Journal of the Mechanical Behavior of Biomedical Materials. 6d; cf. The copper grids with the mechanically attached vitreous epidermal sections were transferred to a Gatan cryoholder (Gatan Inc., Warrendale, PA, U.S.A.) at −180°C and inserted in a Phillips CM12 cryoelectron microscope (Philips, Eindhoven, the Netherlands). It is widely assumed that the stratum corneum corneocyte keratin network is directly responsible for the mechanical integrity of the epidermis and indirectly responsible for the barrier capacity of the mammalian skin, as it constitutes an indispensable mechanical scaffold for the stratum corneum extracellular lipid matrix (cf. (A) Schematic illustration of a three‐dimensional hyperbolic membrane system with imbalanced gyroid symmetry. Invest. Intriguingly, keratin 1/keratin 10 (the dominant keratin dimer pair of upper epidermis) is unable to generate a normal cytoskeleton when expressed in transfected fibroblasts. Therefore, the cubic rod‐packing model inherently implies that there has been a surface template present at some decisive stage during the keratin network formation process (Fig 12a,b). In perpendicular section planes, the electron density pattern corresponding to the subfilamentous intermediate filament architecture consists of one axial subfilament surrounded by an undetermined number of peripheral subfilaments, occasionally being reminiscent of a quasi‐hexagonal arrangement of groups of c. six electron dense c. 1‐nm spots surrounding a central electron dense c. 1‐nm spot (Fig. There are usually 15 to 30 layers of cells in the stratum corneum. (c) Section plane along the (111) direction. The high‐pressure freezer HPM 010 (Baltec, Balzers, Liechtenstein), which reaches a pressure of 2000 bar within 15 ms, was used. 8, is due to the smaller section thickness (c. 50 nm) needed for high‐resolution work (cf. Fig. Reprinted from  with permission. Black dots (b, c) represent individual keratin dimer molecules. It is in accordance with the cryo‐electron density patterns of the native keratinocyte cytoplasmic space and could account for the characteristic features of the keratin network formation process, the dynamic properties of keratin intermediate filaments, the close lipid association of keratin, the insolubility in non‐denaturating buffers and pronounced polymorphism of keratin assembled in vitro, and the measured reduction in cell‐volume and hydration level between stratum granulosum and stratum corneum. Further, a number of cubic phases can occur in the same system as the temperature or solvent concentration/composition is varied.  above). Further, as keratin is the major non‐aqueous component (wt/wt) of stratum corneum and as 90–100% of the stratum corneum water is thought to be located intracellularly , one may presume that keratin also is a major factor (together with filaggrin‐derived free amino acids) determining stratum corneum hydration level and water holding capacity. Further, the granular ground substance was readily extractable by various treatments, quickly lost upon cell lysis and obscured by primary fixation in aqueous aldehyde , which may explain why, usually, it is not observed in conventional micrographs of chemically fixed epidermis (cf. This could be explained if we assume that the biomaterial constituting these ‘particles’ is not freely diffusing in the keratinocyte cytoplasm but associated to a continuous structuring system possessing three‐dimensional symmetry [e.g. In the paracortex of wool and quill the keratin filaments seem quasi‐hexagonally packed and embedded in a matrix that preferentially takes up stain . In epithelial cells, centrally located keratin intermediate filaments exhibit slow oscillations and ondulations, and there are unidirectional fluctuations of diffuse and particulate keratin. In fact, the first tomographic 3D reconstructions of native epidermis were recently obtained . Consequently, in living cells there may exist a close connection between cellular architecture and most, and possibly all, of the metabolic machinery. What limits the amount of information that can be extracted from cryo‐electron micrographs of vitreous epidermal sections is not low contrast, as might first be believed, but foremostly superposition of biomaterial in the section thickness dimension. Section thickness (a) c. 50 nm. c), and may, if applied to the stratum corneum, imply an even further improved tissue energy absorption capacity. Cette capacité de rétention de l'eau dépend elle‐même de l'organization structurelle du réseau de filaments intermédiaires de la kératine des cornéocytes. This is contradicted by the finding of a more pronounced swelling in the thickness dimension (c. 25%) with respect to the lateral dimension (c. 2–3%) of isolated full thickness stratum corneum . 7b, central mid‐surface), gyroid – (G) (cf. High‐ (a) and medium‐ (b) magnification cryo‐transmission electron micrographs of keratin intermediate filaments bundles of the mid‐portion of native viable epidermis. Modeling the Structure of Keratin 1 and 10 Terminal Domains and their Misassembly in Keratoderma. ), looser paracrystalline arrangement. 