Ganglion cells with intraretinal axon collaterals
have been described in monkey (Usai et al., 1991), cat
(Dacey, 1985), and turtle (Gardiner & Dacey, 1988)
retina. Using intracellular injection of horseradish peroxidase
and Neurobiotin in in vitro whole-mount preparations
of human retina, we filled over 1000 ganglion cells, 19
of which had intraretinal axon collaterals and wide-field,
spiny dendritic trees stratifying in the inner half of
the inner plexiform layer. The axons were smooth and thin
(∼2 μm) and gave off thin (<1 μm), bouton-studded
terminal collaterals that extended vertically to terminate
in the outer half of the inner plexiform layer. Terminal
collaterals were typically 3–300 μm in length,
though sometimes as long as 700 μm, and were present
in clusters, or as single branched or unbranched varicose
processes with round or somewhat flattened lobular terminal
boutons 1–2 μm in diameter. Some cells had a
single axon whereas other cells had a primary axon that
gave rise to 2–4 axon branches. Axons were located
either in the optic fiber layer or just beneath it in the
ganglion cell layer, or near the border of the ganglion
cell layer and the inner plexiform layer. This study shows
that in the human retina, intraretinal axon collaterals
are associated with a morphologically distinct ganglion
cell type. The synaptic connections and functional role
of these cells are not yet known. Since distinct ganglion
cell types with intraretinal axon collaterals have also
been found in monkey, cat, and turtle, this cell type may
be common to all vertebrate retinas.