Subretinal macrophages in the developing eye of eutherian mammals and marsupials

Blood-borne mononuclear cells invade the developing retina via the hyaloid vasculature at the optic nerve head. Following removal of apoptotic cell de bris they give rise to the network of resident microglia. The population of cells recently described in the peripheral subretinal space of developing human eyes may represent a further population of macrophages destined to be come microglia. The aim of the present study was to confirm the presence of subretinal macrophages in the developing eye in other mammalian species an d perform preliminary immunophenotypic analysis in rat tissues. The range o f species chosen included eutherian mammals (rat and rabbit) and marsupials (wallaby and opossum). Ocular tissues from a range of developmental stages were studied by scanning electron microscopy and transmission electron mic roscopy. Distinctive networks of dendriform and pleomorphic macrophages wer e observed by scanning electron microscopy in the peripheral subretinal spa ce of D2 rabbits, newborn and D2 rats and D75 wallaby. Transmission electro n microscopic studies of D2 rabbit, newborn and D2 rat and all ages of Nort h American opossum revealed cells with the ultrastructural features of macr ophages in the peripheral subretinal space, cilio-retinal junction and betw een ciliary epithelial cells. Preliminary immunoperoxidase studies using a panel of anti-leukocyte monoclonal antibodies on frozen sections of rat ocu lar tissues (newborn, D2 and D4) revealed ED1(+) Ox42(+) ED2(+) but Ox6(-) cells in the peripheral subretinal space, peripheral retina and ciliary bod y epithelia. The data confirms that subretinal macrophages are a feature of the developing eye in a broad range of mammalian species and immunophenoty pic evidence leads the author to postulate that these cells arise from the ciliary body vasculature and may migrate into peripheral neural retina and mature into resident microglia.