Introduction

The word stroma is generally used in anatomy and histology to signify the supporting connective tissue associated with the dominant functional tissue (the parenchyma) in an organ. There is perhaps no other organ, however, for which the original meaning (στρoμα=mattress, that which one rests or lies upon) of the word stroma applies as appropriately as for the stroma of the bone marrow. Here, maturing precursors of blood cells rest directly upon surfaces provided by the ‘stromal’ cells. This is a notable peculiarity of the bone marrow stroma: it is largely made of cells and cell surfaces, rather than of physically conspicuous extracellular matrix components, such as the collagenous scaffolds holding parenchymal tissues together in most other organs. This reflects the special nature and function of the bone marrow stroma with respect to haematopoiesis, i.e. not just a system of physical support, but the repository of a host of cell-derived cues and signals driving the commitment, differentiation and maturation of haematopoietic cells.

Different definitions of the bone marrow stroma result in different concepts of its identity and cellular composition, and in some confusion. Anatomically, the stroma of the mammalian post-natal bone marrow is the three-dimensional network of cell surfaces holding maturing blood cells together in the extravascular space. Four main cell types comprise this network: macrophages, adipocytes, osteogenic cells near bone surfaces, and cells commonly referred to as ‘reticular’ cells.