Journal of Neurocytology 13, 329-338 (1984) A4: an antigenic marker cells of neural tube-derived R. H. MILLER, B. P. WILLIAMS, J. COHEN and M. C. RAFF MRC NeuroimmunologyProject,Departmentof Zoology, University CollegeLondon, GowerStreet, London WCIE 6BT, UK Received 6 December 1983; revised 30 January 1984; accepted 8 February 1984 Summary The A4 monoclonal antibody was originally found to bind to the surface of the majority of neurons in rat CNS cultures, but not to PNS neurons or non-neural cells. It was subsequently shown to bind to immature oligodendrocytes and their precursor cells but not to the most mature oligodendrocytes. In the present study, we have used immunofluorescence assays on cell suspensions and cul~res and on semi-thin, frozen tissue sections to show that protoplasmic and fibrous astrocytes and most ependymal cells are also A4§ Taken together, these results suggest that in adult rats A4 is expressed exclusively by cells of the CNS and that all cell types derived from the neural tube are A4§ at least at some time in their development. While neurons, astrocytes and ependymal cells continue to express the antigen in adults, most oligodendrocytes appear to lose it as they mature. The finding that macrophages in CNS cell suspensions and cultures are A4- suggests that microglial cells are not derived from the neural tube. Introduction We have previously described a monoclonal antibody, A4, which recognizes a cell-surface antigen that is present on the great majority of neurons in the rat CNS but not on neurons (or glial cells) of the PNS or on non-neural cells (Cohen & Selvendran, 1981). We subsequently found that the antibody reacts with oligodendrocyte precursor cells in embryonic brain and with newly formed oligodendrocytes in neonatal brain, but not with the majority of more mature oligodendrocytes (Abney et al., 1983). In the present study, we have re-examined the cell-type distribution of the A4 antigen in CNS cell suspensions and cultures and in semi-thin frozen sections of a wide variety of neural and non-neural tissues and show that fibrous and protoplasmic astrocytes and most ependymal cells are A4 § Taken together with our previous findings, these results suggest that A4 is expressed by all CNS cell types that develop from the neural tube, at least at some time during their development. The finding that macrophages in cell suspensions and cultures of embryonic and neonatal CNS are A4- suggests that microglial cells are not derived from the neural tube. 0300-4868/84 $03.00 + .12 9 1984 Chapman and Hall Ltd.