Leydig Cells

The steroidogenic interstitial cells of the testis, Leydig cells are the principal source of testosterone in men and the male counterpart of the ovarian theca cell. Located in the peritubular and perivascular connective tissue between seminiferous tubules, they convert cholesterol to testosterone through a well-defined cascade of mitochondrial and smooth-endoplasmic-reticulum enzymes, operating under the control of luteinizing hormone (LH) from the pituitary. With aging, Leydig cell steroidogenic capacity declines substantially — driven more by reduced per-cell enzyme activity than by simple cell loss — producing the lower testosterone and compensatory LH rise characteristic of the primary hypogonadal pattern of andropause. The mechanisms center on impaired StAR-mediated cholesterol transport into mitochondria, declining TSPO-dependent mitochondrial dynamics, and oxidative damage to the steroidogenic machinery ¹².

Identity and markers

Leydig cells are identified by their anatomical position in the testicular interstitium and by co-expression of the nuclear receptor NR5A1 (SF-1), the master transcription factor of steroidogenic cell identity, along with the full complement of steroidogenic enzymes. There is no single universally specific surface marker, but INSL3 (insulin-like peptide 3) is the most specific secreted product of mature Leydig cells and serves as a circulating biomarker of Leydig cell functional mass ³.

Marker	Mouse	Human	Notes
NR5A1 / SF-1	Nr5a1+	NR5A1+	Master steroidogenic TF; required for Leydig cell differentiation
LHCGR	Lhcgr+	LHCGR+	LH/CG receptor; the upstream signaling input for steroidogenesis
StAR	Star+	STAR+	Mitochondrial cholesterol transporter; rate-limiting for steroidogenesis
CYP11A1 (P450scc)	Cyp11a1+	CYP11A1+	Cleaves cholesterol side-chain → pregnenolone
CYP17A1	Cyp17a1+	CYP17A1+	17α-hydroxylase/17,20-lyase; converts pregnenolone → DHEA → androstenedione
HSD3B (3β-HSD)	Hsd3b1+	HSD3B+	Converts pregnenolone → progesterone; DHEA → androstenedione
HSD17B3 (17β-HSD3)	Hsd17b3+	HSD17B3+	Converts androstenedione → testosterone; testis-specific isoform
INSL3	Insl3+	INSL3+	Relaxin-family peptide; produced exclusively by mature Leydig cells; serum level reflects functional Leydig mass

Anatomy and niche

Leydig cells occupy the interstitial space of the testis: the connective tissue compartment between seminiferous tubules, in close proximity to peritubular myoid cells and testicular capillaries. The vascular proximity is functionally important — both for delivery of LH from the circulation and for the rapid export of testosterone, which diffuses to Sertoli cells within the tubule (where it is required for spermatogenesis at supraphysiological local concentrations) as well as entering the circulation.

The extracellular matrix (ECM) stiffness of the testicular interstitium is an underappreciated regulator of the stem Leydig cell progenitor pool. Recent work shows that high ECM stiffness — which accumulates with age — activates Piezo1-mediated calcium influx, triggers mitochondrial dysfunction and excessive ROS production, and promotes Gli1 degradation via the ubiquitin-proteasome pathway, suppressing stem Leydig cell proliferation and differentiation ⁴. needs-human-replication (findings are in mouse and in-vitro hydrogel models; ECM stiffness measurement in aging human testis is not established at equivalent resolution)

Steroidogenic cascade

The Leydig cell steroidogenic pathway is initiated by LH binding to LHCGR, a G-protein-coupled receptor that activates adenylyl cyclase → cyclic AMP (cAMP) → protein kinase A (PKA). The downstream cascade proceeds as follows:

1. Cholesterol mobilization: PKA phosphorylates StAR (steroidogenic acute regulatory protein) and promotes assembly of the mitochondrial translocator protein (TSPO)-based cholesterol-trafficking scaffold; cholesterol is transported from the outer to the inner mitochondrial membrane ⁵.
2. Side-chain cleavage: CYP11A1 (P450scc) at the inner mitochondrial membrane cleaves the cholesterol side-chain → pregnenolone. This is the committed, rate-limiting enzymatic step.
3. Conversion to androgens (smooth ER): Pregnenolone → progesterone (via HSD3B); → 17α-hydroxyprogesterone (via CYP17A1); → androstenedione (via CYP17A1 lyase activity); → testosterone (via HSD17B3).

