Magnetic resonance imaging of fetal bone marrow for quantitative definition of the human fetal stem cell compartment.

Magnetic resonance imaging (MRI) can be used to distinguish bone marrow (BM) from cartilage and may therefore be used to measure BM volume in intact bones. We used MRI to measure the total human fetal BM volume in intact fetuses during the second trimester of pregnancy and determined the contribution of the individual bones to the total compartment. The total BM volume ranged from 934 microL at 17 to 18 weeks to 4563 microL at 22 to 23 weeks of gestation. The largest contributor to the total BM volume was the spine, constituting 26.4% +/- 2.7% of the total volume. By analyzing leukocyte content and percentages of CD34+ cells, lymphocytes, granulocytes, and monocytes of determined volumes, absolute numbers of these cell populations in BM could be measured. The cellular composition of the BM compartment did not significantly change throughout the second trimester of gestation. Absolute white blood cell counts per fetus increased from 111 x 10(6) at 16 to 17 weeks to 1229 x 10(6) at 21 to 22 weeks. The absolute numbers of CD34+ cells increased from 25 x 10(6) at 16 to 17 weeks to 256 x 10(6) at 21 to 22 weeks. Similar analysis of liver and spleen revealed comparable absolute numbers of CD34+ cells in BM and liver throughout the second trimester of gestation. In fetal liver, CD34+ cells differentiate into red cells, myeloid cells, and platelets, while lymphopoiesis mainly occurs in BM or spleen. Combining MRI and cell counts provides a method to quantify specific cell populations in fetal compartments. This study may enable better evaluation of fetal diagnostics and therapies.