BILD 2: Multicellular Life - Reproduction and Hormonal Cycles

Distinguishing Reproduction Types: Differentiate between asexual and sexual reproduction.
Sex Determination: Discuss the multi-faceted contributions to sex determination, including environmental factors, sex chromosomes, sex-determination genes, hormone levels, and anatomical features in humans and other organisms.
The HPG Axis: Diagram the hypothalamic-pituitary-gonadal (HPG) axis and explain the roles of negative and positive feedback in regulating the secretion of estrogen, progesterone, and testosterone.
Cycle Predictions: * Predict how perturbations in levels of estrogen, progesterone, GnRH, FSH, and LH affect the ovarian and uterine (menstrual) cycles. * Predict how perturbations in sexual organ anatomy or hormone levels affect human reproductive function and fertility.

General Pathway: 1. Hypothalamus: Secretes GnRH (Gonadotropin-releasing hormone). 2. Anterior Pituitary: Responds to GnRH by secreting LH (Luteinizing hormone) and FSH (Follicle-stimulating hormone). 3. Gonads: Respond to LH and FSH by producing sex hormones and gametes.
Typical Regulation in Testes: * Testosterone Feedback: High levels of testosterone exert negative feedback on both the hypothalamus and the anterior pituitary, inhibiting the release of GnRH, LH, and FSH.
Typical Regulation in Ovaries: * Hormonal Output: The ovaries produce Estrogen and Progesterone. * Feedback Dynamics: * Low levels of estrogen result in negative feedback. * High levels of estrogen (without progesterone) can lead to positive feedback. * Key Rule: If both estrogen and progesterone are present at high levels, the effects of progesterone dominate, resulting in negative feedback on the HPG axis.

FSH and LH Function in Testes: * Leydig Cells: Specifically stimulated by LH to produce testosterone. * Sertoli Cells: Stimulated by both FSH and testosterone. These cells are essential for supporting and nurturing developing sperm cells.
Sperm Development: Mature sperm are released from the Sertoli cells and can then exit the testes.
Clinical Infertility Case Study: * Scenario: A cis-hetero couple presents with infertility. The man has typical-sized testes and typical testosterone levels. Biopsy reveals the presence of Leydig and Sertoli cells, but no sperm. * Diagnosis/Solution: The correct prescribed hormone would be FSH. * Reasoning: Since the man has typical testosterone levels, he must have functional Leydig cells and LH. Since Sertoli cells are present but sperm is not being produced, the missing component is FSH, which is required alongside testosterone to stimulate Sertoli cells for sperm development.
Testosterone Supplementation Effects: * In individuals with very low testosterone, injections can increase sperm count. * In individuals with typical testosterone levels, exogenous testosterone generally suppresses sperm production because it triggers negative feedback on the HPG axis, reducing the body's internal production of GnRH, LH, and FSH.

Ovarian Cycle (Events in the Ovaries): * Involves the development of follicles, ovulation (release of an egg), and hormone secretion. * Anatomy: Oocytes (eggs) are contained in small sacs called follicles. * Oocyte Premature Formation: All oocytes a person will ever have are produced before birth. * Follicular Phase: FSH stimulates follicles to grow and mature. A larger, more mature follicle produces more estrogen. * Ovulation: Triggered by a sudden surge in LH. * Luteal Phase: Following ovulation, the remaining follicle becomes the corpus luteum, which secretes progesterone and estrogen.
Uterine Cycle (Events in the Uterus): * Menstrual Flow Phase (Days 0–5): Shedding of the endometrium (uterine lining). * Proliferative/Secretory Phases: Thickening of the endometrium.
Hormonal Responses of the Uterus: * Thickening: Stimulated by progesterone and/or estrogen. Progesterone is more effective at thickening the lining than estrogen. Higher hormone levels correlate with a thicker lining. * Shedding (Menstruation): Triggered by a drop in progesterone and/or estrogen levels. The uterus does not "know" if an egg was released or fertilized; it responds exclusively to hormone levels.

Feedback Loops: * Negative Feedback: Low levels of estradiol (estrogen) inhibit the anterior pituitary. * Positive Feedback: High levels of estradiol (the peak) stimulate the hypothalamus and anterior pituitary, leading to the LH surge. * Combined Negative Feedback: The combination of estradiol and progesterone (secreted by the corpus luteum) inhibits the hypothalamus and anterior pituitary.
Cycle Timeline (Approximate 28 Days): * Days 0–14 (Follicular Phase): FSH and LH stimulate follicle growth; estrogen levels rise. * Day 14 (Ovulation): Estrogen peak causes the LH surge, which triggers the release of the egg. * Days 15–28 (Luteal Phase): The corpus luteum produces high levels of progesterone and estrogen. If pregnancy does not occur, the corpus luteum degenerates. * The "Big Drop": The degeneration of the corpus luteum causes a sharp decline in estrogen and progesterone, which triggers the menstrual flow and removes the inhibition on GnRH, allowing the next cycle to begin.

Question on Chronic High LH: How would constant high levels of LH (due to certain medical conditions) affect the ovarian cycle? * Answer: They are much less likely to ovulate. Ovulation requires a surge (a sudden, rapid increase) of LH. If LH is always high, a surge cannot occur.
Question on Failed Egg Release: If an oocyte accidentally sticks to a follicle during ovulation and dies, how is the uterine cycle affected? * Answer: It does not affect the cycle, provided hormone levels remain normal. The uterine lining thickens and sheds based on endocrine signals (estrogen/progesterone) independently of the physical presence of the egg.
The Contraceptive Pill: * Combined pills (estrogen + progesterone) or progesterone-only pills increase the thickness of cervical mucus and prevent ovulation. * Mechanism: By maintaining constant, moderate levels of these hormones, they exert negative feedback on the HPG axis, preventing the LH surge and thus preventing ovulation.
Discussion Topic: Diabetes: Students were asked to record their thoughts on the causes and definition of diabetes on index cards for later use.