Macrophages appear to develop elsewhere

A recent study has found that macrophages appear to be possibly created before birth, and not just from blood stem cells. A TUM scientist has found 2 different lineages of macrophages that continue into adult life and come from different places. During a study sabbatical, Dr. Christian Schulz found a lineage of macrophages that come from the yolk sac of the mother and become part of the fetus, instead developing from the red blood stem cells like all other macrophages are believed to do, while studying mice.

 http://www.biologynews.net/archives/2012/05/03/new_path_of_origin_for_macrophages.html