Three-dimensional ultrastructure of anionic sites of the glomerular basement membrane by a quick-freezing and deep-etching method using a cationic tracer

Elucidation of the control mechanism of dynamic tissues using freezing techniques

The preparation of histological specimens from animals and humans is a multi-step process comprising tissue collection, fixation, and dehydration, followed by paraffin embedding. Each process can be achieved using different methods and substances. For example, dehydration may not be required depending on the substance used for embedding. The freezing technique described in the present study can be used for tissue collection and fixation. Tissues obtained using "in vivo cryotechnique (IVCT)" reflect blood flow and protein localization in body fluids at the time of tissue collection, making it an indispensable method in histological analyses of the future. This study utilized the IVCT to capture histological images of dynamic objects from multiple viewpoints and elucidate the mechanism underlying their movement control at the molecular level.

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.

Anionic sites in articular cartilage revealed by polyethyleneimine staining

Articular cartilage is a unique tissue that contains neither blood vessels nor nerves, and that performs mechanical loading during joint movement. These properties are endowed by abundant glycosaminoglycans (GAGs), which are capable of retaining water-soluble substances. The GAGs attach to core proteins and form proteoglycans. Although many studies have focused on proteoglycans and collagen fibrils in cartilage, little is known about the nature of the negative charge of GAGs. Recently, we investigated this subject using a cationic dye, polyethyleneimine (PEI), with several different techniques such as pre-embedding, post-embedding, and quick-freezing and deep-etching methods. In addition, we investigated whether the anionic charge is altered at low pH, using PEI and cationic colloidal gold (CCG) labeling. The shapes of PEI-positive structures revealed by the pre-embedding method varied at different pHs. Three-dimensional analysis using the quick-freezing and deep-etching method demonstrated that meshwork structures composed of fine filaments were decorated with tiny PEI granules. Additionally, the meshwork structure was broken down after chondroitinase ABC digestion. These data indicate that the large PEI deposits observed in pre-embedding preparations are, at least in part, artificial images, and that the meshwork structure consists of chondroitin sulfate-retaining anionic sites. Low pH conditions changed PEI or CCG labeling patterns, showing that negative charges of GAGs in articular cartilage are altered under environmental pH conditions. These findings demonstrate that binding capacities of anionic sites to water-soluble or ionic substances are greatly affected by pH alterations without actually decreasing the number of anionic sites. Therefore, to understand cartilage dynamics and the pathogenesis of joint diseases in greater detail, alterations of anionic charge during mechanical loading or under pathological conditions should be examined in future studies.

Proteoglycans in articular cartilage revealed with a quick freezing and deep etching method

OBJECTIVES

To clarify the three dimensional ultrastructure of proteoglycans, and their relationship with other matrix components in articular cartilage.

METHODS

Specimens from rat femoral heads were examined using three techniques: (1) Histochemical staining with cationic polyethyleneimine (PEI), using a pre-embedding or a postembedding method. Some tissues were pretreated with chondroitinase ABC or hyaluronidase. (2) Quick freezing and deep etching (QF-DE). Some specimens were fixed with paraformaldehyde and washed in buffer solution before quick freezing; others were frozen directly. (3) Ultrathin sections were studied after conventional preparation.

RESULTS

Proteoglycans were observed as aggregated clumps with PEI staining by the pre-embedding method, but as fine filaments by the postembedding method. They were lost with enzyme digestion; this was also demonstrated by the QF-DE method. The ultrastructure was well preserved by the QF-DE method when fixation and washing procedures were included, but not without these procedures. A fine mesh-like structure was connected to the cell membrane in the pericellular matrix. Filamentous structures suggestive of aggrecans were observed among collagen fibrils. They had side chains, approximately 50 nm in length, which branched from the central filaments at intervals of 10-20 nm, and were occasionally linked to other structures. Many thin filaments were also attached to the collagen fibrils.

CONCLUSIONS

The QF-DE method incorporating paraformaldehyde fixation and buffer washing procedures revealed three dimensional, extended structures suggestive of proteoglycans.

Dynamic structure of glomerular capillary loop as revealed by an in vivo cryotechnique

Morphological studies using immersion or perfusion fixation methods do not reveal the ultrastructure of functioning kidneys with normal circulation. A simple apparatus was developed for freezing the kidneys in vivo without stopping the blood supply, and the ultrastructure of the glomerular capillary loops was examined under different haemodynamic conditions. Mouse kidneys were frozen under normal blood flow conditions; others were frozen in the same way after ligation of the abdominal aorta at a point caudal to the renal arteries. They were then processed for the freeze-substitution or deep-etching method. Good ultrastructural preservation was obtained within about 5 microM depth from the frozen tissue surface. Functioning glomeruli with normal blood flow possessed open capillary lumens, different shapes of foot processes and atypical basement membranes with low density. Moreover, heterogeneity in width between foot processes was identified on the replica membranes. Under the acute conditions used to increase blood supply into the kidneys, the spaces between the flat foot processes became more widely dilated and the basement membrane was seen to be three-layered. The ultrastructure of glomeruli in functioning kidneys has been demonstrated for the first time by this "in vivo cryotechnique."

