Histopathology of bone spicule pigmentation in retinitis pigmentosa

To evaluate bone spicule pigmentation, a fundus feature in retinitis pigmentosa (RP) formed by migration of pigment-containing cells to perivascular sites in the inner retina.

The authors performed light and electron microscopy, including immunocytochemistry, on the retinas from ten patients with RP and five normal donors.

The pigment-containing cells in regions of bone spicule pigmentation were derived from the retinal pigment epithelium (RPE). The translocated cells were remarkably polarized with a number of specializations characteristic of RPE cells in situ, but they did not contain lipofuscin granules and were not immunoreactive for cellular retinaldehyde-binding protein. The cells were linked by junctional complexes and formed epithelial layers around retinal vessels and next to the inner limiting membrane. Adjacent Müller cell processes contained glial fibrillary acidic protein-positive filaments and formed microvilli and intermediate junctions, resembling those in the external limiting membrane. Vascular endothelial cells adjacent to the translocated RPE cells were thin and fenestrated, resembling the choriocapillaris, and were separated from the pigmented cells by a layer of extracellular matrix similar in organization to Bruch membrane. Thickening of the matrix layer caused narrowing and occlusion of the vascular lumina.

The lack of lipofuscin granules and cellular retinaldehyde-binding protein immunoreactivity in the translocated RPE cells is probably related to the loss of photoreceptors. The development of fenestrations in the endothelial cells correlates with the leakiness of retinal vessels to fluorescein observed in some cases of RP. Narrowing and occlusion of vascular lumina by thickening of the surrounding layer of extracellular matrix may contribute to the loss of inner retinal neurons found in RP. These changes in the RPE, blood vessels, glia, and inner neurons warrant consideration in designing therapies to restore vision to degenerate retinas.