In situ hybridization analysis of HCA expression in fetal BM. (A) Eleven and one-half–week fetal BM logette hybridized with a DIG-labeled HCA probe. HCA⁺ cells (dark blue staining) can be seen running along both sides of the central arteriole. (B) Adjacent section labeled with the same CD34 antibody as in (A) and counterstained with Gill’s hematoxylin. Endothelial cells lining the central arteriole and the primary logette express CD34. The HCA⁺ cells in the upper section roughly correspond to rows of flattened cells located among developing blood progenitors (arrows).