Second harmonic generation microscopy of early embryonic mouse hearts.

The understanding of biomechanical regulation of early heart development in genetic mouse models can contribute to improved management of congenital cardiovascular defects and embryonic cardiac failures in humans. The extracellular matrix (ECM), and particularly fibrillar collagen, are central to heart biomechanics, regulating tissue strength, elasticity and contractility. Here, we explore second harmonic generation (SHG) microscopy for visualization of establishing cardiac fibers such as collagen in mouse embryos through the earliest stages of development. We detected significant increase in SHG positive fibrillar content and organization over the first 24 hours after initiation of contractions. SHG microscopy revealed regions of higher fibrillar organization in regions of higher contractility and reduced fibrillar content and organization in mouse Mlc2a model with cardiac contractility defect, suggesting regulatory role of mechanical load in production and organization of structural fibers from the earliest stages. Simultaneous volumetric SHG and two-photon excitation microscopy of vital fluorescent reporter EGFP in the heart was demonstrated. In summary, these data set SHG microscopy as a valuable non-bias imaging tool to investigate mouse embryonic cardiogenesis and biomechanics.