Substructure of cytoplasmic dense bodies and changes in distribution of desmin and α‐actinin in developing smooth muscle cells

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Abstract
The substructure of assembling cytoplasmic dense bodies (CDBs) and changes in the distribution of desmin and α‐actinin during development of smooth muscle were studied in gizzard samples from 10‐ and 16‐day embryos and from 1‐ and 7‐day post‐hatch chickens. CDBs in these cells lack the density of CDBs in mature or adult smooth muscle cells and, thus, allow observations of the changes inside CDBs. The random filament orientation seen in younger embryonic cells is first modified to include relatively small patches of IFs that are somewhat straighter and are approaching a side‐by‐side arrangement. As development proceeds, the IFs in these arrays become straighter, are parallel over longer lengths of the IFs and later acquire the density characteristic of mature CDBs. Anti‐desmin labeling in embryonic 10‐ and 16‐day cells showed that desmin intermediate filaments (IFs) were located in the myofilament compartment but were concentrated in or near assembling CDBs. Anti‐desmin labeling shifted to the perimeter of CDBs after hatching. Cross sections, longitudinal sections, and stereo pairs all show that IF profiles are present inside unlabeled assembling CDBs. Anti‐α‐actinin labeling was directly on CDBs and was often associated with the cross‐connecting filaments (CCFs) (average diameter of 2–3nm) inside CDBs. We propose, based on these data, that desmin IFs, α‐actinin‐containing CCFs, and actin filaments are the principal components of the substructure of assembling CDBs. We also present a proposed model for CDB assembly.