Porcine Leydig cells: central regulators of boar reproductive development and fertility

Leydig cells within the interstitium of the testis produce the sex steroids (e.g., testosterone and 17β-estradiol), which are essential to male fertility. Unlike most species, pigs have three distinct phases of Leydig cell development and function. The first transient phase occurs during early fetal life, leading to the production of testosterone, which is critical for sex differentiation and early testicular development. The second transient phase in Leydig cell development occurs during the perinatal/neonatal period; during this developmental window, gonadal steroids further masculinize the brain and regulate testicular development. The final phase of Leydig cell development begins prior to puberty, between 100 and 130 d of age. The adult Leydig cell population remains active throughout adulthood. Steroidogenesis within adult Leydig cells is classically stimulated by luteinizing hormone synthesized and released from the anterior pituitary gland. Porcine Leydig cells produce androgens, as well as relatively high concentrations of estrogens, via the Δ5 pathway. Steroidogenesis is reliant on both acute (mobilization of cholesterol and activity of steroid acute regulatory protein [StAR]) and chronic (upregulation of gene expression, organelle expansion, and cell proliferation) responses. Many proteins and enzymes are involved in driving steroidogenesis within Leydig cells, including StAR, CYP11A1, CYP17A1, 3β-HSD, 17β-HSD, and CYP19A3. The production of gonadal steroids by adult Leydig cells is essential for numerous reproductive processes, including spermatogenesis, sperm maturation within the epididymis, accessory sex gland function, and libido. Thus, Leydig cells are critical regulators of boar fertility.