Leydig cells are the testosterone-producing factories of the male body, tucked between the sperm-generating tubules of the testis. They respond to signals from the brain’s pituitary gland and convert cholesterol into testosterone through a multi-step biochemical process. But these cells do more than supply a single hormone, and their story involves two distinct cell populations, a surprising dependence on neighboring immune cells, and a vulnerability to aging, environmental chemicals, and genetic mutations that can quietly reshape male development.
Where Leydig Cells Sit in the Testis
The bulk of testicular tissue is devoted to seminiferous tubules, the coiled structures where sperm develop. Leydig cells occupy the narrow spaces between those tubules, a zone called the interstitium. They tend to cluster together, and each cluster is threaded with tiny blood capillaries that run right through its center, placing every Leydig cell within arm’s reach of the bloodstream.1Scientific Reports. Low testosterone in ApoE/LDL receptor double-knockout mice is associated with rarefied testicular capillaries together with fewer and smaller Leydig cells That arrangement is not decorative. Testosterone has to travel through the blood to reach distant targets like muscle, bone, and the brain. Without intimate capillary access, Leydig cells could not function as an endocrine gland.
Surrounding these clusters, the interstitial space also contains connective tissue, immune cells, and a recently appreciated population of elongated cells with thin, spindly projections that wrap around both the Leydig cell clusters and the nearby blood vessels.2Scientific Reports. Reappraising the microscopic anatomy of human testis: identification of telocyte networks in the peritubular and intertubular stromal space The picture that emerges is not of isolated hormone-producing cells but of a tightly organized neighborhood where structure and function go hand in hand.
How Testosterone Gets Made
Testosterone production starts with a signal from the pituitary gland. Luteinizing hormone, or LH, travels through the blood and docks on receptors covering the surface of Leydig cells. That binding event kicks off an internal signaling cascade that ultimately switches on the genes and enzymes needed to manufacture testosterone.3PubMed. Luteinizing hormone regulates testosterone biosynthesis via the cAMP/PKA-CREB pathway in Leydig cells of Tibetan sheep
The raw material is cholesterol. For the process to begin, cholesterol has to be shuttled from the outer membrane of a mitochondrion to its inner membrane, a step carried out by a protein called StAR. This transfer is the bottleneck of the whole operation: if StAR is missing or impaired, very little testosterone gets made regardless of how much cholesterol is available.4PubMed Central. Role of the steroidogenic acute regulatory protein in health and disease Once inside the mitochondrion, an enzyme clips the side chain off cholesterol to produce a precursor called pregnenolone. From there, a series of additional enzymes convert pregnenolone step by step into testosterone. Each enzyme in the chain is a potential weak link: studies using environmental toxicants, for example, have shown that disrupting any one of these enzymes can impair testosterone output at that specific step.5Endocrinology. Di(n-Butyl) Phthalate Impairs Cholesterol Transport and Steroidogenesis in the Fetal Rat Testis through a Rapid and Reversible Mechanism
This assembly-line design means testosterone synthesis is not a single on-off switch. It is a chain of events that can be dialed up by LH stimulation, dialed down by feedback from circulating testosterone, or thrown off by anything that damages a single link in the chain.
Two Generations of Leydig Cells
One of the more surprising details about Leydig cells is that there are actually two distinct populations that appear at different points in life. Fetal Leydig cells emerge during early development in the womb. Their testosterone is critical for masculinizing the reproductive tract, driving the formation of internal male structures and the development of external genitalia. Adult Leydig cells, by contrast, arise around puberty and are the population responsible for testosterone production throughout the rest of life.
Although fetal and adult Leydig cells do the same basic job, they are genuinely different cell types, with distinct gene expression profiles and regulatory quirks. Researchers cannot simply apply what they know about one population to the other.6PubMed Central. Fetal Leydig cells: What we know and what we don’t Still, the two populations are not entirely independent. Evidence suggests they share a common pool of precursor cells, and disruptions to fetal Leydig cell development can ripple forward, causing adult Leydig cell dysfunction years later.7PubMed Central. Development of fetal and adult Leydig cells That link has implications for environmental health, because exposures during pregnancy do not just affect the fetus in the short term; they can permanently alter the trajectory of the adult hormone system.
