DIFFERENT MACROPHAGE POPULATIONS DEVELOP FROM EMBRYONIC FETAL AND ADULT HEMATOPOIETIC TISSUES/

Immunohistochemical and ultrastructural studies have indicated the exi stence of a distinct ''fetal macrophage'' type, differing from monocyt e-derived macrophages. In order to characterize macrophages of differe nt ontogenetic origins on the molecular level, we examined their surfa ce-marker and marker-gene expression patterns. We found that macrophag es derived from chicken embryos express the lysozyme gene at significa ntly lower levels than macrophages derived from adult chicken. The sam e was observed when expression of the chicken lysozyme gene was analyz ed in transgenic mice. In three independent mouse lines, mature macrop hages derived from embryonic or fetal hematopoietic tissues expressed the transgene at drastically lower levels than macrophages derived fro m the bone marrow spleen, or peritoneal cavity of adult mice. Macropha ges obtained by in vitro differentiation of mouse embryonic stem cells (a process resembling early embryonic hematopoiesis) displayed the em bryo-specific low transgene expression level. Experiments determining the developmental potential of myeloid precursors in culture and immun ophenotypic analyses revealed differences between embryo-derived and a dult myeloid progenitor populations. In summary, our results provide f urther evidence for the existence of dissimilar embryonic/fetal and ad ult macrophage types and describe the first molecular marker for their distinction.