Reproduction
609
Transport of Sperm
From the seminiferous tubules, the sperm pass through the
rete testis and efferent ducts into the epididymis and from
there to the vas deferens. The vas deferens and the portion
of the epididymis closest to it serve as a storage reservoir for
sperm until ejaculation.
Movement of the sperm as far as the epididymis results
from the pressure that the Sertoli cells create by continuously
secreting fl uid into the seminiferous tubules. The sperm them-
selves are normally nonmotile at this time.
During passage through the epididymis, the concentra-
tion of the sperm increases dramatically due to fl uid absorp-
tion from the lumen of the epididymis. Therefore, as the
sperm pass from the end of the epididymis into the vas def-
erens, they are a densely packed mass whose transport is no
longer facilitated by fl uid movement. Instead, peristaltic con-
tractions of the smooth muscle in the epididymis and vas def-
erens cause the sperm to move.
The absence of a large quantity of fl uid accounts for the
fact that
vasectomy
,
the surgical tying-off and removal of a
segment of each vas deferens, does not cause the accumula-
tion of much fl uid behind the tie-off point. The sperm, which
are still produced after vasectomy, do build up, however, and
eventually dissolve, with their chemical components absorbed
into the bloodstream. Vasectomy does not affect testosterone
secretion because it does not alter the function of the Leydig
cells. The next step in sperm transport is ejaculation.
Erection
The penis consists almost entirely of three cylindrical, vascular
compartments running its entire length. Normally the small
arteries supplying the vascular compartments are constricted
so that the compartments contain little blood and the penis
Basement
membrane
Smooth muscle-like cells
A
Tight
junctions
BC
D
E
F
Sertoli cells
Lumen
G
Figure 17–9
Relation of the Sertoli cells and germ cells. The Sertoli cells form a ring (barrier) around the entire tubule. For convenience of presentation,
the various stages of spermatogenesis are shown as though the germ cells move down a line of adjacent Sertoli cells; in reality, all stages
beginning with any given spermatogonium take place between the same two Sertoli cells. Spermatogonia (A and B) are found only in the basal
compartment (between the tight junctions of the Sertoli cells and the basement membrane of the tubule). After several mitotic cycles (A to B),
the spermatogonia (B) give rise to primary spermatocytes (C). Each of the latter crosses a tight junction, enlarges (D), and divides into two
secondary spermatocytes (E), which divide into spermatids (F), which in turn differentiate into spermatozoa (G). This last step involves loss of
cytoplasm by the spermatids.
Adapted from Tung.
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