The migration of cardiogenic cells and the formation of a tubular heart in newt embryos were examined mainly by scanning electron microscopy (SEM). Cardiogenic cells are known to localize at the border region of lateral mesoderm migrating in the space between the ectoderm and the endoderm. They initially (before stage 20 or mid-neurula) appeared to attach to the basal surface of the ectoderm, whereas later (after stage 22 or late neurula) they changed their scaffold to the endoderm. On the scaffold cell surface, very fine fibrils of extracellular matrix (ECM) were found. These fibrils were proved to be composed partly of fibronectin by the immunofluorescence method as well as by immunoSEM using latex bead-labeled antibody, suggesting their seemingly important role in migration of cardiogenic cells. At stage 26 or the early tail bud stage, when the tips of bilateral cardiogenic areas begin to fuse under the foregut, several free vasoformative cells are seen there and the mesodermal sheet itself splits into two layers to produce a coelomic cavity. The splanchnic wall of the coelomic or pericardial cavity was recognized to form a trough consisting of cobblestone-like myocardial cells not yet covered with the epicardium.
Hirakow, R.; Komazaki, S.; and Hiruma, T.
"Early Cardiogenesis in the Newt Embryo,"
Scanning Microscopy: Vol. 1
, Article 50.
Available at: https://digitalcommons.usu.edu/microscopy/vol1/iss3/50