Human Physiology, 14th edition (2016)

Interstitial tissue

with Leydig cells

Seminiferous

tubule

(a)

Sertoli cell

Spermatozoa

(23 chromosomes)

Cytoplasmic

droplets

Spermatids
(23 chromosomes)

Secondary
spermatocytes
(23 chromosomes)

Primary
spermatocytes
(46 chromosomes)

Spermatogonia
(46 chromosomes)

Lumen of

seminiferous

tubule

Wall of

seminiferous

tubule

(b)

Reproduction 717

The Sertoli cells help to make the seminiferous tubules an

immunologically privileged site (protected from immune attack)

through another mechanism as well. As described in chapter 15,

the Sertoli cells produce FAS ligand, which binds to the FA S

receptor on the surface of T lymphocytes. This triggers apopto-

sis (cell suicide) of the T lymphocytes and thus helps to prevent

immune attack of the developing sperm. Immunological protec-

tion of the germinal cells of the testis is also helped by their usual

lack of MHC class-1 or class-2 molecules to interact with the

T cell receptors (chapter 15; see fig. 15.14).

In the process of spermiogenesis (conversion of spermatids

to spermatozoa), most of the spermatid cytoplasm is eliminated.

This occurs through phagocytosis by Sertoli cells of the “residual

bodies” of cytoplasm from the spermatids ( fig. 20.17 ). Phagocy-

tosis of residual bodies may transmit regulatory molecules from

germ cells to Sertoli cells. The Sertoli cells, in turn, provide mol-

ecules needed by the germ cells. It is known, for example, that

the X chromosome of germ cells is inactive during meiosis. Since

this chromosome contains genes needed to produce many essen-

tial molecules, it is believed that these molecules are provided by

the Sertoli cells during this time.

Sertoli cells secrete a protein called androgen-binding

protein (ABP) into the lumen of the seminiferous tubules. This

protein, as its name implies, binds to testosterone and thereby

concentrates it within the tubules. Production of ABP is stimu-

lated by FSH, and FSH receptors are found only in Sertoli cells.

Thus, all effects of FSH in the testes must be mediated by Sertoli

cells. These effects include production of ABP, FSH-induced

stimulation of spermiogenesis, and other paracrine interactions

between Sertoli and Leydig cells in the testis.

At the conclusion of spermiogenesis, spermatozoa enter the

lumen of the seminiferous tubules. A spermatozoon ( fig.  20.18 )

contains a head, consisting mostly of a nucleus with DNA and an

overlying cap known as an acrosome (section 20.6; see fig. 20.38 ),

and a flagellum tail. The flagellum has a characteristic “9  1  2”

microtubule structure (chapter 3, section 3.1) called an axoneme.

The axoneme runs through the entire flagellum, which is divided

cell cytoplasm ( fig. 20.16 b ). Where there are no developing germ

cells between them, the adjacent Sertoli cells are tightly joined

together by junctional complexes that must be formed, broken,

and re-formed as the developing sperm move toward the lumen.

Figure 20.17 The processing of spermatids into

spermatozoa (spermiogenesis). As the spermatids develop

into spermatozoa, most of their cytoplasm is pinched off as

residual bodies and ingested by the surrounding Sertoli cell

cytoplasm.

Figure 20.16 A photomicrograph and diagram of the

seminiferous tubules. ( a ) A cross section of the seminiferous

tubules also shows surrounding interstitial tissue. ( b ) the stages of

spermatogenesis are indicated within the germinal epithelium of

a seminiferous tubule. The relationship between Sertoli cells and

developing spermatozoa can also be seen.