Overview
This lecture covers testicular endocrinology in three parts: the hormones produced by the testis and how they are regulated (the HPG axis, steroidogenic pathways, hormone transport in blood, the roles of testosterone/DHT/oestradiol, with clinical examples of hypogonadism and enzyme deficiencies); the compartmentalisation of the testis and the blood-testis barrier (including how Sertoli and Leydig cells signal to control spermatogenesis); and the endocrinology of the reproductive ducts and accessory glands (efferent ducts, epididymis, seminal vesicles, prostate), all of which depend on androgenic stimulation.
The hypothalamic-pituitary-gonadal (HPG) axis
- The hypothalamus secretes gonadotropin-releasing hormone (GnRH) in pulses; GnRH pulse frequency determines the rate of gonadotropin release.
- GnRH travels via portal vessels to gonadotrophs in the anterior pituitary, which release luteinising hormone (LH) and follicle-stimulating hormone (FSH), LH itself also released in pulses.
- LH acts on Leydig cells, stimulating testosterone production.
- FSH acts on Sertoli cells (within the seminiferous tubule), stimulating inhibin synthesis.
- Negative feedback: testosterone inhibits GnRH, LH and FSH release; inhibin inhibits FSH release only, not LH or GnRH.
- Diurnal/pulsatile pattern: less frequent but larger LH pulses occur during sleep/early morning. Serum testosterone shows a diurnal peak in the early morning and a trough in the evening; young men show a higher-amplitude diurnal rhythm than elderly men (MESOR 18.14 nmol/l vs 15.70 nmol/l).
Hypogonadism
- Primary hypogonadism: the gonad itself fails to produce steroids. Loss of negative feedback means LH and/or FSH are usually elevated.
- Secondary (hypogonadotrophic) hypogonadism: the defect is at the hypothalamus or pituitary (e.g. a pituitary tumour reducing LH/FSH output), so FSH and LH are normal or low, and gonadal steroid output falls as a result.
- Kallmann syndrome: a hypogonadotrophic (secondary) hypogonadism, often with anosmia, caused by absence of hypothalamic GnRH neurons. In embryonic life these neurons normally migrate to the hypothalamus; mutations in at least 5 genes can disrupt this (e.g. the anosmin gene, a guidance molecule for GnRH neuron migration).
- Presents with absent or delayed puberty, lack of secondary sex characteristics, sometimes micropenis.
- Prevalence 1:10,000 to 1:60,000; a Y chromosome increases incidence roughly 5-fold (reason unclear).
- Because GnRH (and so LH/FSH) is absent, none of the downstream hormone output (testosterone/oestrogen, sperm/egg cells) or pubertal changes occur: in males, voice breaking, muscle growth, genital enlargement, facial/pubic/axillary hair; in females, hip broadening, menstruation, breast development, pubic/axillary hair.
Steroidogenesis: pathways and key enzymes
- Steroid synthesis proceeds: cholesterol -> pregnenolone -> progestagens -> androgens -> oestrogens.
- Two pathways are distinguished by the position of a double bond in the steroid ring: the Δ4 pathway and the Δ5 pathway. The Δ5 pathway is the predominant pathway in the testis, localising histologically to the interstitial (Leydig cell) regions.
- What matters for reproductive biology is the classes of steroid hormone, not memorising the entire pathway.
- Two key enzymes: aromatase converts androgens to oestrogens; 5α-reductase converts testosterone to dihydrotestosterone (DHT).
- The full pathway groups steroids into classes built stepwise from cholesterol via pregnenolone: progestagens, corticosteroids, androgens and oestrogens.
Steroid hormone transport
- Steroids are lipophilic and so insoluble in aqueous fluids such as blood; they are carried by proteins with hydrophobic binding regions, for which steroids have a greater affinity than for aqueous fluid.
- Main carrier proteins: albumin and sex hormone binding globulin (SHBG), both produced in the liver; androgen binding protein (ABP), produced in the Sertoli cell.
- Percentage of steroid bound to each carrier (Essential Reproduction Table 1.7):
- Albumin: progestagens 48%, androgens 32%, oestrogens 63%, cortisol 20%.
- Cortisol-binding globulin: progestagens 50%, androgens 1%, cortisol 70% (oestrogen figure not given).
- SHBG: androgens 66%, oestrogens 36% (progestagen and cortisol figures not given).
- Free steroid: progestagens 2%, androgens 1%, oestrogens 1%, cortisol 10%.
- Steroids dissociate (“fall off”) their carrier proteins periodically, generating free steroid; this dissociation is necessary for the steroid to leave the blood and enter target cells.
