Inferior Extensions of the Atrioventricular Node

The pathways for excitation of the atrioventricular node enter either superiorly, as the so-called ‘fast’ pathway, or inferiorly as the ‘slow’ pathway. However, knowledge of the specific anatomical details of these pathways is limited. Most of the experimental studies that established the existence of these pathways were conducted in mammalian hearts, which have subtle differences to human hearts. In this review, the authors summarise their recent experiences investigating human cardiac development, correlating these results with the arrangement of the connections between the atrial myocardium and the compact atrioventricular node as revealed by serial sectioning of adult human hearts. They discuss the contributions made from the atrioventricular canal myocardium, as opposed to the primary ring. Both these rings are incorporated into the atrial vestibules, albeit with the primary ring contributing only to the tricuspid vestibule. The atrial septal cardiomyocytes are relatively late contributors to the nodal inputs. Finally, they relate our findings of human cardiac development to the postnatal arrangement.

Comparative flower morphology of Agapanthus africanus and A. praecox (Amaryllidaceae)

The structure of Agapanthus africanus and A. praecox flowers was studied on permanent cross-sectional and longitudinal sections using a light microscope. The genus Agapanthus belongs to the subfamily Agapanthoideae, the family Amaryllidaceae, which is characterized by the presence of the upper ovary, septal nectaries and fruit – fleshy capsule. Micromorphological studies of the flower are considered as a way for detection of unknown plant features, adjustment of plants to specialized ways of pollination and determining the first stages of morphogenesis of fruit, and further use these features in taxonomy. 10 flowers of A. africanus and A. praecox were sectioned using standard methods of Paraplast embedding and serial sectioning at 20 micron thickness. Sections were stained with Safranin and Astra Blau and mounted in Eukitt. It was found that in the studied species the tepals have single-bundle traces. The vascular system of the superior ovary consists of a three bundle dorsal vein, of the ventral roots complex, which are reorganized into paired ventral bundles of the carpel, which form traces to ovules. For the first time, the following gynoecium zones were detected in A. africanus: a synascidiate structural zone with a height of about 560 μm and a fertile symplicate structural zone with a height of about 380 μm and a hemisymplicate zone of 2580 μm. In A. praecox gynoecium, there is a synascidiate structural zone with a height of 200 μm and a symplicate structural zone of 600 μm and a hemisymplicate zone of 620 μm. Septal nectaries appear in the hemisymplicate zone and open with nectar fissures at the base of the column, with a total septal nectar height of 2880 μm in A. africanus and 820 μm in A. praecox. The ovary roof is 300 µm in A. africanus and 200 µm in A. praecox. Triple dorsal bundles of carpels in A. africanus have been identified, which could be considered as adaptation of different stages of morphogenesis of fruit to dehiscence. The new data obtained by the vascular anatomy of the flower and the presence of different ovary zones significantly add to the information about anatomical and morphological features of the studied species, which can be further used in the taxonomy of the family Amaryllidaceae.

Niwaki Instead of Random Forests: Targeted Serial Sectioning Scanning Electron Microscopy With Reimaging Capabilities for Exploring Central Nervous System Cell Biology and Pathology

Ultrastructural analysis of discrete neurobiological structures by volume scanning electron microscopy (SEM) often constitutes a “needle-in-the-haystack” problem and therefore relies on sophisticated search strategies. The appropriate SEM approach for a given relocation task not only depends on the desired final image quality but also on the complexity and required accuracy of the screening process. Block-face SEM techniques like Focused Ion Beam or serial block-face SEM are “one-shot” imaging runs by nature and, thus, require precise relocation prior to acquisition. In contrast, “multi-shot” approaches conserve the sectioned tissue through the collection of serial sections onto solid support and allow reimaging. These tissue libraries generated by Array Tomography or Automated Tape Collecting Ultramicrotomy can be screened at low resolution to target high resolution SEM. This is particularly useful if a structure of interest is rare or has been predetermined by correlated light microscopy, which can assign molecular, dynamic and functional information to an ultrastructure. As such approaches require bridging mm to nm scales, they rely on tissue trimming at different stages of sample processing. Relocation is facilitated by endogenous or exogenous landmarks that are visible by several imaging modalities, combined with appropriate registration strategies that allow overlaying images of various sources. Here, we discuss the opportunities of using multi-shot serial sectioning SEM approaches, as well as suitable trimming and registration techniques, to slim down the high-resolution imaging volume to the actual structure of interest and hence facilitate ambitious targeted volume SEM projects.

AFRL Additive Manufacturing Modeling Series: Challenge 4, 3D Reconstruction of an IN625 High-Energy Diffraction Microscopy Sample Using Multi-modal Serial Sectioning

High-energy diffraction microscopy (HEDM) in-situ mechanical testing experiments offer unique insight into the evolving deformation state within polycrystalline materials. These experiments rely on a sophisticated analysis of the diffraction data to instantiate a 3D reconstruction of grains and other microstructural features associated with the test volume. For microstructures of engineering alloys that are highly twinned and contain numerous features around the estimated spatial resolution of HEDM reconstructions, the accuracy of the reconstructed microstructure is not known. In this study, we address this uncertainty by characterizing the same HEDM sample volume using destructive serial sectioning (SS) that has higher spatial resolution. The SS experiment was performed on an Inconel 625 alloy sample that had undergone HEDM in-situ mechanical testing to a small amount of plastic strain (~ 0.7%), which was part of the Air Force Research Laboratory Additive Manufacturing (AM) Modeling Series. A custom-built automated multi-modal SS system was used to characterize the entire test volume, with a spatial resolution of approximately 1 µm. Epi-illumination optical microscopy images, backscattered electron images, and electron backscattered diffraction maps were collected on every section. All three data modes were utilized and custom data fusion protocols were developed for 3D reconstruction of the test volume. The grain data were homogenized and downsampled to 2 µm as input for Challenge 4 of the AM Modeling Series, which is available at the Materials Data Facility repository.