Journal of Neuroscience, Vol 10, 1600-1614, Copyright © 1990 by Society for Neuroscience
Reticular astrocytes in the fish optic nerve: macroglia with epithelial characteristics form an axially repeated lacework pattern, to which nodes of Ranvier are apposed
A Maggs and J Scholes
MRC Cell Biophysics Unit, King's College, London, England.
Astroglia in lower vertebrate optic nerves are unusual: as shown recently
(Giordano et al., 1989; Rungger-Brandle et al., 1989), they express
abundant Type II cytokeratin, not glial fibrillary acidic protein (GFAP),
the cytoskeletal marker for astrocytes elsewhere. To determine the
implications for the glial cells of these epithelial-type cytoskeletons,
which are linked up by desmosomal junctions, we analyzed the tissue
patterning of fish optic nerve astroglia, which we term reticular
astrocytes on account of their uniquely specialized arrangement. The
processes of the reticular astrocytes fasciculate extensively with one
another in a pattern stabilized by the desmosomes, forming a network laid
out in thin planar sheets, or partitions. These are arranged transversely,
that is at right angles to the optic fibers and are repeated at regular
intervals of about 15 microns longitudinally throughout the optic nerve.
They merge periodically forming a 3-dimensional framework whose pattern we
speculate provides a flexible tissue skeleton for the optic nerve, capable
of accommodating eye movements. Virtually all fibers in mature regions of
the optic nerve are myelinated, and we show that nodes of Ranvier mostly
occur in register with the partitions, displaying perinodal astrocytic
associations resembling those found at CNS nodes in mammals. This
clustering may account for the unexpectedly high observed incidence of
neighbor pairs of nodes. Among other peculiarities associated with the
reticular astrocytic network, one is that up to 20% of all cells comprise
foamy macrophages not found elsewhere in the CNS.