The male gonads, testes, or testicles, begin their development
high in the abdominal cavity, near the kidneys. During the last two
months before birth, or shortly after birth, they descend through
the inguinal canal into the scrotum, a pouch that extends below the
abdomen, posterior to the penis. Although this location of the testes,
outside the abdominal cavity, may seem to make them vulnerable to
injury, it provides a temperature about 3° C below normal body temperature.
This lower temperature is necessary for the production of viable sperm.The
scrotum consists of skin and subcutaneous tissue. A vertical septum,
or partition, of subcutaneous tissue in the center
divides it into two parts, each containing one testis. Smooth muscle
fibers, called the dartos muscle, in the subcutaneous tissue contract
to give the scrotum its wrinkled appearance. When these fibers are
relaxed, the scrotum is smooth. Another muscle, the cremaster muscle,
consists of skeletal muscle fibers and controls the position of the
scrotum and testes. When it is cold or a man is sexually aroused,
this muscle contracts to pull the testes closer to the body for warmth.
Structure
Each testis is an oval structure about 5 cm long and 3 cm in diameter.
A tough, white fibrous connective tissue capsule, the tunica albuginea,
surrounds each testis and extends inward to form septa that partition
the organ into lobules. There are about 250 lobules in each testis.
Each lobule contains 1 to 4 highly coiled seminiferous tubules that
converge to form a single straight tubule, which leads into the
rete testis. Short efferent ducts exit the testes. Interstitial
cells (cells of Leydig), which produce male sex hormones, are located
between the seminiferous tubules within a lobule.
Spermatogenesis
Sperm are produced by spermatogenesis within the seminiferous tubules.
A transverse section of a seminiferous tubule shows that it is packed
with cells in various stages of development. Interspersed with these
cells, there are large cells that extend from the periphery of the
tubule to the lumen. These large cells are the supporting, or sustentacular
cells (Sertoli's cells), which support and nourish the other cells.
Early in embryonic development, primordial germ cells enter the
testes and differentiate into spermatogonia, immature cells that
remain dormant until puberty. Spermatogonia are diploid cells, each
with 46 chromosomes (23 pairs) located around the periphery of the
seminiferous tubules. At puberty, hormones stimulate these cells
to begin dividing by mitosis. Some of the daughter cells produced
by mitosis remain at the periphery as spermatogonia. Others are
pushed toward the lumen, undergo some changes, and become primary
spermatocytes. Because they are produced by mitosis, primary spermatocytes,
like spermatogonia, are diploid and have 46 chromosomes.
Each primary spermatocytes goes through the first meiotic division,
meiosis I, to produce two secondary spermatocytes, each with 23
chromosomes (haploid). Just prior to this division, the genetic
material is replicated so that each chromosome consists of two strands,
called chromatids, that are joined by a centromere. During meiosis
I, one chromosome, consisting of two chromatids, goes to each secondary
spermatocyte. In the second meiotic division, meiosis II, each secondary
spermatocyte divides to produce two spermatids. There is no replication
of genetic material in this division, but the centromere divides
so that a single-stranded chromatid goes to each cell. As a result
of the two meiotic divisions, each primary spermatocyte produces
four spermatids. During spermatogenesis there are two cellular divisions,
but only one replication of DNA so that each spermatid has 23 chromosomes
(haploid), one from each pair in the original primary spermatocyte.
Each successive stage in spermatogenesis is pushed toward the center
of the tubule so that the more immature cells are at the periphery
and the more differentiated cells are nearer the center.
Spermatogenesis (and oogenesis in the female) differs from mitosis
because the resulting cells have only half the number of chromosomes
as the original cell. When the sperm cell nucleus unites with an
egg cell nucleus, the full number of chromosomes is restored. If
sperm and egg cells were produced by mitosis, then each successive
generation would have twice the number of chromosomes as the preceding
one.
The final step in the development of sperm is called spermiogenesis.
In this process, the spermatids formed from spermatogenesis become
mature spermatozoa, or sperm. The mature sperm cell has a head,
midpiece, and tail. The head, also called the nuclear region, contains
the 23 chromosomes surrounded by a nuclear membrane. The tip of
the head is covered by an acrosome, which contains enzymes that
help the sperm penetrate the female gamete. The midpiece, metabolic
region, contains mitochondria that provide adenosine triphosphate
(ATP). The tail, locomotor region, uses a typical flagellum for
locomotion. The sperm are released into the lumen of the seminiferous
tubule and leave the testes. They then enter the epididymis where
they undergo their final maturation and become capable of fertilizing
a female gamete.
Sperm production begins at puberty and continues throughout the
life of a male. The entire process, beginning with a primary spermatocyte,
takes about 74 days. After ejaculation, the sperm can live for about
48 hours in the female reproductive tract.
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