Journal of Neuroscience, Vol 6, 487-497, Copyright © 1986 by Society for Neuroscience
Development of ultrastructural specializations during the formation of acetylcholine receptor aggregates on cultured myotubes
AJ Olek, A Ling and MP Daniels
The ultrastructure of cultured rat myotubes was examined at stages in the
initial assembly of acetylcholine receptor (AChR) aggregates in order to
elucidate the role of cell-surface specializations in aggregate formation.
Within 4-6 hr, embryonic brain extract (EBX) induces the formation of sites
of AChR density elevated 5-9 X above that of surrounding regions, and the
appearance of these aggregates is preceded by the formation of clouds of
punctate microaggregates (Olek et al., 1983). A video image-intensification
system was used to monitor this redistribution of fluorescently labeled
AChR, and sites of aggregation were mapped on identified myotubes. After
processing the cultures for electron microscopy, thin sections were taken
through identified aggregate sites at various stages in assembly.
Specializations, including a basal lamina, mound-shaped plasma membrane
contours with occasional deep infoldings, and a subjacent dense
cytoskeletal specialization, which tended to exclude other cytoplasmic
organelles, were associated with newly formed aggregates found 4-6 hr after
adding EBX to the cultures. Analysis of random thin sections through
EBX-treated and untreated myotubes showed that the extent of
specializations of the basal lamina and cytoplasm was approximately
threefold greater in cells exposed to EBX for 4 hr, suggesting a
concurrent, and possibly interdependent, organization of such
specializations with AChR aggregate assembly. Examination of sections
through clouds of microaggregates, which formed within 90 min, revealed
mound-shaped plasma membrane contours and underlying cytoplasm depleted of
organelles but relatively little basal lamina and submembrane cytoskeletal
density. These results suggest that the initial stage of AChR aggregate
assembly involves relatively subtle changes in the structure of the cell
cortex and that the evolution of microaggregates to aggregates may require
the formation of additional cytoskeletal and extracellular matrix
structures.
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