Ultrastructure of exogenous peroxidase in cerebral cortex

Protein uptake and transport and the intercellular spaces were studied electron microscopically in normal rabbit, cat and monkey cerebral cortex using horseradish peroxidase (HRP) as a cytochemical tracer. Extensive endocytosis and accumulation of tracer were seen as a result of delivering high concentrations of HRP into the intercellular space followed by long observation experiments, up to 120 h. Incorporation of HRP into neurons and their processes after a single application was studied by following the progress of neuron organelle labeling; the disappearance of tracer from these structures gave new information on the optimal times when HRP could be seen as a cellular tracer in cortical neurons. Neurons and their processes demonstrated a remarkable facility to take up peroxidase in coated invaginations and vesicles and accumulate it in multivesicular and other lysosomal bodies. The first structures labeled were coated vesicles, synaptic vesicles, sacs and tubules. HRP disappeared from the intercellular space after 24 h, and coated vesicles and synaptic vesicles no longer contained it at this time. Sacs and tubules were free of tracer by 48 h. Multivesicular bodies contained HRP after 30 min and lost all label between 72–96 h. Dense bodies became labeled after 1.5 h, and remained labeled until 96–120 h. Evidence of axon transport of the tracer was added to that extant on this pathway.