606
Chapter 17
urethra. The urethra emerges from the prostate gland and
enters the penis. The paired
bulbourethral glands,
lying
below the prostate, drain into the urethra just after it leaves
the prostate.
The prostate gland and seminal vesicles secrete most of
the fl uid in which ejaculated sperm are suspended. This fl
uid,
plus the sperm cells, constitute
semen,
the sperm contributing
a small percentage of the total volume. The glandular secre-
tions contain a large number of different chemical substances,
including (1) nutrients, (2) buffers for protecting the sperm
against the acidic vaginal secretions, (3) chemicals (particu-
larly from the seminal vesicles) that increase sperm motility,
and (4) prostaglandins. The function of the prostaglandins,
which are produced by the seminal vesicles, is still not clear.
The bulbourethral glands contribute a small volume of lubri-
cating mucoid secretions.
In addition to providing a route for sperm from the
seminiferous tubules to the exterior, several of the duct system
segments perform additional functions to be described in the
section on sperm transport.
Spermatogenesis
The various stages of spermatogenesis are summarized in
Figure
17–7
. The undifferentiated germ cells, which are termed
sper-
matogonia
(singular,
spermatogonium
) begin to divide mitoti-
cally at puberty. The daughter cells of this fi rst division then
divide again and again for a specifi ed number of division cycles
so that a clone of spermatogonia is produced from each stem
cell spermatogonium. Some differentiation occurs in addition
to cell division. The cells that result from the fi nal mitotic
division and differentiation in the series are called
primary
spermatocytes,
and these are the cells that will undergo the
fi rst meiotic division of spermatogenesis.
It should be emphasized that if all the cells in the clone
produced by each stem cell spermatogonium followed this
pathway, the spermatogonia would disappear—that is, they
would all be converted to primary spermatocytes. This does
not occur because, at an early point, one of the cells of each
clone “drops out” of the mitosis-differentiation cycle to
remain a stem cell spermatogonium that will later enter into
its own full sequence of divisions. One cell of the clone it pro-
duces will do likewise, and so on. Thus, the supply of undif-
ferentiated spermatogonia does not decrease.
Each primary spermatocyte increases markedly in size
and undergoes the fi rst meiotic division (see Figure 17–7) to
form two
secondary spermatocytes,
each of which contains
23 two-chromatid chromosomes. Each secondary spermato-
cyte undergoes the second meiotic division (see Figure 17–2h
to i) to form
spermatids.
Thus, each primary spermatocyte,
containing 46 two-chromatid chromosomes, produces four
spermatids, each containing 23 one-chromatid chromosomes.
+
+
FSH
LH
Sertoli cell
Interstitial
Leydig cells
Spermatid
Spermatogonium
Seminiferous
tubules
Figure 17–4
Cross-section of an area of testis. The Sertoli cells (stimulated by
FSH to increase spermatogenesis and produce inhibin) are in the
seminiferous tubules, the sites of sperm production. The tubules
are separated from each other by interstitial space (colored light
blue) that contains Leydig cells (stimulated by LH to produce
testosterone) and blood vessels.
Rete
testis
Seminiferous
tubule
Epididymis
Efferent ductules
Vas
deferens
Figure 17–5
Section of a testis. The upper portion of the testis has been removed
to show its interior.
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