Basement membrane of mouse bone marrow sinusoids shows distinctive structure and proteoglycan composition: A high resolution ultrastructural study

Venous sinusoids in bone marrow are the site of a large-scale traffic of ce lls between the extravascular hemopoietic compartment and the blood stream. The wall of the sinusoids consists solely of a basement membrane interpose d between a layer of endothelial cells and an incomplete covering of advent itial cells. To examine its possible structural specialization, the basemen t membrane of bone marrow sinusoids has now been examined by high resolutio n electron microscopy of perfusion-fixed mouse bone marrow. The basement me mbrane layer was discontinuous, consisting of irregular masses of amorphous material within a uniform 60-nm-wide space between apposing endothelial ce lls and adventitial cell processes. At maximal magnifications, the material was resolved as a random arrangement of components lacking the "cord netwo rk" formation seen in basement membranes elsewhere. Individual components e xhibited distinctive ultrastructural features whose molecular identity has previously been established. By these morphological criteria, the basement membrane contained unusually abundant chondroitin sulfate proteoglycan (CSP G) revealed by 3-nm-wide "double tracks," and moderate amounts of both lami nin as dense irregular coils and type IV collagen as 1-1.5-nm-wide filament s, together with less conspicuous amounts of amyloid P forming pentagonal f rames. In contrast, 4.5-5-nm-wide "double tracks" characteristic of heparan sulfate proteoglycan (HSPG) were absent. The findings demonstrate that, in comparison with "typical" basement membranes in other tissues, the bone ma rrow sinusoidal basement membrane is uniquely specialized in several respec ts. Its discontinuous nature, lack of network organization, and absence of HSPG, a molecule that normally helps to maintain membrane integrity, may fa cilitate disassembly and reassembly of basement membrane material in concer t with movements of adventitial cell processes as maturing hemopoietic cell s pass through the sinusoidal wall: the exceptionally large quantity of CSP G may represent a reservoir of CD44 receptor for use in hemopoiesis. Anat R ec 264:294-304, 2001. (C) 2001 Wiley-Liss, Inc.