RESUMEN
The development and differentiation of cells in the spinal ganglion were studied by electron microscopy in human fetuses ranging from 12 mm to 260 mm crown rump length. At 12 mm embryo the primitive neuroblasts which had a single process, contained a large numbers of free ribosome and mitochondria but very little rough endoplasmic reticulum. At 30 mm fetus, the primitive spinal ganglion consisted of bipolar neuroblasts, satellite cells and undifferentiated cells. Spindle-shaped bipolar neuroblasts formed spinal ganglion of loosely grouped cells at 50 mm fetus. Two neuroblast cell types, a small cell contained large clumps of rough endoplasmic reticulum at periphery, could be distinguished. At 80 mm fetus, the spinal ganglion constituted of bipolar neuroblast with apparently random distribution of small and large neurons with processes, together with satellite cells and blood vessels. The presences of a large numbers of neurotubules in the Golgi-central region were one of the first sign of further maturation of the neuroblast. During next prenatal stage from 120 mm on fetus, the ganglion cells were large and contained much rough endoplasmic reticulum, neurotubules and extensive Golgi complex. A large number of neuroblasts became transformed into unipolar cells from 180 mm to 260 mm feuts. Nissl bodies appeared during this stage. The ganglion-satellite cell boundary became complicated with increasing age, then enlarging in parallel with the increase in volume of the nerve cell. During next prenatal stage up to 180 mm fetus, the unipolar ganglion cell increased in number and size, and the cytoplsm contained all intracytoplasmic structures which were also found in mature spinal ganglion except for large pigment granules.