Testosterone can subsequently be aromatized to estradiol by CYP19A1 (aromatase) in peripheral tissues, including adipose and the brain. CYP19A1 is expressed at low levels within the testis itself (primarily in Sertoli cells), but the Leydig cell is not the primary aromatase-expressing cell; compare the analogous role of cyp19a1 in granulosa cells and theca cells of the ovary. See the female steroidogenic analog page granulosa-cells.

Autocrine and paracrine modulation

Testosterone production by Leydig cells is further modulated locally:

• Insulin and IGF-1: Amplify LH-stimulated steroidogenesis.
• Estradiol (via intratesticular aromatization): Can inhibit Leydig cell steroidogenesis via direct feedback — estrogen receptor alpha is expressed on Leydig cells.
• Cytokines from macrophages: Resident testicular macrophages in close proximity to Leydig cells can both stimulate (IL-1α) and inhibit (TNF-α, IL-1β) steroidogenesis depending on inflammatory state.
• Sertoli cell factors: Desert hedgehog (DHH) and PDGF from Sertoli/peritubular cells are required for Leydig cell differentiation from progenitors during development; role in adult maintenance is incompletely characterized.

Stem and progenitor Leydig cells

A population of stem Leydig cells (SLCs) — also called adult Leydig cell progenitors — exists in the adult testis and retains the capacity to regenerate mature Leydig cells after experimental ablation (e.g., ethane dimethanesulfonate treatment in rodents). Recent lineage-tracing work using TCF21-Cre mice identified TCF21+ mesenchymal cells as multipotent somatic progenitors capable of generating multiple testicular lineages including Leydig and peritubular myoid cells, and showing residual progenitor activity in the adult interstitium ⁶. SLCs are spindle-shaped, express low or absent levels of mature Leydig markers (low CYP11A1, absent INSL3), and can be induced to differentiate by LH and local growth factors.

The SLC pool is not unlimited. With advancing age, stem Leydig cell number and differentiation capacity decline — at least in part attributable to the age-related increase in ECM stiffness described above ⁴ and to a progressively more oxidative and inflammatory interstitial environment. Whether this progenitor depletion is a contributor to age-related testosterone decline (in addition to the better-characterized per-cell functional decline in existing mature Leydig cells) is an area of active investigation. needs-human-replication

Because SLC regeneration is experimentally demonstrable after ablation, self-renewal: limited is the accurate designation — the progenitor pool exists, but it is not robustly self-sustaining across the adult lifespan and declines with age.

Aging phenotypes

Age-related testosterone decline: number vs. per-cell function

The testosterone decline of aging has contributions from two distinct sources — reduced Leydig cell number and reduced per-cell steroidogenic capacity — and their relative importance has been debated. Key evidence:

• Per-cell functional decline (primary mechanism in rodents): In the Brown Norway rat aging model, individual Leydig cells from old rats produce significantly less testosterone than those from young rats when maximally stimulated ex vivo; this could not be explained by cell-number changes ⁷. This finding established that the steroidogenic defect is intrinsic to the cell, not merely a reflection of fewer cells.
• Number in humans (contested): A stereological study of 26 post-mortem human testes aged 16–80 years found a significant decline in Sertoli cell number with age but no significant age-related decline in Leydig cell number (mean unilateral count ~99 × 10⁶, stable across age groups) ⁸. Older histological studies attributed part of the testosterone decline to reduced Leydig cell number; the discordance may reflect methodological differences (stereology vs. volume-density estimation) or population heterogeneity. contradictory-evidence

Current consensus, based predominantly on the rodent data and supported by per-cell functional studies in human tissue, is that reduced per-cell testosterone output is the dominant mechanism, with progenitor-pool decline potentially contributing at older ages through failure to replenish cells lost to apoptosis ².

