Nevertheless, labeled fibers originating in the graft could at times be
seen crossing the graft–host border. Such bridging was observed at all
survival times, but not in all specimens examined (in 8 of 11), and
only in regions where the host photoreceptor layer was absent. In
fortuitous cases, it was possible to follow a labeled process from the
graft all the way into the host inner plexiform layer. Examples of this
are given in
Figures 2 and 3 . A long thin bridging process is seen
originating in a graft corresponding to PN 12 (28 days after
transplantation;
Fig. 2A ) and a shorter one is seen in
Figure 2B in a
transplant corresponding to PN 20 (36 days after transplantation). As a
result of the degeneration of the host outer layers, NOS-containing
cells and fibers occasionally were found in the grafts relatively near
the host inner plexiform layer. In
Figure 2B 2a bridging process is
seen to run between the host inner plexiform layer and a more distal
area where several immunolabeled structures are observed. Because no
NOS-immunoreactive fibers were seen external to the inner plexiform
layer in a normal rabbit retina
(Fig. 1B) , the bridging process seen in
Figure 2B is also likely to connect structures within the graft with
the host retina. In
Figure 3A , several stained processes are seen
within a graft corresponding to PN 45 (61 days after transplantation).
In addition, a process arising from a NOS-immunolabeled cell located in
the graft is seen to extend toward the host inner plexiform layer.
Serial sections confirmed the location of this cell. Further, a stained
cell body also is seen in the amacrine cell layer of the host retina,
and strong and continuous labeling is noted in the host inner plexiform
layer, reflecting the relative organization of the inner host retina.
The immunolabeled fiber, seen in
Figure 3A 3B 3C 3D 3E 3F 3G 3H 3I 3J 3K 3L 3M 3N 3O 3P 3Q 3R each almost
perpendicularly the host inner plexiform layer, can thus be judged as a
bridging process that originates in the graft. In
Figure 3B 3a stained
cell body is seen in the host retina 106 days after transplantation.
One strongly labeled cell body also is seen in the transplant
(corresponding to PN 90) to project to a region within the graft where
faintly stained fibers are seen. In addition, one large
NOS-immunoreactive cell body is seen to project to the same region
within the graft and to emit a more weakly labeled process toward the
host inner plexiform layer. As mentioned above, a progressive loss of
the outer host retina is normally observed, which with time brings the
graft closer to the host inner layers. Again, a labeled cell is seen in
the host retina in its expected position, next to the inner plexiform
layer. The connecting cell could therefore, judging from its position,
belong to the transplant. However, it is not possible to determine
conclusively in this case whether the bridging cell indeed belongs to
the graft.