Circulatory systems of vertebrate and invertebrate metazoans are very different. Large vessels of invertebrates are constituted of spaces and lacunae located between the basement membranes of endodermal and mesodermal epithelia, and they lack an endothelial lining. Myoepithelial differentation of the coelomic cells covering hemal spaces is a frequent event, and myoepithelial cells often form microvessels in some large invertebrates. There is no phylogenetic theory about the origin of the endothelial cells in vertebrates. We herein propose that endothelial cells originated from a type of specialized blood cells, called amoebocytes, that adhere to the vascular basement membrane. The transition between amoebocytes and endothelium involved the acquisition of an epithelial phenotype. We suggest that immunological cooperation was the earliest function of these protoendothelial cells. Furthermore, their ability to transiently recover the migratory, invasive phenotype of amoebocytes (i.e., the angiogenic phenotype) allowed for vascular growth from the original visceral areas to the well-developed somatic areas of vertebrates (especially the tail, head, and neural tube). We also hypothesize that pericytes and smooth muscle cells derived from myoepithelial cells detached from the coelomic lining. As the origin of blood cells in invertebrates is probably coelomic, our hypothesis relates the origin of all the elements of the circulatory system with the coelomic wall. We have collected from the literature a number of comparative and developmental data supporting our hypothesis, for example the localization of the vascular endothelial growth factor receptor-2 ortholog in hemocytes of Drosophila or the fact that circulating progenitors can differentiate into endothelial cells even in adult vertebrates.

Background: Coronary artery (CA) stems connect the ventricular coronary tree with the aorta. Defects in proximal CA patterning are a cause of sudden cardiac death. In mice lacking Tbx1, common arterial trunk is associated with an abnormal trajectory of the proximal left CA. Here we investigate CA stem development in wild-type and Tbx1 null embryos. Results: Genetic lineage tracing reveals that limited outgrowth of aortic endothelium contributes to proximal CA stems. Immunohistochemistry and fluorescent tracer injections identify a periarterial vascular plexus present at the onset of CA stem development. Transplantation experiments in avian embryos indicate that the periarterial plexus originates in mesenchyme distal to the outflow tract. Tbx1 is required for the patterning but not timing of CA stem development and a Tbx1 reporter allele is expressed in myocardium adjacent to the left but not right CA stem. This expression domain is maintained in Sema3c 2/2 hearts with a common arterial trunk and leftward positioned CA. Ectopic myocardial differentiation is observed on the left side of the Tbx1

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An epithelial‐mesenchymal transition is involved in two main morphogenetic events of cardiac morphogenesis, namely the differentiation of the valvuloseptal tissue from the endocardial endothelium, and the formation of subepicardial mesenchyme from the epicardial mesothelium. We have proposed that the dogfish (Scyliorhinus canicula) is a suitable model for the study of basic processes of cardiac morphogenesis in vertebrates, since the heart of this primitive fish probably outlines the original bauplan of the vertebrate heart. In order to study in this model the endocardial and epicardial epithelial‐mesenchymal transition under scanning electron microscopy, we have used a technique of paraffin‐embedding, partial sectioning, dewaxing and critical‐point drying. Our results showed: 1) A centrifugal pattern of epicardial development from the atrioventricular groove to the sinus venosus and conus arteriosus; 2) A close spatial and temporal relationship between the endocardial and epicardial epithelial‐mesenchymal transition, although the transformation of the endocardium starts earlier and ends later the epicardial transformation; 3) A complex arrangement of the fibrous extracellular matrix which is established prior to the migration of the mesenchymal cells. Subepicardial, but not subendothelial mesenchymal cells, coalesce in unicellular or pluricellular ring‐like structures that probably are related to the origin of the cardiac vessels.