Summary
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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.
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2.
Mononucleated myoblasts have a reduced amount of rough surfaced endoplasmic reticulum and a large number of ribosome clusters (polyribosomes). They contain some microtubules.
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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:
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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
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b)
“thick” filaments, at first wavy, scattered and not always up to their full size (90–120 Å diameter);
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c)
“thin” filaments (∼ 50 Å) which are difficult to visualize in our conditions when thed are scattered.
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4.
“Thick” (100–125 Å) and “thin” (∼ 50 Å) 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.
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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.
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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.
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Firket, H. Ultrastructural aspects of myofibrils formation in cultured skeletal muscle. Z. Zellforsch. 78, 313–327 (1967). https://doi.org/10.1007/BF00325316
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DOI: https://doi.org/10.1007/BF00325316