Dynamic ultrastructure of mouse pulmonary alveoli revealed by an in vivo cryotechnique in combination with freeze-substitution

Light and electron microscopic analyses of the high deformability of adhesive toe pads in White's tree frog, Litoria caerulea

White's tree frog (Litoria caerulea) has large, adhesive toe pads that are among the softest of all known biological structures. To explore the morphological basis for the physical properties of the toe pads, the internal microstructure of the toe pads in L. caerulea was examined using both light and transmission electron microscopy. Three design elements that are distinct from other areas of skin were observed. First, the keratinocytes comprising the adhesive surface of the toe pad all contained keratin filament bundles (tonofibrils) exhibiting structural anisotropy. Specifically, the curved conformation of the hierarchical (branching) tonofibrils was characterized by the formation of anastomoses consisting of tonofibrils beneath the adhesive cell surface and stem keratin filament bundles concentrated in the lower-middle part of the dorsal-side of adhesive cells. Second, the cytoplasm of keratinocytes in the most superficial cell layer contained glycoproteins (stained by periodic acid/Schiff reagent) that are considered to confer high viscoelasticity. Third, the dermis contained large lymph spaces interspersed with elastic fibers and collagen fibers, which were relatively sparsely distributed compared to the dorsal skin of the toe pads. The profiles of these structures were easily deformed by the slight application of pressure. These findings reaffirmed that the unique internal architecture of the toe pads in L. caerulea contributed to their remarkable softness and high deformability, which in turn increased the contact area and provided improved adaptability to the local topography of natural surfaces. J. Morphol. 277:1509-1516, 2016. © 2016 Wiley Periodicals, Inc.

Histochemical analyses and quantum dot imaging of microvascular blood flow with pulmonary edema in living mouse lungs by "in vivo cryotechnique"

Light microscopic imaging of blood vessels and distribution of serum proteins is essential to analyze hemodynamics in living animal lungs under normal respiration or respiratory diseases. In this study, to demonstrate dynamically changing morphology and immunohistochemical images of their living states, "in vivo cryotechnique" (IVCT) combined with freeze-substitution fixation was applied to anesthetized mouse lungs. By hematoxylin-eosin staining, morphological features, such as shapes of alveolar septum and sizes of alveolar lumen, reflected their respiratory conditions in vivo, and alveolar capillaries were filled with variously shaped erythrocytes. Albumin was usually immunolocalized in the capillaries, which was confirmed by double-immunostaining for aquaporin-1 of endothelium. To capture accurate time-courses of blood flow in peripheral pulmonary alveoli, glutathione-coated quantum dots (QDs) were injected into right ventricles, and then IVCT was performed at different time-points after the QD injection. QDs were localized in most arterioles and some alveolar capillaries at 1 s, and later in venules at 2 s, reflecting a typical blood flow direction in vivo. Three-dimensional QD images of microvascular networks were reconstructed by confocal laser scanning microscopy. It was also applied to lungs of acute pulmonary hypertension mouse model. Erythrocytes were crammed in blood vessels, and some serum components leaked into alveolar lumens, as confirmed by mouse albumin immunostaining. Some separated collagen fibers and connecting elastic fibers were still detected in edematous tunica adventitia near terminal bronchioles. Thus, IVCT combined with histochemical approaches enabled us to capture native images of dynamically changing structures and microvascular hemodynamics of living mouse lungs.

Significance of 'in vivo cryotechnique' for morphofunctional analyses of living animal organs

Our final goal of morphological and immunohistochemical studies is that all findings examined in animal experiments should reflect the physiologically functional background. Therefore, the preservation of original components in cells and tissues of animals is necessary for describing the functional morphology of living animal organs. It is generally accepted that morphological findings of various organs were easily modified by stopping their blood supply. There had been a need to develop a new preparation technique for freezing the living animal organs in vivo and then obtaining acceptable morphology and also immunolocalization of original components in functioning cells and tissues. We already developed the 'in vivo cryotechnique' (IVCT) not only for their morphology, but also for immunohistochemistry of many soluble components in various living animal organs. All physiological processes of cells and tissues were immediately immobilized by IVCT, and every component in the cells and tissues was maintained in situ at the time of freezing. Thus, the ischaemic or anoxic effects on them could be minimized by IVCT. Our specially designed cryoknife with liquid cryogen has solved the morphological and immunohistochemical problems which are inevitable with the conventional preparation methods at a light or electron microscopic level. The IVCT will be extremely useful for arresting transient physiological processes and for maintaining any intracellular components in situ, such as rapidly changing signal molecules, membrane channels and receptors.

