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Since embryonic heart development is a complex process and acquisition of human embryonic specimens is challenging, the mechanism by which the embryonic conduction system develops remains unclear. Herein, we attempt to gain insights into this developmental process through immunohistochemical staining and 3D reconstructions. Expression analysis of T‐box transcription factor 3, cytoskeleton desmin, and nucleoskeleton lamin A protein in human embryos in Carnegie stages 11–20 showed that desmin is preferentially expressed in the myocardium of the central conduction system compared with the peripheral conduction system, and is co‐expressed with T‐box transcription factor 3 in the central conduction system. Further, lamin A was first expressed in the embryonic ventricular trabeculations, where the terminal ramifications of the peripheral conduction system develop, and extended progressively to all parts of the central conduction system. The uncoupled spatiotemporal distribution pattern of lamin A and desmin indicated that the association of cytoskeleton desmin and nucleoskeleton lamin A may be a late event in human embryonic heart development. Compared with model animals, our data provide a direct morphological basis for understanding the arrhythmogenesis caused by mutations in human DES and LMNA genes. [ABSTRACT FROM AUTHOR]

The second heart field ( SHF) is indicated to contribute to the embryonic heart development. However, less knowledge is available about SHF development of human embryo due to the difficulty of collecting embryos. In this study, serial sections of human embryos from Carnegie stage 10 ( CS10) to CS16 were stained with antibodies against Islet-1 (Isl-1), Nkx2.5, GATA4, myosin heavy chain ( MHC) and α-smooth muscle actin (α- SMA) to observe spatiotemporal distribution of SHF and its contribution to the development of the arterial pole of cardiac tube. Our findings suggest that during CS10 to CS12, SHF of the human embryo is composed of the bilateral pharyngeal mesenchyme, the central mesenchyme of the branchial arch and splanchnic mesoderm of the pericardial cavity dorsal wall. With development, SHF translocates and consists of ventral pharyngeal mesenchyme and dorsal wall of the pericardial cavity. Hence, the SHF of human embryo shows a dynamic spatiotemporal distribution pattern. The formation of the Isl-1 positive condense cell prongs provides an explanation for the saddle structure formation at the distal pole of the outflow tract. In human embryo, the Isl-1 positive cells of SHF may contribute to the formation of myocardial outflow tract ( OFT) and the septum during different development stages. [ABSTRACT FROM AUTHOR]