Topic 12- Reproduction I (Male)

Outcomes:

• sequence animal reproduction

• compare and contrast types of animal reproduction

• sequence mitosis and meiosis

• compare and contrast mitosis and meiosis

• compare and contrast male reproductive structures

• sequence spermatogenesis

• hypothesize and diagnose the impact of variability on spermatogenesis

• sequence male reproductive endocrinology

• hypothesize and diagnose the impact of variability on male reproductive endocrinology

I. Introduction to Animal Reproduction

A. Asexual Reproduction

• Single parent → 100% of genes passed on.

• Mitosis only, no gametes involved.

Mechanisms of Asexual Reproduction

• Parthenogenesis:

  • Diploid eggs develop into an organism without fertilization.

  • Offspring are genetically identical to the mother.

• Budding, fission, fragmentation:

  • Offspring produced via mitosis.

Advantages of Asexual Reproduction

• Rapid reproduction.

• No need for a mate.

• 100% of genes are passed on.

B. Sexual Reproduction

• Meiosis → Gametes (n)

  • Sperm: Small, motile, flagellated.

  • Egg: Large, non-motile, rich in nutrients.

• Fertilization:

  • Sperm meets egg → Forms a zygote (2n) → Develops into an embryo.

Advantages of Sexual Reproduction

• Genetic variation.

II. Male Reproductive System

A. Introduction

• Functions: Produces and delivers sperm.

• Sperm pathway ("SEVEn UP"):

  • Seminiferous tubules (testes) → Epididymis → Vas deferens → Ejaculatory duct → Urethra → Penis → Exit from body.

B. Testes

• Male gonads located in the scrotum.

• Seminiferous tubules:

  • Site of spermatogenesis (sperm production via meiosis).

Types of Cells in Seminiferous Tubules

1. Spermatocytes: Undergo meiosis.

2. Sertoli cells:

   • Support sperm development.

   • Secrete inhibin (regulates FSH for sperm count control).

   • Secrete androgen-binding protein (ABP) (binds & transports testosterone).

   • Negative feedback mechanism.

3. Leydig cells:

   • Located between seminiferous tubules.

   • Produce testosterone.

C. Epididymis

• Coiled duct in testes responsible for:

  • Sperm transport.

  • Sperm maturation (takes ~3 weeks).

  • Sperm storage.

D. Scrotum

• Surrounds testes and epididymis.

• Functions:

  • Cooling (1-2°C below body temperature).

  • Increases sperm longevity.

E. Penis

• Function: Delivers sperm to the female reproductive tract.

Internal Structure

• Urethra: Runs through the middle, pathway for sperm and urine.

• Erectile tissue:

  • Three parallel columns.

  • Engorge with blood during an erection.

Erection Mechanism

1. Sexual stimulation → Nitric oxide (NO) released from endothelial cells.

2. Smooth muscle relaxation → Arteries dilate and veins compress → Increased blood flow → Erection.

F. Accessory Glands

Produce semen (fluid that carries sperm).

1. Seminal Vesicles (~60% of semen volume)

   • Secretes into vas deferens.

   • Contains:

     • Fructose (energy for sperm).

     • Prostaglandins (stimulate uterine contractions to help sperm reach the egg).

2. Prostate Gland

   • Secretes into urethra.

   • Produces:

     • Anticoagulant enzyme (prevents semen clotting).

     • Citrate (nutrient for sperm).

3. Bulbourethral Glands (Cowper’s glands)

   • 2 glands, one on each side of the urethra.

   • Secretes:

     • Mucus (lubricates and neutralizes vaginal acidity).

     • Some sperm can be caught in this secretion.

G. Sperm Pathway

1. Sexual stimulation → NO release → Erectile tissue engorges.

2. Sperm leaves epididymis → Enters vas deferens.

3. Vas deferens loops over the bladder → Passes by seminal vesicles.

4. Path becomes the ejaculatory duct → Passes through prostate gland.

5. Ejaculatory duct meets urethra → Passes by Bulbourethral glands.

6. Sperm exits through the urethra and is ejaculated out of the penis.

III. Spermatogenesis (Meiosis)

A. Process of Spermatogenesis

• Occurs throughout adult lifetime.

• Takes place in the testes.

• Regulated by hormones.

Developmental Stages

1. Embryonic Stage:

   • Primordial germ cells (2n) → Differentiate into spermatogonial stem cells (2n).

2. At Puberty (In seminiferous tubules/testes):

   • Spermatogonial cells (2n) undergo mitosis → Produce spermatogonia (2n).

   • Spermatogonia (2n) undergo mitosis → Forms 1 primary spermatocyte (2n) & 1 spermatogonia cell (2n).

   • Primary spermatocyte (2n) undergoes meiosis:

     • Meiosis I → 2 secondary spermatocytes (n).

     • Meiosis II → 4 spermatids (n).

   • Spermatids differentiate into mature sperm cells (spermatozoa).

B. Structure of a Sperm Cell

• Takes ~7 weeks to mature.

• Only the nucleus enters the egg during fertilization.

Parts of the Sperm Cell

1. Head:

   • Contains DNA (nucleus).

   • Acrosome: Contains enzymes to penetrate the egg.

2. Midpiece:

   • Contains mitochondria (produces ATP for motility).

3. Tail:

   • Flagellum (propels the sperm).

IV. Male Endocrinology

A. Androgens (Male Sex Hormones)

• Testosterone:

  • Supports spermatogenesis.

  • Triggers puberty & secondary sex characteristics.

  • Low levels → Infertility (< 15 million sperm/ml).

• All chordates have a hypothalamus and pituitary gland for hormonal regulation.

B. Hormonal Signaling Pathways

1. Testosterone levels drop → Hypothalamus secretes GnRH (Gonadotropin-releasing hormone).

2. GnRH signals the anterior pituitary → Releases FSH & LH.

Roles of FSH & LH

• FSH (Follicle-Stimulating Hormone):

  • Stimulates Sertoli cells → Supports sperm maturation.

• LH (Luteinizing Hormone):

  • Stimulates Leydig cells → Produces testosterone.