Labelling of retinal microglial cells following an intravenous injection of a fluorescent dye into rats of different ages.

Retinal microglia were selectively and sequentially labelled in different layers of the retina of postnatal rats following a single intravenous injection of the fluorescent dye, rhodamine isothiocyanate (RhIc). The fluorescent cells were doubly immunostained with OX-42 and ED-1 antibodies that recognise complement type 3 (CR3) receptors and macrophage antigen, respectively. RhIc was first detected in the retinal blood vessels 5 min after injection. At 1 h, a variable number of microglia in the inner layers of the retina, namely, the nerve fibre and ganglion cell layers appeared to emit weak fluorescence. Labelled microglial cells in the inner nuclear and outer plexiform layers were not detected until 1 and 2 d had elapsed following RhIc injection. The number of labelled retinal microglia was progressively increased with time, peaking at 4 d after RhIc injection. The frequency of RhIc labelled cells also increased with age, with the largest number of cells occurring in 7-d-old rats but declined thereafter. In 11 d or older rats, RhIc was confined to the retinal blood vessels. It is concluded that when injected into the circulation, RhIc could readily gain access into the retina tissues due to an inefficient blood-retina barrier in early postnatal stages. It became impeded with maturation of the blood-retina barrier, which was established between 11 and 13 d of age. RhIc that inundated the retinal tissues was thoroughly sequestered by the resident microglial cells. It is therefore suggested that the latter could play a protective role against serum-derived substances that may be deleterious to the developing retina.