Morphological analysis of lamellar structures in mouse type II pneumocytes by quick-freezing and freeze-drying with osmium tetroxide vapor-fixation

The lamellar body is a membranous structure periodically laminating in vesicles that is known as the most distinctive feature of type II pneumocytes by conventional preparation methods for transmission electron microscopy. The quick-freezing and freeze-drying method, followed by osmium tetroxide vapor-fixation (QF-FD-OsV), was performed to examine the in situ morphology of the lamellar body in type II pneumocytes of living mouse lungs. Typical lamellar structures were rarely seen in vesicles of the type II pneumocytes, but amorphous components and dispersed stripes were often detected in the vesicles, as revealed by the QF-FD-OsV method. To clarify how the lamellar body was formed during the conventional preparation steps, lung tissues of mice were treated with different fixation procedures, such as immersion-fixation with osmium tetroxide or perfusion-fixation with glutaraldehyde followed by osmium tetroxide, in combination with alcohol dehydration or QF-FD-OsV. In addition to lamellar bodies of type II pneumocytes in the specimens with alcohol dehydration, some lamellar structures were also formed even with the QF-FD-OsV method. These findings suggest that the labile lamellar body is easily modified and formed during both chemical fixation and alcohol dehydration steps.

Dynamic study of intramembranous particles in human fresh erythrocytes using an “in vitro cryotechnique”

For analyses of dynamic ultrastructures of erythrocyte intramembranous particles (IMPs) in situ, a quick-freezing method was used to stabilize the flow behavior of erythrocytes embedded in vitreous ice. Fresh human blood was jetted at various pressures through artificial tubes, in which the flowing erythrocytes were elongated from biconcave discoid shapes to elliptical ones, and quickly frozen in liquid isopentane-propane cryogen (-193 degrees C). They were freeze-fractured using a scalpel in liquid nitrogen, and routinely prepared for replica membranes. Many IMPs were observed on the protoplasmic freeze-fracture face (P-face) of the erythrocyte membranes. Some control erythrocytes under nonflowing or stationary conditions showed IMPs with their random distribution. However, other jetted erythrocytes under flowing conditions showed variously sized IMPs with much closer distribution. They were also arranged into parallel rows in some parts, and aggregated together. This quick-freezing method enabled for the first time the visualization of time-dependent topology and the molecular alteration of IMPs in dynamically flowing erythrocytes.

Application of cryotechniques with freeze-substitution for the immunohistochemical demonstration of intranuclear pCREB and chromosome territory

Intranuclear localization of signal molecules and chromosome territories has become more attractive in relation to postgenomic analyses of cellular functions. Cryotechniques and freeze-substitution (CrT-FS) have been generally used for electron microscopic observation to obtain better ultrastructure and immunoreactivity. To investigate benefits of applying the CrT-FS method to immunostaining of intranuclear signal molecules and FISH for chromosome territories, we performed an immunohistochemical study of phosphorylated cAMP-responsive element binding protein (pCREB) in mouse cerebellar tissues and a FISH study of chromosome 18 territory in human thyroid tissues using various cryotechniques. The immunoreactivity of pCREB was more clearly detected without antigen retrieval treatment on sections prepared by the CrT-FS method than those prepared by the conventional dehydration method. In the FISH study, more definite probe labeling of the chromosome territory could be obtained on paraffin sections by the CrT-FS method without microwave treatment, although such labeling was not clear even with microwave treatment on sections prepared by the routine dehydration method. The CrT-FS preserved relatively native morphology by preventing shrinkage of nuclei, and produced better immunoreactivity. Because the reduction of routine pretreatments in the present study might reveal more native morphology, the CrT-FS method would be a useful technique for intranuclear immunostaining and FISH.

