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With rapid advances in medical imaging, fetal diagnosis of human CHD is now technically feasible in the first trimester. Although the first human embryologic studies were recorded by Hippocrates in 300–400 BC, present day knowledge of normal human cardiac development in the first trimester is still limited. In 1886, two papers by Dr His described development of the heart based on dissections of young human embryos. Free hand wax models were made that illustrated the external developmental anatomy. These wax plate reconstruction methods were used by many other investigators until the early 1900s1. Subsequently serial histological sections of human embryos have been used to further investigate human cardiac development2–6. Based on analysis of histological sections and scaled reproductions of human embryos, Grant showed a large cushion in the developing heart at 6 6/7 weeks (CS 14) and separate AV valves at 9 1/7 weeks (CS 22)2. At the end of the 8th week (CS 8), separate aortic and pulmonary outflows were observed. Orts-Llorca used three dimensional reconstructions of transverse sections of human embryos to define development of the truncus arteriosus and described completion of septation of the truncus arteriosus in 14–16mm embryos, equivalent to EGA 8 weeks (CS18)5.
Given the complex tissue remodeling associated with cardiac chamber formation and inflow/outflow tract and valvular morphogenesis, the plane of sectioning often limited the information that can be gathered on developing structures in the embryonic heart. These technical limitations in conjunction with limited access to human embryo specimens have meant that much of our understanding of early cardiac development in the human embryo is largely extrapolated from studies in model organisms7–10. With possible species differences in developmental timing and variation in cardiovascular anatomy, characterization of normal cardiac development in human embryos is necessary for clinical evaluation and diagnosis of CHD in the first trimester. This will be increasingly important, as improvements in medical technology allow earlier access to first trimester human fetal cardiac imaging and in utero intervention.
Recent studies have shown the feasibility of using magnetic resonance imaging (MRI) to obtain information on human embryo tissue structure11, 12. MRI imaging data can be digitally resectioned for viewing of the specimen in any orientation, and three-dimensional (3D) renderings can be obtained with ease. Similarly, episcopic fluorescence image capture (EFIC), a novel histological imaging technique, provides registered two-dimensional (2D) image stacks that can be resectioned in arbitrary planes and also rapidly 3D rendered10. With EFIC imaging, tissue is embedded in paraffin and cut with a sledge microtome. Tissue autofluorescence at the block face is captured and used to generate registered serial 2D images of the specimen with image resolution better than MRI. Data obtained by MRI or EFIC imaging can be easily resectioned digitally or reconstructed in 3D to facilitate the analysis of complex morphological changes in the developing embryonic heart. In this manner, the developing heart in every embryo can be analyzed in it entirety with no loss of information due to the plane of sectioning.
Using MRI and EFIC imaging, we conducted a systematic analysis of human cardiovascular development in the first trimester. 2D image stacks and 3D volumes were generated from 52 human embryos from 6 4/7 to 9 3/7 weeks estimated gestational age (EGA), equivalent to Carnegie stages (CS) 13–23. These stages encompass the developmental window during which all of the major milestones of cardiac morphogenesis can be observed. Using the MRI and EFIC imaging data, we constructed a digital atlas of human heart development. Data from our atlas were used to generate charts summarizing the major milestones of normal human heart development through the first trimester. MRI and EFIC images obtained as part of this study can be viewed as part of an online Human Embryo Atlas. To view the Human Embryo Atlas content, visit http://apps.nhlbi.nih.gov/HumanAtlas/home/login.aspx?ReturnUrl=%2fhumanatlas%2fDefault.aspx.
