Venule. Light micrograph (LM) of red blood cells (erythrocytes, red) in a venule in the pituitary. These vessels arise from capillaries and are characterized by a widening diameter and the acquisition of fibroblasts and mesenchymal cells (collectively pericytes) that associate with the external aspect of the venule wall. Magnification: x100 when printed at 10 centimetres wide.