Dimension	Status
Pathway conserved in humans?	yes (LH → cAMP → StAR → CYP11A1 cascade is conserved)
Phenotype conserved in humans?	yes (age-related testosterone decline + compensatory LH rise is well-documented in human cohorts)
Replicated in humans?	partial (mechanisms primarily characterized in rodent; human histological/biochemical data consistent but less detailed)

Impaired StAR-mediated cholesterol transport

The rate-limiting step in the steroidogenic decline is impaired delivery of cholesterol to the inner mitochondrial membrane, mediated by StAR. In aged rat Leydig cells compared to young, StAR protein levels are reduced by ~47% at middle age and ~74% at old age; simultaneously, CYP11A1 (P450scc) protein is reduced by ~38% and ~54% at the same time points ⁹. Since StAR is required for cholesterol transport and CYP11A1 performs the committed enzymatic step, parallel reductions in both proteins compound to produce a substantial testosterone biosynthetic deficit. The decline in StAR is partly attributable to elevated COX-2 activity in aged Leydig cells, which tonically suppresses StAR gene expression via a prostaglandin E2-mediated mechanism ¹⁰. needs-human-replication (molecular detail characterized in rodent; direct human Leydig cell biochemistry is limited by tissue availability)

Mitochondrial dysfunction and TSPO

The TSPO (translocator protein, formerly peripheral benzodiazepine receptor) is a key component of the mitochondrial cholesterol-trafficking complex. TSPO levels decline in aging Leydig cells, and this decline compromises mitochondrial function and membrane dynamics beyond cholesterol transport per se. In cell-culture models of TSPO depletion, mitochondrial fusion is impaired; restoring mitochondrial fusion (using the fusion promoter M1 or OPA1 overexpression) rescues both mitochondrial health and steroidogenic output. Critically, treating aged primary rat Leydig cells with M1 improves mitochondrial function and increases androgen production, establishing mitochondrial fusion as a therapeutic lever ⁵. This mechanistic link positions Leydig cell aging squarely within the broader mitochondrial-dysfunction hallmark.

Impaired LH responsiveness

Despite normal or elevated circulating LH in older men (the compensatory pituitary response to declining testosterone), aged Leydig cells show reduced cAMP accumulation per unit LH — consistent with reduced LHCGR expression or downstream signaling efficiency, not merely substrate limitation ². This explains the “primary hypogonadal” pattern of andropause: low testosterone with elevated LH, analogous to primary ovarian insufficiency in women. It also means that exogenous LH or hCG stimulation produces submaximal testosterone responses in aged Leydig cells — the functional defect extends downstream of the receptor.

INSL3 as a biomarker of Leydig cell functional mass

INSL3 (insulin-like peptide 3, also called relaxin-like factor) is produced constitutively by mature Leydig cells — not pulse-secreted in response to LH like testosterone — making it a more stable readout of Leydig cell functional mass than the highly variable testosterone measurement ³. Key properties:

• Serum INSL3 levels are highly Leydig-cell-specific: in states of Leydig cell loss or insufficiency, INSL3 falls proportionally.
• INSL3 declines more steeply with age than testosterone in community-dwelling men, consistent with loss of functional Leydig mass exceeding what testosterone (partially compensated by higher LH drive) reveals ¹¹.
• In the European Male Aging Study (EMAS) cohort, lower INSL3 was independently associated with greater age-related morbidity burden (bone mineral density loss, sexual dysfunction, fatigue, metabolic syndrome components) beyond what testosterone alone predicted ¹².
• A reference range and threshold have been established from EMAS and FAMAS cohort data: serum INSL3 of ≤0.4 ng/mL is proposed as the threshold for Leydig Cell Insufficiency (LCI) and clinical primary hypogonadism (representing the lower 95% CI for men aged 60–65 years); the eugonadal reference range for men aged >35 years is 0.4–2.3 ng/mL ¹³.