- Mechanism of action: free testosterone diffuses across the target cell membrane and binds the intracellular androgen receptor (AR); the AR-coactivator complex translocates to the nucleus and binds androgen response elements (AREs) on DNA to drive transcription. Membrane receptors for steroids also exist.
- Comparative example (spotted hyena): females are the dominant sex, with larger body size and a peniform clitoris, driven by increased androgen exposure despite androgen production being in the normal range. Hyena SHBG carries a mutation (W14) giving it high affinity for steroids; a further SHBG gene mutation (non-L) reduces SHBG secretion, which may shift androgen transport onto lower-affinity carriers such as albumin. This shows that changes in carrier protein levels can alter hormone signalling even without a change in hormone production.
Testicular steroid hormones and their roles
- Testosterone: synthesised in Leydig cells (stimulated by LH); around 10 mg secreted into blood and lymph per day.
- Dihydrotestosterone (DHT): a metabolite of testosterone, produced by Sertoli cells (stimulated by FSH) via 5α-reductase. DHT is more active than testosterone, with 2-3x greater affinity for the androgen receptor, and drives male pattern baldness.
- Oestradiol: produced in small amounts by Leydig cells (mainly in adults) and Sertoli cells (mainly prepubertally) via aromatase acting on testosterone. Promotes sexual behaviour and efferent ductule function.
- Functional summary: testosterone is aromatised to oestrogen and maintains sexual behaviour in castrates; DHT is not aromatised and does not maintain sexual behaviour in castrates; oestradiol (already an oestrogen) does maintain sexual behaviour in castrates.
- Overall roles: testosterone and DHT induce primary and secondary sex characteristics, regulate gonadotropin secretion by negative feedback, and support spermatogenesis. Oestradiol is required for efferent duct function (and so fertility) and promotes sexual behaviour.
- Supporting evidence: stilbestrol (an oestrogen analogue) given to castrated male pigs restored sexual desire and erectile ability (Dinusson et al. 1951); oestradiol injected into castrated male rats restored mounting behaviour, an effect also produced by testosterone (Södersten 1973).
Clinical example: 5α-reductase deficiency
- Testosterone level is normal, but DHT cannot be produced because 5α-reductase is deficient.
- XY individuals with this deficiency can be assigned female sex at birth, with variable testicular and androgenic development from puberty onward (testosterone can still act directly at puberty even without DHT).
- Rare in the general population overall, though notably prevalent in some populations (“Guevedoces”).
Testicular compartments and the blood-testis barrier
- The testis is divided into four compartments: Vascular (blood vessels), Interstitial (Leydig cells), Basal and Adluminal.
- Sertoli (“nurse”) cells span both the basal and adluminal compartments.
- The blood-testis barrier separates the basal compartment (containing spermatogonia, alongside Leydig cells, myoid cells and the basement membrane on the interstitial side) from the adluminal compartment (containing spermatocytes, round spermatids and spermatozoa).
- The barrier is formed by tight, adherens and gap junctions between adjacent Sertoli cells, sited between the basal and adluminal compartments.
- Two major functions: (1) prevents an immune response to sperm antigens, and (2) allows creation of a specific chemical microenvironment for sperm development.
Warning
FSH, inhibin and paracrine control of spermatogenesis
- FSH binds FSH receptors on Sertoli cells, which then produce growth factors that stimulate spermatogonial mitosis; FSH also stimulates Sertoli cells to produce inhibin.
- Inhibin acts directly on the pituitary gland to inhibit FSH release only (not LH).
- Testosterone supports the later stages of spermiogenesis and prevents early (premature) spermiation.
- Across spermatogenesis, FSH influences the early stages (spermatogoniogenesis and early meiosis) while testosterone influences the later meiotic and spermiogenic stages.
- Sertoli cells also produce paracrine growth factors/cytokines: glial-derived neurotrophic factor (GDNF), which promotes spermatogonial proliferation (shown by reduced proliferating-cell staining in Gdnf knockout mice), and activin, which supports Sertoli cell survival/growth, germ cell proliferation, and the spermatocyte’s transition across the blood-testis barrier.
- Leydig cells produce oxytocin, which promotes smooth muscle contractility in the seminiferous tubules and epididymis.
- Androgen-binding protein (ABP): produced by Sertoli cells; 80% enters the tubule lumen and 20% enters the interstitial compartment. ABP and SHBG arise from the same gene but differ in post-translational modification. ABP carries androgen via the reproductive tract (not the bloodstream) to the rete testis, efferent ductules and epididymis.
Reproductive ducts: efferent ducts and epididymis
- Duct pathway: seminiferous tubule -> rete testis -> efferent ducts (ductuli efferentes, about 12 convoluted ducts) -> epididymis (head/caput, body/corpus, tail/cauda) -> ductus (vas) deferens -> ejaculatory duct -> urethra.
