A digital Human Embryo Atlas spanning CS13 to 23 was constructed using data obtained by MR and EFIC imaging. Embryos were first scanned by MR microscopy and then processed for EFIC imaging. EFIC yielded higher resolution images, and thus was superior for analyzing the smaller early stage human embryos from CS13–18. At these developmental stages, MR datasets gave good external contour views of the embryo, but fine structures of the inner anatomy could not be resolved. For example, endocardial cushions in the heart or Rathke’s pouch in the forming hypophysis were observed in the EFIC but not MR images. Imaging of phantoms suggests the optical resolution of EFIC is approximately 5–6 um (Lo, unpublished observations). At this resolution, it is possible to see fine structures, such as the smaller branches of blood vessels or developing tubules and collecting ducts in the developing kidney. While such structures may also be observed by standard paraffin histology, the advantage with EFIC imaging is the ability to digitally reslice the images for viewing structures in different imaging planes and also as 3D reconstructions.
For embryos older than CS18, EFIC imaging was problematic. The larger size of the later stage embryos required prolonged processing time for paraffin infiltration and embedding, and as a result, the tissue autofluorescence required for EFIC imaging was markedly reduced. This is likely due to the 30–40 year immersion in fixative of these historical specimens, and not merely the larger size of these later stage embryos. For comparison, mouse fetuses or newborn mice of similar size to CS23 human embryos do not present problems for EFIC imaging (Rosenthal et al., 2004
). Nevertheless, MR imaging of these later stage human embryos yielded good quality imaging data that delineated fine details of internal anatomy. Thus with the combination of MR and EFIC imaging, we were able to construct a Human Embryo Atlas that spanned the entirety of CS13 to 28.
MR imaging has one important advantage in that it is noninvasive and nondestructive, and has been applied for the past 20 years to analyze embryonic development in different animal models (Bone et al., 1986
; Smith et al., 1992
; Smith et al., 1994
; Smith et al., 1996
). With recent advances in MR imaging (Matsuda et al., 2003
), resolution of 35 μm/pixel or better is now possible. However, the cost of MR instrumentation and the technical support required makes this imaging modality out of reach for many investigators. In contrast, the standard sliding microtome used in EFIC imaging is commonly available in many pathology laboratories. The low light digital camera and epifluorescence stereomicroscope required for capturing EFIC images are also standard equipment available in many biomedical research laboratories. In addition, EFIC imaging is currently still higher in resolution and thus superior for imaging small specimen.
The ability to digitally resection MR and EFIC imaging data in arbitrary planes has tremendous advantage for analyzing the dynamic changes in anatomic structures in the developing human embryo. In the online Human Embryo Atlas, 2D image stacks in three histological planes are available for viewing or download as QuickTime movies. These serial 2D image stacks can be digitally resectioned in any plane to optimize viewing of different tissue and anatomic structures. 3D reconstructions are also easily generated for further interrogating the relative position of tissues and organs in the developing embryo.
This Human Embryo Atlas will be useful for the clinician and biomedical scientists studying human embryonic development and the etiology of human congenital anomalies. In addition, this atlas will be a valuable teaching tool for learning embryology and human developmental anatomy. We note embryology is often minimally covered in medical education (less than 20 hours; (Drake et al., 2002
). This web accessible Human Embryo Atlas should serve as a valuable tutorial of human embryology (Watt et al., 1996
; Aiton et al., 1997
; Carlson, 2002
; Puerta-Fonolla et al., 2004
; Arroyo-Jimenez Mdel et al., 2005
; Yamada et al., 2006
), especially as more detailed annotation of the atlas is developed. To this end, we have constructed the Human Embryo Atlas website with a function for users to propose new annotations or revisions of existing annotations. This user interface will allow the biomedical community as a whole to contribute to the development and further refinement of the digital Human Embryo Atlas, making it a more accurate and valuable tool for teaching and research.
Given that much of our present understanding of human embryonic development is extrapolated from insights gained from studying the mouse embryo, access to human embryo data is necessary to ascertain possible differences between mouse vs. human embryonic development. Moreover, availability of the Human Embryo Atlas will be helpful for elucidating the pathogenesis of human congenital anomalies. Since most of the human embryos in the Kyoto collection were derived from pregnancies terminated for socioeconomic reasons under the Maternity Protection Law of Japan (Yamada et al., 2004
; Yamada et al., 2006
), they are expected to represent normal embryonic population in the uterus. Most of the embryos included in this study were normal in appearance. As a pilot, we also analyzed a few embryos with preaxial digit duplications. However, the data collected from the latter embryos showed no detectable change in internal anatomic structures. Nevertheless, the body of data collected from this study will provide a solid basis for launching a more broad based study of human embryos with congenital anomalies.
In the future, expansion of the Atlas with the analysis of more human embryos will help to further refine the temporal profile of human embryonic development. Analysis of imaging data from human embryos with congenital anomalies may yield novel insights into the developmental origin of human birth defects. Overall, this Human Embryo Atlas is a unique resource that provides morphologic data of human development that can facilitate clinical evaluation of congenital anomalies, and accelerate basic research investigations into developmental mechanisms that underlie human congenital anomalies.