608
Chapter 17
The fi nal phase of spermatogenesis is the differentia-
tion of the spermatids into spermatozoa (sperm). This process
involves extensive cell remodeling, including elongation, but
no further cell divisions. The head of a sperm (
Figure 17–8
)
consists almost entirely of the nucleus, which contains the
genetic information (DNA). The tip of the nucleus is covered
by the
acrosome,
a protein-fi lled vesicle containing several
enzymes that play an important role in fertilization. Most of
the tail is a fl agellum—a group of contractile fi laments that
produce whiplike movements capable of propelling the sperm
at a velocity of 1 to 4 mm per min. Mitochondria form the
midpiece of the sperm and provide the energy for movement.
The entire process of spermatogenesis, from primary
spermatocyte to sperm, takes approximately 64 days. The typi-
cal human male manufactures approximately 30 million sperm
per day.
Thus far, spermatogenesis has been described without
regard to its orientation within the seminiferous tubules or the
participation of
Sertoli cells,
the second type of cell in the
seminiferous tubules, with which the developing germ cells
are closely associated. Each seminiferous tubule is bounded
by a basement membrane. Each Sertoli cell extends from the
basement membrane all the way to the lumen in the center of
the tubule and is joined to adjacent Sertoli cells by means of
tight junctions (
Figure 17–9
). Thus, the Sertoli cells form an
unbroken ring around the outer circumference of the seminif-
erous tubule. The tight junctions divide the tubule into two
compartments—a basal compartment between the basement
membrane and the tight junctions, and a central compartment,
beginning at the tight junctions and including the lumen.
The ring of interconnected Sertoli cells forms the
Sertoli
cell barrier
(blood-testes barrier), which prevents the move-
ment of many chemicals from the blood into the lumen of the
seminiferous tubule and helps retain luminal fl
uid. This ensures
proper conditions for germ-cell development and differentiation
in the tubules. The arrangement of Sertoli cells also permits dif-
ferent stages of spermatogenesis to take place in different com-
partments and, therefore, in different environments.
Mitotic cell divisions and differentiation of spermatogonia
to yield primary spermatocytes take place entirely in the basal
compartment. The primary spermatocytes then move through
the tight junctions of the Sertoli cells (which open in front of
them while at the same time forming new tight junctions behind
them) to gain entry into the central compartment. In this central
compartment, the meiotic divisions of spermatogenesis occur,
and the spermatids differentiate into sperm while contained in
recesses formed by invaginations of the Sertoli cell plasma mem-
branes. When sperm formation is complete, the cytoplasm of the
Sertoli cell around the sperm retracts, and the sperm are released
into the lumen to be bathed by the luminal fl
uid.
Sertoli cells serve as the route by which nutrients reach
developing germ cells, and they also secrete most of the fl
uid
found in the tubule lumen. This fl uid has a highly character-
istic ionic composition. It also contains
androgen-binding
protein (ABP),
which binds the total testosterone secreted
by the Leydig cells and crosses the Sertoli cell barrier to enter
the tubule. This protein maintains a high concentration of
total testosterone in the lumen of the tubule. The dissociation
of free testosterone from ABP continuously bathes the devel-
oping spermatocytes and Sertoli cells in testosterone.
Sertoli cells do more than infl uence the ionic and nutri-
tional environment of the germ cells. In response to FSH
from the anterior pituitary and to local testosterone produced
in the Leydig cell, Sertoli cells secrete a variety of chemical
messengers. These function as paracrine agents to stimulate
proliferation and differentiation of the germ cells.
In addition, the Sertoli cells secrete the protein hormone
inhibin and paracrine agents that affect Leydig cell function. The
many functions of Sertoli cells, several of which remain to be
described later in this chapter, are summarized in
Table 17–2
.
Head
Midpiece
Flagellum
(tail)
Acrosome
Cell
membrane
Nucleus
(a)
(b)
Mitochondria
Figure 17–8
(a) Diagram of a human mature sperm. (b) A close-up of the head
drawn from a different angle. The acrosome contains enzymes
required for fertilization of the ovum.
Table 17–2
Functions of Sertoli Cells
1. Provide Sertoli cell barrier to chemicals in the plasma
2. Nourish developing sperm
3. Secrete luminal fl
uid, including androgen-binding protein
4.
Respond to stimulation by testosterone and FSH to secrete
paracrine agents that stimulate sperm proliferation and
differentiation
5.
Secrete the protein hormone inhibin, which inhibits FSH
secretion from the pituitary
6.
Secrete paracrine agents that infl uence the function of
Leydig cells
7. Phagocytize defective sperm
8.
Secrete, during embryonic life, Müllerian-inhibiting
substance (MIS), which causes the primordial female duct
system to regress
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