Introduction

Bone marrow is a highly compartmentalised structure in which haemopoiesis occurs within distinct anatomical niches. Endothelial cells of the bone marrow microvasculature are thought to play a central role in regulating cellular traffic between the intravascular and extravascular (haemopoietic) compartments of the bone marrow (Tavassoli, 1979). The volume of this cellular traffic is highly variable, according to demands such as blood loss or infection. Normal haemopoiesis requires intimate contact between marrow stromal cells and haemopoietic progenitor cells (HPCs). Maturing HPCs detach from the stroma, traverse the sinusoidal endothelium and enter the circulation.

Circulating HPCs are capable of selective ‘homing’ to the bone marrow (Tavassoli & Hardy, 1990). This process is essential to the clinical practice of bone marrow transplantation, in which intravenously transplanted progenitors return to the extravascular compartment of the bone marrow and restore multilineage haemopoiesis. Homing is also thought to occur under normal conditions of haemopoiesis. The homing process is thought to occur in two stages. In the first step, circulating HPCs interact with the luminal surface of sinus-lining endothelium. The cells then migrate through the endothelial layer and enter the extravascular compartment, where they interact with stromal cells, extracellular matrix components, and growth factors.

Interactions between various stromal components of the bone marrow, and HPCs are well documented, and the role of these interactions in progenitor cell homing has been the subject of a large number of studies. By contrast, little is known about the role of microvascular endothelium of the bone marrow in the homing process.