Characterizing Retinal Sheet Transplants In Vivo Using Manganese-Enhanced MRI (MEMRI)

Abstract

Purpose: : To test the usefulness of manganese-enhanced MRI (MEMRI), a non-invasive measure of retinal function and structure, for studying retinal sheet transplants in retinal degenerate (RD) rats in vivo which can restore visual responses in RD models.

Methods: : Donor retinal progenitor sheets (size 1 mm²) of rat fetuses expressing human placental alkaline phosphatase (hPAP) were transplanted unilaterally to the subretinal space of pigmented RD S334ter line-3 rats (24-29 d). At 7 months post surgery, treated and control eyes of selected dark adapted rats (n =5) were studied by high resolution MEMRI (25 µm in plane). After MEMRI, eyes were fixed and retinal sections were stained with antibodies against hPAP (donor marker), together with antibodies against various retinal cell markers.

Results: : Untreated RD rat eyes on MEMRI examination had retinal thickness of 119±11 µm (mean ± S.E.M.). There were no differences between superior and inferior retina in the implant-free eyes (115±10 µm vs. 120 ±14 µm; p>0.05). In treated rat eyes, the transplant (grown to 2-3 mm²) was clearly demonstrated on MEMRI. Importantly, manganese uptake within the transplant was different (P < 0.05) than in the surrounding RD host retina and the control fellow eye. In the superior retina between 1200 to 2700 µm from the optic nerve, the transplant regions had a retinal thickness of 163±12 µm which was significantly different from that of control eyes or non-implanted regions (p<0.05). Transplants with areas of correct lamination and photoreceptor outer segments also demonstrated normal appearing dark adaptation on MEMRI examination.

Conclusions: : These initial studies highlight the feasibility of MEMRI for studying RD rats with restricted areas of retinal sheet transplants. Additional experiments are now underway to determine whether or not light / dark functioning of the transplants on MEMRI corresponds to correct lamination and photoreceptor outer segment integrity on histological examination.

Keywords: imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound) • retinal degenerations: cell biology • imaging/image analysis: non-clinical