Fig. 8. Model of YSL function during early cardiac morphogenesis. (A) Diagram represents transverse section of myocardial precursors (blue) within the lateral plate mesoderm (dotted line) during myocardial migration. Myocardial cells migrate between the endodermal layer (green) and the extra-embryonic YSL (yellow). (B) Model of mtx1 function in the YSL in myocardial migration. mtx1 is expressed exclusively in the extra-embryonic YSL. mtx1 is probably responsible for regulating the differentiation of the YSL as a tissue on which many cell types, including myocardial and endodermal cells, migrate. mtx1 also appears to regulate an unidentified factor X in the YSL, which works in a non-cell-autonomous manner to regulate embryonic natter/fibronectin 1 expression. This pathway is responsible, at least in part, for ECM formation, which itself is essential for early cardiac morphogenesis and body structure maintenance. Other ECM components, such as Laminin, are also probably regulated by mtx1. Solid lines indicate connections supported by the data presented in this study, while dashed lines indicate potential connections suggested by the data.