Budding pouches and associated bubbles: 3D visualization of exo-membrane structures in plasmodium falciparum gametocytes

Plasmodium falciparum gametocytes have unique morphology, metabolism, and protein expression profiles in their asexual stages of development. In addition to the striking changes in their appearance, a wide variety of “exo-membrane structures” are newly formed in the gametocyte stage. Little is known about their function, localization, or three-dimensional structural information, and only some structural data, typically two-dimensional, have been reported using conventional electron microscopy or fluorescence microscopy. For better visualization of intracellular organelle and exo-membrane structures, we previously established an unroofing technique to directly observe Maurer’s clefts (MCs) in asexual parasitized erythrocytes by removing the top part of the cell’s membrane followed by transmission electron microscopy. We found that MCs have numerous tethers connecting themselves to the host erythrocyte membrane skeletons. In this study, we investigated the intracellular structures of gametocytes using unroofing-TEM, Serial Block Face scanning electron microscopy, and fluorescence microscopy to unveil the exo-membrane structures in gametocytes. Our data showed “balloon/pouch”-like objects budding from the parasitophorous vacuole membrane (PVM) in gametocytes, and some balloons included multiple layers of other balloons. Furthermore, numerous bubbles appeared on the inner surface of the erythrocyte membrane or PVM; these were similar to MC-like membranes but were smaller than asexual MCs. Our study demonstrated P. falciparum reforms exo-membranes in erythrocytes to meet stage-specific biological activities during their sexual development.

Intraperitoneal dye injection method for visualizing the functioning lymphatic vascular system in zebrafish and medaka

A hierarchically organized lymphatic vascular system extends throughout the vertebrate body for tissue fluid homeostasis, immune trafficking, and the absorption of dietary fats. Intralymphatic dye injection and serial sectioning have been the main tools for visualizing lymphatic vessels. Specific markers for identifying the lymphatic vasculature in zebrafish and medaka have appeared as new tools that enable the study of lymphangiogenesis using genetic and experimental manipulation. Transgenic fishes have become excellent organisms for visualizing the lymphatic vasculature in living embryos, but this method has limited usefulness, especially in later developmental stages. The functional lymphatic endothelium predominantly takes up foreign particles in zebrafish and medaka. We utilized this physiological activity and lymph flow to label lymphatic vessels. Intraperitoneal injection of trypan blue is useful for temporal observations of the lymphatic ducts, which are essential for large-scale genetic screening, while cinnabar (HgS) injection allows identification of the lymphatic endothelium under electron microscopy, avoiding artefactual damage. This injection method, which is not high in cost and does not require high skill or special devices, is applicable to any live fish with functioning lymphatic vessels, even mutants, with high reproducibility for visualizing the entire lymphatic vascular system.

Three-dimensional ultrastructural study of oil and astaxanthin accumulation during encystment in the green alga Haematococcus pluvialis

Haematococcus pluvialis is a freshwater species of green algae and is well known for its accumulation of the strong antioxidant astaxanthin, which is used in aquaculture, various pharmaceuticals, and cosmetics. High levels of astaxanthin are present in cysts, which rapidly accumulate when the environmental conditions become unfavorable for normal cell growth. It is not understood, however, how accumulation of high levels of astaxanthin, which is soluble in oil, becomes possible during encystment. Here, we performed ultrastructural 3D reconstruction based on over 350 serial sections per cell to visualize the dynamics of astaxanthin accumulation and subcellular changes during the encystment of H. pluvialis. This study showcases the marked changes in subcellular elements, such as chloroplast degeneration, in the transition from green coccoid cells to red cyst cells during encystment. In green coccoid cells, chloroplasts accounted for 41.7% of the total cell volume, whereas the relative volume of astaxanthin was very low (0.2%). In contrast, oil droplets containing astaxanthin predominated in cyst cells (52.2%), in which the total chloroplast volume was markedly decreased (9.7%). Volumetric observations also demonstrated that the relative volumes of the cell wall, starch grains, pyrenoids, mitochondria, the Golgi apparatus, and the nucleus in a cyst cell are smaller than those in green coccid cells. Our data indicated that chloroplasts are degraded, resulting in a net-like morphology, but do not completely disappear, even at the red cyst stage.