Cytoskeletal Architecture of the Matrix Cell and Neuroblast in the Neural Tube of the Chick Embryo

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Cytoskeletal architecture of the matrix cell and neuroblast in the wall of midbrain of 4-6 day-old chick embryos was examined by electron microscopy and immunohistochemistry. The matrix cell, the undifferentiated stem cell later producing neurons and glial cells in the central nervous system, is characterized ultrastructurally by abundant free ribosomes and a poorly developed cytomembrane system. A few microtubules running in random directions are observed in the matrix cell body. In the cell processes, microtubules are oriented longitudinally, and linked with each other by cross-bridges, presumably composed of microtububeassociated proteins (MAPs). The cell processes contain abundant cytoplasmic filaments including a large amount of actin filaments which adhere to the plasma membrane of junctional complexes located immediately below the inner surface of the neural tube.
In the neuroblast which has been differentiated from the matrix cell, the cytomembranous organelles, especially rough endoplasmic reticulum are markedly better developed than in the matrix cell; microtubules are more numerous in the cell body. The cell process contains many microtubules with cross-bridges and a few intermediate filaments, which are relatively characteristic of the cytoskeleton of the neuroblast.
Phalloidin-staining and immunohistochemistry showed that the neuroblast was richer in F-actin, β-tubulin, MAP1, MAP2, tau, calspectin, and synapsin I than the matrix cell. As the matrix cell differentiates into the neuroblast, both the cytoskeletal and cytomembranous systems proved to develop features, characteristic of a neuron.