PRODUCTION OF THE TRANSFORMING-GROWTH-FACTOR-BETA BINDING-PROTEIN ENDOGLIN IS REGULATED DURING CHICK HEART DEVELOPMENT

The early embryonic heart consists of two cell types. The cells form a n inner epithelial tube of endocardium within an outer tube of myocard ium separated by a cell-free extracellular matrix. A crucial process i n heart development is the production of cushion mesenchyme in the atr ioventricular (AV) canal and outflow tract (OT), Cushion mesenchyme di fferentiates from the endocardium in response to signaling molecules p roduced by the adjacent myocardium, In chicken hearts, both transformi ng growth factor-beta 3 (TGF-beta 3) and TGF-beta 2 are present and ha ve been identified as being important in the production of cushion mes enchyme. We were interested in how the signals from these two similar molecules may be differentiated during early heart development. To thi s end, we examined the expression of endoglin, a TGF-beta receptor mol ecule, in the developing chick heart. Endoglin is typically located on endothelial cell layers and binds tightly to TGF-beta 1 and TGF-beta 3 but not well to TGF-beta 2. We show that during the formation of the primitive heart tube, endoglin is found at relatively high levels in both presumptive myocardium and endocardium. However, as myocardium di fferentiates and development proceeds, endoglin expression is progress ively reduced. At stage 20 in the heart, endoglin expression is most r eadily seen in the AV canal and the OT. This pattern of expression is similar to the reported TGF-beta 3 expression patterns in the heart. D ev. Dyn. 1998;213:237-247. (C) 1998 Wiley-Liss, Inc.