Figure 32: In ischemic osteonecrosis, backup pressure greatly dilates the marrow veins, causing release of red blood cells and serous fluids (plasmostasis) into the marrow, as seen here.  Fat is being replaced by a wispy network of reticulin and collagen fibers.  Note the small clear spaces in the stroma toward the left.  These are evidence of past fat necrosis and are called either fatty microvesicles or fatty microcysts.  The bone is still alive in this photo.  Normal marrow veins should not be dilated more than 5-7 red blood cell diameters.