INSL3 is not yet in routine clinical practice for hypogonadism assessment, but its superior stability (circadian variation is minimal, unlike testosterone) and Leydig-cell-specificity make it a strong candidate to complement or replace testosterone as the primary biomarker of Leydig cell health in research settings. needs-human-replication (clinical utility in guiding treatment decisions is not established by randomized outcome data)

Relationship to hallmarks of aging

Hallmark	Role in Leydig cell aging
mitochondrial-dysfunction	TSPO decline impairs cholesterol-trafficking scaffold and mitochondrial dynamics; reduced mitochondrial fusion → impaired steroidogenesis; oxidative damage to CYP11A1 and StAR; central mechanism of per-cell testosterone decline
altered-intercellular-communication	Reduced testosterone output disrupts HPG axis feedback (LH rises compensatorily); impaired paracrine testosterone delivery to Sertoli cells compromises spermatogenesis; age-related inflammatory microenvironment from testicular macrophages further suppresses Leydig function
stem-cell-exhaustion	Stem Leydig cell progenitor pool declines with age (ECM stiffness → Piezo1/ROS → Gli1 degradation); reduced replenishment of the mature Leydig cell compartment; TCF21+ mesenchymal progenitor pool homeostasis disrupted with age

Therapeutic relevance

Leydig cell function is a target for andropause therapy and male reproductive aging research:

• Testosterone replacement therapy (TRT): The most commonly implemented treatment for clinical hypogonadism in older men. Restores androgen levels but suppresses endogenous LH and further atrophies residual Leydig cell steroidogenic capacity through feedback. Not addressed here in detail — see andropause.
• hCG/LH analogs: Stimulate residual Leydig cell function rather than bypassing it; preserve testicular volume and some spermatogenesis. Used in secondary hypogonadism; less studied in primary age-related decline.
• Mitochondrial fusion restoration: The TSPO/OPA1/M1 pathway in Garza 2022 provides proof-of-concept that targeting Leydig mitochondrial dynamics could restore testosterone output without exogenous hormone replacement. Preclinical only. long-term-unknown
• ECM stiffness modulation / stem Leydig cell expansion: Huang 2025 shows that SLC expansion in low-stiffness conditions in vitro can be used to prime cells before transplantation; reintroducing SLCs pre-treated with soft matrix conditions restored testosterone in an aged-mouse model. Extremely early stage. needs-human-replication long-term-unknown
• COX-2 inhibition: Reduced COX-2 → prostaglandin E2 tonic suppression of StAR expression is a proposed mechanism; NSAIDs might partially rescue StAR expression in aged Leydig cells. No human data; potential off-target testicular effects of chronic NSAID use would require characterization. no-mechanism needs-human-replication

Limitations and gaps

• Human mechanistic data are sparse: Most molecular detail on StAR decline, TSPO, and mitochondrial dynamics comes from rat (Brown Norway) models. Direct measurement of StAR and CYP11A1 protein levels in aging human Leydig cells is limited by tissue availability. needs-human-replication
• Leydig cell number in humans (contested): Stereological data (Petersen 2015) show no significant age-related decline in Leydig cell number in humans, while older histological estimates suggested a decline. Resolution requires larger stereological datasets across multiple populations. contradictory-evidence
• Stem Leydig cell contribution to age-related decline: The relative contribution of SLC progenitor failure vs. per-cell functional decline in existing mature Leydig cells to total testosterone reduction is not quantified in humans. needs-replication
• INSL3 clinical translation: Whether INSL3-guided clinical decisions (e.g., to differentiate primary vs. secondary hypogonadism, or to stratify aging men for intervention) improve outcomes over testosterone-based assessment alone has not been tested in randomized trials. needs-human-replication
• Paracrine inflammatory contribution: Testicular macrophage-derived cytokines modulate Leydig function, but the relative contribution of age-related testicular inflammation vs. cell-intrinsic defects to testosterone decline is not disaggregated in human studies. no-mechanism

See also

• testis — the tissue context (implicit stub)
• testosterone — the primary androgen produced by Leydig cells; implicit stub
• lh — the pituitary gonadotropin driving Leydig cell steroidogenesis via LHCGR; implicit stub
• andropause — the clinical phenotype of age-related testosterone decline; implicit stub
• granulosa-cells — the female steroidogenic analog (theca cell is the direct homolog; granulosa cells cooperate with theca in the two-cell model; granulosa page covers the female perspective)
• cyp19a1 — aromatase; expressed in testis at low level; converts testosterone → estradiol; primary expression in ovarian granulosa/theca
• mitochondrial-dysfunction — hallmark central to Leydig cell aging via TSPO/StAR axis
• altered-intercellular-communication — hallmark manifest in disrupted HPG axis and paracrine androgen signaling
• stem-cell-exhaustion — hallmark engaged by stem Leydig cell progenitor pool decline