Embryos from the Kyoto collection, at the Congenital Anomaly Research Center at the Kyoto University in Japan, were collected after termination of pregnancies for socioeconomic reasons under the Maternity Protection Law of Japan. Embryos were derived from normal pregnancies without any clinical presentations. The specimens were in fixative for an estimated duration of 30 to over 40 years, making them unsuitable for immunohistochemistry or any molecular/cellular analysis. This collection represents a random sample of the total intrauterine population of Japan13–16. During accessioning into the Kyoto collection, the embryos were examined and staged according to the criteria of Carnegie Staging proposed by O'Rahilly17. For this study, 52 embryos from the Kyoto collection (see Table S1 in electronic supplement) were donated to the Carnegie collection of normal human embryos archived at the National Museum of Health and Medicine of the Armed Forces Institute of Pathology (http://nmhm.washingtondc.museum/collections/hdac/Carnegie_collection.htm). Each embryo’s age was determined using post conceptional ages previously reported14, which were then converted to estimated gestational age or menstrual age by adding 14 days, and reported in weeks.
High resolution MRI and EFIC images were obtained from 52 human embryos from 6 4/7 to 9 3/7 weeks of gestation (CS 13–23). These specimens from the Kyoto collection were imaged by MRI and EFIC during preparation for accessioning into the Carnegie collection (Table S1 in online supplement). Human embryos in formalin were treated with 1:20 Magnevist (Berlex, Montville, NJ)/10% formalin solution for 3 or more days, then rinsed and prepared in 5–30 mm tubes, depending on embryo size, with fixative or low melting agar. Samples that diffused gadolinium into the media were further soaked in plain fixative two or more days and re-imaged. Imaging was performed at the NIH Mouse Imaging Facility on a 7.0T Bruker vertical bore MRI system with 150 G/cm gradients (Bruker, Billerica, MA) and 5 to 30 mm microimaging birdcage coils (Bruker, Billerica, MA). Some larger samples were also imaged on a 7.0T, 16mm horizontal bore Bruker Paravision system with 39G/cm gradients and a 38-mm birdcage coil. MRI was acquired with Paravision 3.0.2 operating systems. Samples were imaged using a 3D rapid gradient echo (SNAP) sequence with TR 30–40ms, TE 3.3–4.0ms, 20–90 averages, acquisition time approximately 12 to 50 hours, matrices 256×128×128 to 512×512×512 (see Table S2 in online supplement). Over the whole collection, MRI resolution ranged from 29×35×35 to 117×105×105 µm3. Resolution was proportionate to the sample size with the smallest embryos having the highest resolution data sets. Individual image data sets are three dimensional and near-isotropic, with all three voxel dimensions being within 10 microns of each other in an individual data set. Most data sets are in the range 35×35×35 to 60×60×60 µm3. The resolution of each data set is listed in Table S2 in the online supplement.
In preparation for EFIC, embryos stored in 10% phosphate buffered formalin were dehydrated and embedded in a mixture of paraffin wax (70.4%), Vybar (24.9%), stearic acid (4.4%) and red aniline dye Sudan IV (0.4%) using techniques previously described10, 18. The embedded embryos were then sectioned using a sliding microtome (Leica SM 2500) to obtain sections of 5–8 microns in thickness. The block face was sequentially photographed using epifluorescent illumination with a 100W mercury lamp and a Leica MZ16A stereomicroscope equipped with 425 nm/480 nm excitation/emission filters. Images were captured using an ORCA-ER digital camera (Hamamatsu).
MRI images originally recorded in DICOM were converted into TIFF format using ImageJ (http://rsb.info.nih.gov/ij/). The EFIC 2D image stacks were captured and exported as TIFF files. Both the EFIC and MRI data were processed using OpenLab (Improvision Inc). 3D reconstructions and quick time virtual reality (QTVR) movies were generated using Volocity (Improvision Inc). The 2D image stacks also were digitally resectioned using Volocity (Improvision Inc) to view internal and external cardiac structures in planes similar to standard echocardiographic imaging planes used clinically. In EFIC images, each pixel was a square with length dimensions ranging from 2.34 to13.4 microns/pixel edge. Thus, pixel dimensions ranged from 5.48 µm2 to 179.56 µm2. For each embryo, we generated serial 2D image stacks, and 3D reconstructions. From this analysis, we were able to delineate all of the major milestones of human heart development, including chamber formation, septation of the atria, ventricles, and truncus arteriosus, and valvular morphogenesis.
