Micrgolia are fundamental elements within the central nervous system sustaining tissue homeostasis by phagocyting tissue debris and mediating various processes to either neurons or glial cells. According to recent notion microglia differentiate from primitive myeloid cells emerging from the yolk-sac mesoderm. The progenitors populate the intraembryonic mesenchyme and subsequently the neuroepithelium. Different tracing approaches were used concerning the homing of microglial precursors, but the stages of differentiation are still poorly mapped. In our previous works we reported, that chicken microglia express CD45 (protein tyrosine phosphatase receptor type C), CD44 (hyaluronate receptor), MHC class II (major histocompatibility complex II) antigens. As a novel finding we demonstrated the expression of ChB6 (formerly known as Bu-1; chicken B-cell specific surface glycoprotein) by avian microglia.

In the present immunolabelling work we report the sequential maturation of microglial cells. The expression of the above mentioned antigens is timely regulated. Patterning was initiated by the homing of CD45+ cells showing subsequent CD44+ and MHC II+ phenotype. Microglial cells undergoing ramification started to express ChB6 from the 8-10th day of embryonic development. In the subventricular zone of the telencephalon the cells preserved a less ramified phenotype showing intense immunoreactions and interestingly these areas overlap the main locations of glio- and neuroblasts. Our result shows that besides the hematopoietic cell markers the B-cell specific ChB6 marker represents useful tools to monitor the microglial cells in the developing and mature chick central nervous system.