- Efferent ducts connect the rete testis to the head of the epididymis and absorb fluid from the testis. They are lined by alternating ciliated and non-ciliated epithelial cells, with ciliary action thought to mix and propel sperm toward the epididymis, and are surrounded by smooth muscle. Oestradiol is critical for their function.
- The epididymis accumulates, stores and matures spermatozoa; sperm become self-motile and capable of fertilisation here, a maturation process requiring androgenic stimulation. Epithelial cell height decreases progressively from head to tail.
- Head: absorbs fluid to concentrate sperm; slow, rhythmic smooth muscle contractility.
- Body: further fluid absorption and high protein secretion; secretions here modify sperm to induce motility and add cell-surface proteins needed for fertilisation.
- Tail: highest density of stored spermatozoa; sympathetic innervation drives contractions during ejaculation; secretions here preserve sperm.
- Epididymal epithelial cells also secrete proteins and epididymosomes (extracellular vesicles released from an apical bleb) that interact with luminal spermatozoa.
- Clinical correlate: in obstructive azoospermia (sperm cannot pass down the ductus deferens), sperm for assisted reproduction is best aspirated from the tail of the epididymis, reflecting the highest sperm density found there.
- Testosterone withdrawal experiments: castrated rats (low testosterone) show no sperm and a flattened epididymal epithelium; prepubertal rats (also low testosterone) show round immature germ cells (“round sperm”) in the lumen rather than mature elongated spermatozoa, indicating androgen is required for spermiogenesis to complete and for normal epididymal epithelial structure.
- Human semen assessment includes progressive motility: the proportion of sperm swimming in a forward direction, distinguishing normal from abnormal samples.
Accessory glands
- Accessory sex glands (seminal vesicles, prostate gland, bulbourethral gland) produce the large majority of seminal fluid volume. [flag: slide text is cut off, reading “Produce 95% of the seminal fluid in the.” with no further word given]
- Accessory gland growth and secretion are androgen-dependent: androgens drive hyperplasia and promote glandular function.
- Glandular secretions maintain sperm viability and motility and aid sperm transport through the female reproductive tract, but are not required for fertilisation itself.
- Prostate gland: secretory epithelial cell size and activity depend on androgenic stimulation. This dependence underlies benign prostatic hyperplasia (BPH), which affects roughly 50% of ageing men and is occasionally treated with androgen deprivation therapy. Histologically, prostate glands (G) may contain rounded eosinophilic concretions called corpora amylacea (CA).
Self-test
- Describe the steps of the HPG axis from GnRH release to testosterone and inhibin production, including which pituitary hormone acts on which testicular cell type.
- Explain how testosterone and inhibin differ in their negative feedback targets.
- Distinguish primary hypogonadism from secondary (hypogonadotrophic) hypogonadism in terms of LH/FSH levels.
- Describe the underlying defect in Kallmann syndrome and explain why secondary sexual characteristics fail to develop.
- Name the two pathways of steroidogenesis and state which predominates in the testis.
- Describe the actions of aromatase and 5α-reductase, including their substrates and products.
- Distinguish testosterone, dihydrotestosterone and oestradiol in terms of the cell type that produces them and the enzyme responsible.
- Explain the functional difference between DHT and oestradiol regarding aromatisation and the maintenance of sexual behaviour in castrates.
- Describe how a steroid hormone bound to a plasma carrier protein reaches its nuclear receptor and alters transcription.
- List the four testicular compartments and identify which cell type spans two of them.
- Describe the composition and two major functions of the blood-testis barrier.
- Predict what would happen to FSH secretion if Sertoli cell inhibin production were lost, and explain why LH would be unaffected.
- Describe the roles of the paracrine factors GDNF, activin and oxytocin in testicular function.
- Trace the path of a sperm from the seminiferous tubule to the ejaculatory duct, naming each structure in order.
- Describe how epididymal epithelial function changes from head to tail, and relate this to the site chosen for sperm aspiration in obstructive azoospermia.
- A castrated rat and a prepubertal rat both have low testosterone but different epididymal luminal contents. Explain the difference.
- Explain why accessory gland secretions are androgen-dependent and describe the clinical condition that can result from ongoing prostatic androgen stimulation in older men.
- A patient has normal testosterone but no dihydrotestosterone and was assigned female at birth despite an XY karyotype. Which enzyme is deficient, and why does this explain the phenotype?
- Integrative: explain how loss of Leydig cell function versus loss of Sertoli cell function would each disrupt spermatogenesis and testosterone/inhibin feedback differently.