More Than Testosterone
Testosterone gets all the headlines, but Leydig cells produce at least one other hormone that plays a distinct and critical role during fetal life. Insulin-like peptide 3 (INSL3) is a small hormone produced exclusively by testicular Leydig cells. In the developing fetus, INSL3 causes the gubernaculum, a ligament connecting the testis to the lower abdomen, to thicken. This thickening anchors the testes near the inguinal region while the rest of the abdominal organs grow upward and away, setting the stage for the testes to eventually descend into the scrotum.8PubMed Central. Expression and Role of INSL3 in the Fetal Testis
Without adequate INSL3, testicular descent can fail, a condition known as cryptorchidism. In adult life, INSL3 continues to circulate and is sometimes used as a biomarker of Leydig cell health, since its levels reflect Leydig cell function independently of the feedback loops that regulate testosterone.
Teamwork with Other Testicular Cells
Leydig cells do not operate in isolation. They maintain complex signaling relationships with at least three other cell types in the testis: macrophages, peritubular cells, and Sertoli cells. Each of these partnerships serves a different purpose.9Springer Nature Switzerland. Leydig Cell Interaction with Macrophages, Peritubular Cells, and Sertoli Cells
The macrophage connection is especially interesting. Testicular macrophages are immune cells that live right next to Leydig cell clusters. A specific subtype of these macrophages actively promotes Leydig cell development, proliferation, and testosterone production.10Biomedical Journal. Macrophages in the male genital tract This is unusual because macrophages are typically thought of as defenders against infection, not as hormonal regulators. In the testis, though, they seem to play a nurturing role, helping to maintain the Leydig cell population and supporting its endocrine output. Sertoli cells, which form the structural scaffolding of the seminiferous tubules and nurse developing sperm, are also essential for Leydig cell maintenance throughout life. And peritubular cells, which wrap around the outside of the tubules, influence Leydig cell function and contribute to the regulation of blood flow in the interstitial space.
These partnerships mean that damage to one cell type can cascade through the system. An insult that primarily targets Sertoli cells or macrophages, for instance, may indirectly impair testosterone production by undermining the support network that Leydig cells depend on.
Testosterone’s Role in Sperm Production
Beyond its well-known effects on muscle, bone density, libido, and mood, testosterone produced by Leydig cells is essential for sperm production happening right next door in the seminiferous tubules.11PubMed Central. The Role of Testosterone in Spermatogenesis: Lessons From Proteome Profiling of Human Spermatozoa in Testosterone Deficiency Sertoli cells within the tubules have androgen receptors that respond to locally produced testosterone, and this local signaling is what drives the maturation of sperm cells through their elaborate developmental stages. The testosterone concentrations required inside the testis are far higher than what circulates in the general bloodstream, which is why the proximity of Leydig cells to the tubules matters so much. A man can have a normal blood testosterone level and still have impaired spermatogenesis if the local testicular environment is disrupted.
What Happens with Aging
Testosterone levels gradually decline in men starting around the fourth decade of life, and Leydig cells are at the center of that decline. Research in aging animal models has shown that old Leydig cells produce significantly less testosterone than young ones. One reason appears to be oxidative stress within the cell’s mitochondria. In aged Leydig cells, mitochondria generate substantially more reactive oxygen species than in young cells, while the total volume of mitochondria in each cell actually shrinks.12PubMed. Age-related increase in mitochondrial superoxide generation in the testosterone-producing cells of Brown Norway rat testes: relationship to reduced steroidogenic function? Since mitochondria are where the very first step of testosterone synthesis takes place, this combination of more damage and less capacity is a plausible mechanism for the age-related drop in production.
An intriguing experiment tested whether the act of producing testosterone itself is what wears Leydig cells down over time. Researchers suppressed endogenous Leydig cell activity in young rats by giving them exogenous testosterone, essentially putting the cells into a resting state.13PubMed Central. Long-term suppression of Leydig cell steroidogenesis prevents Leydig cell aging The idea was to see whether resting cells aged more slowly than working ones. The result suggested that chronic steroidogenic activity does contribute to the cumulative damage that makes older Leydig cells less productive, somewhat like an engine that wears out faster the more hours it runs.
