Ultrastructure of mouse mononuclear phagocytes in bone marrow colonies grown in vitro

Recently a new method was developed to culture bone marrow cells in a liquid medium on a glass surface. Two kinds of colonies develop in these cultures, namely mononuclear phagocyte and granulocyte colonies. The study of the ultrastructure of the cells in the mononuclear phagocyte colonies was the primary aim of the present study. The architecture of the mononuclear phagocyte colonies appeared to be quite different from that of the granulocyte colonies, since in the latter, the cells lie close together in dense clusters, whereas in mononuclear phagocyte colonies the cells are more loosely dispersed with the highest cell density at the center and stellate orientation of the cells at the periphery. However, both kind of colonies grow entirely separate from each other and mixed colonies are not observed. Electron microscopy showed that there are three types of cell in the mononuclear phagocyte colonies, i.e., monoblasts, promonocytes, and macrophages. The ultrastructure of the promonocytes and macrophages of the colonies is similar to that of the same types of cell isolated directly from the mouse. The monoblast, the most immature cell of mononuclear phagocyte colonies has not been characterized before. The ultrastructure of this cell is clearly distinct from that of the promonocyte in having a round contour without pseudopods, a nuclear to cytoplasmic ratio greater than one, a cytoplasm that contains many polyribosomes, a few small granules, and a small Golgi complex surrounded by a few short strips of rough endoplasmic reticulum.