Bovine megakaryocyte integrins: their association with extracellular matrix in vivo

Abstract

In an effort to define the megakaryocyte microenvironment and the megakaryocyte integrin receptors which might interact with this environment, we undertook a detailed immunofluorescence and immunogold study of bovine bone marrow. Examination of bovine bone marrow using antibodies to laminin, fibronectin and type IV collagen revealed a highly specialised microenvironment with all matrix proteins being present at the basement membranes of fat-cells, vascular sinuses and blood vessels, as well as at the interface with megakaryocytes. In addition, elements of the marrow stroma were heavily labelled by antibody to fibronectin. Lighter labelling was also observed with antibodies to type IV collagen. Immunofluorescence studies were conducted using antibodies to the ‘late antigen’ (VLA) subgroup of the integrin super family which are receptors for mature proteins. Specifically, antibodies to the a antibodies to the α subunit of VLA-2 (collagen), VLA-5 (fibronectin) and VLA-6 (laminin) demonstrated that all of these integrin α subunits were diffusely present throughout the megakaryocyte. Antibody staining with the common β1 subunit for these integrins revealed intense staining of megakaryocyte cytoplasm. Confocal examination of β1 stained marrow demonstrated a clear punctate distribution with equal intensity from the perinuclear zone through to the peripheral zone. These data, as well as in vitro data generated from our laboratory and others, suggest that the specialised megakaryocyte microenvironment and its interaction with the cell's integrins may localise megakaryocytes to the abluminal side of the vascular sinus, thus positioning them for further interaction with the sinus.