In response to hormonal, infectious, or environmental (hypoxia, hemodynamic stress, etc.) stimuli, the fibroblast is activated and secretes chemokines, cytokines and matricellular proteins involved in the recruitment of monocytes, lymphocytes and progenitor cells. With time, fibroblasts upregulate adhesion molecule expression, which promotes retention of leukocytes and progenitor cells within the adventitia. Some of the newly recruited cells can differentiate into fibroblasts and myofibroblasts, which perpetuate the cycle, thus leading to persistent inflammation and structural vascular remodeling. (PA = pulmonary artery).