Development of glycine-accumulating neurons in retinal transplants

Previous studies have shown that the fetal retina not only survives transplantation but also continues to develop and differentiate in the host eye. Several structural and functional proteins have been demonstrated in the transplanted retinas, and the presence of such proteins has been taken as evidence for the capability of retinal transplants to function. Glycine is an important inhibitory neurotransmitter and is found in a large number of the retinal neurons. Uptake of glycine rather than de novo synthesis is the main source of glycine in glycinergic neurons. The present study examined whether glycine-accumulating neurons develop normally in rabbit retina transplants. Embryonic day (E) 15 rabbit retinas were transplanted into the eyes of adult rabbits of the same strain. Transplants were allowed to survive for various times so that the grafts attained the equivalent ages of (donor age + survival time) E 19, 22 and 29 and postnatal days (PN) 2, 5, 9, 12, 19 and 58. On formaldehyde-fixed cryostat sections of these transplants, glycine-accumulating neurons were demonstrated by immunohistochemistry by using an antibody against one of the glycine transporters: GLYT1. Immunoreactivity was first detected 2 days before birth and increased with age until it reached its mature level at PN 19. The immunoreactivity was found in cells belonging to the inner retinal layers, and in plexiform layers of the transplant equivalent to the normal inner and the outer plexiform layers. In places these cells integrated well with similar cells in the host. In the host retina, the immunoreactivity was found in proximal cell layers of the inner nuclear layer, in certain bipolar cells, and in the inner and the outer plexiform layers. The immunoreactivity was preserved even in the degenerated retina overlying the retinal graft. In conclusion, the present study demonstrates that glycine-accumulating neurons develop, integrate and survive in retinal transplants.