| Summary: | In the eutherian mammal seminiferous epithelium, the cytoplasm of Sertoli cells
adjacent to certain regions of intercellular attachment exhibits unusual structural
complexes. At these sites, a layer of non-contractile, hexagonally packed actin filaments
is positioned between the Sertoli cell plasma membrane and an underlying cistern of
endoplasmic reticulum. The filament layer along with the endoplasmic reticulum and the
plasma membrane involved with attachment are referred to as an "ectoplasmic
specialization". Ectoplasmic specializations are found apically at sites of attachment to
spermatids and basally at sites of attachment to neighboring Sertoli cells. Similarly
structured complexes are not found in other cell types nor in Sertoli cells of other
vertebrate species. Ectoplasmic specializations of eutherian mammals have been
hypothesized to be a highly specialized form of actin-related adhesion junction. If this is
true, then non-mammalian homologues of this junction should be present and, in
general, should more closely resemble typical actin-related adhesion junctions found in
other cell types than do mammalian ectoplasmic specializations. I have tested this
prediction by conducting a systematic comparative survey of the morphology and
composition of actin-related adhesion junctions in Sertoli cells of non-mammalian
vertebrates. My results indicate that vertebrate Sertoli cells use actin-related adhesion
junctions as a common mechanism for adhesion to elongating spermatids and to
neighboring Sertoli cells. In non-mammalian vertebrate classes, the junctions appear to
possess contactile properties and, in general, more closely resemble the actin-related
adhesion junctions of other cell types then do eutherian ectoplasmic specializations.
Structural differences in the junctions between classes indicate that different strategies
for intercellular adhesion have evolved at these sites within various vertebrate classes.
These data suggest that eutherian mammal ectoplasmic specializations represent a
modified form of actin-related adhesion junction. I speculate on what the precursors of the present-day eutherian junctions may have been like as well as on how and why
structural adaptations may have occurred at these sites.
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