Abstract
Signaling pathways mediated by receptor tyrosine kinases (RTK) and mitogen‐activated protein kinase (MAPK) activation have multiple functions in the developing cardiovascular system. The localization of diphosphorylated extracellular signal regulated kinase (dp‐ERK) was monitored as an indicator of MAPK activation in the forming heart and vasculature of avian embryos. Sustained dp‐ERK expression was observed in vascular endothelial cells of embryonic and extraembryonic origins. Although dp‐ERK was not detected during early cardiac lineage induction, MAPK activation was observed in the epicardial, endocardial, and myocardial compartments during heart chamber formation. Endocardial expression of dp‐ERK in the valve primordia and heart chambers may reflect differential cell growth associated with RTK signaling in the heart. dp‐ERK localization in the epicardium, subepicardial fibroblasts, myocardial fibroblasts, and coronary vessels is consistent with MAPK activation in epicardial‐derived cell lineages. The complex temporal–spatial regulation of dp‐ERK in the heart supports diverse regulatory functions for RTK signaling in different cell populations, including the endocardium, myocardium, and epicardial‐derived cells during cardiac organogenesis. Developmental Dynamics 230:773–780, 2004. © 2004 Wiley‐Liss, Inc.