158
Another intriguing feature of spermatogenesis is the distinct ordering of cell
associations along the length of the seminiferous tubules (segments), often referred
to as the ‘wave of the seminiferous epithelium’ [9]. A segment is defined as a longitudinal portion of seminiferous tubule corresponding to a single cell association or
stage (for review see [10, 11] . A wave encompasses all 14 segments in rat, 12 in
mouse, and 6 in human (for review see reference [12], but the number of discernible
stages in human has been suggested to be increased from 6 to 12, based on acrosome development [13]. At a given point in the tubules, the chronological succession of all stages constitutes a cycle of the seminiferous epithelium. The duration of
a cycle corresponds to the interval of time separating two waves of entry of spermatogonia into spermatogenesis. A complete spermatogenesis requires four to five
successive cycles. In the longitudinal direction of the tubule, the spermatogenic
process is also coordinated because the cell associations succeed one another in an
order corresponding to the numerical order of the stages of the cycle. That succession corresponds to the spermatogenetic wave [14].
10.2.2 The Sertoli Cell-Germ Cell Communication System
Due to its unique strategic position within the seminiferous tubules, the Sertoli cell
is an orchestrator of the testicular microenvironment. It will play a key role in the
paracrine control of spermatogenesis. The BTB established by the sustentacular
Sertoli cells will divide the seminiferous epithelium into a basal and an adluminal
compartment, thus fulfilling several functions such as the promotion of the seminiferous epithelium architecture; the formation of the environment required for both
meiosis and spermiogenesis by preventing a numerous bloodborne substances
entering the organ through the testicular blood and lymph streams from penetrating
into the depths of the seminiferous epithelium and reaching germ cells; a segregation of the Sertoli cell plasma membrane into distinct regions (for review, see references [15, 16].
Sertoli cell also produce the seminiferous tubule fluid and secrete hundreds of
proteins, peptides and steroids, most of which remain to be characterized. The
tubule fluid is essential for the nutrition of germ cells, but also for transport of
chemical substances, the release of spermatozoa and their transport to the epididymis. Most, if not all Sertoli cell products are destined to support the germ cell lineage through the entire spermatogenic process. These physical and chemical
microenvironments are required for the completion of each of the different steps of
spermatogenesis. Sertoli cell production can be classified as follows: transport or
soluble binding proteins, components of the extracellular matrix, components of
Sertoli cell membranes and junctional complexes, proteases, energy metabolites
and factors involved in germ cell division, differentiation and metabolism.
Interestingly, the seminiferous tubules are also considered an immune sanctuary as
tight junctions between Sertoli cells keep surface antigens against developing germ
cells from escaping into the bloodstream and prompting autoimmune responses [17].
C. Pineau
Another intriguing feature of spermatogenesis is the distinct ordering of cell
associations along the length of the seminiferous tubules (segments), often referred
to as the ‘wave of the seminiferous epithelium’ [9]. A segment is defined as a longitudinal portion of seminiferous tubule corresponding to a single cell association or
stage (for review see [10, 11] . A wave encompasses all 14 segments in rat, 12 in
mouse, and 6 in human (for review see reference [12], but the number of discernible
stages in human has been suggested to be increased from 6 to 12, based on acrosome development [13]. At a given point in the tubules, the chronological succession of all stages constitutes a cycle of the seminiferous epithelium. The duration of
a cycle corresponds to the interval of time separating two waves of entry of spermatogonia into spermatogenesis. A complete spermatogenesis requires four to five
successive cycles. In the longitudinal direction of the tubule, the spermatogenic
process is also coordinated because the cell associations succeed one another in an
order corresponding to the numerical order of the stages of the cycle. That succession corresponds to the spermatogenetic wave [14].
10.2.2 The Sertoli Cell-Germ Cell Communication System
Due to its unique strategic position within the seminiferous tubules, the Sertoli cell
is an orchestrator of the testicular microenvironment. It will play a key role in the
paracrine control of spermatogenesis. The BTB established by the sustentacular
Sertoli cells will divide the seminiferous epithelium into a basal and an adluminal
compartment, thus fulfilling several functions such as the promotion of the seminiferous epithelium architecture; the formation of the environment required for both
meiosis and spermiogenesis by preventing a numerous bloodborne substances
entering the organ through the testicular blood and lymph streams from penetrating
into the depths of the seminiferous epithelium and reaching germ cells; a segregation of the Sertoli cell plasma membrane into distinct regions (for review, see references [15, 16].
Sertoli cell also produce the seminiferous tubule fluid and secrete hundreds of
proteins, peptides and steroids, most of which remain to be characterized. The
tubule fluid is essential for the nutrition of germ cells, but also for transport of
chemical substances, the release of spermatozoa and their transport to the epididymis. Most, if not all Sertoli cell products are destined to support the germ cell lineage through the entire spermatogenic process. These physical and chemical
microenvironments are required for the completion of each of the different steps of
spermatogenesis. Sertoli cell production can be classified as follows: transport or
soluble binding proteins, components of the extracellular matrix, components of
Sertoli cell membranes and junctional complexes, proteases, energy metabolites
and factors involved in germ cell division, differentiation and metabolism.
Interestingly, the seminiferous tubules are also considered an immune sanctuary as
tight junctions between Sertoli cells keep surface antigens against developing germ
cells from escaping into the bloodstream and prompting autoimmune responses [17].
C. Pineau