Haemopoietic phagocytes in the early differentiating avian retina

The existence of specialised phagocytic cells is described in regions of the retinal neuroepithelium undergoing intense cell death during early differentiation of the avian embryo retina (2.5-5 days of incubation). These results were obtained using routine techniques for light microscopy, acid phosphatase histochemistry and immunocytochemical staining with antibodies MB-1 and QH-1, both specific for quail endothelial cells and all blood cells except mature erythrocytes. Specialised phagocytes were distinguishable from neuroepithelial cells on the basis of morphological criteria: in the former, the nucleus was not oval in shape and was not oriented perpendicular to basement membrane neuroepithelium. The cytoplasm of the specialised phagocytes was often filled with dead cell fragments. In contrast to neuroepithelial cells, the specialised phagocytes showed acid phosphatase activity and were labelled with both MB-1 and QH-1 antibodies in normal quail embryos and chick----quail yolk sac chimeras. Moreover, some acid phosphatase positive and MB-1/QH-1 positive cells also appeared in the presumptive vitreous body, at the edges of the optic cup and in the surrounding mesenchyme. As the vitreal cells and the specialised phagocytes of the neural retina were immunolabelled in chick----quail yolk sac chimeras, we conclude that they are derived from haemopoietic cells in the yolk sac. Some images suggest that these cells enter the vitreous body from the surrounding mesenchyme and traverse the basement membrane of the neuroepithelium in the optic disc region to give rise to the specialised phagocytes of the retinal neuroepithelium.