837
CHAPTER TWENTY-TWO
Reproductive System
Many toxic chemicals that affect sperm hamper their ability to
swim, so the cells cannot transmit the toxin to an egg. An exception
is cocaine, which attaches to thousands of binding sites on human
sperm cells, without apparently harming the cells or impeding their
movements. Sperm can ferry cocaine to an egg, but it is not known
what harm, if any, the drug causes. We do know that fetuses exposed
to cocaine in the uterus may suF
er a stroke, or, as infants, be unable
to react normally to their surroundings.
PRACTICE
10
Review the events of spermatogenesis.
11
Explain the function of the sustentacular cells in the seminiferous
tubules.
12
Describe the structure of a sperm cell.
Male Internal Accessory Organs
The internal accessory organs of the male reproductive sys-
tem are specialized to nurture and transport sperm cells.
These structures include the two epididymides, two ductus
deferentia, two ejaculatory ducts, and urethra, as well as
Spermatogenesis occurs continually in a male, starting
at puberty. The resulting sperm cells collect in the lumen of
each seminiferous tubule, then pass through the rete testis to
the epididymis, where they accumulate and mature.
Structure of a Sperm Cell
Spermiogenesis
is the process of sperm cell maturation. A
mature sperm cell is a tiny, tadpole-shaped structure about
0.06 millimeter long. It consists of a fl
attened head, a cylin-
drical midpiece (body), and an elongated tail.
The oval
head
of a sperm cell is primarily composed of a
nucleus and contains highly compacted chromatin consisting
of 23 chromosomes. A small protrusion at its anterior end,
called the
acrosome,
contains enzymes, including hyaluroni-
dase, that aid the sperm cell in penetrating the cell layers
surrounding the oocyte during fertilization
(f
g. 22.10)
.
The
midpiece
of a sperm has a central, filamentous
core and many mitochondria organized in a spiral. The
tail
(fl agellum) consists of several microtubules enclosed in an
extension of the cell membrane. The mitochondria provide
ATP for the lashing movement of the tail that propels the
sperm cell through fluid. The micrograph in
Figure 22.11
shows a few mature sperm cells.
(23 chromosomes, each
with 2 chromatids)
Paired homologous
chromosomes
(46 chromosomes,
each with 2 chromatids)
(23 chromosomes, each
with 2 chromatids)
(23 chromosomes,
each chromatid now an
independent chromosome)
First meiotic
division
Primary
spermatocyte
Secondary
spermatocyte
Second meiotic
division
Spermatids
Sperm cells
FIGURE 22.8
During spermatogenesis four sperm
cells result from meiosis of a primary spermatocyte.
Two representative homologous chromosome pairs
are shown.
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