Transient expression of transferrin receptors and localisation of iron in amoeboid microglia in postnatal rats

The expression of transferrin receptors marked by the monoclonal antibody OX-26 and the localisation of iron were studied in amoeboid microglial cells in postnatal rats. Transferrin receptors were vigorously expressed in amoeboid microglia in rats ranging from 1 to 10 d of age but were undetectable in older rats. Thin serial sections showed that the OX-26 positive amoeboid microglial cells were also immunoreactive for OX-42 and ED1. Using Perls' medium, this study showed the presence of a considerable amount of iron in amoeboid microglial cells in 1-10 d rats. Most iron-containing cells were round but their number had diminished by 2 and 3 wk of age, when the iron was localised instead in some branched cells which were identified as either oligodendrocytes or ramified microglia cells. There has been much speculation on the functional significance of transferrin receptors on amoeboid microglia in postnatal rats. It is suggested that the receptors facilitate the acquisition of iron necessitated for various functions of amoeboid microglia in the developing brain. The presence of iron in some oligodendrocytes suggests their involvement in mediating iron mobilisation and storage. Its localisation in some ramified microglia in older rats indicates the possible role of these cells in sequestration and detoxification of iron in the central nervous system.