Regulation of Blood–Testis Barrier (BTB) Dynamics during Spermatogenesis via the “Yin” and “Yang” Effects of Mammalian Target of Rapamycin Complex 1 (mTORC1) and mTORC2 - International Review of Cell and Molecular Biology - page 307

and Mruk, 2010b ). The ES that is localized to the apical compartment and
is the only anchoring device between Sertoli cells and spermatids (steps
8-19 in the rat testis) is the apical ES. ES is associated with an extensive
actin filaments arranged in hexagonal bundles with unipolar orientation
that lie perpendicular to the Sertoli cell plasma membrane ( Mruk et al.,
2008 ; Yan et al., 2007 ). Interestingly, these actin filaments are noncontractile
in nature, thus they are not likely to be involved in germ cell movement as
developing germ cells are immobile cells per se, lacking all the cell move-
ment apparatus (e.g. lamellipodia) and Sertoli cells inside the seminiferous
epithelium are also not actively motile cells per se ( Mruk et al., 2008 ; Yan
et al., 2007 ). As the actin filament bundles at the ES are restricted only to the
Sertoli cell, but not in elongating/elongated spermatids, the ultrastructural
features of the apical ES and basal ES are essentially identical except that
actin filament bundles are found on both sides of Sertoli cells at the basal
ES, but restricted only to the Sertoli cell at the apical ES ( Cheng and Mruk,
2010b ). Interestingly, the protein composition in both apical and basal ESs
is quite different ( Cheng and Mruk, 2010b ). For instance, JAM-C, nectin-3,
β1-integrin, laminin-α3,-β3,-γ3 are restricted to the apical ES, and JAM-A
and -B are limited to the basal ES, whereas other proteins, such as CAR, are
found in both apical and basal ES ( Cheng and Mruk, 2010b ). At the apical
ES, other than AJ proteins that are usually found in epithelia/endothelia
(e.g. N-cadherin, β-catenin, nectin-2), TJ proteins, GJ proteins, and focal
adhesion complex (FAC, which is an anchoring junction at the cell-matrix
interface) proteins are also found, making this a hybrid junction ( Mruk
et al., 2008 ; Wong et al., 2008 ; Yan et al., 2007 ).
2.2.1. Basal ES
The basal ES is restricted to adjacent Sertoli cells near the basement mem-
brane at the site of the BTB, which is typified by the bundles of actin fila-
ments sandwiched in-between cisternae of endoplasmic reticulum and the
two opposing plasma membranes of Sertoli cells ( Cheng and Mruk, 2010b ).
While the ultrastructural features of basal ES are indifferent from the api-
cal ES, their constituent proteins are quite different as the basal ES shares
some similarity with conventional AJ. For instance, constituent adhesion
molecules at the basal ES are members of the cadherins and nectins family.
2.2.1.1. Cadherins
Being one of the major constituent proteins of AJs, the importance of cad-
herins is well demonstrated by the embryonic lethality of mice lacking