404 SECTION IVEndocrine & Reproductive Physiology
The primary spermatocytes undergo meiotic division, reducing
the number of chromosomes. In this two-stage process, they di-
vide into secondary spermatocytes and then into spermatids,
which contain the haploid number of 23 chromosomes. The
spermatids mature into spermatozoa (sperms). As a single
spermatogonium divides and matures, its descendants remain
tied together by cytoplasmic bridges until the late spermatid
stage. This apparently ensures synchrony of the differentiation
of each clone of germ cells. The estimated number of spermatids
formed from a single spermatogonium is 512. In humans, it
takes an average of 74 d to form a mature sperm from a primi-
tive germ cell by this orderly process of spermatogenesis.
Each sperm is an intricate motile cell, rich in DNA, with a
head that is made up mostly of chromosomal material (Figure
25–14). Covering the head like a cap is the acrosome, a lyso-
some-like organelle rich in enzymes involved in sperm pene-
tration of the ovum and other events involved in fertilization.
The motile tail of the sperm is wrapped in its proximal por-
tion by a sheath holding numerous mitochondria. The mem-
branes of late spermatids and spermatozoa contain a special
small form of angiotensin-converting enzyme called germi-
nal angiotensin-converting enzyme. The function of this
enzyme in the sperms is unknown, although male mice in
which the function of the angiotensin-converting enzyme
gene has been disrupted have reduced fertility.
The spermatids mature into spermatozoa in deep folds of
the cytoplasm of the Sertoli cells (Figure 25–13). Mature sper-
matozoa are released from the Sertoli cells and become free in
the lumens of the tubules. The Sertoli cells secrete androgen-
binding protein (ABP), inhibin, and MIS. They do not syn-
thesize androgens, but they contain aromatase (CYP19), the
enzyme responsible for conversion of androgens to estrogens,
and they can produce estrogens. ABP probably functions to
maintain a high, stable supply of androgen in the tubular
fluid. Inhibin inhibits FSH secretion.
FSH and androgens maintain the gametogenic function of
the testis. After hypophysectomy, injection of LH produces a
high local concentration of androgen in the testes, and this
maintains spermatogenesis. The stages from spermatogonia
to spermatids appear to be androgen-independent. However,
the maturation from spermatids to spermatozoa depends on
androgen acting on the Sertoli cells in which the developing
spermatozoa are embedded. FSH acts on the Sertoli cells to
facilitate the last stages of spermatid maturation. In addition,
it promotes the production of ABP.
An interesting observation is that the estrogen content of the
fluid in the rete testis (Figure 25–11) is high, and the walls of
the rete contain numerous ERα estrogen receptors. In this
region, fluid is reabsorbed and the spermatozoa are concen-
trated. If this does not occur, the sperm entering the epididymis
are diluted in a large volume of fluid, and infertility results.Further Development of Spermatozoa
Spermatozoa leaving the testes are not fully mobile. They con-
tinue their maturation and acquire motility during their passage
through the epididymis. Motility is obviously important in vivo,
but fertilization occurs in vitro if an immotile spermatozoon
from the head of the epididymis is microinjected directly into
an ovum. The ability to move forward (progressive motility),
which is acquired in the epididymis, involves activation of a
unique protein called CatSper, which is localized to the princi-
pal piece of the sperm tail. This protein appears to be a Ca2+ ion
channel that permits cAMP-generalized Ca2+ influx. In addi-
tion, spermatozoa express olfactory receptors, and ovaries pro-
duce odorant-like molecules. Recent evidence indicates that
these molecules and their receptors interact, fostering move-
ment of the spermatozoa toward the ovary (chemotaxis).
Ejaculation of the spermatozoon involves contractions of
the vas deferens mediated in part by P2X receptors, ligand-
gated cation channels that respond to ATP (see Chapter 7),
and fertility is reduced in mice in which these receptors are
knocked out.
Once ejaculated into the female, the spermatozoa move up
the uterus to the isthmus of the uterine tubes, where they slow
down and undergo capacitation. This further maturation
process involves two components: increasing the motility of
the spermatozoa and facilitating their preparation for the
acrosome reaction. However, the role of capacitation appears
to be facilitatory rather than obligatory, because fertilization is
readily produced in vitro. From the isthmuses the capacitated
spermatozoa move rapidly to the tubal ampullas, where fertil-
ization takes place.FIGURE 25–14 Human spermatozoon, profile view. Note the acrosome, an organelle that covers half the sperm head inside the plasma
membrane of the sperm. (Reproduced with permission from Junqueira LC, Carneiro J: Basic Histology: Text & Atlas, 10th ed. McGraw-Hill, 2003.)
5 μm
End piece5 μm
Middle piece5 μm
Head50 μm
Principal pieceMitochondria Acrosome