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Footnotes

Footnotes

1. zirkin-2018-leydig-cells-review · doi:10.1093/biolre/ioy059 · Zirkin BR, Papadopoulos V · review · Biology of Reproduction 2018;99(1):101–111 · PMID:29566165 · comprehensive review of Leydig cell formation, function, and regulation; covers the LH → cAMP → StAR → steroidogenic cascade, age-related functional decline, and the stem Leydig cell concept; 640 citations (citation percentile 100) · archive status: bronze OA; pending download ↩

2. midzak-2009-leydig-aging-testosterone · doi:10.1016/j.mce.2008.07.016 · Midzak AS, Chen H, Papadopoulos V, Zirkin BR · review · Molecular and Cellular Endocrinology 2009;299(1):23–31 · PMID:18761053 · detailed mechanistic review of per-cell steroidogenic decline in aged Leydig cells; covers StAR, TSPO, mitochondria, LH responsiveness; 208 citations · archive status: closed access; not_oa; no-fulltext-access ↩ ↩² ↩³

3. ivell-2013-insl3-leydig-biomarker · doi:10.1095/biolreprod.113.108969 · Ivell R, Wade JD, Anand-Ivell R · review · Biology of Reproduction 2013;88(6):147 · PMID:23595905 · establishes INSL3 as a constitutively secreted, Leydig-cell-specific biomarker of functional Leydig mass; contrasts with pulsatile testosterone; reviews physiological roles and clinical utility; 188 citations · archive status: green OA; pending download ↩ ↩²

4. huang-2025-ecm-stiffness-stem-leydig · doi:10.1016/j.celrep.2025.116207 · Huang J, Sun L, Yin Y, Yang H, Zeng K, Huang P, Liang X, Wu J, Xiang AP, Wang J · in-vivo (mouse) + in-vitro (hydrogel models) · Cell Reports 2025;44(9):116207 · PMID:40884795 · high ECM stiffness in aging testis triggers Piezo1-mediated Ca²⁺ influx → mitochondrial dysfunction + excessive ROS → Gli1 degradation via ubiquitin-proteasome pathway → SLC proliferation and differentiation impaired → pool depletion + testosterone decline; pretreatment of SLCs in low-stiffness matrix in vitro potentiates expansion and testosterone restoration in aged mouse model · model: aged mouse + hydrogel SLC culture · archive status: gold OA; PDF not downloaded (Cell.com redirect loop at time of verification) needs-human-replication ↩ ↩²

5. garza-2022-mitochondrial-dynamics-leydig · doi:10.1096/fj.202201026R · Garza S, Chen L, Galano M, Cheung G, Sottas C, Li L, Li Y, Zirkin BR, Papadopoulos V · in-vitro + in-vivo (rat) · The FASEB Journal 2022;36(12):e22637 · PMID:36349989 · TSPO depletion impairs mitochondrial membrane dynamics and cholesterol trafficking in Leydig cells; restoring mitochondrial fusion via M1 or OPA1 overexpression rescues steroidogenesis; M1 treatment of aged rat Leydig cells increased androgen production · archive status: hybrid OA; pending download ↩ ↩²

6. shen-2021-tcf21-leydig-progenitor · doi:10.1038/s41467-021-24130-8 · Shen YC, Shami AN, Moritz L et al. · in-vivo lineage tracing (mouse) + in-vitro · Nature Communications 2021;12:3875 · PMID:34162856 · TCF21+ mesenchymal cells are bipotent somatic progenitors in fetal and adult testis; can generate Leydig and peritubular myoid cells; retain residual progenitor activity as reserve somatic progenitors following injury; provide the cellular basis for adult Leydig cell regeneration · archive status: gold OA; pending download ↩