Cell biology of kidney glomerulus

It has been accepted that some artifacts are inevitably produced by the conventional preparation steps for electron microscopy, including fixation, dehydration, embedding, ultrathin sectioning, and staining. Therefore, conventional ultrastructural findings on kidney glomeruli are hardly thought to be correlated with the physiological functions of kidneys in vivo. In this chapter, two preparation techniques, the quick-freezing and deep-etching (QF-DE) method or the quick-freezing and freeze-substitution (QF-FS) method, are presented and shown to be useful for clarifying the ultrastructures of kidney glomeruli more closely to structures in vivo with fewer artifacts. Moreover, the ultrastructures of glomerular capillary loops have been demonstrated by a new "in vivo cryotechnique," that shows that hemodynamic factors should be considered in the morphological study of glomerular functions.

Further observations on the morphological alterations of the glomerular capillary wall of the rat kidney caused by chemical and physical agents: standard procedures versus quick-freezing and freeze-substitution

The purpose of the present study was to pin-point which of the various preparatory steps required by different histological procedures (conventional, cryosubstitution, freezing followed by cryosubstitution or replication) induces alterations in the fine structure of the endothelial and epithelial surface coat (SC) and of the interposed glomerular basement membrane (GBM). Samples of rat kidney cortex prefixed by vascular perfusion, and isolated glomeruli fixed by immersion, were used. The results demonstrate a continuous, homogeneous and amorphous SC and a GBM devoid of laminae rarae when freezing is used and followed either by cryosubstitution and embedding or by fracture, deep etching and replication. Postfixation in OsO4, and especially dehydration in organic solvents at room temperature, generate a filamentous SC and a GBM with laminae rarae. The different morphology may be due to an extraction of glycoconjugates from SC and GBM during postosmication and/or especially to precipitation of these components during dehydration by organic solvents at room temperature. Studies on isolated glomeruli show comparable results and, in addition, demonstrate that the distance of the capillary loop from the surface of the block influences the morphology of SC and GBM. Rinsing the vascular bed before isolation does not induce a loss of SC or GBM components.

Anionic sites in glomerular basement membrane of rats with serum sickness nephritis: quick-freezing and deep-etching study

Serum sickness nephritis was induced in male Fisher 344/JCL rats by injecting egg albumin into the foot pads and peritoneal cavity. The alteration of anionic sites in the glomerular basement membrane (GBM) of the rats with significant proteinuria was studied with a quick-freezing and deep-etching method using polyethyleneimine as a cationic probe. In control rats, anionic sites were located around the fibrils of the lamina rara externa, which radiated perpendicularly from the lamina densa to podocyte cell membranes. In the glomeruli of proteinuric rats, many electron-dense deposits were observed in the subepithelial side of the GBM, where the fibrils of the lamina rara externa were usually obscured and anionic sites around them could not be recognized. However, in some areas, a clear boundary could be observed between deposits and the lamina densa. Electron micrographs of freeze-fractured deposits showed that the fibrils radiated perpendicularly from the lamina densa and that anionic sites around them had been preserved. These results suggest that some of the deposits simply passed through the GBM and masked transiently the fibril structures of the GBM, but others probably destroyed these fibril structures, including anionic sites.

Ultrastructural alteration of glomerular anionic sites in nephrotic patients

The relationship of glomerular anionic sites to proteinuria was examined ultrastructurally in human nephrotic syndrome. The anionic sites were analysed morphometrically in patients with minimal-change nephrotic syndrome (MCNS, 11 patients) and in other glomerulonephritides complicated with nephrotic syndrome (4 patients) by the high-iron diamine-thiocarbohydrazide-silver proteinate method. The anionic sites in MCNS patients in remission (7 patients) were normal. In contrast, the anionic sites in nephrotic patients with MCNS (4 patients) and the other glomerulonephritides were decreased in number. Moreover, smaller and irregularly distributed anionic sites or the greater loss of them from the paramesangial region were observed in the nephrotic patients. The loss of glomerular anionic sites may induce structural alteration of the glomerular basement membrane and mesangial matrix. The loss and structural abnormalities of glomerular anionic sites in nephrotic patients may be one of the mechanisms responsible for massive proteinuria.

Ultrastructural study of the glomerular slit diaphragm in fresh unfixed kidneys by a quick-freezing method

In normal kidneys fixed by perfusion with tannic acid and glutaraldehyde, glomerular slit diaphragms have been reported to consist of highly ordered and isoporous substructures with a zipper-like configuration. We have re-evaluated the ultrastructure of fixed or unfixed glomeruli using quick-freezing and deep-etching (QF-DE) and freeze-substitution (QF-FS) methods. In the fixed slit diaphragms, zipper-like substructures were often observed by the QF-DE method. In contrast, in fresh unfixed glomeruli the slit diaphragms mainly consisted of non-porous substructures. The slit diaphragms were more widely opened in the fixed glomeruli, as examined by the QF-FS method. These results suggest that the foot processes shrink during tissue preparation by conventional methods with chemical fixatives, and that the broadening of slit diaphragms and zipper-like substructures are formed by the pulling apart of adjacent foot processes due to shrinkage.