161
Highly polarized spermatozoa have eliminated most of its cytoplasm and organelles during spermiogenesis. Interestingly, nuclear gene transcription is switched off
when DNA begins to condense in elongating spermatids and testicular sperm are
generally considered transcriptionally inert although they retain a few specific coding and non-coding RNAs [24]. In such context, the post-testicular maturation of
sperm cells is crucial and will take place during its transit through the epididymis
[25]. When the spermatozoon transits from caput to cauda, numerous biochemical
events occur at various subcellular parts of the male gamete, including posttranslational modifications of proteins, proteolytic processing, protein redistribution
and disappearance, and integration of new components (for a review see [23].
Epididymal sperm maturation events are inextricably linked to the complex epididymal luminal microenvironment [22, 26, 27].
It was demonstrated in the rat model that segmentation of the epididymis is a
unique feature that occurs during postnatal development and prior to the appearance
of spermatozoa in the epididymal lumen [28] . This segmentation coincides with dramatic changes in morphology of epididymal cells and with regionalized functions that
are essential to the different steps of sperm maturation [26]. As a consequence, a
plethora of epididymal genes are expressed on a spatially restricted manner along the
organ [29]. This regionalized gene expression has been studied in various species (i.e.,
Human, rat, mouse and boar) and the high degree of gene expression segmentation
appeared to be a common feature shared between these species (for a review see [26].
It is now well described that in vertebrates, spermatozoa leaving the testis are unable
to fertilize the egg. Thanks to numerous works in animal models, it became clear that
the male gamete has to transit through a certain portion of the epididymis in order to
do so (for a review see [Indeed, this strongly evidences a role of the epididymis in
controlling and/or inducing the final maturation of the spermatozoa.
The mammalian epididymis holds an essential role in promoting the functional
maturation of spermatozoa, in addition to their prolonged storage in a viable state.
Both processes are supported by a highly specialized luminal microenvironment
that is created, and maintained, by the combined secretory and absorptive activity of
the lining epithelium. The molecular machinery it employs to regulate the tightly
coupled processes of exocytosis and endocytosis remain poorly understood.
In the epididymis, the dominant number of proteins present in the epididymal fluid
are synthesized by the epithelium in a region-specific manner and secreted into the
tubule lumen [30, 31], for a review see reference [32]. These secretion- absorption
events create a special microenvironment for sperm maturation and storage. Studies in
vitro have also shown that epididymosomes, the vesicles formed after apocrine secretion, exchange material with immature spermatozoa [33, 34], for a review see [35].
Spermatozoa entering the caput epididymis, progress through the corpus and
finally reach the cauda region, where they are stored. The transit through the coils
of the epididymis takes about 2-3 days in humans (it is longer in other species) and
the sperm can be stored in the cauda for another 2–3 days. During their transit in the
epididymis, sperm undergo maturation processes necessary for them to acquire
motility and fertility (Fig. 10.1). The final maturation of the male gamete, named
capacitation, will be completed in the female reproductive tracts.
10 An Overview of Male Reproductive Toxicants: Facts and Opinions
Highly polarized spermatozoa have eliminated most of its cytoplasm and organelles during spermiogenesis. Interestingly, nuclear gene transcription is switched off
when DNA begins to condense in elongating spermatids and testicular sperm are
generally considered transcriptionally inert although they retain a few specific coding and non-coding RNAs [24]. In such context, the post-testicular maturation of
sperm cells is crucial and will take place during its transit through the epididymis
[25]. When the spermatozoon transits from caput to cauda, numerous biochemical
events occur at various subcellular parts of the male gamete, including posttranslational modifications of proteins, proteolytic processing, protein redistribution
and disappearance, and integration of new components (for a review see [23].
Epididymal sperm maturation events are inextricably linked to the complex epididymal luminal microenvironment [22, 26, 27].
It was demonstrated in the rat model that segmentation of the epididymis is a
unique feature that occurs during postnatal development and prior to the appearance
of spermatozoa in the epididymal lumen [28] . This segmentation coincides with dramatic changes in morphology of epididymal cells and with regionalized functions that
are essential to the different steps of sperm maturation [26]. As a consequence, a
plethora of epididymal genes are expressed on a spatially restricted manner along the
organ [29]. This regionalized gene expression has been studied in various species (i.e.,
Human, rat, mouse and boar) and the high degree of gene expression segmentation
appeared to be a common feature shared between these species (for a review see [26].
It is now well described that in vertebrates, spermatozoa leaving the testis are unable
to fertilize the egg. Thanks to numerous works in animal models, it became clear that
the male gamete has to transit through a certain portion of the epididymis in order to
do so (for a review see [Indeed, this strongly evidences a role of the epididymis in
controlling and/or inducing the final maturation of the spermatozoa.
The mammalian epididymis holds an essential role in promoting the functional
maturation of spermatozoa, in addition to their prolonged storage in a viable state.
Both processes are supported by a highly specialized luminal microenvironment
that is created, and maintained, by the combined secretory and absorptive activity of
the lining epithelium. The molecular machinery it employs to regulate the tightly
coupled processes of exocytosis and endocytosis remain poorly understood.
In the epididymis, the dominant number of proteins present in the epididymal fluid
are synthesized by the epithelium in a region-specific manner and secreted into the
tubule lumen [30, 31], for a review see reference [32]. These secretion- absorption
events create a special microenvironment for sperm maturation and storage. Studies in
vitro have also shown that epididymosomes, the vesicles formed after apocrine secretion, exchange material with immature spermatozoa [33, 34], for a review see [35].
Spermatozoa entering the caput epididymis, progress through the corpus and
finally reach the cauda region, where they are stored. The transit through the coils
of the epididymis takes about 2-3 days in humans (it is longer in other species) and
the sperm can be stored in the cauda for another 2–3 days. During their transit in the
epididymis, sperm undergo maturation processes necessary for them to acquire
motility and fertility (Fig. 10.1). The final maturation of the male gamete, named
capacitation, will be completed in the female reproductive tracts.
10 An Overview of Male Reproductive Toxicants: Facts and Opinions