Hormone Regulation of Oogenesis and Spermatogenesis

Hormone Regulation of Oogenesis and Spermatogenesis

Introduction

  • The lecture covers the hormonal regulation of oogenesis (egg production) and spermatogenesis (sperm production).
  • The ovary is not directly connected to the fallopian tubes; there's a space.
  • The corpus luteum, a structure in the ovary after ovulation, will be discussed.
  • The ovum is released during ovulation and enters the fallopian tube, where fertilization typically occurs.
  • It takes about ten days for the ovum to travel from the fimbriae to the uterine cavity.

Oogenesis

Timing of Ova Production
  • Oogenesis begins during fetal development around the seventh week.
  • Females produce approximately 7,000,000 potential ova during fetal development.
  • By birth, this number reduces to about 2,000,000.
Natural Selection of Ova
  • The reduction in ova count resembles natural selection, retaining the best potential ova.
  • Chemical signals during fetal development cause non-responsive oocytes to die.
  • From birth to puberty, there is a further reduction to ~200,000 ova per ovary.
  • Ovaries typically alternate in releasing a mature egg each month.
Maturation Process
  • While a mature egg is released monthly, the maturation process takes about 1.5 years.
  • The process starts with a primordial follicle and progresses through various stages to ovulation.
  • Each month, about 15-20 primordial follicles start maturing.
Follicle Development
  • Primordial follicle: an ovum surrounded by a single layer of squamous-like cells.
  • Primary follicle: multiple layers of granulosa cells surrounding the ova, covered by theca cells.
  • Theca and granulosa cells respond to and produce hormones.
Follicular Stages
  • The maturation sequence includes primordial, primary, secondary, and mature follicles, culminating in ovulation.
  • In January, 15-20 primordial follicles respond to hormonal signals; in February, a new batch starts, and so on.
  • By puberty, a mature ovum is available for ovulation each month.
Meiosis in Ova
  • Ova need to reduce their chromosomes to become haploid through meiosis.
  • All primordial follicles are halted in prophase I of meiosis.
  • A triggering mechanism is required to restart meiosis.
Follicular vs. Luteal Phase
  • Oogenesis and the uterine cycle are divided into follicular and luteal phases.
  • The early follicular phase starts on the first day of menstruation (bleeding).
Hormonal Control: Early Follicular Phase
  • The hypothalamus releases gonadotropin-releasing hormone (GnRH) in pulses (every 60-90 minutes).
  • GnRH travels through the hypothalamic-pituitary portal system to the anterior pituitary.
  • The anterior pituitary releases luteinizing hormone (LH) and follicle-stimulating hormone (FSH).
  • FSH stimulates 15-20 primordial follicles to mature.
  • Older follicles develop multiple layers of granulosa and theca cells.
Theca and Granulosa Cell Interaction
  • Theca cells respond to LH by producing androgens.
  • Granulosa cells respond to FSH by producing aromatase.
  • Aromatase converts androgens into low concentrations of estradiol.
  • Estradiol acts as a paracrine signal stimulating neighboring granulosa cells to enhance mitosis leading to more granulosa cells.
Primary Follicle Selection
  • Some primary follicles respond better to hormonal signals than others.
  • A few days later, follicles develop a fluid-filled cavity called the antrum.
Estradiol Production and Feedback
  • More granulosa cells result in more estradiol production.
  • Medium levels of estradiol enter circulation and have a negative feedback effect on GnRH and FSH production to prevent another maturation cycle from beginning too soon.
  • This ensures only one group of follicles matures per month.
Cervical Mucus Thinning
  • Medium levels of estradiol also cause the mucus at the external os to thin, facilitating sperm passage for potential fertilization.
Late Follicular Phase
  • Many days later, only one or two follicles remain; the rest undergo atresia and die.
  • Remaining follicles develop more granulosa cells and a larger antrum.
  • High levels of estradiol are produced, causing the hypothalamus to increase GnRH production (every 30-60 minutes).
  • Granulosa cells also start producing low levels of progesterone and inhibin.
Hormone Effects
  • Progesterone stimulates the anterior pituitary to release LH and hypothalamus to release GnRH.
  • Inhibin inhibits FSH release from the anterior pituitary.
LH Surge and Ovulation
  • High estradiol and low progesterone levels lead to an LH surge.
  • The LH surge causes the ovum to complete meiosis I and pause at metaphase II.
  • The LH surge triggers ovulation, expelling the ovum from the follicle.
Ovum Protection
  • The ovum is surrounded by granulosa cells (corona radiata) and the zona pellucida for protection.
  • Sperm cells must penetrate these barriers for fertilization.
Corpus Luteum Formation
  • After ovulation, the remaining follicle cells in the ovary form the corpus luteum.
  • It takes 16-18 hours from the LH surge to ovulation.
LH Surge Test Kits
  • LH surge test kits can help track ovulation timing.

