Correct Answer: A. Leydig cells
Testosterone production is the exclusive function of Leydig cells (interstitial cells of Leydig), which are located in the interstitium between seminiferous tubules in the testis. These cells are stimulated by luteinizing hormone (LH) from the anterior pituitary, which binds to LH receptors on the Leydig cell membrane and triggers a cAMP-mediated cascade leading to synthesis and secretion of testosterone. The enzyme 17α-hydroxylase and 17,20-lyase are critical for the final steps of testosterone synthesis from pregnenolone. Leydig cells contain abundant smooth endoplasmic reticulum and mitochondria, reflecting their steroid-synthesizing capacity. Testosterone is then transported via blood to target tissues (prostate, seminal vesicles, external genitalia, bone, muscle, brain) where it exerts androgenic and anabolic effects. In Indian clinical practice, assessment of Leydig cell function is critical in evaluating male hypogonadism, infertility, and delayed puberty—conditions increasingly recognized in Indian adolescent and adult populations. The testosterone level (normal range 300–1000 ng/dL) is measured to diagnose primary hypogonadism (elevated LH, low testosterone) versus secondary hypogonadism (low LH, low testosterone).
Why the other options are wrong
B. Epididymis — The epididymis is a storage and maturation organ for spermatozoa, not an endocrine structure. It secretes glycoproteins and enzymes that aid sperm motility and capacitation, but plays no role in testosterone synthesis. This is a common trap—students confuse reproductive anatomy with endocrine function. C. Sertoli cells — Sertoli cells (sustentacular cells) line the seminiferous tubules and support spermatogenesis through phagocytosis of residual bodies, secretion of anti-Müllerian hormone (AMH), and inhibin production. They respond to FSH but do NOT synthesize testosterone—they are somatic support cells, not steroid-producing cells. D. Seminiferous tubules — Seminiferous tubules are the site of spermatogenesis, containing germ cells and Sertoli cells. While testosterone diffuses into the tubular lumen to support spermatogenesis, the tubules themselves do not produce testosterone—production occurs in the interstitial Leydig cells surrounding them.
High-Yield Facts
- Leydig cells are stimulated by LH (not FSH) and are the sole source of circulating testosterone in males.
- Testosterone synthesis requires 17α-hydroxylase and 17,20-lyase enzymes; deficiency of these causes 17-hydroxylase deficiency syndrome (hypertension, hypokalemia, female phenotype in 46,XY individuals).
- Sertoli cells respond to FSH and produce inhibin and AMH, not testosterone.
- Normal testosterone in adult males is 300–1000 ng/dL; levels <300 ng/dL define hypogonadism in Indian guidelines.
- Primary hypogonadism (testicular failure) shows high LH + low testosterone; secondary hypogonadism (pituitary/hypothalamic) shows low LH + low testosterone.
Mnemonics
LH → Leydig → Testosterone LH stimulates Leydig cells to produce testosterone. FSH stimulates Sertoli cells to produce inhibin and support spermatogenesis. Remember: LH → Leydig, FSH → Follicle (Sertoli). Interstitial Location = Endocrine Function Leydig cells sit in the interstitium (between tubules) → they are endocrine. Sertoli cells sit inside seminiferous tubules → they are somatic support. Location predicts function.
NBE Trap
NBE pairs "seminiferous tubules" with testosterone to trap students who know testosterone supports spermatogenesis but confuse the site of action with the site of synthesis. The tubules are the target, not the source.
Clinical Pearl
In Indian clinical practice, a young man presenting with delayed puberty or infertility should have serum testosterone and LH measured. If both are low, suspect secondary hypogonadism (pituitary adenoma, hemochromatosis—common in Indian males); if testosterone is low but LH is high, suspect primary testicular failure (Klinefelter syndrome, prior chemotherapy). This distinction guides treatment: testosterone replacement for primary, or GnRH/hCG for secondary hypogonadism.
_Reference: Guyton & Hall Physiology Ch. 80 (Testicular Function); KD Tripathi Pharmacology Ch. 61 (Androgens)_