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2.2.2. Apical ES
In rodents, the apical ES, once it appears, is the only anchoring device
between Sertoli cells and elongating spermatids (step 8-19 in rats). Besides
conferring adhesion and structural support to developing spermatids, the
apical ES also confers spermatid polarity during spermiogenesis so that the
heads of developing spermatids are pointing toward the basement mem-
brane, thus, the maximal number of spermatids can be packed in the semi-
niferous epithelium of a tubule ( Wong and Cheng, 2009 ). Although the
actin filament bundles, the hallmark ultrastructure of the ES, are only visible
on the Sertoli cell, not the spermatid, at the apical ES ( Cheng and Mruk,
2010b ; Mruk et al., 2008 ), but the stage-specific expression of cadherins
( Johnson and Boekelheide, 2002 ; Lee et al., 2003 ), nectin-3 ( Ozaki-Kuroda
et al., 2002 ) and laminin-α3, -β3, and γ-3 chains ( Koch et al., 1999 ; Siu and
Cheng, 2004 ; Yan and Cheng, 2006 ) by the spermatids during the epithelial
cycle suggest that spermatids also play a role in establishing the apical ES.
Apical ES is the strongest anchoring devices between Sertoli cells and sper-
matids (steps 8-19), significantly stronger than DSs between Sertoli cells
and spermatids (steps 1-7) ( Wolski et al., 2005 ). This unusual adhesive force
is contributed by a number of factors. For instance, nectin-3 is exclusively
expressed by elongating/elongated spermatids in the testis and this enables
the formation of heterotypic trans -interaction between nectin-3 from germ
cells and nectin-2 from Sertoli cells to yield a strong cell-cell adhesion.
Furthermore, the hybrid nature of the apical ES also supports its adhesive
strength. Among the different junction proteins present at the apical ES, it
is believed that the interaction between laminin-333 (composed of laminin
α3, β3, γ3 chains) from elongating/elongated spermatids and the α6β1-
integrin from Sertoli cells contribute significantly to its adhesive force ( Pal-
ombi et al., 1992 ; Salanova et al., 1995 ; Yan and Cheng, 2006 ). Interestingly,
besides performing the anchoring function at apical ES, the laminin-333-
α6β1-integrin protein complex also participates in regulating BTB integ-
rity at the apical ES-BTB-hemidesmosome axis ( Fig. 6.2 ). It was proposed
that during spermiation, laminin chains at the apical ES was cleaved by
matrix metalloproteinases, such as MMP-2, which was highly expressed at
the apical ES at stage VIII of the epithelial cycle ( Siu and Cheng, 2004 ), to
facilitate the release of mature spermatids at spermiation ( Yan et al., 2008a ).
Some of these fragments of laminin chains, which were shown to regulate
cell-adhesion function in other epithelia ( Yan et al., 2008b ) were shown to
perturb the Sertoli cell TJ-permeability barrier function ( Yan et al., 2008a ).
This functional axis between the apical ES and the BTB was confirmed by
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