Iiisperm Morphology Figure 251

IV. LEYDIG (INTERSTITIAL) CELLS are found in connective tissue between the seminiferous tubules. They contain crystals of Reinke within the cytoplasm.

A. Functions

1. Leydig cells possess LH receptors so that LH from the adenohypophysis stimulates testosterone secretion.

2. Leydig cells secrete testosterone.

a. Testosterone gives rise to two other potent androgens:

(1) Dihydrotestosterone (DHT) is produced from testosterone via the enzyme 5a-reductase.

Head Neck

Middle piece

Acrosomal cap / i

Middle piece

Acrosomal cap / i

• Cell membrane 1 Acrosome

■ Cell membrane

■ Mitochondrial sheath

■ Nine outer dense fibers • Axoneme (cilium with 9+2 arrangement)

• Cell membrane 1 Acrosome

■ Cell membrane

■ Mitochondrial sheath

■ Nine outer dense fibers • Axoneme (cilium with 9+2 arrangement)

Tail

Principal piece

■ Seven outer dense fibers

End piece

■ Cell membrane Axoneme

Figure 25-1. Diagram of human sperm. Regions of the mature sperm are shown on the left. Sections through the head, middle piece, principal piece, and end piece along with their composition are shown on the right.

(2) 5oi-Androstanediol is produced from testosterone via the enzyme 3a-reductase.

b. Aromatization of testosterone and androstenedione within the liver and adipose tissue by P450 aromatase produces significant amounts of estradiol and estrone in males.

C. During fetal life, testosterone is essential in the development of the epididymis, ductus deferens, seminal vesicle, and ejaculatory duct to producc the male phenotype in a genotypic XY fetus. DHT is essential for the development of the penis and scrotum (external genitalia) and prostate gland.

d. During puberty and adult life, androgens are essential for spermatogenesis; function of prostate, seminal vesicle, and bulbourethral glands; appearance of secondary sex characteristics; closure of the epiphyseal growth plate; increase in muscle mass; and lipid metabolism [testosterone increases low-density lipoprotein (LDL) and decreases high-density lipoprotein (HDL)].

6. Between 1% and 2% of circulating testosterone is in the free form; the remainder is bound to a liver-derived sex steroid-binding globulin or albumin.

f. Plasma testosterone levels during the lifespan of a normal male are shown in Figure 25-2.

Capillary

Leydig cell

Life stage

Figure 25-2. (A) Diagram of hormonal control of the male reproductive system. The adenohypophysis (AD) secretes follicle-stimulating hormone (FSH) and luteinizing hormone (LH). FSH binds to FSH receptors (box) on the Sertoli cclls (shaded), which stimulates the synthesis of androgen-binding protein (ABP). The Sertoli cells secrete inhibin (INH), which inhibits FSH secretion (a feedback loop). LH binds to LH receptors (triangle) on the Leydig cells, which stimulates the secretion of testosterone (T). Testosterone circulates in the blood bound to sex steroid-binding globulin (SSBG). Testosterone binds to ABP within Sertoli cells to maintain high levels of testosterone necessary for spermatogenesis. (B) Plasma testosterone levels during the lifespan of a normal male. The peak testosterone levels during in utero development (0-9months) produce the male phenotype in a geno-typic XY fetus. The physiologic significance of the peak at birth is not known. Plasma testosterone levels increase steeply at puberty, plateau in the adult at approximately 600 ng/dl, and begin to decline in old age. (Based on data from Griffin JE et al: The testes. In Metabolic Control and Disease. Edited by Bondy PK, Rosenberg LE. Philadelphia, Saunders, 1980; and Winter JSD, Hughes I A, Reyes FI, et al: Pituitary-gonadal relations in infancy. J Clin Endocrinol Metabol 142: 679, 1976.)

In utero

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Figure 25-2. (A) Diagram of hormonal control of the male reproductive system. The adenohypophysis (AD) secretes follicle-stimulating hormone (FSH) and luteinizing hormone (LH). FSH binds to FSH receptors (box) on the Sertoli cclls (shaded), which stimulates the synthesis of androgen-binding protein (ABP). The Sertoli cells secrete inhibin (INH), which inhibits FSH secretion (a feedback loop). LH binds to LH receptors (triangle) on the Leydig cells, which stimulates the secretion of testosterone (T). Testosterone circulates in the blood bound to sex steroid-binding globulin (SSBG). Testosterone binds to ABP within Sertoli cells to maintain high levels of testosterone necessary for spermatogenesis. (B) Plasma testosterone levels during the lifespan of a normal male. The peak testosterone levels during in utero development (0-9months) produce the male phenotype in a geno-typic XY fetus. The physiologic significance of the peak at birth is not known. Plasma testosterone levels increase steeply at puberty, plateau in the adult at approximately 600 ng/dl, and begin to decline in old age. (Based on data from Griffin JE et al: The testes. In Metabolic Control and Disease. Edited by Bondy PK, Rosenberg LE. Philadelphia, Saunders, 1980; and Winter JSD, Hughes I A, Reyes FI, et al: Pituitary-gonadal relations in infancy. J Clin Endocrinol Metabol 142: 679, 1976.)

600" -500" -400- -300" -200" -100- -

In utero

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