The myocardial plasma membrane during development: influence of glutaraldehyde fixation on the density and size of intramembranous particles.

The ultrastructure of the plasma membrane of mouse myocardial cells has been studied during development (from 10 days post-coïtum to the adult stage) using the freeze-cleave method, and the effect of glutaraldehyde-fixation on this structure has also been evaluated. Blebs, free of intramembranous particles (IMP), were found to be dependent upon the developmental stage and upon fixation with glutaraldehyde. On P and E fracture faces, the presence of IMP-free areas was age-dependent and fixation-independent. No difference was demonstrated in the IMP density during heart development. Fixation with glutaraldehyde retains in the plasma membrane approximately 40% of IMP, probably by preventing an alteration induced by glycerol. Whatever the developmental stage, E face-associated particles are significantly larger than P face-associated particles. The diameter of both types of particle increases slightly but irregularly as myocardial cells mature. Fixation reduces the particle size on both fracture faces in such a way that no significant difference between them can be demonstrated.