Macrophage heterogeneity in human fetal tissue. Fetal macrophages

The immunophenotype of the macrophage population in human fetal tissue was studied, using a panel of monoclonal antibodies against cells of the macrophage/monocyte lineage. Using a double-labelling technique two main populations were observed in tissue from 14 weeks of estimated gestational age (EGA); EBM11+ DR+ and EBM11+ DR- cells of which a small proportion were also RFD7+. Most macrophages were negative with 3.9, an antibody specific for the adhesion molecule P150.95 and LP9 which is specific for a lysosomal enzyme. The exception to this was a small population of positive cells in the thymus. Small numbers of 3.9+ cells were also infrequently observed in tissue at and above 17 weeks of EGA, while occasional RFD9+ cells were only observed in most tissues, before this time. The higher percentage of macrophages were DR+ DQ- DP-, with a few DQ+ cells appearing at 15 weeks of EGA. In the thymus, DQ+ cells outnumbered DP+ cells especially in the medulla. These results indicate the heterogeneous and immature nature of the fetal macrophage population and point to the importance of age, tissue-specific factors and probable immune mediators in macrophage differentiation.