Developmental changes of the endoplasmic reticulum and ribosomes in nerve cells of the spinal ganglia of the domestic fowl,

The changes of the endoplasmic reticulum (ER) and ribosomes in developing nerve cells of the spinal ganglia of the domestic fowl have been studied by light and electron microscopy. The thin rim of cytoplasm encircling the nucleus of the undifferentiated cells contains numerous free ribosomal clusters and a scanty endoplasmic reticulum. The primitive neuroblasts are spindle‐shaped and their two processes contain filamentous structures. In comparison with the undifferentiated cells, primitive neuroblasts have many more free ribosomes and a prominent rough‐surfaced ER consisting of long, interconnected channels often continuous with the nuclear envelope. The remarkable development of the ER is one of the main characteristics of differentiating ganglionic nerve cells. Subsurface cisternae become apparent first in the primitive neuroblasts. The ER and ribosomes of the intermediate neuroblast are confined in the peripheral region of the perikaryon; mitochondria, Golgi complexes, dense bodies, microtubules, neurofilaments, etc. are located more centrally. Units of the smooth‐surfaced ER appear first in the intermediate neuroblasts. In the pseudo‐unipolar nerve cells, the cisternae of the ER and the ribosomes are mainly grouped in discrete areas (Nissl bodies). The rough‐surfaced ER is more developed and highly organized in the intermediate neuroblasts and especially in the pseudo‐unipolar nerve cells than in the primitive neuroblasts. In the pseudo‐unipolar nerve cells, peculiar relationships between ER and Golgi complexes have been observed. Both free and membrane‐attached ribosomes are arranged in clusters (polysomes) in all the developmental stages of the ganglionic nerve cells. The highest number of ribosomes in the membrane‐attached polysomes is at least 13. The electron microscopic images suggest that the ER of the developing neuroblasts might take its origin from the nuclear envelope and afterwards might increase in volume through an intrinsic expansive growth of its membranes.