The cardiac loop or looped heart tube is observed from EGA 6 4/7 to 7 5/7 weeks (CS13 to CS17). 3D reconstruction of the heart at 7 5/7 weeks (CS17) reveals internal structures of the cardiac loop (Fig.1). The only exit for blood from the left sided inflow limb, consisting of the atrial cavity, atrioventricular junction, and the presumptive left ventricle, is the interventricular foramen (also known as primary foramen, primary interventricular foramen, bulboventricular foramen or embryonic interventricular foramen) (double arrow in Fig.1); while the only exit for blood from the right sided outflow limb, consisting of the presumptive right ventricle is the truncus arteriosus (arrowhead in Fig.1). Also of note, the atrioventricular junction (AV in Fig.1) is surrounded by endocardial cushion tissue, which is contiguous with the truncus arteriosus.
The developmental changes seen in the cardiac loop are shown in more detail in Figure 2, with images from embryos at 6 4/7 (CS13) (Fig.2A–E) and 7 5/7 weeks (CS17) (Fig.2F–I). As the looped heart tube matures, the atrial and ventricular chambers expand in size, giving rise to distinct subdivisions recognizable as the primitive left and right atria and presumptive left and right ventricles (Fig.2H). At 6 4/7 weeks (CS13), the endocardial cushions seen lining the atrioventricular junction appear thin with little apparent cellular content. As development progresses, they become filled with dense material (Fig.2B,E,G; Supplemental Movies 8a,b). The interventricular foramen also shows striking changes during this developmental period. It is a wide and open communication at 6 4/7 weeks (CS13) (asterisk in Fig.2C), but as the chambers grow, it becomes a narrow and more distinct opening (foramen), by 7 3/7 to 7 5/7 weeks (CS16–17) (asterisk in Fig.2H,,3C).3C). The superior atrioventricular cushion can be seen (Fig.2G). The inflow consisting of the venous confluence or primitive atrium (Fig.2A) is observed to communicate with the ventricular chamber via the atrioventricular junction (Fig.2B,E,G). The presumptive left ventricle communicates with the presumptive right ventricle via the interventricular foramen (asterisk in Fig. 2C,H). The outflow from the cardiac loop comprises the yet undivided truncus arteriosus (T in Fig. 2D,I) arising from the presumptive right ventricle.
The process of atrial septation is thought to begin with a thin septum primum growing from the posterior wall of the atrium, from a location cranial to the pulmonary vein orifice. It grows towards and eventually fuses with the endocardial cushions19. At 6 6/7 weeks gestation (CS 14), the mesenchymal cap of the primary atrial septum could be seen in contact with the superior atrioventricular cushion. The atrial spine, a mesenchymal structure, was also observed. The atrial spine fuses with the inferior atrioventricular cushion (6 6/7 weeks (CS 14)) (Fig.3A,B; Supplemental Movies 8a,b), and plays an important role in closure of the primary foramen. Although the pulmonary vein orifice was not seen by our imaging, it can be inferred from previous studies that it lies to the left of the atrial spine20. Septum primum can be observed at 6 6/7 weeks of gestation and its developmental progression through 7 5/7 weeks can be seen in Figures 2H, 3A,B, and E (Supplemental Movies 1,8a,8b). Later, septum secundum develops as an infolding of the dorsal wall of the right atrium, completing atrial septation with fenestrations forming the foramen ovale. Both atrial septum primum and secundum were present by 8 weeks (CS18) (Supplemental Movie 2). At this stage, the mesenchymal cap can be seen fused with the now divided superior atrioventricular cushion (Fig.3F). This is consistent with developmental timing suggested by others21, 22.