7. chen-1994-leydig-aging-brown-norway · doi:10.1002/j.1939-4640.1994.tb00498.x · Chen H, Hardy MP, Huhtaniemi I, Zirkin BR · in-vivo (rat) · Journal of Andrology 1994;15(5):381–387 · PMID:7721657 · individual Leydig cells from aged Brown Norway rats produced significantly less testosterone under maximal LH or dbcAMP stimulation; average Leydig cell volume decreased with age; age-associated decline could not be explained by changes in Leydig cell number, serum LH levels, Leydig cell responsiveness to LH, or testicular germ cell content; established per-cell steroidogenic defect downstream of LH receptor–cAMP as primary mechanism · archive status: bronze OA; PDF not downloaded (Wiley paywall) ↩

8. petersen-2015-leydig-sertoli-stereology · doi:10.1111/joa.12261 · Petersen PM, Seierøe K, Pakkenberg B · observational (post-mortem stereology) · Journal of Anatomy 2015;226(2):175–179 · PMID:25545958 · PMC:PMC4304573 · n=26 post-mortem male subjects aged 16–80 yr; optical fractionator (sampling optimized for human testis); mean unilateral Sertoli count 407 × 10⁶ (range 86–665 × 10⁶, CV=0.33); significant decline in Sertoli cell number with age; mean unilateral Leydig count 99 × 10⁶ (range 47–245 × 10⁶, CV=0.48); no significant decline in Leydig cell number with age; contradicts older volume-density-based studies reporting Leydig cell loss · archive status: bronze OA; PDF not downloaded (Wiley paywall); abstract + metadata verified via PMC record ↩

9. luo-2001-leydig-star-p450scc-aging · doi:10.1002/j.1939-4640.2001.tb02165.x · Luo L, Chen H, Zirkin BR · in-vivo (rat) · Journal of Andrology 2001;22(1):149–156 · PMID:11191081 · StAR protein (steady-state Western blot) reduced 47% (middle-aged) and 74% (old) vs. young; CYP11A1/P450scc protein reduced 38% and 54%; StAR mRNA reduced 26% and 52%; P450scc mRNA reduced 29% and 50%; LH-stimulated testosterone production by aged cells was significantly lower than young despite fold-changes being similar; model: Brown Norway rat 4 mo (young), 14 mo (middle-aged), 24 mo (old) · archive status: bronze OA; PDF not downloaded (Wiley paywall) ↩

10. wang-2005-testosterone-biosynthesis-aging-cox2 · doi:10.1016/j.mce.2005.04.009 · Wang X, Stocco DM · review · Molecular and Cellular Endocrinology 2005;238(1-2):1–7 · PMID:15939533 · proposes age-related COX-2 elevation tonically suppresses StAR expression via prostaglandin E2; provides mechanistic link between aging inflammation and impaired cholesterol transport to mitochondria · archive status: closed; not_oa; no-fulltext-access ↩

11. anandivell-2022-insl3-aging-emas · doi:10.1111/andr.13220 · Anand-Ivell R, Heng K, Severn K et al. · observational cohort · Andrology 2022;10(7):1328–1338 · PMID:35770372 · EMAS cohort community-dwelling men; INSL3 declines more steeply with age than testosterone across age groups; age, BMI, and comorbidities independently predict INSL3; validates INSL3 as aging biomarker · archive status: hybrid OA; pending download ↩

12. ivell-2022-insl3-morbidity-emas · doi:10.3389/fendo.2022.1016107 · Ivell R, Heng K, Severn K et al. · observational cohort · Frontiers in Endocrinology 2022;13:1016107 · PMID:36425465 · lower INSL3 associated with greater age-related morbidity burden independent of testosterone; supports clinical utility of INSL3 as functional Leydig mass proxy · archive status: gold OA; pending download ↩

13. anandivell-2024-insl3-lci-threshold · doi:10.1080/13685538.2024.2346322 · Anand-Ivell R, Heng K, Antonio L et al. · review (re-analysis of FAMAS and EMAS cohort data) · The Aging Male 2024;27(1):2346322 · PMID:38676285 · proposes ≤0.4 ng/mL serum INSL3 as threshold for Leydig Cell Insufficiency (LCI) and primary hypogonadism — this represents the lower 95% CI (2 SD) for men aged 60–65 years; eugonadal reference range (serum, men >35 yr) established as 0.4–2.3 ng/mL; distinguishes primary from secondary hypogonadism by INSL3/LH ratio; INSL3 declines ~15% per decade from age 35 · archive status: gold OA; downloaded 2026-06-03 ↩