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Cheng, 2004
). Spermatogenesis is a complex and precisely regulated pro-
cess that produces spermatozoa (haploid, 1n) from spermatogonia (diploid,
2n). Spermatogenesis is also tightly controlled by the hypothalamic-pitu-
itary-testicular hormonal axis. This axis involves the production of gonado-
tropin-releasing hormone (GnRH) from the hypothalamus that induces the
secretion of follicle-stimulating hormone (FSH) and luteinizing hormone
(LH) from the pituitary gland. LH then stimulates the release of testosterone
from Leydig cells via steroidogenesis to support Sertoli and germ cell func-
tion, which together with FSH that exerts its effects exclusively on Sertoli
cells. These hormones together with locally produced hormones (e.g. inhibin,
activin), steroids (e.g. estradiol-17β), and paracrine and autocrine factors (e.g.
cytokines, fragments of laminins and collagens), thereby maintaining sper-
matogenesis in a unique microenvironment in the seminiferous epithelium
(
Carreau and Hess, 2010
;
Cheng and Mruk, 2012
;
O'Donnell et al., 2001
;
Sharpe, 1994
;
Walker, 2011
;
Winters and Moore, 2007
). During spermato-
genesis, a single type A spermatogonium undergoes 10 successive rounds of
mitosis to give rise to 1024 primary spermatocytes, which then enter meiosis
to produce 4096 spermatids theoretically (
Cheng and Mruk, 2012
;
Ehm-
cke et al., 2006
). Spermatids then undergo maturation via spermiogenesis to
form spermatozoa which are to be released into the tubule lumen at sper-
miation (
O'Donnell et al., 2011
). However, it is estimated that the efficiency
of spermatogenesis is only ∼25%, and the majority of germ cells undergo
apoptosis, which is regulated by estrogen produced by Leydig cells, Sertoli
cells and germ cells (
Barratt, 1995
;
Shaha, 2008
;
Tegelenbosch and de Rooij,
1993
). This is to prevent overwhelming the capacity of Sertoli cells since each
Sertoli cell can support ∼30-50 developing germ cells (
Billig et al., 1995
;
Weber et al., 1983
). During spermatogenesis, the seminiferous epithelium can
be organized into 14 stages in rats (stage I-XIV); 12 stages (stage I-XII) in
mice and six stages (I-VI) in humans according to the different developmen-
tal stages of germ cells, in particular, the association of developing spermatids
with Sertoli cells (
de Kretser and Kerr, 1988
;
Hess and de Franca, 2008
;
Mruk
et al., 2008
;
Parvinen, 1982
). Throughout the seminiferous epithelial cycle,
germ cells have to traverse the seminiferous epithelium, from the basal to the
adluminal (apical) compartment, and finally reach the luminal edge of the
seminiferous tubule at spermiation. This timely translocation of germ cells is
synchronized with a series of cyclic junctional restructuring events at the Ser-
toli-Sertoli and Sertoli-germ cell interface (
Cheng and Mruk, 2010b
,
2012
).
These events are tightly regulated and precisely coordinated, their disruption
can perturb spermatogenesis, leading to infertility.
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