Human Reproduction, Development and Ageing - Week 3 Lecture Notes
Topic = Male Reproductive System
Reproductive System Overview
Core function of BOTH sexes: production of viable gametes and successful offspring formation.
Male: create and deliver sperm to female tract for fertilisation.
Female (from previous lecture): produce ovum and provide protected site for fertilisation, implantation and embryonic development.
Lecture 3 focus: Male anatomy, physiology, hormone regulation, spermatogenesis, clinical disorders, and upcoming Literature Review Plan.
Male Hormone Regulation – General Principles
Governed by Hypothalamus–Anterior Lobe of Pituitary (ALP)–Testes axis.
Interaction of positive & negative feedback loops keeps testosterone within a narrow range until late life.
Environmental inputs (light, stress, temperature, pheromones) affect axis via the brain; stress has measurable inhibitory effects.
Key hormones & pulsatility:
GnRH released in pulses every 1–3 h; continuous exposure desensitises receptors.
LH → Leydig cells → testosterone.
FSH → Sertoli cells → spermatogenesis support & inhibin.
Inhibin: Sertoli-derived; specifically suppresses FSH release.
Broad summary loop:
Hypothalamus releases GnRH.
GnRH → ALP → LH & FSH.
FSH promotes spermatogenesis (with testosterone).
LH stimulates Leydig cells to secrete testosterone.
Rising testosterone ⇒ male sexual maturation, libido, negative feedback on GnRH/FSH/LH (with inhibin).
Male Reproductive Anatomy
• Main structures: scrotum, testes, epididymis, vas deferens, ejaculatory ducts, urethra, penis, accessory glands (seminal vesicles, prostate, bulbourethral).
Scrotum
Pouch holding testes outside abdominopelvic cavity; needed because optimal spermatogenesis temperature ≈ 34^{\circ}\text{C} (≈ 3^{\circ}\text{C} below core 37^{\circ}\text{C}).
Pampiniform venous plexus acts as counter-current heat exchanger; scrotum contracts/relaxes in response to temperature, altering surface area.
Contains spermatic cords (vas deferens, nerves, blood & lymph vessels); traverses inguinal canal.
Weakness → inguinal hernia: bowel/omentum protrusion; common in males due to canal.
Testes
Plum-sized (≈4 cm × 2.5 cm).
Covered by two tunics:
Tunica vaginalis (peritoneal-derived, superficial).
Tunica albuginea (fibrous; septa divide testis into ≈250 lobules).
Each lobule houses 1–4 seminiferous tubules (site of sperm production).
Embryology & descent:
Develop high in abdomen; descend through inguinal canal into scrotum between 7 mo gestation and birth.
Failure → cryptorchidism: undescended testes; ↑ risk infertility & malignancy.
Comparative Size & Mating Implications
Testes mass correlates with body mass and mating system:
Large testes in multi-male (polyandrous) species (e.g., chimpanzee) for sperm competition.
Smallest in single-male (monogamous) species.
Humans intermediate; suggests mild/moderate polygyny historically.
Seminiferous Tubules – Histology & Cells
Surrounded by 3–5 layers of contractile myoid cells.
Sertoli (nurse) cells:
Form blood-testis barrier (BTB) via tight junctions (physical, immunological & physiologic).
Feed developing germ cells (lactate instead of glucose); secrete androgen-binding protein (ABP), fluid for sperm transport, inhibin.
Concentrate testosterone locally.
Spermatogenic cells: successive stages from spermatogonia → spermatids.
Leydig (interstitial) cells in interstitium produce testosterone under LH.
Penis
Copulatory organ delivering semen.
Parts: root, shaft, glans (distal expansion), prepuce/foreskin (may be removed by circumcision).
Erectile tissues (vascular sinusoids within CT & smooth muscle):
Corpora cavernosa (paired dorsal columns).
Corpus spongiosum (ventral; surrounds spongy urethra; expands to glans).
Duct System
Epididymis (head, body, tail):
Sperm storage/maturation (~20 days transit); tail stores until ejaculation.
Vas (ductus) deferens:
Continuation from tail; ascends in spermatic cord → inguinal canal → pelvic cavity.
Distal dilation = ampulla; joins seminal vesicle duct → ejaculatory duct → prostatic urethra.
Vasectomy: cutting/ligating vas; sperm still produced but not ejaculated (form of contraception).
Accessory Glands & Semen Composition
Seminal vesicles (≈60 % semen): alkaline, fructose (ATP fuel), prostaglandins (↓ cervical mucus viscosity, ↑ uterine contractions).
Prostate (≈30 %): slightly acidic, enzymes for sperm activation (e.g., PSA, fibrinolysin).
Pathologies: Benign prostatic hyperplasia (BPH) (urethral compression); prostate cancer (2nd leading male cancer death after lung).
Bulbourethral (Cowper) glands (<5 %): clear mucus for urethral lubrication & neutralisation of residual acidic urine.
Semen overall:
Volume per ejaculation: 2–5\,\text{mL}; only ≈10 % cellular sperm.
