The Fate of Leydig Cells in Men with Spermatogenic Failure

Abstract

The steroidogenic cells in the testicle, Leydig cells, located in the interstitial compartment, play a vital role in male reproductive tract development, maintenance of proper spermatogenesis, and overall male reproductive function. Therefore, their dysfunction can lead to all sorts of testicular pathologies. Spermatogenesis failure, manifested as azoospermia, is often associated with defective Leydig cell activity. Spermatogenic failure is the most severe form of male infertility, caused by disorders of the testicular parenchyma or testicular hormone imbalance. This review covers current progress in knowledge on Leydig cells origin, structure, and function, and focuses on recent advances in understanding how Leydig cells contribute to the impairment of spermatogenesis.

Keywords: Leydig cells; estradiol; male infertility; non-obstructive azoospermia; spermatogenic failure; testicular dysgenesis syndrome; testosterone.

Figure 1

(Upper panel): Proposed model of Leydig cell lineage development. In the fetal testis, there is a shared Leydig cell progenitor pool that gives birth to both fetal Leydig cells (FLCs) and adult Leydig cells (ALCs). FLCs form in the fetal testis’ interstitial area. In the neonatal testis progenitor cells develop into stem Leydig cells (SLCs), which is the initial step in ALC subgroup differentiation. The dotted arrows show the hypothetical origin of neonatal Leydig cells (NLCs). NLCs are supposedly derived from either non-degraded FLCs or newly formed SLCs. NLCs slowly regress after the first postnatal year. During puberty, SLCs develop into mature cells via stages of newly progenitor Leydig cells (PLCs), immature Leydig cells (ILCs), and ALCs. A fraction of FLCs remains in the adult testis and accounts for approximately 10% of the total Leydig cell pool. However, it is still unknown how each progenitor is destined to become FLCs or ALCs. (Lower panel): Serum levels of luteinizing hormone (LH) secreted by pituitary and androgens (A) and insulin-like factor 3 (INSL3) produced by Leydig cells at different stages of their development. During the prenatal period fetal LH surges at mid-gestation, then declines and is low or undetectable in cord blood. The pattern of androgens and INSL3 concentrations follow that of LH. At birth, LH, androgens, and INSL3 levels are low and increase during the first weeks and months (minipuberty) to reach peak levels during the third month of life and then gradually decline and remain low at childhood. At puberty, LH, androgens, and INSL3 increase to reach the levels characteristic for adulthood. The main role of androgens and INSL3 in prenatal/neonatal period is induction of internal and external male genitalia differentiation and development. At puberty and then in adulthood, the main androgen action is associated with the development and maintenance of secondary sex characteristics and spermatogenesis, while INSL3 exerts a role in bone metabolism.

Figure 2

Schematic diagram of regulatory axis of testicular steroidogenesis. Sex steroids (predominantly testosterone) are produced upon central gonadotropin-releasing hormone (GnRH)—luteinizing hormone (LH) stimulation. LH binds to LH receptors on Leydig cells in the testicle and activates the pathway for the synthesis of steroid hormones from cholesterol. Different pathways variants are available after conversion to pregnenolone (Δ4 and Δ5). Normal, adult testicular steroidogenesis in men follows steroidal pathway Δ5, with a little amount of testosterone generated via the Δ4 pathway. Small amount of testosterone and androstendione are converted in the Leydig cells to estrogens by enzyme aromatase (CYP19A1) and to dihydrotestosterone (DHT) by 5α-reductase. Testosterone and estradiol act locally in testicle to regulate its function (e.g., spermatogenesis) or are released to blood circulation. Circulating sex steroids form a negative feedback loop to inhibit the secretion of GnRH and LH.

Figure 3

Microphotographs of testicular histology: (a) Section of testicular biopsy from men with obstructive azoospermia; normal human testis—well-developed seminiferous tubules with a clear lumen, lined with seminiferous epithelium presenting complete spermatogenesis (qualitatively and quantitatively); a normal number of Leydig cells (LCs) is observed in the intertubular space; LCs are visible as solitary cells or are grouped in small clusters (2–4 cells) (blue arrow); (b) Section of testicular biopsy from men with non-obstructive azoospermia presenting hypospermatogenesis—in seminiferous tubules a general decrease in germ cell components is observed; a normal number of LCs is observed in the intertubular space (blue arrow); (c) Section of testicular biopsy from men with non-obstructive azoospermia presenting maturation arrest at spermatocyte stage—in seminiferous tubule germ cell development stops during meiosis, a normal number of LCs is observed in the intertubular space (blue arrow); (d) Section of testicular biopsy from men with non-obstructive azoospermia presenting Sertoli cell-only syndrome—significantly decreased tubule diameter, increased thickness of basement membrane (green arrow); seminiferous tubules are lined with mature SCs, no germ cells are present; in the intertubular space, an aggregation of LCs is present to form larger clusters (>5 cells) (C). (e) Section of testicular biopsy from men with non-obstructive azoospermia presenting mixed atrophy–seminiferous tubules presenting different histological pattern i.e., Sertoli cell-only pattern and hypospermatogenesis; in the intertubular space, an aggregation of LCs can be seen to form larger clusters (>5 cells); note the thickened basement membrane in tubules with Sertoli cell-only pattern (green arrow) in (d,e). Hematoxylin and eosin staining; Magnification—×200; scale bar, 50 µm.

Figure 4

Immunohistochemical staining against insulin-like factor 3 in the mature Leydig cells in testicular biopsies from azoospermic men with different histological patterns. Insulin-like factor 3 expression at protein level is visible as a brown color in Leydig cell cytoplasm (used chromogen: diaminobenzidine—DAB); (a) complete spermatogenesis with normal Leydig cells number; (b) Sertoli cell only syndrome with exceedingly large Leydig cell hyperplasia; (c) mixed atrophy and increased Leydig cell number in the intertubular space. Magnification ×200; scale bar, 50 µm.

Figure 5

Estrogen synthesis in testes. (a) In adulthood, the main site of testicular estrogen production is the Leydig cell. Under the LH stimulation, testosterone (T) and androstendione are converted by aromatase (CYP19A1) to estrone and estradiol (E2), respectively. The proper balance between intratesticular T level (ITT) and estradiol level (ITE2) is crucial for normal testicular function (steroidogenesis and spermatogenesis); (b) In testes with impaired spermatogenesis, the dysfunction of Leydig cells is observed. Leydig cell dysfunction is accompanied by increased serum LH levels and decreased T/LH ratio. It was reported that in dysfunctional Leydig cells, the increased activity of aromatase may be observed, which results in elevation of intratesticular E2 levels and thus altered ITT/ITE2 ratio. The increased ITE2 level may be associated with Leydig cell hyperplasia/hypertrophy.