Seminiferous Tubules and Spermatogenesis Quiz

Seminiferous Tubules

• There are approximately 1,000 seminiferous tubules in each testis, where spermatogenesis takes place.
• Between the tubules are interstitial cells (Leydig's cells) that produce testosterone.
• Inside the tubules are Sertoli's cells that support and nourish the sperm.

Spermatogenesis

• Spermatogonia (containing 46 chromosomes) undergo DNA replication and produce primary spermatocytes (with 46 pairs of chromosomes).
• Primary spermatocytes undergo "crossing-over" and meiosis I, resulting in secondary spermatocytes (each containing 46 unique chromosomes).
• Secondary spermatocytes undergo meiosis II, producing spermatids (with 23 unique chromosomes).
• Spermatids transform into spermatozoa (containing 23 unique chromosomes) in a final event called spermiogenesis.

Spermatozoa Structure and Function

• Each spermatozoon consists of a head (containing 23 chromosomes), a mid-piece (storing mitochondria for energy production), and a tail.
• The head is enclosed by a structure called an acrosome, which stores enzymes called acrosin for breaking down the coatings surrounding the egg.

Journey of a Sperm

• At the end of spermatogenesis, spermatozoa are propelled by cilia in the inner walls of rete testis toward the epididymis.
• Inside the epididymis, certain enzymatic reactions occur, allowing spermatozoa to mature and become functional but not yet able to fertilize the egg.

Fertilization

• Spermatozoa travel from the vagina, through the cervix, into the uterus and uterine tube, and eventually reach the outer coatings of the ovum.
• The acrosome on the head of spermatozoa is worn off, releasing acrosin enzyme, allowing some sperm to reach the inner coating called zone pellucida.
• One of the sperm fuses with the ovum, causing rapid electrical depolarization at the cell membrane of the ovum, preventing other sperm from entering.

Mechanism of Penile Erection

• Sexual stimulation leads to parasympathetic neurons releasing nitric oxide, causing dilation of small arterioles of the penis.
• Blood accumulates within the vascular spaces in erectile tissue of the penis, causing it to swell and become erect.

Mechanism of Emission and Ejaculation

• Intense sexual stimulation leads to sympathetic impulses, causing:
  • Peristaltic contractions in the testicular ducts, epididymis, vas deferens, and ejaculatory ducts.
  • Rhythmic contractions in the bulbourethral, prostate, and seminal vesicles.
  • Rhythmic contractions in the erectile columns of the penis.
• Emission: semen moves into the urethra.
• Ejaculation: semen is forcefully expelled from the urethra.

Hormonal Control of Male Reproductive Function

• Hypothalamic and pituitary hormones regulate male reproductive function:
  • GnRH (Gonadotropin-Releasing Hormone) stimulates the anterior pituitary gland to release gonadotropins (FSH and LH).
  • FSH stimulates spermatogenesis.
  • LH (ICSH) stimulates the interstitial cells to produce testosterone.
  • Inhibin prevents over-secretion of FSH.

Brain-Testicular Axis

• The hypothalamus releases GnRH, which stimulates the anterior pituitary gland to release gonadotropins (FSH and LH).
• FSH stimulates spermatogenesis, and LH stimulates the interstitial cells to produce testosterone.

Male Sex Hormones

• Testosterone is converted into dihydrotestosterone in some organs, stimulating cells in these organs.

Hormonal Control of Female Secondary Sex Characteristics

• Hypothalamic and pituitary hormones regulate female reproductive function:
  • GnRH stimulates the anterior pituitary gland to release gonadotropins (FSH and LH).
  • FSH stimulates the maturation of a follicle.
  • Granulose cells of the follicle produce and secrete estrogen.
  • Estrogen is responsible for the development and maintenance of most female secondary sex characteristics.

Ovarian Cycle

• A series of events in the ovarian cortex produce a mature ovum and sex hormones.
• The cycle lasts about 28 days:
  1. Hypothalamus secretes LHRH, which stimulates the anterior pituitary gland to release FSH.
  2. About 20-25 primary follicles develop into secondary follicles.
  3. The anterior pituitary gland releases a surge of LH, which stimulates ovulation.
  4. Follicular and thecal cells become corpus luteum cells, which secrete estrogen and progesterone.
  5. Estrogen and progesterone stimulate uterine wall development and inhibit secretion of FSH and LH.
  6. If the egg is not fertilized, the corpus luteum degenerates, and the uterine lining disintegrates, producing a menstrual flow.

Menstrual Cycle

• The menstrual cycle is regulated by hormonal changes:
  1. The hypothalamus releases GnRH, which stimulates the anterior pituitary gland to release FSH and LH.
  2. FSH stimulates the maturation of a follicle, and LH stimulates ovulation.
  3. Estrogen and progesterone stimulate uterine wall development and inhibit secretion of FSH and LH.
  4. If the egg is not fertilized, the corpus luteum degenerates, and the uterine lining disintegrates, producing a menstrual flow.

Birth Control

• Birth control is the voluntary regulation of conception.
• Contraception is any method used to prevent fertilization of the ovum.
• Common contraceptive methods and their success rates:
  • Abstinence (100%)
  • Vasectomy (99%)
  • Tubule ligation (99%)
  • Birth control pills (98%)
  • Intrauterine devices (IUDS) (95%)
  • Condom (90%)
  • Diaphragm and/or foam (80%)
  • Withdrawal method (or coitus interruptus) (75%)