Cytodifferentiation and synaptogenesis in the neostriatum of fetal and neonatal rhesus monkeys.

Cytodifferentiation and synaptogenesis in the neostriatum (caudate nucleus and putamen) were analyzed by the Golgi impregnation method and electron microscopy in 14 fetuses and 8 postnatal rhesus monkeys. During the second fetal month the neostriatum consists primarily of simple, mostly bipolar, immature cells and a small number of undefined profiles ending with growth cones. The first morphologically defined synapses appear in the putamen at embryonic day 60 ( E60 ) and in the head of the caudate nucleus at E65 . Synaptic density in both structures is less than one per 1000/micron2 of neuropil at this stage; synapses are characterized by asymmetric junctions between axonal profiles and immature dendritic shafts, accumulation of an intermembrane web and aggregation of round clear vesicles in presynaptic profiles. During the third fetal month neuronal cell bodies and glial cells enlarge, and axonal and dendritic processes in Golgi preparations become more complex. Although the basic morphology of synapses remains unchanged, their density increases to 9/1000 micron2 in the putamen and 3.7/1000 micron2 in the caudate. During the fourth fetal month the four principal cell classes of the neostriatum emerge. Spines on the shafts of dendrites are followed closely by the appearance of axospinous synapses. Synaptic density in the putamen is still significantly higher (10.1/1000 micron2) than in the caudate (5.4/1000 micron2), but by the end of the fifth fetal month ( E150 ) it is the same (80/1000 micron2) in both structures. A dramatic increase in synaptic density to 125/1000 micron2 occurs before term ( E165 ) with the emergence of the first asymmetric synapses as well as symmetric synapses with flat or pleomorphic vesicles that terminate predominately on dendritic shafts. Synaptic density continues to increase after birth, reaching a plateau of approximately 190/1000 micron2 at the end of the first postnatal month. Throughout postnatal development the proportions of symmetric and asymmetric synapses on the smooth dendritic shafts undergo systematic fluctuations which may reflect the ingrowth of various afferents as well as local cytological differentiation including the formation of cellular compartments.