1032
UNIT 5
Continuity
27
earliest primary spermatocytes. Te
adluminal compartment
lies internal to the tight junctions and includes the meiotically
active cells and the tubule lumen (see Figure 27.8c).
Te tight junctions between the sustentocytes form the
blood
testis barrier
. Tis barrier prevents the membrane antigens of
differentiating sperm from escaping through the basal lamina
into the bloodstream where they would activate the immune sys-
tem. Because no sperm are formed until puberty, they are absent
when the immune system is being programmed to recognize
a person’s own tissues early in life. Te spermatogonia, which
are recognized as “self,” are outside the barrier and for this rea-
son can be influenced by bloodborne chemical messengers that
prompt spermatogenesis. Following mitosis of the spermatogo-
nia, the tight junctions of the sustentocytes open to allow type B
daughter cells to pass into the adluminal compartment—much
as locks in a canal open to allow a boat to pass.
In the adluminal compartment, spermatocytes and sper-
matids are nearly enclosed in recesses in the sustentocytes (see
Figure 27.8), anchored there by a particular glycoprotein on the
spermatogenic cell.
nucleus (Figure 27.9a
5
and
6
). Like a lysosome produced by
the Golgi apparatus, the acrosome contains hydrolytic enzymes
that, in this case, enable the sperm to penetrate and enter an egg.
Te sperm
midpiece
contains mitochondria spiraled tightly
around the microtubules of the tail. Te long
tail
is a typical
flagellum produced by one centriole (actually a basal body) near
the nucleus. Te mitochondria provide the metabolic energy
(A±P) needed for the whiplike movements of the tail that will
propel the sperm along its way in the female reproductive tract.
Role of the Sustentocytes
Troughout spermatogenesis, de-
scendants of the same spermatogonium remain closely attached
to one another by cytoplasmic bridges (see Figure 27.8c). Tey
are also surrounded by and connected to nonreplicating sup-
porting cells called
sustentocytes
or
Sertoli cells
, which extend
from the basal lamina to the tubule lumen.
Te sustentocytes, bound to each other laterally by tight
junctions, divide the seminiferous tubule into two compart-
ments. Te
basal compartment
extends from the basal lamina
to their tight junctions and contains spermatogonia and the
Centrioles
Spermatid
nucleus
Golgi apparatus
Acrosomal
vesicle
Mitochondria
Approximately 24 days
Excess
cytoplasm
Nucleus
Acrosome
Microtubules
Flagellum
Tail
Midpiece
Head
(a)
(b)
1
2
3
4
5
6
7
Figure 27.9
Spermiogenesis:
transformation of a spermatid into a
functional sperm. (a)
During the stepwise
process of spermiogenesis:
1
the Golgi
apparatus packages the acrosomal enzymes,
2
the acrosome forms at the anterior end
of the nucleus and the centrioles gather at
the opposite end of the nucleus,
3
micro-
tubules elaborate to form the flagellum,
4
mitochondria multiply and take up
position around the proximal portion of the
flagellum, and
5
excess cytoplasm sloughs
off.
6
Structure of an immature sperm that
has just been released from a sustentocyte.
7
Structure of a fully mature sperm.
(b)
Scanning electron micrograph of mature
sperm (900
3
).
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