The fine structure of the axons and growth cones of the human fetal cerebral cortex.

The fine structure of axon cylinders and growth cones has been examined in serial sections through the marginal zone of the human cerebral cortex from 10 to 22 weeks of gestation. Neurotubules, mitochondria, agranular reticulum, coated vesicles, and synaptic vesicles both of the translucent and dense core variety are found throughout the axon cylinders. At periodic intervals along the axons, bulged-out regions display accumulations of either synaptic vesicles or transversely oriented neurotubules. Prominent, growth cone varicosities are encountered at the distal segment of the axon cylinders. In these, mitochondria, channels of both tubular and dilated agranular reticulum, dense core vesicles, and multivesicular bodies lie in a finely filamentous matrix. Processes characterized by their content of either agranular reticulum and synaptic vesicles or a finely filamentous meshwork are infrequently continuous with these varicosities. Dense membrane thickenings are associated with some varicosities and their processes. The ultrastructural features of the human fetal growth cones have been compared to growth cones previously described in other mammalian and submammalian species.