Evidence to Suggest that the Human Fetal Spleen Is Not a Hematopoietic Organ*

The human fetal spleen commonly is regarded as an organ of hematopoiesis. Because of the authors' interest in myelofibrosis with myeloid metaplasia (MMM) and because the myeloid metaplasia commonly is regarded as a reactivation of embryonic sites of blood formation, spleens from 48 fetuses and stillborn infants were studied, in an attempt to evaluate splenic hematopoiesis (myelopoiesis). The authors employed immunohistologic and cytochemical technics to identify granulocytic, erythroid, and megakaryocytic cells, in contrast to previous studies that have relied solely on conventional morphology. The authors found surprisingly little evidence of hematopoiesis. Virtually no hematopoietic cells of the dividing cell pool were identified, in spite of the fact that the technic used is capable of detecting such immature forms. The results suggests that the spleen is not a significant organ of hematopoiesis in the human fetus but that immature hematopoietic cells found there merely reflect trapping of circulating precursors in the fetal blood. These findings have significant implications for the pathophysiology of MMM.