Which phase of the cell cycle does not have a fixed duration?

G2 phase (preparation for mitosis)

A major causal factor in some cases of hypogonadism is:

Reduced secretion of gonadotropin-releasing hormone (GnRH)

Excess secretion of testicular activin by Sertoli cells

Hypersecretion of pituitary LH and FSH as the result of increased GnRH

Failure of the hypothalamus to respond to testosterone

Disruption of the hypothalamic-pituitary portal system will lead to

High circulating levels of PRL, low levels of LH and FSH, leading to ovarian atrophy.

Increased follicular development due to elevated circulating levels of PRL.

Ovulation with subsequent increase in circulating progesterone levels.

Increased FSH levels due to reduced ovarian inhibin levels.

What is the mechanism by which hyperprolactinemia causes amenorrhea?

Inhibition of GnRH pulse secretion

Inhibition of adrenal steroidogenesis

It causes hypogonadotropic hypogonadism

It leads to decreased ovarian function due to low FSH and LH levels

Spermatogenesis and Sperm Function for INI-CET: High-Yield Physiology Lessons

Intro & Site - The Sperm Factory

Spermatogenesis: Process of sperm cell (spermatozoa) development from primordial germ cells (spermatogonia).
Site: Seminiferous Tubules (ST) within the testes.
- Sertoli Cells: Located within ST.
  - "Nurse cells": Provide structural support, nourishment, and phagocytosis for developing sperm.
  - Form Blood-Testis Barrier (BTB).
  - Secrete: Androgen Binding Protein (ABP), Inhibin, Anti-Müllerian Hormone (AMH).
- Leydig Cells (Interstitial cells): Located between ST, in the interstitium.
  - Produce Testosterone (stimulated by Luteinizing Hormone - LH).
Total Duration: Approximately 74 days in humans.
Onset: Begins at puberty and continues throughout adult life, though efficiency may decline with age.

⭐ Spermatogenesis is highly temperature-sensitive, requiring a temperature 2-3°C lower than core body temperature, maintained by the scrotum.

Testis anatomy, seminiferous tubule, spermatogenesis

Stages & Spermiogenesis - From Stem to Swimmer

Spermatogenesis, the ~74-day process from spermatogonium to spermatozoon, occurs in three phases within seminiferous tubules:

Spermatocytogenesis (Proliferation & Differentiation):
- Spermatogonia (Type A dark - stem; Type A pale - renewing; Type B - differentiating) undergo mitosis.
- Type B spermatogonia → Primary Spermatocytes (diploid, 4N DNA after S-phase).
Meiosis (Reduction Division):
- Primary Spermatocytes → Meiosis I (prolonged prophase I, ~24 days) → Two Secondary Spermatocytes (haploid, 2N DNA).
  
  ⭐ Each primary spermatocyte yields four spermatids.
- Secondary Spermatocytes → Meiosis II (rapid) → Four Spermatids (haploid, 1N DNA).
Spermiogenesis (Metamorphosis of Spermatids to Spermatozoa): No cell division.
- Golgi Phase: Acrosomal vesicle formation from Golgi apparatus.
- Cap Phase: Acrosomal vesicle spreads over nucleus (acrosomal cap).
- Acrosome Phase: Nuclear condensation, cell elongation, flagellum formation.
- Maturation Phase: Excess cytoplasm shed as residual bodies (phagocytosed by Sertoli cells).

Spermatogenesis stages and seminiferous tubule structure

Hormonal Control - The Master Controllers

Governed by the Hypothalamic-Pituitary-Gonadal (HPG) axis.

Hypothalamus: Secretes Gonadotropin-Releasing Hormone (GnRH).
Anterior Pituitary:
- LH (Luteinizing Hormone): Stimulates Leydig cells → produce Testosterone (T).
  - T: Essential for spermatogenesis (acts on Sertoli cells), 2° sexual traits, negative feedback on GnRH & LH.
- FSH (Follicle-Stimulating Hormone): Stimulates Sertoli cells → support spermatogenesis, produce Androgen-Binding Protein (ABP) & Inhibin B.
  - ABP: Binds T, ↑ its concentration in seminiferous tubules, vital for sperm maturation.
  - Inhibin B: Selectively inhibits FSH secretion from pituitary.

⭐ Pulsatile GnRH secretion is crucial; continuous GnRH leads to downregulation of pituitary receptors.

HPG axis and spermatogenesis

Sperm Structure & Function - Built for Speed

Diagram of human spermatozoon structure

Structure:
- Head:
  - Acrosome: Cap-like, contains enzymes (hyaluronidase, acrosin) for oocyte penetration. 📌 Mnemonic: "HyAc" for Hyaluronidase, Acrosin.
  - Nucleus: Haploid (23,X or 23,Y), highly condensed chromatin.
- Midpiece:
  - Mitochondrial sheath: Provides ATP for motility ("powerhouse").
  - Axoneme: Core of microtubules (9+2 arrangement).
- Tail (Flagellum):
  - Principal & end pieces: Generates motility for forward progression.
Key Functional Processes:
- Motility: Propels sperm towards oocyte.
- Capacitation:
  - Final maturation in female reproductive tract (essential for fertilization).
  - Removal of decapacitating factors, ↑$Ca^{2+}$ influx, hyperactivated motility.
- Acrosome Reaction:
  - Triggered by sperm binding to zona pellucida (ZP3).
  - Release of acrosomal enzymes to digest zona pellucida.

⭐ Capacitation, a crucial step for sperm to acquire fertilizing ability, occurs over several hours in the female genital tract, primarily in the fallopian tube.

High-Yield Points - ⚡ Biggest Takeaways

Spermatogenesis occurs in seminiferous tubules; duration ~74 days.

Sertoli cells are "nurse cells"; Leydig cells secrete testosterone.

Spermiogenesis: spermatids mature into spermatozoa (no cell division).

Acrosome reaction, triggered by zona pellucida, is essential for oocyte penetration.

Capacitation in the female reproductive tract enables fertilization.

Normal sperm: >15 million/mL count, >40% progressive motility.

Optimal temperature for spermatogenesis is 2-3°C below core body temperature.

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