Human Physiology, 14th edition (2016)

Reproduction 713

rise in FSH. In rams and bulls, a water-soluble (and, therefore,

peptide rather than steroid) product of the seminiferous tubules

specifically suppresses FSH secretion. This hormone, produced

by the Sertoli cells, is called inhibin. The seminiferous tubules

The testes consist of two parts, or “compartments”—the semi-

niferous tubules, where spermatogenesis occurs, and the intersti-

tial tissue, which contains the testosterone-secreting Leydig cells

( fig. 20.11 ). The seminiferous tubules account for about 90% of

the weight of an adult testis. The interstitial tissue is a thin web

of connective tissue that fills the spaces between the tubules. The

most abundant cells in the interstitial tissue are the Leydig cells,

but the interstitial tissue is also rich in blood and lymphatic cap-

illaries to transport the hormones of the testes.

With regard to gonadotropin action, the testes are strictly

compartmentalized. Cellular receptor proteins for FSH are located

exclusively in the seminiferous tubules, where they are confined

to the Sertoli cells. LH receptor proteins are located exclusively in

the interstitial Leydig cells. Secretion of testosterone by the Ley-

dig cells is stimulated by LH but not by FSH. Spermatogenesis in

the tubules is stimulated by FSH. The apparent simplicity of this

compartmentation is an illusion, however, because the two com-

partments interact with each other in complex ways.

Control of Gonadotropin Secretion

Castration of a male animal results in an immediate rise in FSH

and LH secretion. This demonstrates that hormones secreted

by the testes exert negative feedback control of gonadotropin

secretion. If testosterone is injected into the castrated animal,

the secretion of LH can be returned to the previous (precastra-

tion) levels. This provides a classical example of negative feed-

back—LH stimulates testosterone secretion by the Leydig cells,

and testosterone inhibits pituitary secretion of LH ( fig. 20.12 ).

The amount of testosterone that is sufficient to suppress

LH, however, is not sufficient to suppress the postcastration

Epididymis

Spermatic cord

Efferent ductules

Rete testis within

mediastinum testis

Plane of

section

(a)

Testis

Seminiferous
tubules

Interstitial

(Leydig) cells

Germinal

epithelial cells

(b)

Spermatozoa

Basement membrane

Lumen of

seminiferous tubule

Figure 20.11 The seminiferous tubules. ( a ) A sagittal section of a testis and ( b ) a transverse section of a seminiferous

tubule.

Figure 20.12 The anterior pituitary and testes. The

seminiferous tubules are the targets of FSH action; the interstitial

(Leydig) cells are targets of LH action. Testosterone secreted by

the Leydig cells inhibits LH secretion; inhibin secreted by the

tubules may inhibit FSH secretion.

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• –

Interstitial

Leydig cells

Sensor

Integrating center

Hypothalamus Effector

GnRH

Anterior

pituitary

FSH LH

Testes

Testosterone

Interstitial

(Leydig) cells

Seminiferous

tubules

Inhibin