Cytokine transcripts expressed by microglia in vitro are not expressed by ameboid microglia of the developing rat central nervous system.

Because of morphological similarities between ameboid microglia in the developing central nervous system (CNS), brain macrophages in the injured CNS, and cultured microglia in vitro, it is thought that these cell types are functionally equivalent. To investigate the validity of this assumption, we have compared mRNA levels of interleukin-1alpha and -1beta (IL-1alpha and IL-1beta), tumor necrosis factor-alpha and -beta (TNF-alpha and TNF-beta), transforming growth factor-beta1 (TGF-beta1), and macrophage colony-stimulating factor (M-CSF) in the postnatal day 4 (P4) supraventricular corpus callosum (SVCC) with those in unstimulated cultured microglia. Control tissues included spleen, cortex, hippocampus, and cerebellum. Our analyses have shown that while IL-1alpha, IL-1beta, TNF-alpha, TNF-beta, and TGF-beta1 transcripts are abundantly expressed by cultured microglia, they are very low to virtually undetectable in the SVCC. These data strongly suggest that ameboid microglia, which are concentrated in the SVCC, are unlikely to be a significant source of these cytokines. Our study, which shows clear differences in the functional status of cultured microglia vs. ameboid microglia in vivo, stresses the importance of using caution when interpreting in vitro findings in terms of the in vivo functions of microglia.