HPA Axis, HPG Axis, and the Human Menstrual Cycle: Exhaustive Lecture Notes

Learning Outcomes and Overview

The following objectives formulate the basis of the HPA/HPG and Menstrual Cycle study guide: - Describe oogenesis and the formation of female gametes. - Outline the development of the pituitary gland during embryogenesis. - Draw and label a diagram illustrating the connections between the hypothalamus and the pituitary gland. - Explain how pituitary function is controlled and describe the role of negative feedback at the hypothalamic and pituitary levels. - List the anterior and posterior pituitary hormones. - Describe the endocrine changes leading to ovulation. - Draw and label a diagram of the major hormonal changes throughout the human and primate menstrual cycle (noting they are functionally the same). - Describe the specific roles of GnRH (gonadotropin-releasing hormone), LH (luteinizing hormone), FSH (follicle-stimulating hormone), estrogen, and progesterone in controlling ovarian and uterine cycles.

Oogenesis: The Formation of Female Gametes

Definition: Oogenesis is the formation of gametes (ova) within the ovaries.
Temporal Development: Unlike spermatogenesis, which begins at puberty in males, oogenesis starts before birth during fetal life (embryogenic life, around months $3$ to $4$ of human development).
Oocyte Population Statistics: - Fetal Life: Approximately $7,000,000$ to $8,000,000$ oocytes are present. - Birth: The number drops drastically to between $1,000,000$ and $2,000,000$ oocytes. - Puberty: The number further decreases to $300,000$ to $400,000$ oocytes. - Fertility Life: Only about $400$ to $500$ ova are actually used (ovulated). The biological purpose of this decline is the selection of the most "competent" ova for potential fertilization.
Stages of Oogenesis: - Primordial Germ Cells: These cells migrate from the yolk sac to the ovaries during early fetal development. - Oogonia: Once in the ovaries, primordial germ cells differentiate into oogonia ( $2n$ , diploid stem cells). They divide mitotically to produce millions of germ cells. - Primary Oocytes: Most oogonia degenerate before birth, but some develop into larger cells called primary oocytes. - Meiotic Arrest: Primary oocytes enter Prophase I of Meiosis I during fetal development but do not complete it; they remain dormant/arrested until the onset of puberty. - Secondary Oocytes: At puberty, Meiosis I is completed, resulting in a secondary oocyte ( $1n$ , haploid) and a first polar body. The secondary oocyte begins Meiosis II but halts in metaphase until fertilization occurrs.
Meiotic Outcomes: One oogonium results in a single functional ovum and three polar bodies (which degenerate). This is distinct from spermatogenesis, which yields four sperm cells per parent cell.

Follicular Development and Anatomy

Follicle Definition: Tissues surrounding and enclosing the oocyte comprised of follicular cells, basement membranes, collagen fibers, and stromal cells. Their primary role is to protect the oocyte.
Follicular Stages: - Primordial Follicle: Contains the primary oocyte; dormant stage. - Primary Follicle: Developing follicle containing the primary oocyte. - Secondary Follicle: Enlarging follicle, still containing the primary oocyte. - Tertiary (Graafian/Mature) Follicle: Characterized by the presence of follicular fluid and extreme enlargement. It contains the secondary oocyte and is ready for ovulation.
Ovulation: The mature Graafian follicle ruptures, expelling the secondary oocyte into the pelvic cavity, where it is typically swept into the fallopian tubes.
Post-Ovulatory Structures: - Corpus Hemorrhagicum: The ruptured follicle immediately after ovulation, often containing a blood clot. - Corpus Luteum: Known as the "yellow body" ( $lutea$ = yellow). It is rich in lipids/fats, which serve as precursors for progesterone. It is essential for establishing and maintaining pregnancy. - Corpus Albicans: Known as the "white body" ( $albicans$ = white). If fertilization does not occur within approximately $14\,\text{days}$ , the corpus luteum undergoes luteolysis (regression) and becomes a mass of fibrous scar tissue.

