Feature Of The Week 4/17/11: Researchers Study Fundamental Aspects of Embryonic Heart Development Using OCT
Researchers from the University of South Florida, Western Reserve University, and the Massachusetts Institute of Technology use 4-D OCT to study fundamental aspects of embryonic heart development during the morphogenesis of a single chamber heart to a four chamber heart. Below, courtesy of Dr. Kersti Linask, is a visual, audio, and text summary of that work. To hear narration click the speaker symbol in the lower right hand corner and adjust volume.
Cells that will become cardiac cells first arise in two bilateral embryonic fields in the anterior part of the early vertebrate embryo. Due to intersecting activator and inhibitory growth and regulatory factor signaling pathways, only a ventral subpopulation of mesenchymal cells eventually forms the heart. Cardiac precursor cells arise from the middle layer of the embryo called the mesoderm. These cells demonstrate a mesenchymal to epithelial cell transformation and sort out to form two polarized epithelial compartments that begin to show phenotypic characteristics of cardiomyocytes, i.e. they organize myofibrils and display electrical activity (Linask, 1992; Linask et al., 1997). The differentiation proceeds in an anterior to posterior wave across the heart fields at the same time as the cells move to the embryonic midline (Linask and Lash, 1986) where the two bilateral compartments fuse to form a single, straight, heart tube that soon begins to beat rhythmically. The first intrinsic heartbeats are driven by the sodium-calcium exchanger Ncx1 (Linask et al., 2001).
Subsequently, the single vertebrate heart tube begins to loop or bend and always does so in a rightward direction. This important process begins with the anterior part of the heart specifying direction of the bend. In the 1990’s many of the regulatory genes and pathways defining left-right sidedness or laterality in the embryo were determined (Harvey, 1998). Our laboratory was focused on the morphoregulatory processes of cardiogenesis to answer how gene expression translated to control the mechanics of looping. We demonstrated that the laterality genes regulated the expression of flectin which we identified later as cytoskeletal nonmuscle myosin IIB in the heart and this regulation also occurred in a left-right manner (Lu et al., 2008). To drive directionality of looping, we showed that asymmetric cell proliferation in the dorsal mesocardial folds and in the ventral foregut endoderm helped to define the direction of the heart tube bending to the right (Linask et al., 2005). This occurs as cells continue to proliferate and add to the length of the heart tube, both from the anterior, outflow and posterior, inflow, regions. Once directionality of looping is defined, the bending will continue in that direction. This direction of looping is maintained by the heart being enclosed by the closely apposed splanchnopleural membrane.
The looping process is a critical event in cardiac morphogenesis. Anything that will disrupt this process has the capability of producing heart defects. The eventual outcome of looping is the formation of a four-chambered, trabeculated heart with a parallel blood flow circuit that has formed from a single heart tube displaying peristaltic flow (Manner et al., 2010). Looping is completed with positioning the initial posterior atrial regions, anterior to the ventricles and dorsal to the outflow tract. Perfect chamber alignment is essential to allow for correct signaling to take place for the formation of normal valves, ventricular and atrial septa, and to allow for the heart to become correctly connected to the embryonic vasculature that has been forming concomitantly within the embryo and external to it.
Research from a number of laboratories supports that the stresses of blood flow regulate looping and help to drive it to completion (Groenendijk et al., 2004; Hove et al., 2003; Sedmera, 2005). It is not by chance alone that looping begins right after the onset of rhythmic heart contractions and initiation of blood flow. As the bending of the heart tube deepens, an increasing volume of blood enters the heart. The blood flow and the associated stresses on the endocardium are somehow transmitted across the cardiac jelly, a large compartment filled with extracellular matrix proteins, to the myocardial wall (Linask and Vanauker, 2007). We hypothesized that these endocardial stresses are differential and can control signaling and gene expression in a region-specific way to regulate further developmental events such as valve and trabeculae formation. Optical coherence microscopy (OCM) and optical coherence tomography (OCT) allowed us to visualize for the first time the structural and functional aspects of looping in real time and to do so in one cardiac cycle. This allowed defining specific areas along the heart tube that would be important to the transmission of forces and signaling to help drive looping. Our manuscript highlighted the structural complexity of the lumen of the “simple” tubular heart; that it is not just a tube within a tube. Our analysis revealed heterogeneity of the subendocardial structure including the matrix-mediated tethers that are not randomly placed.
Understanding the interrelatedness of blood flow, formation of trabeculae, and mechanotransduction that involves the cytoskeleton, fine cellular extensions, and cardiac cilia provided a new mechanistic understanding of this central defining event of cardiogenesis.
For more information see recent Article and references below. Courtesy Kersti Linask.
Citations and for Further Reading
Groenendijk, B., Hierck, B., Gittenberger-de-Groot, A., Poelmann, R., 2004. Development-related changes in the expression of shear stress responsive genes KLF-2, ET-1, and NOS-3 in the developing cardiovascular system of chicken embryos. Dev. Dyn. 230, 57-68.
Harvey, R. P., 1998. Links in the left/right axial pathway. Cell. 94, 273-276.
Hove, J. R., Koster, R. W., Forouhar, A. S., Acevedo-Bolton, G., Fraser, S. E., Gharib, M., 2003. Intracardiac fluid forces are an essential epigenetic factor for embryonic cardiogenesis. Nature. 142, 172-177.
Linask, K. K., 1992. N-cadherin localization in early heart development and polar expression of Na, K-ATPase, and integrin during pericardial coelom formation and epithelialization of the differentiating myocardium. Dev. Biol. 151, 213-224.
Linask, K. K., Han, M., Cai, D. H., Brauer, P. R., Manisastry, S. M., 2005. Cardiac Morphogenesis: Matrix metalloproteinase coordination of cellular mechanisms underlying heart tube formation and directionality of heart looping. Dev. Dynamics. 233, 739-753.
Linask, K. K., Han, M. D., Artman, M., Ludwig, C. A., 2001. Sodium-calcium exchanger (NCX-1) and calcium modulation. NCX protein expression patterns and regulation of early heart development. Dev. Dynamics. 221, 249-264.
Linask, K. K., Knudsen, K. A., Gui, Y. H., 1997. N-Cadherin-Catenin Interaction: Necessary component of cardiac cell compartmentalization during early vertebrate heart development. Dev. Biol. 185, 148-164.
Linask, K. K., Lash, J. W., 1986. Precardiac cell migration:Fibronectin localization at mesoderm-endoderm interface during directional movement. Dev. Biol. 114, 87-101.
Linask, K. K., Vanauker, M., 2007. A role for the cytoskeleton in heart looping. ScientificWorldJournal. 7, 280-98.
Lu, W., Seeholzer, S. H., Han, M., Arnold, A. S., Serrano, M. C., Garita, B., Philp, N. J., Farthing, C., Steele, P., Chen, J., Linask, K. K., 2008. Cellular nonmuscle myosins NMHC-IIA and NMHC-IIB and vertebrate heart looping. . Dev. Dyn. 237, 3577-3590.
Manner, J., Wessel, A., Yelbuz, T. M., 2010. How does the tubular embryonic heart work? Looking for the physical mechanism generating unidirectional blood flow in the valveless embryonic heart tube. Dev Dyn. 239, 1035-46.
Sedmera, D., 2005. Form follows function: developmental and physiological view on ventricular myocardial architecture. Eur. J. Cardiothorac. Surg. 28, 526-528.