Detection of injected fluorescence-conjugated IgG in living mouse organs using “in vivo cryotechnique” with freeze-substitution

In this experiment, we performed the "in vivo cryotechnique" in tandem with fluorescence microscopy. The fluorescein isothiocyanate (FITC)-conjugated goat anti-rabbit immunoglobulin (IgG) antibody (FITC-IgG) was directly injected into mouse livers or kidneys, which were then frozen in vivo by pouring an isopentane-propane mixture (-193 degrees C) cooled in liquid nitrogen over these living organs. The organs were subsequently freeze-substituted in acetone containing paraformaldehyde at about -80 degrees C, then gradually brought up to a room temperature, infiltrated with 30% sucrose and refrozen. Some well-frozen areas 300-400 mum below the frozen tissue surface were cryocut into several slices. The slices were observed under the fluorescence microscope. By examining the distribution of FITC-IgG in the frozen livers, some aspects of functional blood circulation in the liver, such as the concept of the liver lobule, were reconfirmed. This also confirmed that the blood flow in the liver after the FITC-IgG injection was normal. The subsequent preparation of the specimens with immunohistochemistry, using the tetramethylrhodamine (TRITC)-conjugated anti-mouse IgG antibody, allowed us to visualize the localizations of both the original mouse IgG and the injected goat IgG in the cryosections with different color images. The experimental protocol presented demonstrates the in situ localization of the various proteins labeled with fluorescent probes, and it can, in conjunction with immunohistochemistry, localize proteins in cells and tissues.

Effects of anoxia on serum immunoglobulin and albumin leakage through blood–brain barrier in mouse cerebellum as revealed by cryotechniques

The purpose of this study was to examine time-dependent topographical changes of leaking proteins from blood vessels in the mouse cerebellum to assess the effect of normal blood circulation on the blood-brain barrier (BBB). The distribution of leaking serum proteins was immunohistochemically examined by various cryotechniques including our "in vivo cryotechnique". The cryofixed cerebellar tissues were processed for the freeze-substitution method, and finally embedded in the common paraffin wax. Serial de-paraffinized sections were immunostained by anti-mouse immunoglobulin-G (IgG) or albumin antibody. By combination of the "in vivo cryotechnique", in which normal blood flow into the cerebellum was always kept in vivo, with the freeze-substitution method, serum IgG and albumin were clearly localized inside of cerebellar blood vessels. To examine abnormal leakage of blood vessels as a model of anoxia, some cerebellar tissues were partially removed from brains in the mouse skull and quickly frozen in the isopentane-propane within a minute. In such resected cerebellar tissues, serum IgG and albumin were diffusely immunostained in large areas around the blood capillaries, probably because of easy leakage of the serum components through the immediately changed BBB. To the contrary, no serum protein could be identified outside blood capillaries under living conditions of the anesthetized mice. The present combination method, both "in vivo cryotechnique" and freeze-substitution, for immunohistochemistry enabled us to examine the in vivo localization of serum components in mouse brains due to alteration of the BBB.

On defining total lung capacity in the mouse

Maximal lung volume or total lung capacity in experimental animals is dependent on the pressure to which the lungs are inflated. Although 25-30 cmH2O are nominally used for such inflations, mouse pressure-volume (P-V) curves show little flattening on inflation to those pressures. In the present study, we examined P-V relations and mean alveolar chord length in three strains (C3H/HeJ, A/J, and C57BL/6J) at multiple inflation pressures. Mice were anesthetized, and their lungs were degassed in vivo by absorption of 100% O2. P-V curves were then recorded in situ with increasing peak inflation pressure in 10-cmH2O increments up to 90 cmH2O. Lungs were quickly frozen at specific pressures for morphometric analysis. The inflation limbs never showed the appearance of a plateau, with lung volume increasing 40-60% as inflation pressure was increased from 30 to 60 cmH2O. In contrast, parallel flat deflation limbs were always observed, regardless of the inflation pressure, indicating that the presence of a flat deflation curve cannot be used to justify measurement of total lung capacity in mice. Alveolar size increased monotonically with increasing pressure in all strains, and there was no evidence of irreversible lung damage from these inflations to high pressures. These results suggest that the mouse lung never reaches a maximal volume, even up to nonphysiological pressures >80 cmH2O.