Testicular Function and Androgen Physiology

Testicular function couples two compartments under gonadotropin control: the Leydig cells, which under luteinizing hormone (LH) synthesize testosterone, and the seminiferous tubules, where Sertoli cells support spermatogenesis under follicle-stimulating hormone (FSH) and the high local testosterone concentrations the Leydig cells provide. Androgen physiology describes the production, transport, and action of testosterone and its more potent metabolite dihydrotestosterone.

Definition

Testicular function is the gonadotropin-regulated activity of the testis in producing androgens (Leydig cells, under LH) and sperm (seminiferous epithelium, under FSH and intratesticular testosterone), with androgens acting through the androgen receptor and feeding back on the reproductive axis.

Scope

The topic covers Leydig-cell steroidogenesis and LH control, Sertoli-cell support of germ cells, the complementary roles of testosterone and FSH in spermatogenesis, androgen feedback on the hypothalamic-pituitary axis, and androgen action through the androgen receptor. It is a physiology reference topic and does not provide clinical guidance.

Core questions

• How do LH and FSH divide control over the testis?
• What are the distinct contributions of testosterone and FSH to spermatogenesis?
• How does intratesticular testosterone support the seminiferous epithelium?
• How do androgens act on target tissues and feed back on the axis?

Key concepts

• Leydig cells and LH-driven steroidogenesis
• Sertoli cells and the blood-testis barrier
• Intratesticular testosterone
• Follicle-stimulating hormone action
• Testosterone and dihydrotestosterone
• Androgen receptor
• Inhibin and steroid feedback

Key theories

Complementary testosterone and FSH control of spermatogenesis

Testosterone and FSH have independent and partly complementary roles in driving and maintaining spermatogenesis through their actions on Sertoli cells, so that qualitatively complete spermatogenesis depends on adequate intratesticular androgen with FSH enhancing sperm output.

Mechanisms

LH acts on Leydig cells to stimulate testosterone synthesis, generating intratesticular androgen concentrations far above those in blood. FSH and this local testosterone act on Sertoli cells, which form the blood-testis barrier and provide the structural and metabolic support that germ cells require to complete spermatogenesis. Testosterone and FSH have distinct, complementary actions: intratesticular testosterone is essential for completing meiosis and spermiogenesis, while FSH supports Sertoli-cell number and function and enhances sperm output (McLachlan et al., 1996; Walker, 2011; Ramaswamy & Weinbauer, 2014). Circulating testosterone, after conversion to dihydrotestosterone in some tissues, signals through the androgen receptor, a nuclear receptor whose defects cause androgen-insensitivity phenotypes (Quigley et al., 1995). Testosterone and Sertoli-cell inhibin feed back on the hypothalamus and pituitary to restrain LH and FSH.

Clinical relevance

These mechanisms provide the physiological background for understanding male reproductive function and the consequences of disrupted androgen production or action. The entry is educational reference on how the testis and androgens work; it is non-prescriptive and is not a basis for diagnosis or treatment.

History

Classical endocrinology established the two-compartment model of the testis, with LH controlling Leydig-cell androgen production and FSH acting on the seminiferous epithelium. Later work using selective hormone withdrawal and replacement clarified the independent roles of testosterone and FSH in spermatogenesis (McLachlan et al., 1996), and molecular studies defined androgen-receptor signalling and the spectrum of androgen-receptor defects (Quigley et al., 1995).

Debates

How essential is FSH versus testosterone for spermatogenesis?

Whether FSH is strictly required or mainly quantitatively enhances sperm output, relative to the essential role of intratesticular testosterone, has been debated; the prevailing view assigns testosterone an indispensable role with FSH optimizing Sertoli-cell capacity and sperm number.

Key figures

• Robert McLachlan
• William H. Walker
• David de Kretser
• Frank French

Seminal works

• mclachlan-1996
• quigley-1995

Frequently asked questions

Why is intratesticular testosterone so much higher than blood testosterone?

LH-stimulated Leydig cells produce testosterone within the testis, and the seminiferous epithelium requires these high local concentrations to support spermatogenesis, so intratesticular levels greatly exceed circulating levels.

What are the separate roles of LH and FSH in the testis?

LH stimulates Leydig-cell testosterone production, while FSH acts on Sertoli cells to support the germinal epithelium; together with intratesticular testosterone they sustain spermatogenesis.