Expression and distribution of various antigens of developing microglial cells in the rat telencephalon.

The distribution of microglia during the early stages of postnatal development in the rat was studied on rat brain from day of birth to postnatal day 90 (P90), using immunohistochemical methods with a panel of monoclonal antibodies that recognized the complement type 3 receptor (OX-42), macrophage antigen of unknown function (ED1), and the major histocompatibility complex (MHC) class I (OX-18) or class II (OX-6) antigens. Starting from the day of birth, ameboid microglia can be differentiated with positive immunoreactivity to OX-42, OX-18, and ED1. Labeled cells were localized mainly in the developing white matter. After P21, only positive reaction to OX-42 was present, and those cells had the typical morphology of the resting microglial cells that were located either in the white or grey matter. The changes in the appearance of different antigens are correlated with the morphological differentiation and transformation of ameboid microglial cells that are to become ramified microglia, present in the adult animals.