Male Reproductive System: Structure, Function, and Physiology (Seeley's Anatomy & Physiology, Ch. 28, Part A)

28.1 Functions of the Reproductive System

Overview of Reproductive System Functions

• Gametogenesis: Specialized organs produce gametes—sperm cells in males and oocytes (eggs) in females. This process ensures genetic diversity and continuity of species.

• Fertilization: The reproductive system facilitates the union of sperm and oocyte. Male ducts nourish and transport sperm to the female tract, where fertilization occurs.

• Development and Nourishment: The female system supports embryonic and fetal development in the uterus and provides nourishment (milk) after birth.

• Production of Reproductive Hormones: Hormones regulate reproductive system development, function, and secondary sexual characteristics, as well as reproductive behavior.

28.2 Meiosis

Cell Division for Sexual Reproduction

• Meiosis: Specialized cell division producing four genetically unique haploid cells from one diploid cell. Essential for gamete formation and genetic variation.

• Diploid (2n): Cells with 46 chromosomes (normal for human somatic cells).

• Haploid (n): Cells with 23 chromosomes (sperm and eggs).

Key Steps in Meiosis:

• Meiosis I: Homologous chromosomes separate, reducing chromosome number by half (reduction division). Crossing over during prophase I increases genetic diversity.

• Meiosis II: Sister chromatids are separated, similar to mitosis.

Equation: $2n \rightarrow n$

28.3 Anatomy of Male Reproductive System

Scrotum and Perineum

• Scrotum: Two-chambered sac containing the testes, separated by the raphe. Dartos and cremaster muscles regulate temperature for optimal sperm development.

• Perineum: Diamond-shaped region between thighs, divided into the urogenital triangle (anterior; contains penis and scrotum) and anal triangle (posterior).

Testes

• Testes: Small, oval organs where sperm develop. Located outside the body cavity for temperature regulation.

• Functions: Exocrine (sperm secretion) and endocrine (testosterone production).

• Coverings: Tunica vaginalis (serous membrane) and tunica albuginea (thick connective tissue dividing testis into lobules).

• Seminiferous tubules: Site of sperm production.

• Interstitial (Leydig) cells: Secrete testosterone.

Descent of Testes

• Testes descend from the abdominal cavity through the inguinal canal to the scrotum, guided by the gubernaculum.

• Pass through inguinal rings (openings in the abdominal wall).

• Cryptorchidism: Failure of one or both testes to descend, leading to impaired sperm development.

Spermatogenesis

Formation of Sperm Cells

• Spermatogenesis: Production of spermatozoa in seminiferous tubules.

• Spermatids (after second meiotic division) develop an acrosome (enzyme cap) and flagellum (tail).

• Sustentacular (Sertoli) cells: Nourish sperm, form the blood-testis barrier, and produce hormones.

• Interstitial cells: Produce testosterone; Sertoli cells convert it to dihydrotestosterone (DHT) and estrogen, which are active in sperm formation.

Duct System

Pathway of Sperm

• Sperm move from seminiferous tubules → tubuli recti → rete testis → efferent ductules → epididymis.

• Epididymis: Site of sperm maturation (head, body, tail); takes 1–2 days. Stereocilia increase surface area for absorption.

• Ductus (vas) deferens: Transports sperm from epididymis to the abdominal cavity; forms part of the spermatic cord. The distal end is the ampulla.

• Ejaculatory duct: Formed by joining ductus deferens and seminal vesicle; ends in the prostatic urethra.

Urethra

Passageway for Urine and Semen

• Extends from the urinary bladder to the distal end of the penis.

• Three parts:

  • Prostatic urethra: Passes through prostate; receives ducts from prostate.

  • Membranous urethra: Passes through the perineum (urogenital diaphragm).

  • Spongy (penile) urethra: Passes through the penis; receives mucus from urethral glands.

Penis

Structure and Function

• Functions: Urination and delivery of sperm into the vagina (erection).

• Three columns of erectile tissue:

  • Corpora cavernosa (2): Form the crus, attached to coxae.

  • Corpus spongiosum (1): Surrounds spongy urethra, forms the bulb.

  • Glans penis: Enlargement of corpus spongiosum.

  • Prepuce (foreskin): Covers glans; removed during circumcision.

Accessory Glands

Seminal Vesicles, Prostate, Bulbourethral Glands

Gland	Contribution to Semen	Secretion Characteristics
Seminal vesicles	~60%	Fibrinogen, high pH
Prostate gland	~30%	Thin, milky, high pH, clotting factors, fibrinolysin
Bulbourethral glands	~5%	Mucous secretion, neutralizes vaginal pH

Semen

Composition and Function

• Mixture of sperm and secretions from accessory glands.

• Emission: Discharge of semen into prostatic urethra.

• Ejaculation: Forceful expulsion of semen from urethra, caused by peristalsis.

• Temporary coagulation (fibrinogen → fibrin), then liquefaction (fibrinolysin). Sperm swim up the vagina.

28.4 Physiology of Male Reproduction

Hormonal Regulation

• At puberty, increased GnRH stimulates release of LH and FSH from the anterior pituitary.

• FSH: Promotes sperm formation.

• LH: Stimulates interstitial cells to produce testosterone.

Puberty in Males

• Before birth: Placenta secretes chorionic gonadotropin to stimulate fetal testosterone production.

• From birth to puberty: Minimal testosterone secretion.

• At puberty: Pituitary becomes less sensitive to testosterone inhibition; increased GnRH, LH, and FSH.

• Results: Sperm cell formation and increased testosterone secretion.

Effects of Testosterone

• Development of male genitalia and reproductive ducts.

• Essential for spermatogenesis and testicular descent.

• Secondary sexual characteristics: Hair growth, skin changes, increased sebaceous secretion, laryngeal hypertrophy (voice deepening), increased metabolic rate, red blood cell count, protein synthesis, and bone growth.

Male Sexual Behavior and the Male Sex Act

Neural and Hormonal Control

• Testosterone is required to initiate and maintain sexual behavior.

• Sex act involves reflexes: erection, mucus secretion, emission, ejaculation, and orgasm.

Emission and Ejaculation

• Emission: Accumulation of sperm and glandular secretions in the urethra, controlled by sympathetic spinal centers.

• Ejaculation: Rhythmic contractions (somatic motor output) expel semen; sympathetic constriction of the bladder sphincter prevents mixing of semen and urine.

Erection Reflex

• Involves sensory input, integration in the CNS, and motor output to erectile tissue and glands.

Additional info: While these notes focus on the male reproductive system, the processes of meiosis and spermatogenesis are foundational to genetics, as they underlie gamete formation, genetic variation, and inheritance patterns. The hormonal regulation of reproduction also connects to gene expression and developmental genetics.