Towards the end of the looped heart tube stages of development (7 3/7 and 7 5/7 weeks, CS16,17), distinct separation of presumptive LV and RV chambers is evident. The beginning of the muscular interventricular septum can be seen at these stages, but ventricular septation is not yet complete (Fig.2H,3C,D; Supplemental Movies 9a,b). By 8 weeks (CS18), the muscular ventricular septum can be seen extending from the floor of the ventricular chamber towards the crux of the heart (Fig.3F). This leaves open a relatively large interventricular foramen which allows communication between the ventricles (Supplemental Movies 1, 2). Recent lineage tracing experiments in mice have suggested that the muscular interventricular septum is comprised of cells originating from the ventral aspect of the primitive ventricle, with closure of the ventricular foramen mediated by dorsal migration of this precursor cell population; these cells likely represent a subpopulation of cells derived from the secondary heart field23. Immunohistochemical analysis of human fetal cardiac tissue showed myocytes expressing G1N2 antigen localized in a ring around the junction between the future right and left ventricles24. In later developmental stages, G1N2 expressing cells are found in the area clinically termed the inlet ventricular septum, but not in the subaortic outflow septum.
At 8 weeks (CS 18), the ventricular septum at the level of the left ventricular outflow is closed (Fig.3G), but part of the inlet ventricular septum at the level of the atrioventricular valves remains open (arrowhead in Fig.3F; Supplemental Movies 3,4,6). The inlet and membranous portions of the ventricular septum are fully closed at 9 1/7 weeks (CS22), completing ventricular septation (Fig.3H; Supplemental Movie 5). The area clinically termed the inlet ventricular septum has been shown in prior studies to originate from the embryonic right ventricle25. In agreement with previous reports on human development, our data showed the final portion of the ventricular septum to close included what likely comprises a combination of the membranous and inlet ventricular septum. These findings suggest that an arrest in development of the ventricular septum could result in ventricular septal defects similar to those observed clinically.
Atrioventricular valve morphogenesis begins at the looped heart tube stages, with large endocardial cushions prominently seen at the center of the cardiac loop (asterisks in Fig.3A,,4A).4A). The atrioventricular canal is divided by the endocardial cushions, which form on the posterior (dorsal) and anterior (ventral) walls of the atrioventricular canal. These cushions eventually divide the atrioventricular canal into right and left atrioventricular orifices2, 19. A well delineated atrioventricular junction can be seen at 7 3/7 weeks gestation (CS16) (Fig.4B,C;Supplemental Movies 10a,b). At 7 3/7 and 7 5/7 weeks (CS16 and CS17), the atrioventricular junction was still undivided. A few days later, by 8 weeks gestation (CS18), separate atrioventricular valves can be seen (arrowheads in Fig.4D,E), with left sided mitral and right sided tricuspid valves forming. An embryo at 8 weeks (CS18) is approximately 10 mm in size, correlating well with the embryonic stage at which fusion of the endocardial cushions is thought to occur26. The valve leaflets however appear thick at this stage (see Supplemental Movie 4). By 9 1/7 weeks (CS22), the atrioventricular valve leaflets are thinner and more mature in appearance (Fig.3H;Supplemental Movie 5). At 7 3/7 weeks (CS16), distinct posterior and anterior cushions are not observed, the inferior atrioventricular cushion is observed and this timing is consistent with previous reports of human embryonic development.22
The major developmental processes occurring at the level of the truncus arteriosus consist of septation into two separate arterial channels, and semilunar valve morphogenesis. The truncus arteriosus is formed largely from cells derived from the secondary heart field27. Septation of the truncus arteriosus is dependent on activity of the secondary heart field and migrating neural crest cells28, 29, and is achieved with in growth of ridges. In the proximal truncus arteriosus, we observed truncal cushions in the form of swellings at 7 1/7 weeks (CS15) (arrowhead in Fig.5A). This forming aorticopulmonary septum undergoes a gradual spiraling course that ultimately completes truncus arteriosus septation into separate aorta and pulmonary arteries28. At 7 3/7 weeks (CS16), this spiraling course of the forming aorticopulmonary septum is evident as a spiraling in the orientation of the lumen along the proximodistal axis of the truncus arteriosus (Fig.5D–F). The truncus arteriosus remains as a single channel proximally (Fig.5D–E), but distally, it divides into two separate channels (Fig.5F; Supplemental Movie 7). Smooth muscle derived from the secondary heart field and from cardiac neural crest cells plays a crucial role in the septation and alignment of the truncus arteriosus28.
