Ultrastructural aspects of the organization of somites and the early differentiation of myotomes in the embryo of the common toad Bufo bufo L

Brustis, J.J.

Archives de Biologie 90(3): 261-272


ISSN/ISBN: 0003-9624
PMID: 121682
Accession: 068521145

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Unsegmented mesoderm cells are separated by important intercellular lacunae but remain in reciprocal contact by filopods and particular junctions. The constituting cells of the newly isolated somites have the same characteristics as those of the unsegmented paraxial mesoderm. The earliest modifications associated with cell differentiation are first observed in the middle zone of the somite and then progress towards the dorsal and ventral zones. The cells bordering the intersomitic gaps (A type cells) lengthen towards the center of the somite; ones that are situated more deeply (B type) become fusiform. The 1st myofilaments are synthesized near the myoblast nuclei. The fusion between A and B cells results in the formation of giant plurinucleated cells whose length is equal to that separating 2 consecutive intersomitic boundaries. The myofilament synthesis and the typical differentiation of the myofibrils which started in the middle zone spreads towards the septal zones. A central-peripheral polarity of differentiation is defined in the somite. Two reverse polarities work simultaneously in the genesis of myoblasts and muscle fibers: while the myofilaments first appear in the middle zone of the myoblasts and progress towards their septal ends, the fusion between myoblasts starts in the septal zone and later extends to the central cells.