836
UNIT SIX
(sper
mah-to-jen
e
˘-sis) is arrested at this stage
(f g. 22.8)
.
At puberty, mitosis resumes, and new spermatogonia form.
Testosterone secretion increases, and the primary spermato-
cytes then reproduce by meiosis. Each primary spermatocyte
divides to form two
secondary spermatocytes.
Each of these
cells, in turn, divides to form two
spermatids,
which mature
into sperm cells
(f
g. 22.9)
. Meiosis reduces the number of
chromosomes in each cell by one half.
The spermatogonia are located near the wall of the semi-
niferous tubule. As spermatogenesis proceeds, cells in more
advanced stages are pushed along the sides of sustentacular
cells toward the lumen of the seminiferous tubule.
Near the base of the epithelium, membranous processes
from adjacent sustentacular cells fuse by tight junctions
(F g. 22.9). The sustentacular cells and their tight junctions
form the
blood-testis barrier,
which prevents some substances
from reaching the developing sperm. The blood-testis bar-
rier helps maintain a favorable environment by isolating the
developing sperm from the male’s immune system, which
might otherwise view the sperm as abnormal cells.
Sperm have fascinated biologists for centuries. Anton van
Leeuwenhoek was the F
rst to view human sperm under a microscope
in 1678, concluding that they were parasites in semen. By 1685, he
had modified his view, writing that sperm contain a preformed
human being and are seeds requiring nurturing in a female to start
a new life.
to the lumen of the seminiferous tubule. The sustentacular
cells support, nourish, and regulate the spermatogenic cells,
which give rise to sperm cells (spermatozoa).
In the male embryo, undifferentiated spermatogenic
cells are called
spermatogonia.
Each spermatogonium has
46 chromosomes (23 pairs) in its nucleus, the usual number
for human body cells. Spermatogonia are located within the
seminiferous tubules, adjacent to the inside surface of the
basement membrane surrounding each seminiferous tubule.
Hormones stimulate the spermatogonia to become
active. Some of the cells undergo mitosis (see chapter 3, pp.
99–100). Each cell division gives rise to two new cells, one
(type A) of which maintains the supply of undifferentiated
cells, the other (type B) of which differentiates to become a
primary spermatocyte.
Sperm production or
spermatogenesis
FIGURE 22.6
Structure of the testis. (
a
) Sagittal section of a testis. (
b
) Cross section of a seminiferous tubule.
Plane of section
Epididymis
Ductus deferens
Rete testis
Testis
Tunica
albuginea
Seminiferous tubules
Interstitial cells
(Cells of Leydig)
Spermatogonia
Sperm cells
Basement membrane
Lumen of seminiferous
tubule
(a)
(b)
Basement
membrane
Seminiferous
tubule
Spermatogenic
cells
Spermatogonia
Sperm cells
Interstitial cells
(cells of Leydig)
FIGURE 22.7
Light micrograph of a seminiferous tubule (250×).
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