Previous work in this laboratory has shown that endoderm cells in the heart forming region (HFR endoderm) of the chicken embryo induce terminal cardiac differentiation in explanted precardiac mesoderm cells. Immunostaining patterns indicating that HFR endoderm cells express Drosophila decapentaplegic (dpp)-like antigens prompted a degenerate polymerase chain reaction (PCR) screen to identify cDNAs in the dpp subgroup of the transforming growth factor-beta (TGF-beta) family. Among 50 clones of PCR products that have been sequenced, over half have identity with bone morphogenetic protein-2 (BMP-2). No other TGF-beta cDNAs have been detected, suggesting that BMP-2 is the major dpp subgroup protein synthesized by HFR endoderm cells. However, BMP-2 protein did not promote survival of either precardiac or non-precardiac mesoderm cells in culture. Whereas FGF-4 supports cardiogenesis in precardiac mesoderm, it did not induce cardiogenesis in nonprecardiac mesoderm, although explant viability was maintained. In contrast to the isolated effects of these growth factors, treatment of non-precardiac mesoderm with combined BMP-2 and FGF-4 induced cardiogenesis in the majority of explants, as revealed by the formation of a rhythmically contractile multicellular vesicle that expresses sarcomeric alpha-actin. These findings suggest that BMP-2 and FGF-4 possess respective differentiative and proliferative activities, the combination of which specifies cells to the cardiac lineage.

The early embryonic heart is composed of two cylindrical epithelial layers, an inner endothelium and an outer myocardium. The cardiac jelly (CJ), an acellular accumulation of extracellular matrix (ECM), fills the space between the two epithelia. During development of the heart, a portion of the endothelial cells of the atrioventricular (AV) region differentiate into mesenchyme cells in a temporally and spacially specific manner. Although contiguous with those in the AV region, endothelial cells lining the ventricle never form mesenchyme in situ. At present, the mechanisms controlling the biphasic differentiation of the endothelium and the subsequent migration of cardiac mesenchymal cells are poorly understood. Although the CJ lies between two epithelial and is spatially equivalent to a basement membrane (BM), it has not traditionally been considered to be organized into a BM-like structure. The potential significance of this observation to developmental biology lies in the possibility that BM or their individual components (i.e., fibronectin (FN), laminin (LM), type IV collagen, and heparin sulfate proteoglycan (HSPG] may function as the regulatory site of epithelial differentiation and morphogenesis. A cryofixation technique was developed in order to determine the in situ immunohistochemical distribution of the BM components in the CJ. Results indicated that the CJ exists as the fusion between a larger myocardially derived BM having a lamina densa and an extended reticular lamina and an attenuated, endothelial-associated BM composed only of a lamina densa. Except for FN, the individual BM components were not all present during early stages, but instead appeared in a sequential manner, suggesting that all components of an adult-type BM are not required to initiate the assembly of a structural and functional BM during development. In the AV canal and outflow tract (OT), FN appeared as a progressively expanding gradient of material with the greatest density nearer the myocardium.

The heart is a remarkably complex organ. Teaching its details to medical students and clinical trainees can be very difficult. Despite the complexity, accurate recognition of these details is a pre‐requisite for the subsequent understanding of clinical cardiologists and cardiac surgeons. A recent publication promoted the benefits of virtual reconstructions in facilitating the initial understanding achieved by medical students. If such teaching is to achieve its greatest value, the datasets used to provide the virtual images should themselves be anatomically accurate. They should also take note of a basic rule of human anatomy, namely that components of all organs should be described as they are normally situated within the body. It is almost universal at present for textbooks of anatomy to illustrate the heart as if removed from the body and positioned on its apex, the so‐called Valentine situation. In the years prior to the emergence of interventional techniques to treat cardiac diseases, this approach was of limited significance. Nowadays, therapeutic interventions are commonplace worldwide. Advances in three‐dimensional imaging technology, furthermore, now mean that the separate components of the heart can readily be segmented, and then shown in attitudinally appropriate fashion. In this review, we demonstrate how such virtual dissection of computed tomographic datasets in attitudinally appropriate fashion reveals the true details of cardiac anatomy. The virtual approach to teaching the arrangement of the cardiac components has much to commend it. If it is to be used, nonetheless, the anatomical details on which the reconstructions are based must be accurate. Clin. Anat. 32:288–309, 2019. © 2019 The Authors. Clinical Anatomy published by Wiley Periodicals, Inc. on behalf of American Association of Clinical Anatomists.

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