Studies on the Organ Structure of Bone Marrow, with Special Reference to the Fine Structure of Its Vascular System

The structure of bone marrow, especially the relation between its vascular system and myeloid cells, has been studied in femora of rabbits mainly by an injection method with India ink and by the silver-impregnation method of SUZUKI. The results of the investigation are as follows:1. Walls of the blood-sinusoids in bone marrow are lined by delicate cytoplasmic membrane composed of littoral cell-like endothelial cells, whose cytoplasm shows net-like pattern in relief with meshes of different shapes and sizes. The spaces in these meshes are generally shut up with extremely thin cytoplasmic membranes, but in some spots they remain open to show small pores through which matured erythrocytes could pass.2. By the modes of ramification the blood-sinusoids in bone marrow can be classified into two types: the first net-like type occupies the spaces between osseal trabeculae near the epiphysial plate and is connected by blood capillaries with arterioles, and the second straight type comming out of the former lies in meta- and diaphysial part of bone marrow.3. Two different types of arterio-venous communication are found in bone marrow: the one is connected by capillaries, another communicate through the arteriovenous anastomosis. The anastomosis combines arterioles with venules in the subcortical part of bone marrow, while capillaries convey the blood from arterioles into blood-sinusoids of the first type near epiphysial line.4. In bone marrow of the spongial and diaphysial parts of bone, there often occur blood-capillaries with blind ends at which figures of angioblast or endothelium in degeneration or regeneration can be observed.5. It is very important that many small pores communicating the blood-stream with perivascular spaces of bone marrow can be observed in the cytoplasms of endothelial cells lining the small arteries. The calibers of these pores are so small that a single erythrocyte can not pass through them except blood-plasma. This sort of perforation of the wall of small artery seems to cause the elevation of fluid-pressure in the perivascular spaces wihtin bone marrow.6. On the section stained with Azan-staining the cytoplasm of the endothelium of the blood-sinusoid appears blue in color as these of normoblast before the enuclation stage. These data suggest that the cytoplasms of the endothelium of the blood-sinusoid and of the normoblast may have the same densities in structure.7. According to MATSUSHITA the cytoplasm of the normoblast, youngest erythrocyte, becomes much rigid in structure, forming elastic cytoplasmic membrane in its surface, immediately after the enuclation, and stained green to orange in color by Azan-staining. These data and the peculiar structure of the blood-vessels in bone marrow make it possible to interprete the mechanism by which only matured erythrocytes come into the peripheral blood-stream. In other words, the rigidity of the matured erythrocyte may overcome the softness of the delicate cytoplasmic membrane of the blood-sinusoid and it passes through the membrane or the pores into the peripheral blood-stream.