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Testicular growth and maturation are largely influenced by testosterone, which
is produced by the testicle both before and after birth.1 Testosterone
production is regulated centrally by the hypothalamic-pituitary-testicular
axis as well as intragonadally. Testosterone effects include embryologic
male genital differentiation, neonatal imprinting of androgen-dependent
target tissue, maturation of the genitalia at puberty, growth of skeletal
muscle, deepening of the voice from laryngeal growth, epiphyseal
cartilage growth during puberty, male hair growth and distribution,
erythropoiesis, stimulation of sebaceous glands, and male social
behavior.1,2
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Developmental Anatomy
and Histology
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The adult testicle is ovoid in shape with a volume of 15 to 25
mL, an average length of 4.6 cm (range 3.6–5.5 cm), and
an average width of 2.6 cm (range 2.1–3.2 cm). Each testis
is surrounded by a capsule made up of 3 layers: the outer visceral
layer of the tunica vaginalis, the tunica albuginea, and the inner
layer of the tunica vasculosa. Contained by the capsule, each testis
is divided into 250 lobules by fibrous septae, with 1 to 4 seminiferous
tubules in each lobule. The tubules account for 90% of
testicular mass, and the interstitium accounts for the remaining
10%. The interstitium consists of Leydig cells, blood vessels,
lymphatic channels, macrophages, and mast cells. Leydig cells are
the major source of testosterone and are closely applied to the
outer wall of the seminiferous tubule. Each seminiferous tubule
is approximately 60 cm in length and 150 to 175 μm in
diameter.3 The tubule is the site of spermatogenesis
and contains 2 cell types, Sertoli cells and germ cells.4,5
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Gonadal Differentiation
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Although the sex of the embryo is determined at conception, the
potential male and female gonads do not differ morphologically until
the seventh week of development. Initially, they appear as gonadal
ridges, into which the primordial germ cells migrate in the sixth
week of growth. Primitive sex cords develop before incorporation
of the germ cells and are the progenitors of the seminiferous tubules.1 By the
fourth month, the primitive germ cells and Sertoli cells can be
identified in the tubules (Fig. 65-1). Leydig
cells are abundant during the fourth to sixth month and assist in
influencing the sexual differentiation of the genital ducts and
external genitalia by means of testosterone production. The fetal
testes produce inducer substances, which promote growth of the mesonephric
or wolffian duct and inhibit development of the paramesonephric
or müllerian duct. The mesonephric duct persists (except
for the most cranial portion, the appendix epididymis) and gives
rise to the epididymis, ductus deferens, and seminal vesicle. The
paramesonephric duct completely degenerates except for a small portion
at the cranial end, which persists as the appendix testis. Descent
of the testes from their abdominal origin to their final location
in the scrotal sac begins in the seventh or eighth month.1,5 It
is typically complete shortly before birth. However, ...