Recent developments in understanding the pathogenesis of aplastic anemia.

Bone marrow failure in aplastic anemia (AA) could result from abnormalities of hematopoietic stem cells, abnormal control of hematopoiesis, or abnormalities of the hematopoietic environment. Bone marrow transplantation, in vitro marrow culture techniques, and studies in animal models of marrow failure have provided insights on the possible pathogenetic mechanisms underlying AA. Studies in man and in murine models suggest that most often AA results from injuries to hematopoietic stem cells. Despite the intriguing report of abnormal regulatory cells in congenitally anemic mice, instances of marrow failure due to defective humoral or cellular control of hematopoiesis have not been identified in man. In vitro studies employing allogeneic marrow targets have suggested that immune suppression of hematopoiesis may occasionally mediate AA in man. Marrow failure due to abnormalities of the hematopoietic microenvironment has been suggested by experience with bone marrow transplantation, but no direct study of this possibility has been reported. Based on available evidence, it seems likely that AA will prove to be many diseases that share common clinical and morphologic features.