RED CELL & FIBRIN. RED CELL & FIBRIN This scanning electron micrograph (SEM) depicted a closer view of a number of red blood cells found enmeshed in a fibrinous matrix on the luminal surface of an indwelling vascular catheter, Magnified 7766x. In this instance, the indwelling catheter was a tube that was left in place creating a patent portal directly into a blood vessel. Some of the erythrocytes are grouped in a stack known as a ´Rouleaux formation´. Note the biconcave cytomorphologic shape