Bartelings and Gittenberger de Groot6 suggested that in 7 3/7 weeks (CS16) embryos septation begins at the ventriculo-arterial junction and progresses proximal to distal in the truncus arteriosus. However, our findings show septation of the truncus arteriosus occurring in the opposite direction, being complete distally in the 7 3/7 week (CS16) embryo, at a time when the proximal truncus arteriosus is still undivided. This would suggest that the direction of septation is distal to proximal. This is supported by Kirby28, who described the proximal truncus arteriosus closing zipper-like from distal to proximal towards the ventricles. Our data also support both the timing and direction of septation proposed by Anderson et al29. They described septation of the truncus arteriosus initiating distally and progressing proximally with the presence of distal septation and the absence of proximal septation at 7 3/7 week (CS 16). Moore19 described bulbar ridges at the fifth week post conception, equivalent to 7 weeks gestation. Assuming the bulbar and truncal cushions are forming at the same time, our finding of truncal cushions in the outflow at 7 1/7 weeks (CS15) also corroborates these investigators’ timeframe.
The process of semilunar valve morphogenesis, similar to atrioventricular valve morphogenesis, began earlier with the formation of truncal cushion tissue which was observed in the outflow starting at 7 1/7 weeks (CS15). At 8 weeks (CS18), distinct pulmonary and aortic valves can be seen (Fig.5B,C). These valve leaflets, as well as atrioventricular valve leaflets, are initially thick. They undergo a process of thinning as the valve leaflets continue to form and mature; a process that continues well after the formation of distinct valve leaflets (Fig. 6). By 9 1/7 weeks (CS22), all of the major structures of the heart are formed, with the last developmental milestone being completion of the inlet ventricular septum (see above).
As rapid advances in technology provide first trimester human fetal cardiac imaging and opportunities for in utero intervention continue to advance, there is increasing need for data documenting human cardiac development in the first trimester. Using a large data set generated by MRI and EFIC imaging, the major developmental milestones of human cardiac morphogenesis were delineated spanning EGA 6 4/7–9 3/7 weeks. A summary timeline is provided in Figure 6 for the temporal profile of atrial and ventricular septation, outflow septation and valvular morphogenesis. In addition, Figures 7 and and88 were generated as reference guides to aid clinical practice. They contain thumbnail images of cardiac structures seen at each developmental milestone of cardiac morphogenesis. Full size images and Quicktime movies of the 2D serial image stacks of these embryos can be viewed via the online supplement (Movies 1–7) and the web based Human Embryo Atlas (http://apps.nhlbi.nih.gov/HumanAtlas/). A deeper understanding of human cardiovascular development, including this large dataset and the reference guides generated, may ultimately aid in clinical practice and facilitate prenatal diagnosis of CHD and appropriate counseling of families.
This work was supported by NIH grant ZO1-HL005701. The Kyoto collection was supported by Japanese Ministry of Education, Culture, Sports, Science and Technology (Grant 19390050); Japanese Ministry of Health, Labor and Welfare (Grant: 17A-6) and Japan Science Technology Agency (BIRD grant). S.Yamada was supported by Kyoto University Foundation.
Preeta Dhanantwari: None; Elaine Lee: None; Anita Krishnan: None; Rajeev Samtani: None; Shigehito Yamada: None; Stasia Anderson: None; Elizabeth Lockett: None; Mary Donofrio: None; Kohei Shiota: None; Cecilia Lo: None;
Linda Leatherbury: Research Grant: Comparison of Human Cardiac Development in the First Trimester with Mouse: Analysis with High Resolution MRI and EFIC.