The Hypothalamic-Pituitary-Gonadal (HPG) Axis

Definition: An integrated system that regulates gamete production, hormone secretion, and the reproductive cycle via signals between the hypothalamus, pituitary gland, and gonads.
Pituitary Development: - Anterior Pituitary (Adenohypophysis): Derived from oral ectoderm (specifically Rathke's pouch). It is composed of endocrine cells. - Posterior Pituitary (Neurohypophysis): Derived from neural ectoderm. It is an extension of the hypothalamus (neural cells).
Hormonal Secretion (Anterior/Adenohypophysis): Uses the mnemonic "FLAT PIG": - F (FSH): Follicle Stimulating Hormone; acts on ovaries/testis. - L (LH): Luteinizing Hormone; stimulates ovulation and testosterone/progesterone production. - A (ACTH): Adrenocorticotropic hormone; acts on the adrenal cortex to release cortisol/corticosteroids. - T (TSH): Thyroid Stimulating Hormone; triggers thyroxine release from the thyroid. - P (Prolactin): Acts directly on mammary glands for lactation. - I: Ignored for the mnemonic. - G (GH): Growth Hormone; acts on liver and tissues to release IGF-1.
Hormonal Storage (Posterior/Neurohypophysis): Does not synthesize hormones; it stores and releases hormones produced in the hypothalamus (specifically the paraventricular and supraoptic nuclei): - ADH (Vasopressin). - Oxytocin.

Hypothalamic-Hypophyseal Portal System

Mechanism: A specialized circulatory system consisting of primary and secondary capillary plexuses connected by portal vessels.
Function: Releasing hormones from the hypothalamus reach the anterior pituitary directly through this portal system. This prevents the dilution of hormones that would occur if they entered the general systemic circulation.
Real-World Comparison: The speaker compares this to the Hepatic Portal System, where nutrients from the GI tract bypass systemic circulation to go directly to the liver.

Hormonal Feedback and the Menstrual Cycle

GnRH Dynamics: Gonadotropin-releasing hormone (GnRH) is released from the hypothalamus in a "pulsatile manner" ( $4$ to $8$ pulses per day).
Negative Feedback (Multi-loop): - Long Loop: Hormones from target glands (e.g., progesterone, estrogen) inhibit both the anterior pituitary and the hypothalamus. - Short Loop: Pituitary hormones inhibit the hypothalamus. - Inhibin: A hormone released by the developing follicle that specifically inhibits FSH secretion without affecting LH.
Positive Feedback (The LH Surge): - Occurs only just before ovulation when estrogen levels are extremely high/surging. - Surge-level estrogen stimulates the "surge center" in the brain, causing a massive release of GnRH, which triggers a sharp LH surge from the anterior pituitary. - High LH causes the follicle to rupture (ovulation).
Menstrual Cycle Phases: - Follicular Phase: Characterized by rising estrogen levels produced by the growing follicles; occurs before ovulation. - Luteal Phase: Occurs after ovulation; the corpus luteum produces high levels of progesterone to maintain the endometrium and prepare for implantation. If no fertilization occurs, progesterone drops, leading to menstruation.

Questions & Discussion

Q: Where does oogenesis take place in mammals? - A: The ovary.
Q: At which stage are primary oocytes arrested before puberty? - A: Prophase I of Meiosis I.
Q: What is the outcome of one primary oocyte after the completion of oogenesis? - A: One functional ovum and three polar bodies.
Q: When is Meiosis II completed in mammalian oogenesis? - A: At fertilization (after the sperm penetrates).
Q: Which connection exists between the hypothalamus and the anterior pituitary? - A: The hypothalamic-hypophyseal portal system.
Q: Negative feedback regulation involves what? - A: Target gland hormones inhibiting the hypothalamus and pituitary gland.
Q: Can you repeat these lectures? I am lost. - A: The lecturer acknowledges the complexity and the need for a tutorial to bridge the gap between endocrine and reproduction lectures.