Answers
Reveal answers
- GnRH is released from the hypothalamus in pulses and travels via the pituitary portal vessels to gonadotrophs in the anterior pituitary, causing release of LH and FSH. LH acts on Leydig cells to stimulate testosterone production; FSH acts on Sertoli cells to stimulate inhibin synthesis.
- Testosterone inhibits GnRH release from the hypothalamus and LH and FSH release from the pituitary; inhibin inhibits FSH release only, not LH or GnRH.
- In primary hypogonadism the defect is in the gonad itself, so feedback is lost and LH/FSH are usually elevated; in secondary (hypogonadotrophic) hypogonadism the defect is at the hypothalamus/pituitary, so FSH and LH are normal or low.
- Kallmann syndrome results from absence of hypothalamic GnRH neurons, often due to failure of GnRH neuron migration in embryonic life (e.g. anosmin gene mutations). Without GnRH there is no LH/FSH release, so the gonads are never stimulated to produce sex steroids, and none of the downstream pubertal changes occur.
- The Δ4 and Δ5 pathways (differing in the position of a double bond in the steroid ring); the Δ5 pathway predominates in the testis.
- Aromatase converts androgens to oestrogens; 5α-reductase converts testosterone to dihydrotestosterone.
- Testosterone is produced by Leydig cells (LH-stimulated); dihydrotestosterone is produced by Sertoli cells from testosterone via 5α-reductase (FSH-stimulated); oestradiol is produced by both Leydig cells (mainly in adults) and Sertoli cells (mainly prepubertally) from testosterone via aromatase.
- Testosterone and oestradiol are both aromatised (or already oestrogenic) and both maintain sexual behaviour in castrates; DHT cannot be aromatised to oestrogen and does not maintain sexual behaviour in castrates.
- The steroid dissociates from its carrier protein (e.g. albumin or SHBG) to become free steroid, diffuses across the target cell membrane, binds its nuclear hormone receptor (e.g. the androgen receptor) in the cytoplasm, and the receptor-coactivator complex translocates to the nucleus where it binds response elements (e.g. AREs) on DNA to drive transcription.
- Vascular, Interstitial, Basal and Adluminal; the Sertoli cell spans the basal and adluminal compartments.
- Formed by tight, adherens and gap junctions between adjacent Sertoli cells, located between the basal and adluminal compartments. It prevents an immune response against sperm antigens and creates a specific chemical microenvironment for sperm development.
- FSH secretion would rise, since loss of inhibin removes its selective negative feedback on FSH at the pituitary; LH would be unaffected because inhibin does not act on LH, only testosterone does.
- GDNF (from Sertoli cells) promotes spermatogonial proliferation; activin (from Sertoli cells) supports Sertoli cell survival/growth, germ cell proliferation and the spermatocyte’s transition across the blood-testis barrier; oxytocin (from Leydig cells) promotes smooth muscle contractility in the seminiferous tubules and epididymis.
- Seminiferous tubule -> rete testis -> efferent ducts -> epididymis (head, body, tail) -> ductus (vas) deferens -> ejaculatory duct -> urethra.
- Epithelial cell height decreases from head to tail; the head absorbs fluid to concentrate sperm, the body absorbs fluid and secretes proteins that confer motility and fertilising ability, and the tail stores the highest density of mature sperm and preserves them, contracting under sympathetic drive at ejaculation. Because sperm density is highest in the tail, it is the preferred site for aspiration in obstructive azoospermia.
- In the castrated rat, testosterone is too low to support spermatogenesis at all, so no sperm are present; in the prepubertal rat, low testosterone means spermiogenesis has not yet been driven to completion, so round immature germ cells (“round sperm”) are seen instead of mature elongated spermatozoa. Both illustrate that androgen is required for spermiogenesis to complete normally.
- Accessory gland (seminal vesicle, prostate, bulbourethral) growth and secretory activity depend on androgenic stimulation, which drives glandular hyperplasia and function; in older men, ongoing androgen stimulation of the prostate can lead to benign prostatic hyperplasia (BPH), seen in around 50% of ageing men.
- 5α-reductase is deficient, so testosterone cannot be converted to DHT; testosterone alone is not sufficient to virilise the external genitalia in utero, so the individual can be assigned female at birth, but testosterone (without DHT) can still drive some androgenic development at puberty.
- Loss of Leydig cell function would abolish testosterone production, removing LH negative feedback (LH rises) and removing androgen support for later spermiogenesis and duct/gland function, disrupting fertility despite intact FSH/inhibin signalling. Loss of Sertoli cell function would abolish inhibin (FSH rises), abolish DHT production and the ABP/nutritive support Sertoli cells give developing germ cells, and disrupt the blood-testis barrier, directly halting spermatogenesis even if Leydig cell testosterone output were normal.