The retrograde intraaxonal transport of horseradish peroxidase in the chick visual system: A light and electron microscopic study

Abstract

Retrograde transport of horseradish peroxidase (HRP) from the region of retinal genglion cell axon terminals back to the cell bodies has been studied by light and electron microscopy. After injection of HRP into the chick optic tectum, it was taken up by axon terminals and unmyelinated axons as well as by other processes and cell bodies of the outer tectal layers. Subsequently the HRP was obseved in vesicles, multivesicular bodies, cup‐shaped organelles and small tubules within axons in the stratum opticum, optic tract, optic nerve and optic fiber layer of the retina with accumulation in the retinal ganglion cell bodies. Pinocytosis of extracellular HRP along the axon shaft was rare. After a short postinjection interval, HRP was found in organelles within the axons of the optic nerve but not in the extracellular spaces. After larger injections or longer postinjection intervals, extracellular HRP diffused from the injection site to the back of the eye, but none was found in the extracellular spaces of the retina; ganglion cells were the only cells of the retina which contained HRP product. HRP disappeared from the cell bodies 3–4 days after transport. These findings suport the concept of intraaxonal retrograde movement of HRP. Within axons the vesicles carrying HRP frequently were partially or completely surrounded by a regualr array of microtubules. Doses of colchicine greater than 5–10 µ/eye administered 4 days before tectal injection of HRP interfered with the uptake and/or transport of HRP. HRP also moved in an anterograde direction in membrane‐bound vesicles within the ganglion cell axons, although apparently more slowly and in smaller quantities than in the retrograde direction. The localization of HRP in neurons of the isthmo‐optic nucleus following intravitreal injections has also been studied. The marker enzyme was found in neuronal cell bodies 4 hours after injection, indicating a rate of retrograde transport of at least 84 mm/day in these neurons.