Intraventricular macrophages in the lateral ventricles with special reference to epiplexus cells: a quantitative analysis and their uptake of fluorescent tracer injected intraperitoneally in rats of different ages.

The labelling of epiplexus cells associated with the choroid plexus in the lateral ventricles was examined in rats of different ages with the fluorescent dye, rhodamine isothiocyanate (RhIc). A quantitative study was also attempted; this showed that the number of epiplexus cells and their related cells, namely supraependymal and free-floating cells, increased with age. The mean absolute number of epiplexus cells ranged from approximately 700 in the newborn to approximately 2200 in rats of 17 d of age; thereafter it remained unchanged. The number of free-floating cells also increased substantially but showed considerable individual variation. Following i.p. injection, the tracer was rapidly taken up by the epiplexus cells. This provided strong support for their phagocytic nature. In the newborn (1 d) and developing (13 d, 17 d) rats, RhIc-labelled epiplexus cells were first observed 3 h after the injection. In adult rats, labelled cells were not observed until 12 h after injection. In either case, the fluorescence in the epiplexus cells gradually increased with time. It is suggested from this study that the blood-CSF barrier in the choroid plexus in postnatal rats is incomplete, thereby allowing a rapid transvascular diffusion of the injected RhIc into the blood circulation. The fluorescent dye which enters the ventricle by way of the choroid epithelium is subsequently taken up by the epiplexus cells. Such an unimpeded passage, however, is reduced in the adult rats, probably due to the maturation of the blood capillaries as well as the choroid epithelium.