The effects of subcutaneous injections of glucocorticoids on amoeboid microglia in postnatal rats.

Subcutaneous injections of glucocorticoids into postnatal rats resulted in a drastic reduction in the number of amoeboid microglial cells in the corpus callosum as shown by their labelling with the monoclonal antibodies of the OX-series, ED1, lectin and rhodamine isothiocynate (RhIc). In rats receiving 2 or 3 injections of glucocorticoids and killed at the age of 4 or 7 days, between 40 to 60% of the callosal amoeboid microglial cells were depleted when compared with the corresponding control animals. The cells that survived the glucocorticoid treatments became ramified, while those in the controls of the same age group remained round or amoeboidic. In rats killed at 2 or 3 weeks of age, the microglia became extremely ramified with a concomitant diminution in their immunostaining, particularly in the glucocorticoid-injected rats. In rats receiving glucocorticoid injections along with RhIc, the RhIc-laden amoeboid microglia appeared round and amoeboidic and were intensely stained with OX-42, suggesting their activation and upregulation of complement type 3 receptors when compared with rats receiving only glucocorticoids. Compared with the control, cellular proliferation continued in rats given glucocorticoid injection as indicated by the occurrence of many bromodeoxyuridine-labelled cells in the corpus callosum at the age of 6 days. Ultrastructural studies confirmed the presence of mitotic cells identified as amoeboid microglia because of their labelling with isolectin. A striking ultrastructural feature in glucocorticoids-injected rats was the wide occurrence of amoeboid microglial cells that had ingested a variable number of lectin-labelled cells. It is concluded from this study that the drastic reduction of amoeboid microglia after glucocorticoid injections can be attributed to the suppression of their precursor cells, monocytes. Another possible explanation is the acceleration of their degeneration process, probably greatly enhanced by glucocorticoids; the degenerating amoeboid microglia were readily eliminated by the surviving amoeboid microglial cells through endocytosis. Glucocorticoids also accelerated the maturation process of the persisting amoeboid microglia to become ramified in form.