In vitro detection of cells with monocytic potentiality in the wall of the chick embryo aorta.

Our previous investigations in 3- to 4-day avian chimeras have revealed that the wall of the aorta is a site from which hemopoietic stem cells can be obtained. In the present work using an in vitro clonal assay, we searched for cells with monocytic potentiality in this location as well as in the remainder of the embryo's body. In each experimental series thoracic segments from 30 chick embryo aortae were dissociated by a pancreatin treatment and plated in agar medium containing chicken serum and fibroblast-conditioned medium. Eighty to 620 macrophage colonies developed when 50,000 cells from 4-day aortae were plated, somewhat fewer when 3-day cells were plated (19-110). By contrast no progenitors were detected when cells were plated from 3- or 4-day embryos after their aorta had been removed. The cell composition and morphology of colonies deriving from aorta cells, their growth requirement and kinetics of development were identical to these of colonies deriving from young chicken bone marrow cells, cultured in the same conditions. The presence of macrophage progenitors in the wall of the 3- or 4-day embryo aorta and their absence in the rest of the embryo argues for a specific role of that region in embryonic hemopoiesis, namely that this is the location where intraembryonic hemopoietic stem cells emerge from the mesoderm at that period of development.