Single-Cell Expression Profiling Reveals a Dynamic State of Cardiac Precursor Cells in the Early Mouse Embryo

In the early vertebrate embryo, cardiac progenitor/precursor cells (CPs) give rise to cardiac structures. Better understanding their biological character is critical to understand the heart development and to apply CPs for the clinical arena. However, our knowledge remains incomplete. With the use of single-cell expression profiling, we have now revealed rapid and dynamic changes in gene expression profiles of the embryonic CPs during the early phase after their segregation from the cardiac mesoderm. Progressively, the nascent mesodermal gene Mesp1 terminated, and Nkx2-5+/Tbx5+ population rapidly replaced the Tbx5low+ population as the expression of the cardiac genes Tbx5 and Nkx2-5 increased. At the Early Headfold stage, Tbx5-expressing CPs gradually showed a unique molecular signature with signs of cardiomyocyte differentiation. Lineage-tracing revealed a developmentally distinct characteristic of this population. They underwent progressive differentiation only towards the cardiomyocyte lineage corresponding to the first heart field rather than being maintained as a progenitor pool. More importantly, Tbx5 likely plays an important role in a transcriptional network to regulate the distinct character of the FHF via a positive feedback loop to activate the robust expression of Tbx5 in CPs. These data expands our knowledge on the behavior of CPs during the early phase of cardiac development, subsequently providing a platform for further study

Analysis of spatial and temporal gene expression patterns in blastula and gastrula stage chick embryos

AbstractStudies on the genetic basis of rostral–caudal specification, neural induction, and head development require knowledge of the relevant gene expression patterns. Gaps in our understanding of gene expression have led us to examine the detailed spatiotemporal expression patterns of 19 genes implicated in early development, to learn more about their potential role in specifying and patterning early developmental processes leading to head formation. Here, we report the expression patterns of these markers in blastula- and gastrula-stage chick embryos, using whole-mount in situ hybridisation. Nodal, Fgf8, Bmp7, Chordin, Lim1, Hnf3β, Otx2, Goosecoid, Cerberus, Hex, Dickkopf1, and Crescent are all already expressed by the time the egg is laid. When the primitive streak has reached its full length, a later group of genes, including Ganf, Six3, Bmp2, Bmp4, Noggin, Follistatin, and Qin (BF1), begins to be expressed. We reassess current models of early rostral patterning based on the analysis of these dynamic spatiotemporal expression patterns

The presence of an embryonic opercular flap in amniotes

The operculum is a large flap consisting of several flat bones found on the side of the head of bony fish. During development, the opercular bones form within the second pharyngeal arch, which expands posteriorly and comes to cover the gill-bearing arches. With the evolution of the tetrapods and the assumption of a terrestrial lifestyle, it was believed that the operculum was lost. Here, we demonstrate that an embryonic operculum persists in amniotes and that its early development is homologous with that of teleosts. As in zebrafish, the second pharyngeal arch of the chick embryo grows disproportionately and comes to cover the posterior arches. We show that the developing second pharyngeal arch in both chick and zebrafish embryos express orthologous genes and require shh signalling for caudal expansion. In amniotes, however, the caudal edge of the expanded second arch fuses to the surface of the neck. We have detailed how this process occurs and also demonstrated a requirement for thyroid signalling here. Our results thus demonstrate the persistence of an embryonic opercular flap in amniotes, that its fusion mirrors aspects of amphibian metamorphosis and gives insights into the origin of branchial cleft anomalies in humans