EXTRAMEDULLARY HEMATOPOIESIS IN FETAL AND ADULT HUMAN SPLEEN - A QUANTITATIVE IMMUNOHISTOLOGICAL STUDY

Haemopoietic cells were assessed in spleens from normal adults, adults with splenic extramedullary haemopoiesis due to chronic myeloprolifer ative disorders and fetuses of 17-21 weeks' gestation. A variety of an tigens expressed by developing granulocytes and erythrocytes were demo nstrated immunohistochemically. The relative proportions of early and late precursor cells of these two lineages were quantified. There was no significant haemopoiesis in normal adult spleen, while there was ab undant (predominantly granulocytic) haemopoiesis in patients with chro nic myeloproliferative disorders. Fetal spleens contained numerous lat e erythroid precursors but few early erythroid or granulocytic cells. The relative numbers of early and late haemopoietic cells in adult chr onic myeloproliferative disorders and fetal spleens showed statistical ly significant differences. Our findings indicate that haemopoiesis in the spleens of adult patients with these disorders differs fundamenta lly from that occurring in fetal life. They support the view that the human spleen does not have a significant role in fetal haemopoiesis, b ut that it filters circulating nucleated erythroid precursors and is p ermissive of their terminal differentiation only. Our results also fav our the view that adult splenic haemopoiesis originates by displacemen t of precursor cells from the bone marrow rather than by activation of stem cells which have lain dormant in the spleen since fetal life.