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E. Fahl, G. B. Jaissle, C. A. May, S. van de Pavert, A. Wenzel, E. Claes, U. Wolfrum, J. Wijnholds, P. Humphries, M. W. Seeliger; Bone Spicule Formation in Retinitis Pigmentosa: Insights From a Mouse Model. Invest. Ophthalmol. Vis. Sci. 2008;49(13):2199.
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Bone spicules are a hallmark of retinal degenerations of the Retinitis Pigmentosa (RP) group. In this work, we examined the process of bone spicule formation in a murine homologue of autosomal-recessive RP, the rhodopsin knockout (rho-/-) mouse.
rho-/- mice aged 3 to 16 months, representing the range from early to late stages of degeneration, were examined morphologically by light and electron microscopy of retinal sections and retinal whole mounts. The results were compared to fundus images of human RP patients.
Following the selective loss of the photoreceptor cells (i. e. both rods and cones), inner retinal vessels located in the outer plexiform layer made direct contact with the apical side of the retinal pigment epithelium (RPE). This situation caused RPE cells to leave their united cell structure and to migrate along these vessels towards the inner retina. Ultrastructurally, the migrating RPE cells directed their basal lamina towards the vessel and sealed it by tight junction linkage. Similar to the RPE-Choroid interface, the perivascular extracellular matrix comprised basal lamina, and vessels adjacent to the RPE cells developed endothelial fenestrations. In whole mounts, the arrangement of pigmented cell clusters outlining retinal capillaries correlated well with the bone spicule pigmentations in humans. The structure of the inner retina remained well preserved even in very late stages.
The following conclusions may be drawn from this study: (1) Bone spicules only form in areas devoid of photoreceptors, (2) Direct contact between the inner retinal vessels and the RPE apparently triggers the migration of RPE cells, (3) Bone spicules are made of RPE cell pigment and reflect the capillary network at the time of formation.The substantial similarity of the disease process between mouse and human makes the rho-/- mouse a valuable model for further insights in the dynamics of bone spicule formation.
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