di_fiore_chapter_18___male_reproductive_system
, Location of the testes and the accessory male reproductive organs, with emphasis on the inter-
nal organization of the testis, the different phases of spermiogenesis, and the structure of a mature sperm.
408
,410 PART II — ORGANS
stem cells
for storage and maturation. During sexual exci-
spermatogenesis, or sperm production.
Equally important in lowering testicular temp
and nonproliferating
, Formation of Sperm: Spermatogenesis
The process of sperm formation is called spermatogenesis. This includes mitotic divisions of
spermatogenic cells, which produce replacement stem cells and other spermatogenic cells that
eventually give rise to primary spermatocytes and secondary spermatocytes. Both primary and
secondary spermatocytes undergo meiotic divisions that reduce the number of chromosomes
and the amount of DNA. Division of secondary spermatocytes produces cells called spermatids
that contain 23 single chromosomes (22!X or 22!Y). Spermatids do not undergo any further
divisions, but instead are transformed into sperm by a process called spermiogenesis.
Once the spermatogenic cells in the germinal epithelium differentiate, they are held
together by intercellular bridges during further differentiation and development. The
intercellular bridges are broken when the developed spermatids are released into the
seminiferous tubules as mature sperm.
Transformation of Spermatids: Spermiogenesis
Spermiogenesis is a complex morphologic process by which the spherical spermatids are
transformed into elongated sperm cells. During spermiogenesis, the size and shape of the
spermatids are altered, and the nuclear chromatin condenses. In the Golgi phase, small
granules accumulate in the Golgi apparatus of the spermatid and form an acrosomal granule
within a membranebound acrosomal vesicle. During the acrosomal phase, both the acrosomal
vesicle and acrosomal granule spread over the condensing spermatid nucleus at the anterior tip
of the spermatid as an acrosome. The acrosome functions as a specialized type of lysosome and
contains several hydrolytic enzymes, such as hyaluronidase and protease with trypsinlike
activity, that assist the sperm in penetrating the cells (corona radiata) and the membrane (zona
pellucida) that surround the ovulated oocyte. During the maturation phases, the plasma
membrane moves posteriorly from the nucleus to cover the developing flagellum (sperm tail).
The mitochondria migrate to and form a tight sheath around the middle piece of the flagellum.
The final maturation phase is characterized by the shedding of the excess or residual cytoplasm
of the spermatid and release of the sperm cell into the lumen of the seminiferous tubule. Sertoli
cells then phagocytose the residual cytoplasm.
The mature sperm cell is composed of a head and an acrosome that surrounds the anterior
portion of the nucleus, a neck, a middle piece characterized by the presence of a compact
mitochondrial sheath, and a main or principal piece (Overview Figure 18).
Excurrent Ducts
Newly released sperm pass from the seminiferous tubules into the intertesticular excurrent
ducts that connect each testis with the overlying epididymis. These excurrent ducts consist of
the straight tubules (tubuli recti) and the rete testis, the epithelial-lined spaces in the
mediastinum testis. From the rete testis, the sperm enter approximately 12 short tubules, the
ductuli efferentes (efferent ducts), which conduct sperm from the rete testis to the initial
segment or the head of the epididymis.
The extratesticular duct that conducts the sperm to the penile urethra is the ductus
epididymis, which is continuous with the ductus (vas) deferens and ejaculatory ducts in the
prostate gland. During sexual excitation and ejaculation, strong contractions of the smooth
muscle that surrounds the ductus epididymis expel the sperm (Overview Figure 18).
, Location of the testes and the accessory male reproductive organs, with emphasis on the inter-
nal organization of the testis, the different phases of spermiogenesis, and the structure of a mature sperm.
408
,410 PART II — ORGANS
stem cells
for storage and maturation. During sexual exci-
spermatogenesis, or sperm production.
Equally important in lowering testicular temp
and nonproliferating
, Formation of Sperm: Spermatogenesis
The process of sperm formation is called spermatogenesis. This includes mitotic divisions of
spermatogenic cells, which produce replacement stem cells and other spermatogenic cells that
eventually give rise to primary spermatocytes and secondary spermatocytes. Both primary and
secondary spermatocytes undergo meiotic divisions that reduce the number of chromosomes
and the amount of DNA. Division of secondary spermatocytes produces cells called spermatids
that contain 23 single chromosomes (22!X or 22!Y). Spermatids do not undergo any further
divisions, but instead are transformed into sperm by a process called spermiogenesis.
Once the spermatogenic cells in the germinal epithelium differentiate, they are held
together by intercellular bridges during further differentiation and development. The
intercellular bridges are broken when the developed spermatids are released into the
seminiferous tubules as mature sperm.
Transformation of Spermatids: Spermiogenesis
Spermiogenesis is a complex morphologic process by which the spherical spermatids are
transformed into elongated sperm cells. During spermiogenesis, the size and shape of the
spermatids are altered, and the nuclear chromatin condenses. In the Golgi phase, small
granules accumulate in the Golgi apparatus of the spermatid and form an acrosomal granule
within a membranebound acrosomal vesicle. During the acrosomal phase, both the acrosomal
vesicle and acrosomal granule spread over the condensing spermatid nucleus at the anterior tip
of the spermatid as an acrosome. The acrosome functions as a specialized type of lysosome and
contains several hydrolytic enzymes, such as hyaluronidase and protease with trypsinlike
activity, that assist the sperm in penetrating the cells (corona radiata) and the membrane (zona
pellucida) that surround the ovulated oocyte. During the maturation phases, the plasma
membrane moves posteriorly from the nucleus to cover the developing flagellum (sperm tail).
The mitochondria migrate to and form a tight sheath around the middle piece of the flagellum.
The final maturation phase is characterized by the shedding of the excess or residual cytoplasm
of the spermatid and release of the sperm cell into the lumen of the seminiferous tubule. Sertoli
cells then phagocytose the residual cytoplasm.
The mature sperm cell is composed of a head and an acrosome that surrounds the anterior
portion of the nucleus, a neck, a middle piece characterized by the presence of a compact
mitochondrial sheath, and a main or principal piece (Overview Figure 18).
Excurrent Ducts
Newly released sperm pass from the seminiferous tubules into the intertesticular excurrent
ducts that connect each testis with the overlying epididymis. These excurrent ducts consist of
the straight tubules (tubuli recti) and the rete testis, the epithelial-lined spaces in the
mediastinum testis. From the rete testis, the sperm enter approximately 12 short tubules, the
ductuli efferentes (efferent ducts), which conduct sperm from the rete testis to the initial
segment or the head of the epididymis.
The extratesticular duct that conducts the sperm to the penile urethra is the ductus
epididymis, which is continuous with the ductus (vas) deferens and ejaculatory ducts in the
prostate gland. During sexual excitation and ejaculation, strong contractions of the smooth
muscle that surrounds the ductus epididymis expel the sperm (Overview Figure 18).
