Electron microscopical study on the development of the satellite cell sheath in spinal ganglia

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Abstract
The spinal ganglia of the fowl have been studied from the third incubation day to the adult.The neuroblasts are the first cells that differentiate in the ganglionic anlage. Satellite cells, probably arising from the undifferentiated elements scattered among the neuroblasts, become detectable during the fifth incubation day. They appear starshaped with attenuated cytoplasmic expansions intervening between adjacent neuroblasts. At this time the neuroblasts largely outnumber the satellite cells.From the sixth incubation day connective tissue spaces, appearing as large intercellular gaps containing an amorphous material, become evident between satellite cells. During the second half of the incubation period each nerve cell is completely enveloped by a satellite cell sheath, which is in turn entirely surrounded by a connective tissue space. The satellite cells change in shape from star‐shaped to flat elements. By this time and even more so in adult life, satellite cells outnumber nerve cells.The satellite cells increase progressively in number, up to the ninth incubation day mainly through differentiation of undifferentiated cells; but later on by mitotic reproduction of fully differentiated satellite cells.The neuron‐satellite cell boundary becomes more and more complicated with time, thus enlarging in parallel with the increase in volume of the nerve cell. Adhering junctions may be observed along this boundary.The significance of the ergastoplasm, centrioles and cilia of the satellite cells is briefly discussed. Numerous adhering and scarce occluding junctions, some of which are temporary, link satellite cells.