Evaluation of subretinal macrophage-like cells in the human fetal eye

In many ocular diseases, macrophages are found in the subretinal space and probably play an important role in maintaining, the disease process. Several issues concerning these cells are still unclear, such as their route of entry or their relation to the retinal pigment epithelium. The authors have found that the human fetal eye contains macrophage-like cells in the peripheral subretinal space. Their localization, distribution, and ultrastructural features are evaluated in 33 eyes from 17 specimens (12 to 22 weeks gestational age) by light microscopy and scanning and transmission electron microscopy. Subretinal macrophage-like cells occurred predominantly in the region of the ciliary folds. They were observed within the peripheral neural retina, beneath the retinal and ciliary pigmented epithelia, under Bruch's membrane, and in the choroid. The distribution suggests that one of the main entry pathways is through the vascular bed of the ciliary body. There were approximately nine subretinal space macrophages per 0.1 mm² distributed in a regular fashion on the retinal pigment epithelial surface close to the ciliary folds, however, the incidence decreased further posteriorly where they were very rare. Their shapes varied from large flat cells with several long processes to more spherical cell bodies with a few membrane ruffles. There was also evidence that some of these cells had recently phagocytosed cell debris, including retinal pigment epithelial premelanosomes. Morphologically, these cells closely resemble supraependymal and epiplexus cells, the macrophage populations found on the cerebral ventricles, an environment that corresponds anatomically to the subretinal space. Subretinal space macrophages in the fetus may be functionally important for the removal of degenerating cells but may also be an additional source of blood-derived monocytes, besides those derived from retinal vessels, destined to mature into microglia.