Ultrastructural aspects of myofibrils formation in cultured skeletal muscle.

Abstract

1. Tissue cultures of chick embryo muscles are prepared for electron microscopy after identification with the light microscope of the cells concerned. A new device for flat face embedding is briefly described. 2. Mononucleated myoblasts have a reduced amount of rough surfaced endoplasmic reticulum and a large number of ribosome clusters (polyribosomes). They contain some microtubules. 3. Differentiation of myofilaments proper begins only in plurinucleated sarcoblasts (myotubes). The number of ribosomes decreases while scattered filaments accumulate. These are of three types: a) microtubules, mainly abundant at first, perhaps related to the numerous cell centers, have a general orientation along the main axis of the fiber. They decrease in number as appear b) “thick” filaments, at first wavy, scattered and not always up to their full size (90–120 A diameter); c) “thin” filaments (∼ 50 A) which are difficult to visualize in our conditions when thed are scattered. 4. “Thick” (100–125 A) and “thin” (∼ 50 A) filaments associate in bundles of myofilaments where they regularly alternate. The first bundles are homogenous along their length and show no sign of sarcomeric differentiation. 5. Z substance appears in or near the membrane of the extending T system. This Z substance develops into more compact Z bodies that align themselves more or less on neighbouring small bundles of myofilaments. In their immediate neighbourhood, the thick filaments disappear so that a very narrow and later widening I band occurs. At the same time bigger and denser granules than ribosomes, probably glycogen, appear between the bundles. Z bodies decrease later to form a Z line, neighbouring bundles joining up. The structure of the adult muscle myofibrils is finally reached. 6. These results are compared to others in the litterature and an attempt is made to give a synthetic picture of the myofibrils formation in Vertebrate skeletal muscle.