Benign B-Cell Precursors (Hematogones) Are the Predominant Lymphoid Population in the Bone Marrow of Preterm Infants

Bone marrow (BM) findings in 3rd-trimester stillborns and full-term living neonates have been previously described. However, there is no information regarding BM composition in living preterm infants. Specifically, it is unknown whether the BM lymphocytosis seen in full-term infants at 1–4 weeks of age also occurs in preterm infants. Furthermore, the lineage of these cells has never been investigated. We used a panel of immunohistochemical stains to characterize the BM composition in 11 neonates (8 living and 3 deceased). Unlike in the other age groups, immature B cells (hematogones) were the most common lymphoid population, accounting for 10–60% (mean 34%) of all cells. In two additional cases (both living patients), flow cytometry revealed a level of 3.8% of immature B cells in a