Sperm density: 20–150\times10^{6}\,\text{mL}^{-1} (WHO: normal ≥15\times10^{6}; oligospermia below).
pH 7.3–7.5 (alkalinity counteracts vaginal acidity).
Of ≈375\times10^{6} sperm/ejaculate, only ~200 reach the oocyte.
Spermatogenesis & Spermiogenesis
Begins at puberty (~age 14) and continues life-long; average production ≈4\times10^{8} sperm day⁻¹.
Phases within seminiferous tubules:
Mitosis of spermatogonia (stem cells): each division yields Type A (basal lamina renewer) & Type B (primary spermatocyte) daughter cells.
Meiosis I: primary spermatocyte → two secondary spermatocytes (haploid).
Meiosis II: secondary spermatocytes → four early spermatids.
Spermiogenesis: metamorphic phase; spermatids → spermatozoa.
Nuclear condensation, acrosome formation (from Golgi; digestive enzymes for zona pellucida penetration), mitochondrial sheath in midpiece (ATP for motility), flagellum formation, excess cytoplasm shed.
One primary spermatocyte yields four genetically unique sperm.
Sertoli Cell Functions – Expanded Significance
Nutrient/metabolite provision (lactate, ions, ABP).
Immunoprotection (BTB prevents autoimmune response to late germ cells).
Paracrine signalling for germ cell differentiation.
Mechanical support & phagocytosis of residual cytoplasm.
Secretion of inhibin & Müllerian-inhibiting substance (AMH) in fetal testes.
Endocrine Axis – Detailed Map & Feedback Loops
GnRH pulsatile → anterior pituitary.
LH binds Leydig cell receptors (cAMP pathway) → testosterone secretion.
FSH binds Sertoli cells (cAMP) → ABP ↑ testosterone local concentration, stimulates spermatogenic support.
Testosterone & inhibin exert negative feedback:
Testosterone on hypothalamus (↓ GnRH) & pituitary (↓ LH).
Inhibin specifically on pituitary (↓ FSH).
Developmental hormone timeline (graph):
High Leydig activity fetal weeks 11–20 (genital differentiation).
Quiescence post-birth until “mini-puberty” (months 1–6) and true puberty.
Semen Analysis & Clinical Relevance
Volume: 3–5\,\text{mL}; evaluate viscosity, liquefaction time.
Concentration: normal mean 75\times10^{6}\,\text{mL}^{-1}; WHO lower reference 15\times10^{6}.
Motility: ≥40 % progressive within 60 min.
Morphology: ≥4 % normal forms (strict Kruger); <20 % abnormal of normals.
Only ~0.00005 % of sperm reach oocyte → underscores quality > quantity in fertilisation.
Male Reproductive Disorders (Selected)
Benign prostatic hyperplasia (BPH): almost universal in ageing men; obstructive LUTS, weak stream, nocturia.
Prostate cancer: PSA screening; metastasises to bone; 2nd most common male cancer death.
Cryptorchidism: undescended testis; ↑ infertility & testicular cancer risk.
Inguinal hernia: protrusion via inguinal canal; surgical repair.
Testicular torsion: twisting of spermatic cord; acute ischemia.
Epididymitis: infection/inflammation; often STI-related in young men.
Hypogonadism/Low testosterone: ↓ libido, infertility, metabolic effects; may require TRT.
Erectile dysfunction: vascular, neurogenic, endocrine or psychogenic.
Phimosis: tight foreskin; may impede hygiene/sexual function.
Infertility/Sterility: multifactorial – hormonal, genetic (e.g., Y-microdeletions), obstructive, varicocele.
Ethical, Philosophical & Practical Implications
Managing male fertility (vasectomy, ART) raises issues of consent, reversibility, societal norms.
Screening (PSA) debates: balance early detection vs overdiagnosis.
Use of testosterone supplementation: performance vs therapy; potential abuse.
Cryptorchidism surgery timing to preserve fertility—guideline evolution.
Literature Review Plan (Assessment Guidance)
Conducted Week 4 lab session.
Bring: Reference document, LRP template, preliminary research notes.
Provided: Topic allocation list, guidance material.
Expected deliverables:
Identify scope of allocated topic.
List 3–4 key findings, gaps, theories, or studies discovered so far.
Explain how these link to the critical elements/questions of your review.
Prepare to discuss methodology and relevance to human reproduction.
Connections to Previous & Future Lectures
Builds on Lecture 1 (Meiosis & Gametogenesis) – understanding meiosis crucial for spermatogenesis stages.
Precedes Fertilisation & Pregnancy lecture; sperm transport, capacitation, acrosome reaction will be revisited.
Endocrinology principles will parallel female cycle (contrast cyclic vs constant hormone patterns).
Real-World & Clinical Correlations
Knowledge underpins contraceptive strategies (vasectomy, hormonal suppression).
Assisted Reproductive Technology (ART) relies on semen analysis, sperm preparation, and endocrine manipulation (later lecture).
Understanding temperature sensitivity of spermatogenesis influences lifestyle advice (tight clothing, hot baths, laptop on lap, varicoceles).
Environmental disruptors (phthalates, heat) act via endocrine axis – public health considerations.