What is the anatomy of the male human body?

Male Anatomy Overview

The male reproductive system comprises the testes, ductal system (epididymis, vas deferens, ejaculatory duct, urethra), and sex accessory organs (seminal vesicles, prostate, bulbourethral glands) that collectively produce sperm and supporting fluids for reproduction 1, 2.

Primary Reproductive Organs

Testes

• Dual function: The testes serve both reproductive and endocrine roles by synthesizing spermatozoa and producing androgens (primarily testosterone) under hypothalamic-pituitary-gonadal (HPG) axis regulation 1, 2.
• Testosterone production timing: Testicular testosterone production begins at 7-8 weeks of gestation, with peak fetal levels occurring at 14-16 weeks (reaching adult male range), which is essential for male genital differentiation 3.
• Postnatal surge: Male infants experience a critical testosterone surge from birth to 1-3 months of life, then levels decrease to prepubertal values by 4-6 months 3.
• Leydig cells: These specialized testicular cells first appear between days 16-18 after conception in rats and are responsible for testosterone secretion 4.

Ductal System

• Epididymis: Site of sperm maturation where spermatozoa gain motility and fertilizing capacity 1, 5.
• Vas deferens: Muscular duct that transports mature sperm from the epididymis during ejaculation 1.
• Ejaculatory duct and urethra: Final conduits through which semen is expelled during ejaculation 1, 2.

Sex Accessory Glands

• Seminal vesicles: Contribute approximately 60-70% of seminal fluid volume, providing fructose and prostaglandins 1.
• Prostate gland: Secretes alkaline fluid containing enzymes and zinc that supports sperm viability 1.
• Bulbourethral glands: Produce pre-ejaculatory fluid that lubricates the urethra 1.

Penis and Erectile Structures

Erectile Bodies

• Corpora cavernosa: Paired erectile tissues that fill with blood during erection, providing penile rigidity 2.
• Corpus spongiosum: Surrounds the urethra and forms the glans penis 2.
• Mechanism: Penile tumescence results from relaxation of smooth-muscle cells in erectile tissue and endothelium of penile arteries, with additional rigidity achieved through ischiocavernosus muscle contraction 2.

Neural Control

• Spinal nuclei: Specific spinal centers integrate peripheral genital stimuli and central sexual cues to trigger erection 2, 6.
• Autonomic and somatic pathways: Both systems coordinate to control erectile function, with parasympathetic activation promoting erection and sympathetic/somatic systems mediating ejaculation 2, 6.

Ejaculatory Function

Two-Phase Process

• Emission phase: Secretion of semen components by sex glands under autonomic control 2.
• Expulsion phase: Forceful ejection of semen through rhythmic contractions of the bulbospongiosus muscle 2.
• Spinal generator: A specialized spinal mechanism integrates sensory input and triggers ejaculation when excitatory threshold is reached 2.

Hormonal Regulation

Hypothalamic-Pituitary-Gonadal Axis

• Primary hypogonadism: Results from testicular dysfunction with elevated gonadotropins (hypergonadotropic) 4.
• Secondary hypogonadism: Caused by impairment of the HPG axis with low gonadotropins (hypogonadotropic) 4.
• Normal testosterone range: Adult males maintain serum testosterone levels of 300-1,000 ng/dL 7.

Androgen Receptor Mechanism

• Genomic effects: Testosterone binds to androgen receptors (AR), functioning as a ligand-activated transcription factor that modulates gene expression 5.
• Tissue-specific action: AR expression varies across reproductive tissues, with testosterone undergoing local conversion to dihydrotestosterone (DHT) in some tissues for enhanced androgenic effects 5.
• Aromatization: Testosterone can be converted to estradiol by aromatase enzyme in brain and other tissues, contributing to sexual differentiation 4.

Brain Sexual Differentiation

Critical Periods

• Prenatal window: Weeks 8-24 of gestation represent the primary period when androgens masculinize the developing nervous system 4.
• Postnatal window: Months 1-6 of life constitute a second critical period for potential brain masculinization, with testosterone levels significantly elevated in males compared to females 4, 3.

Structural Dimorphism

• Sexually dimorphic nucleus: The preoptic area of the hypothalamus is several times larger in males than females, influenced by early testosterone exposure 4.
• Estrogen hypothesis: Testosterone enters brain cells and is converted to estradiol by aromatase, with estradiol then binding estrogen receptors to promote masculine neural differentiation 4.

Common Pathological Conditions

Developmental Disorders

• Klinefelter syndrome (47,XXY): Most common chromosomal cause of male infertility, characterized by testicular atrophy and elevated FSH 4.
• Y-chromosome microdeletions: Found in 5% of males with sperm concentrations 0-1 million/mL 4.
• Cryptorchidism: Undescended testes requiring early correction to preserve fertility potential 4.

Acquired Conditions

• Obstructive azoospermia: Suspected when testes are normal size with dilated epididymides and FSH <7.6 IU/L 4.
• Non-obstructive azoospermia: Indicated by testicular atrophy and FSH >7.6 IU/L, reflecting spermatogenic failure 4.
• Ejaculatory duct obstruction: Presents with low semen volume (<1.4 mL) and acidic pH (<7.0) 4.