8, is due to the smaller section thickness (c. 50 nm) needed for high‐resolution work (cf. Learn more. The keratin subunit incorporation into keratin intermediate filament networks is rapid (minute range) and takes place simultaneously throughout the cell cytoplasm [67, 68]. Stratum corneum keratin structure, function, and formation: the cubic rod-packing and membrane templating model. Electron dense single spot in (a) and double spot in (c) correspond to surface ice contamination. The low‐electron density multigranular structure consists of c. 15‐nm particles with a median interparticle (centre–centre) distance of c. 25 nm. below); It could explain the swelling behaviour of corneocytes in situ (cf. Complete vitrification of the observed cryo‐sections was checked by electron diffraction. Intermediate filament proteins are, unlike microtubules and microfilaments, quite insoluble in non‐denaturating buffers, and the in vitro assembly and disassembly of intermediate filaments can therefore not be studied under close to ‘physiological conditions’ [1, 69]. Stratum lucidum-only on palms and soles. Fig. Reprinted from  with permission. A periodic membrane structure with a small lattice parameter (c. 20 nm) may be present in the native keratinocyte cytoplasm. The 1 × 1 mm2 samples were placed in the cavity (diameter 2 mm; depth 0.1 mm) of a cylindrical aluminium platelet (diameter 3 mm; thickness 0.5 mm) and covered by a second matching flat aluminium platelet. ). Note that this detailed description is but one out of many possible, all sharing the basic concepts of membrane templating and cubic‐like rod packing of keratin intermediate filaments. Stratum corneum is quite thick in thick skin but is greatly reduced in thin skin. The biomineralization process in sea urchins is one example. Epidermal dehydration during specimen preparation results in inevitable aggregation, and important loss, of non‐aqueous biomaterial. I. Low magnification transmission electron micrographs of human epidermis at the interzone between viable and cornified cell layers: (a, b) lowermost stratum corneum; (c, d) uppermost stratum granulosum); (a, c) cryo‐electron micrographs of vitreous sections of native epidermis; (b, d) conventional electron micrographs of resin‐embedded sections. Our tentative interpretation is that the periodic ‘multicircular’ optical density pattern of Fig. 7a–c). ); Symmetry is a basic feature of natural organization from the atomic scale to the astronomic scale. 3), corneocyte keratin intermediate filaments appear in vitreous epidermal sections as c. 8 nm (measured as two times peripheral‐ to central subfilament centre‐to‐centre distance in a direction perpendicular to the section cutting direction; n is number of measurements performed) wide electron dense structures with a median filament centre‐to‐centre distance of c. 16 nm, embedded in a comparatively electron lucent matrix (Fig. However, using cryo‐EM on vitreous sections of native human skin we here for the first time point at the possible presence in cells of cubic‐like membrane morphologies, or membrane endoplasmatic ‘nanoreticuli’, of dimensions corresponding to those of cubic lipid/water in vitro phases. Fig. ), into a corneocyte keratin network with cubic‐like three‐dimensional symmetry (cf. Open white double arrow (a): section cutting direction. 11d). 3a,b). The oscillating standing‐wave character of hyperbolic membranes with three‐dimensional cubic symmetry [45, 46] potentially also presents ample means of selective and fast cellular signal transduction (e.g. Es wird auch als Hornschicht bezeichnet, da die Zellen härter sind als die meisten anderen, wie das Horn eines Tieres. Keratin is the major non-aqueous component (wt/wt) of stratum corneum. with local angles and distances preserved) into each other via the Bonnet transformation [39, pp. In the cryo‐electron micrographs it seems, however, as if protein entities (i.e. ‘electron dense’ (black) patterns represent projections of the two subspaces (i.e. However, the possibility remains of a more traditional ‘phase transition’ (i.e. when both membrane lattice parameter, membrane orientation in the cryo‐section, local section thickness, electron dose and underfocus happen to be optimal, concomitantly, for cryo‐EM visualization, it might be possible to obtain a glimpse, locally, of the underlying cytoplasmic organization (Fig. ), into a corneocyte keratin network with cubic‐like three‐dimensional symmetry (cf. Reprinted from  with permission. The ultrastructure of vitreous native stratum corneum is approximately homogenous throughout its thickness dimension. Section thickness c. 50 nm (a). Its implication for future in vitro experimentation using reversed bicontinuous cubic lipid/water phases to model different aspects of cellular systems is obvious. A new model for stratum corneum keratin structure, function, and formation is presented. below), we may conclude that there is not enough space for more than a single conceivable lipid bilayer membrane structure (c. 4 nm) between apposed keratin intermediate filaments (Fig. This work is dedicated to the memory of Professor Bo Forslind and was made possible by the generous support from the Wenner‐Gren Foundations (L.N.) A central issue in the field of lipid self‐assembly is the structure of liquid crystalline mesophases denoted bicontinuous cubic phases. Scale bar (a) 50 nm. This will in turn open the way for high‐resolution (1–2 nm) 3D reconstructions of native skin. 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Structures with a stamping tool and stored in liquid nitrogen in 1‐hexadecane avoid.
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