Abstract
The primitive heart tube is composed of an outer myocardial and an inner endocardial layer that will give rise to the cardiac valves and septa. Specification and differentiation of these two cell layers are among the earliest events in heart development, but the embryonic origins and genetic regulation of early endocardial development remain largely undefined. We have analyzed early endocardial development in the zebrafish using time-lapse confocal microscopy and show that the endocardium seems to originate from a region in the lateral plate mesoderm that will give rise to hematopoietic cells of the primitive myeloid lineage. Endocardial precursors appear to rapidly migrate to the site of heart tube formation, where they arrive prior to the bilateral myocardial primordia. Analysis of a newly discovered zebrafish Scl/Tal1 mutant showed an additional and previously undescribed role of this transcription factor during the development of the endocardium. In Scl/Tal1 mutant embryos, endocardial precursors are specified, but migration is severely defective and endocardial cells aggregate at the ventricular pole of the heart. We further show that the initial fusion of the bilateral myocardial precursor populations occurs independently of the endocardium and tal1 function. Our results suggest early separation of the two components of the primitive heart tube and imply Scl/Tal1 as an indispensable component of the molecular hierarchy that controls endocardium morphogenesis.
Highlights
The primitive heart tube is the first functional organ in the vertebrate embryo and is composed of a myocardial tube lined by an inner endothelial layer called the endocardium
Our results suggest a separate origin for the two components of the primitive heart tube and show an indispensable role for Scl/Tal1 during endocardial morphogenesis
Our data strongly suggest the loss of a fourth vascular endothelial growth factor (VEGF) receptor within the mammalian lineage, as we identified an orthologue of vegfr4 in the genome of the opossum Monodelphis domestica, tightly linked to the Cdx4 gene
Summary
The primitive heart tube is the first functional organ in the vertebrate embryo and is composed of a myocardial tube lined by an inner endothelial layer called the endocardium. Significant progress has been made towards elucidating the morphogenetic events and transcriptional control underlying patterning of the myocardium [1]. The morphogenetic events and the transcription factors involved in early development of the endocardium remain largely undefined. The specific embryonic origin of the future endocardial cells and their relationship with the future myocardial cells is still unclear [2]
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