The development and differentiation of cell sin the spinal ganglion were studied by electron microscopy in human fetuses ranging from 12mm to 260mm crown rump lenght. At 12mm 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 30mm 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 50mm fetus. Two neuroblast cell types, a small cell contained large clumps of rough endoplasmic reticulum at peripehry, could be distinguished.
At 80mm 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 120mm 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 180mm to 260mm feuts. Nissl bodies appeared during this stage. The ganglion-satellite cell boundary became complicated with increaseing age, thus enlarging in
parallel
with the increase in volume of the nerve cell.
During next prenatal stage up to 180mm 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.
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