Stromal cells are the cells that comprise the backbone of a bone marrow fragment. They provide the support matrix as well as some of the nutrients necessary for the growth of all cellular precursors found in the bone marrow.
Stromal cells appear similar to macrophages and tend to be found in sheets. Usually, they are deep in the heart of a fragment. When the differential is counted in the areas adjacent to the fragments, these cells may be improperly identified and should not be counted. While stromal cells may be present in clusters, it is important to recognize that these cells are normal bone marrow elements and are not tumor cells. In addition to stromal cells is adipose tissue or adipocytes (fat cells).
In aplastic bone marrows, or bone marrows with decreased cellularity, they may be more apparent. However, since they are considered tissue cells, they are not included in the differential.
The top image to the right demonstrates a typical stromal clump. The bottom image shows stromal cells mixed with phagocytic macrophages.