Environmental Chemicals That Target Leydig Cells
Leydig cells are sensitive to a range of environmental pollutants. Phthalates, a family of chemicals used as plasticizers in consumer products, have been shown in animal studies to impair cholesterol transport and reduce the expression of key steroidogenic enzymes in fetal Leydig cells, leading to decreased testosterone output.5Endocrinology. Di(n-Butyl) Phthalate Impairs Cholesterol Transport and Steroidogenesis in the Fetal Rat Testis through a Rapid and Reversible Mechanism Bisphenol A (BPA), another widely used industrial chemical, has been found to alter the proliferation and differentiation of fetal Leydig cells and to affect serum testosterone levels after birth in animal models.14PubMed. Bisphenol A alters differentiation of Leydig cells in the rabbit fetal testis
More recently, researchers have turned their attention to per- and polyfluoroalkyl substances (PFAS), sometimes called “forever chemicals.” A PFOS alternative compound was shown to impair testosterone synthesis in Leydig cells by disrupting the cellular recycling process known as lipophagy, leading to an abnormal accumulation of lipid droplets inside the cells.15Environmental Science & Technology. 6:2 Chlorinated Polyfluoroalkyl Ether Sulfonate (6:2 Cl-PFAES), a PFOS Alternative, Impairs Testosterone Synthesis in Leydig Cells by Targeting Lipophagy via the SIRT1–FOXO1–RAB7 Axis: An In Vitro and In Vivo Study Since cholesterol stored in lipid droplets is the raw material for testosterone, anything that jams the recycling machinery can effectively starve the steroidogenic pathway even when plenty of cholesterol is available.
The vulnerability of fetal Leydig cells to these exposures is especially concerning given the evidence that fetal-stage disruptions can propagate into adult Leydig cell dysfunction. Most of this work comes from animal models, and translating precise dose-response relationships to humans is difficult. But the consistency of findings across different chemical classes and species has made Leydig cell disruption a central theme in reproductive toxicology.
Genetic Disorders of Leydig Cells
Leydig cell hypoplasia is a rare condition caused by mutations in the gene encoding the LH receptor (LHCGR). Because LH is the primary signal telling Leydig cells to grow and produce testosterone, loss-of-function mutations in this receptor leave the cells unable to respond. The clinical result depends on how severe the mutation is.
In type 1 Leydig cell hypoplasia, the receptor is completely nonfunctional. Individuals with a 46,XY chromosome complement may be born with female or ambiguous external genitalia because fetal testosterone production was effectively absent. Diagnosis can sometimes happen in early childhood, particularly when an inguinal swelling raises suspicion.16PubMed Central. Leydig cell hypoplasia type 1 diagnosed in early childhood with inactivating mutation in LHCGR gene In type 2, the receptor retains partial function. Affected individuals typically have a male phenotype but with micropenis, small testes, and delayed or absent puberty. Different mutations in the LHCGR gene produce these varying degrees of severity, including compound heterozygous mutations where each copy of the gene carries a different defect.17PubMed. Aberrant transcription of the LHCGR gene caused by a mutation in exon 6A leads to Leydig cell hypoplasia type II
Leydig Cell Tumors
Leydig cell tumors are the most common type of testicular sex cord-stromal tumor, though they are still uncommon in the big picture. In a long-term clinical series of 24 patients, the average age at diagnosis was about 38, and the tumors were found in various ways: some were felt as a palpable lump, some were discovered incidentally on imaging, and a few were investigated because the patient developed breast enlargement (gynecomastia), a classic sign that the tumor was producing excess estrogen or disrupting the testosterone-to-estrogen balance.18PubMed. Long-term followup and clinical characteristics of testicular Leydig cell tumor: experience with 24 cases
The reassuring finding from that series was that all tumors were benign, and after an average follow-up of nearly ten years, there was no recurrence or progression. Malignant Leydig cell tumors do exist but account for a small minority of cases. Because frozen section examination during surgery correctly identified the tumor in most cases, the clinical trend has shifted toward testis-sparing surgery in selected patients, removing just the tumor while preserving the remaining testicular tissue and its hormone-producing capacity.
