Fibroglial proliferation in pars planitis.

In a 51-year-old patient with long-standing pars planitis, light and electron microscopical examination of the intravitreal 'snowbank' revealed a fibrovascular layer adjacent to the hyperplastic nonpigmented epithelium of the pars plana and an extensive fibroglial proliferation within the vitreous base. The fibrovascular layer consisted of well-differentiated capillaries, probably emanating from the peripheral retina, interspersed with aggregated vitreous fibrils. The fibroglial portion of the 'snowbank' was composed of fibrous astrocyte-like cells which had secreted basement membranes and larger diameter collagen fibrils. This fibroglial tissue was in direct continuity with an ultrastructurally similar preretinal membrane. No significant choroiditis or cyclitis could be demonstrated. It is hypothesized that, in pars planitis, a primary inflammatory process of the peripheral retina and vitreous base may stimulate the observed preretinal and intravitreal fibroglial proliferation.