Characterization of the macrophages associated with the tunica vasculosa lentis of the rat eye.

PURPOSE The tunica vasculosa lentis (TVL) is a transient vascular network surrounding the developing lens that regresses prenatally in humans and postnatally in rodents. Macrophage-like cells, sometimes referred to as hyalocytes, have been postulated to play a role in the regression of this vascular system; however, the precise identity of these cells is still unclear. The aim of the present investigation was to provide phenotypic data on these cells in combination with their three-dimensional distribution on the lens surface during the period when regression of the TVL is taking place. To this end the authors have used a novel combination of silver-enhanced immunogold labeling and environmental scanning electron microscopy (ESEM). METHODS The eyes of Wistar rats at various pre- and postnatal ages (E20, postnatal days [D]0, 2, 5, 7, and 10) were studied by either conventional scanning electron microscopy (SEM) or ESEM. The latter was used to study specimens that had been incubated with various antileukocyte monoclonal antibodies (ED1, ED2, Ox6, Ox42), followed by immunolabeling with gold-conjugated secondary antibody visualized by silver enhancement. RESULTS Conventional SEM of the developing lens revealed a pattern of radial and interconnecting vessels in the TVL similar to previous studies. In addition, large numbers of cells with the morphologic characteristics of macrophages were noted on the lens surface closely associated with the vessels. The gradual attenuation and regression of vessels was noted over the course of the time period investigated. Immunolabeled specimens examined by ESEM revealed that most of the macrophage-like cells were indeed ED1(+) and ED2(+) (both pan-macrophage markers) and MHC class II(-) (Ox6) and CD11b/18(-) (Ox42): a phenotype characteristic of macrophages. This phenotype altered little between E20 and D10. CONCLUSIONS The cells surrounding the developing lens that are postulated to play a role in regression of the TVL have the morphologic and immunophenotypic characteristics of resident tissue macrophages similar to those previously identified in the adult rodent uveal tract and the vitreous (hyalocytes). This phenotype differs from that of dendritic cells and microglia; however, it is postulated that lens-associated macrophages are ideally located to act as a source of retinal microglia after completion of their role in TVL regression.