Stem Leydig Cells and Regenerative Medicine
Researchers have identified a population of stem Leydig cells in both rodent and human testes. These multipotent precursors can differentiate into mature Leydig cells, and interestingly, they also have the capacity to become fat cells, bone cells, and cartilage cells under experimental conditions.19PubMed. Stem Leydig cells: Current research and future prospects of regenerative medicine of male reproductive health This versatility is part of what makes them a target for regenerative medicine. The idea is that if you could transplant or activate stem Leydig cells in men with hypogonadism (low testosterone), you might be able to restore endogenous testosterone production rather than relying on lifelong exogenous hormone replacement.20PubMed Central. Leydig cell stem cells: Identification, proliferation and differentiation
Exogenous testosterone therapy, while effective at raising blood testosterone levels, suppresses the brain’s signals to the testes, which causes Leydig cells to go dormant and sperm production to plummet. For men who want both normal testosterone and fertility, that trade-off is a real problem. A stem cell approach that rebuilt the Leydig cell population from within would theoretically sidestep this issue by restoring the natural feedback loop. The research is still at an early stage, mostly in animal models, but the isolation of these stem cells from human tissue is an encouraging step.
Seasonal Leydig Cells in Other Species
Leydig cell biology is not uniform across the animal kingdom. In seasonal breeders, these cells undergo dramatic cyclical changes. The European hedgehog offers a vivid example. During late winter hibernation, Leydig cells begin to enlarge and testosterone levels climb, peaking a full month before the seminiferous tubules reach their maximum development. Throughout spring and summer, the hedgehog’s Leydig cells are large, numerous, and highly active, supporting peak testosterone levels and full spermatogenesis. Then in late summer, Leydig cells regress, testosterone drops, and the entire reproductive apparatus winds down for the dormant season.21General and Comparative Endocrinology. An endocrine and histophysiological study of the testicular annual cycle in the hedgehog (Erinaceus europaeus L.)
This cycle illustrates a point that is easy to miss when studying human Leydig cells: in many species, these cells are not a fixed population running at a steady state. They can proliferate, hypertrophy, and atrophy on a seasonal schedule dictated by photoperiod and other environmental cues. Understanding how seasonal breeders activate and deactivate their Leydig cells without permanent loss of function may eventually inform efforts to rejuvenate aging or damaged Leydig cells in humans.
Cellular Housekeeping and Testosterone Output
Recent research has highlighted the role of internal cellular recycling processes in sustaining testosterone production. Leydig cells, like most cells, use a process called autophagy to break down and recycle damaged organelles and stored lipids. A specialized form of this recycling, lipophagy, breaks down lipid droplets to liberate the cholesterol needed for steroidogenesis. New data suggest that the interactions between mitochondria, peroxisomes, and specialized membrane contact sites within the cell are not just correlated with testosterone production but are genuinely required for it.22Current Opinion in Endocrine and Metabolic Research. Regulation of Leydig cell steroidogenesis: intriguing network of signaling pathways and mitochondrial signalosome
When these recycling pathways are disrupted, whether by environmental chemicals, aging, or genetic factors, lipid droplets pile up inside the cell and cholesterol cannot reach the mitochondria where steroidogenesis begins. The PFAS-alternative compound described earlier provides a concrete example: by suppressing key regulators of the lipophagy pathway, it caused lipid droplets to accumulate and testosterone synthesis to fall.15Environmental Science & Technology. 6:2 Chlorinated Polyfluoroalkyl Ether Sulfonate (6:2 Cl-PFAES), a PFOS Alternative, Impairs Testosterone Synthesis in Leydig Cells by Targeting Lipophagy via the SIRT1–FOXO1–RAB7 Axis: An In Vitro and In Vivo Study The emerging view is that healthy testosterone output depends not just on having the right enzymes but on keeping the cell’s internal logistics running smoothly, a dimension of Leydig cell biology that was largely invisible until the last decade or so.
A Long Road to Recognition
The cells were first described in 1850 by the German anatomist Franz Leydig, but their function remained a mystery for over a century. In 1903, two French researchers provided the first substantial evidence that these interstitial cells acted as an endocrine gland controlling male sexual characteristics, but their case was circumstantial. Throughout the early twentieth century, opinion swung back and forth. By the late 1920s, many reproductive biologists suspected that the seminiferous tubules themselves, not the interstitial cells, were the true source of male hormone.23PubMed Central. A History of Leydig Cell Research Testosterone was identified as the male hormone in the 1930s, and Leydig cells gradually regained favor as the likely source. But direct proof did not arrive until 1958, from histochemistry, and 1965, from biochemistry. For a cell type so central to male development and physiology, it took a remarkably long time for the scientific community to agree on what it actually does.