Microvacuola

The vacuoles measure from a few microns to a few millimeters or more in size and are organized in a dispersed branching fractal pattern.5 Most of these sequences have a pseudogeometric shape of a polygon. However, they are organized differently according to their function.

All of the vacuoles are situated within a pseudopolygonal fibrillar framework containing a gel. The major role of this framework is to make sure that when stimulated, the structures can move freely without anything else moving around them. The vacuolar structure must be resistant, adapt to the physical forces applied to it, and keep its shape. In other words, its role is to ensure the dynamics of movement and to resist the shocks that this movement creates. The structure also has a memory so that it returns to its initial position between movements (Figure 4-2).

The sides of the vacuoles, which are intertwined, are composed of collagen fibers, mostly type I (23%), III, IV, and VI. They are organized on several levels in different directions, but have no regular, basic pattern. Their diameter ranges from a few to several dozen micrometers, and they vary in length, thus giving an overall disorganized, chaotic appearance. Magnification reveals lateral modifications of the collagen fibers in the vacuole, suggesting the linking of proteoglycan chains. These vacuoles contain a highly hydrated proteoglycan gel (70%) that can change shape during movement but whose volume remains constant. Their lipid content (4%) is high.

Proteoglycans are proteins that are glycosylated by covalent attachment of highly anionic glycosaminoglycans (sulfated polysaccharides). As a result of their strong negative charge, glycosaminoglycans attract counter-ions and water molecules into the tissue. This ability endows proteoglycans with their unique physical characteristics, allowing them to fill the intravacuolar spaces and to change shape when required.

The bonds between the collagen fibrils and the proteoglycan-enriched vacuolar fluid might be composed of type VI collagen, a unique collagen that occurs in the form of beaded filaments and that is often found at the interface between type I collagen and the surrounding extracellular matrix. Type VI collagen is composed of both globular domains and a short, triple-helical domain (60 nm long), and it assembles to form a structure resembling a pearl necklace. Collagen I fibrils also interact specifically with small proteoglycans, such as decorin, whereas large proteoglycans provide hydration and swelling

Figure 4-2. MVCAS under the electron microscope. a) Histological and collagenous continuity between the epitendon and MVCAS. b) Sketch of this organization, in vacuoles. c) 3D tissue supports. d) 3D vacuola. (A.D.F. Video-Productions with special permission)

Figure 4-2. MVCAS under the electron microscope. a) Histological and collagenous continuity between the epitendon and MVCAS. b) Sketch of this organization, in vacuoles. c) 3D tissue supports. d) 3D vacuola. (A.D.F. Video-Productions with special permission)

pressure, often in association with the nonsul-fated glycosaminoglycan hyaluronan. Owing to their capacity to attract water molecules, large proteoglycan molecules allow MVCAS to resist compression, unlike collagen fibrils, which resist tension by extending and retracting under mechanical stress. The collagen framework and the intravacuolar spaces give form and stability to the tissue.

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