovum and three polar bodies produced from single primary oocyte
  • Polar bodies typically degenerate and do not contribute to reproduction

Ovarian Follicle Structure and Function

Follicle Development Stages

• Primordial follicles contain primary oocyte surrounded by single layer of flattened granulosa cells
• Primary follicles develop from primordial follicles
  • Growing oocyte and cuboidal granulosa cells
  • Formation of zona pellucida around oocyte
• Secondary follicles have multiple granulosa cell layers and theca layer
  • Theca interna produces androgens (testosterone)
  • Theca externa provides structural support
• Antral follicles form fluid-filled cavity (antrum)
  • Specialized cumulus cells surround oocyte
• Mature (Graafian) follicles are large, preovulatory structures
  • Fully developed antrum and cumulus oophorus supporting oocyte

Follicle Functions

• Provide nurturing environment for oocyte development
• Produce hormones crucial for ovarian and menstrual cycles
  • Estrogen (primarily estradiol) synthesized by granulosa cells
  • Inhibin produced by granulosa cells
  • Androgens produced by theca cells (precursors for estrogen)
• Facilitate oocyte maturation and preparation for ovulation
• Respond to and produce signaling molecules for intercellular communication

Hormonal Regulation of Ovarian Cycle

Hypothalamic-Pituitary-Gonadal Axis

• Gonadotropin-releasing hormone (GnRH) from hypothalamus stimulates anterior pituitary
  • Pulsatile GnRH release essential for proper gonadotropin secretion
• Anterior pituitary releases follicle-stimulating hormone (FSH) and luteinizing hormone (LH)
  • FSH promotes follicular development and estrogen production
  • LH triggers ovulation and corpus luteum formation
• Ovarian hormones (estrogen, progesterone, inhibin) provide feedback to hypothalamus and pituitary

Follicular Phase Regulation

• Rising FSH levels stimulate follicle development
• Granulosa cells produce estrogen and inhibin
  • Estrogen exerts negative feedback on FSH production
  • Inhibin suppresses FSH secretion from pituitary
• High estrogen levels trigger positive feedback on LH production
  • Results in LH surge, initiating ovulation

Luteal Phase Regulation

• Corpus luteum produces progesterone and estrogen
  • Prepare uterus for potential implantation
• Progesterone and estrogen exert negative feedback on GnRH, FSH, and LH
• If pregnancy occurs, human chorionic gonadotropin (hCG) maintains corpus luteum
• Without pregnancy, corpus luteum degenerates
  • Decreased hormone levels trigger menstruation and new cycle

Ovarian Cycle Phases and Characteristics

Follicular Phase

• Begins with menstruation, lasts until ovulation (typically 14 days in 28-day cycle)
• Early follicular phase:
  • FSH stimulates follicle development
  • Multiple follicles begin to grow and produce estrogen
  • Estrogen levels gradually rise
• Late follicular phase:
  • Dominant follicle emerges, continues to grow
  • High estrogen production from dominant follicle
  • Endometrial proliferation in uterus

Ovulation

• Occurs at end of follicular phase, triggered by LH surge
• Mature follicle ruptures, releasing secondary oocyte
• Typically happens around day 14 in a 28-day cycle
• Accompanied by slight increase in body temperature (0.5°C)

Luteal Phase

• Follows ovulation, lasts approximately 14 days
• Early luteal phase:
  • Corpus luteum forms from ruptured follicle
  • Begins producing progesterone and estrogen
• Mid-luteal phase:
  • Progesterone levels peak
  • Uterus prepared for potential implantation
• Late luteal phase:
  • If no pregnancy, corpus luteum degenerates
  • Hormone levels decline, leading to menstruation

Ovarian and Menstrual Cycle Relationship

Cycle Synchronization

• Ovarian cycle hormonal changes drive uterine changes in menstrual cycle
• Follicular phase (ovarian) corresponds to proliferative phase (menstrual)
  • Rising estrogen stimulates endometrial growth
  • Endometrial glands elongate and become more coiled
• Ovulation marks transition between proliferative and secretory phases
• Luteal phase (ovarian) corresponds to secretory phase (menstrual)
  • Progesterone causes endometrium to become secretory
  • Endometrial glands produce glycogen-rich secretions

Cycle Length and Variability

• Menstrual cycle length primarily determined by follicular phase duration
  • Luteal phase remains relatively constant (12-14 days)
• Average cycle length 28 days, but can range from 21-35 days
• Follicular phase variability accounts for differences in cycle length
  • Influenced by factors like stress, nutrition, and overall health

Feedback Mechanisms in Cycle Regulation

Negative Feedback

• Early follicular phase: Low estrogen exerts negative feedback on GnRH, FSH, and LH
  • Prevents premature follicle development
• Luteal phase: Progesterone and estrogen suppress GnRH, FSH, and LH
  • Prevents new follicle development during potential pregnancy

Positive Feedback

• Late follicular phase: High estrogen switches to positive feedback on LH
  • Triggers LH surge, essential for ovulation
• LH surge initiates final oocyte maturation and follicle rupture
  • Also stimulates luteinization of granulosa and theca cells

Cycle Initiation and Reset

• Declining progesterone and estrogen reduce negative feedback
  • Allows FSH levels to rise, initiating new cycle
• If pregnancy occurs, hCG maintains corpus luteum
  • Sustains hormone production, preventing menstruation
• Absence of pregnancy leads to corpus luteum degeneration
  • Hormone decline triggers menstruation and new cycle begins