Luteal Phase

Hormone Production by Corpus Luteum
  • The corpus luteum produces progesterone, estradiol, and inhibin.
  • Progesterone levels increase significantly.
  • Estradiol levels drop slightly.
Negative Feedback
  • High progesterone and medium-high estradiol levels create a negative feedback on GnRH release.
  • Inhibin further decreases FSH levels.
  • This prevents the awakening of new primordial follicles.
Ovum Lifespan & Cervical Mucus Thickening
  • An ovulated ovum survives for only about 24 hours.
  • High levels of progesterone thicken the cervical mucus, preventing pathogen entry.
Corpus Luteum Duration
  • The corpus luteum lasts for 10-12 days if fertilization does not occur; otherwise, it persists for 60-90 days.
  • If no fertilization, the corpus luteum becomes the corpus albicans, ceasing hormone production.
hCG and Corpus Luteum
  • If fertilization occurs, the developing embryo produces human chorionic gonadotropin (hCG).
  • hCG extends the life of the corpus luteum to support embryo development for 60-90 days.
Hormone Level Changes
  • The hormone levels change throughout the cycle: low estradiol early, medium estradiol later, high estradiol with low progesterone and inhibin, then high progesterone with medium-high estradiol and inhibin.
  • Progesterone leads to increased body temperature, increased blood supply, and thickening of the cervical mucus.
Hormone Decline and Menstruation
  • When progesterone and estradiol levels drop, prostaglandins are released.
  • Prostaglandins cause constriction of spiral arteries, leading to the shedding of the stratum functionalis (menstruation).
Cycle Duration and Variability
  • The average menstrual cycle is 28 days, but it varies due to stress or body fat levels.
  • The follicular phase can vary, but the luteal phase is consistently 12 days if there is no pregnancy.
Studying Tips
  • Master each stage of the cycle one at a time.
  • Practice drawing out the cycle to remember hormone sources, targets, and effects.

Spermatogenesis

Testes Anatomy
  • The testes contain interstitial cells (Leydig cells) and seminiferous tubules.
  • Interstitial cells produce testosterone.
  • Seminiferous tubules contain Sertoli cells (sustentacular cells) involved in sperm development.
  • Sertoli cells form the blood-testes barrier.
Cells Within Seminiferous Tubules
  • Spermatogonium (stem cells) undergo mitosis. One cell remain a stem cells and the other differentiates.
  • Primary spermatocytes undergo meiosis I and meiosis II.
  • Spermatids (immature sperm) mature into sperm.
Blood-Testis Barrier
  • Tight junctions between Sertoli cells create a blood-testis barrier
  • Prevent immune cells from attacking developing sperm cells.
  • Developing sperm cells migrate through tight junctions that break down and reseal.
Hormonal Regulation
  • GnRH is released from the hypothalamus in pulses, stimulating the release of LH and FSH from the anterior pituitary.
  • LH targets interstitial cells, stimulating testosterone production.
  • FSH targets Sertoli cells, stimulating the production of androgen-binding protein (ABP) and inhibin and stimulation of spermatogenesis.
Testosterone's Roles
  • Testosterone circulates, passes through Sertoli cells, binds to ABP, and provides negative feedback on GnRH and LH.
Spermatogenesis vs. Spermogenesis
  • Spermatogenesis leads to the formation of spermatids.
  • Spermiogenesis is the maturation of spermatids into sperm, where excess cytoplasm is removed, and the sperm cell gets its mature shape.
Meiosis Comparison
  • Oogenesis involves halting and initiating meiosis in stages.
  • Spermatogenesis is a continuous, non-stop process.
Goal of Gamete Production
  • The goals involve reducing the number of chromosomes by half to create haploid structures and combining these through fertilization to create a diploid embryo