Macroscopic Structure of Tissues

There are a number of techniques that are used to elucidate the structure of tissues at the light and electron microscopic levels. At the light level different techniques are used to process tissue for making standard paraffin sections that are the gold standard of light microscopic sample preparation. The steps used to prepare paraffin sections include fixation, 

Sometimes tissues are frozen and then sectioned so that they can be cut directly without the need for embedding. In other instances tissues are embedded in a polymer such as methacrylate instead of paraffin after fixation. There are several variations that are used to make tissue sections to view under the light microscope. In addition to staining with dyes, fluorescent molecules can be specifically attached to macromolecules using antibodies. 

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Actin and Myosin Motions in Muscle Tissue The relationship between the macroscopic structure of muscle tissue and its key proteins actin and myosin is illustrated in Figure 14.30 (99). 

From the atomic to the macroscopic level chirality is a characteristic feature of biological systems and plays an important role in the interplay of structure and function. Originating from small chiral precursors complex macromolecules such as proteins or DNA have developed during evolution. On a supramolecular level chirality is expressed in molecular organization, e.g. in the secondary and tertiary structure of proteins, in membranes, cells or tissues. On a macroscopic level, it appears in the chirality of our hands or in the asymmetric arrangement of our organs, or in the helicity of snail shells. Nature usually displays a preference for one sense of chirality over the other. This leads to specific interactions called chiral recognition. 

Bone is an extremely dense connective tissue that, in various shapes, constitutes the skeleton. Although it is one of the hardest structures in the body, bone maintains a degree of elasticity owing to its structure and composition. It possesses a hierarchical structure and, as most of the tissues, is nanostructured in fact, it is a nanoscaled composite of collagen (organic extracellular matrix) and hydroxycarbonate apatite, (HCA, bone mineral). This nanostructure is in intimate contact with the bone cells (several microns in size), which result (at the macroscopic level) in the bone tissue. Figure 12.2 shows the bone hierarchical ordering from the bone to the crystalline structure of HCA. 

The immediate or secondary necrosis (by more progressive diffusion into depth or by concomitant toxic effect) explain the macroscopic lesions of the tissues of the various eye structures (eyelids, conjunctiva, cornea, even iris or the ocular lens) that are specific to the chemical bum. 

Although there is a strong molecular and nanoscopic driving force behind our understanding of the functionality of collagen fibrillar structures, the mesoscopic and macroscopic properties of the tissue are where the structural integrity manifests itself. One of the differences in mechanical resistive properties between tissues such as skin and tendon is the feltwork nature of fibril distribution in skin. This allows resistance to strain to occur within a two-dimensional plane. In contrast, tendon is required (normally) to resist strain only along its axis. 

A range of functionalized and unfunctionalized self-assembling fibrous structures have been tested for their biocompatibility and ability to provide cells with a favorable micro- and nanoenvironments for soft tissue engineering. In this section, studies that focus on amyloid fibrils, on peptide amphi-philes, on ionic complementary peptides, and on dipeptide structures are reviewed. Hard tissue engineering, composites, and coating are also explored followed by macroscopic structures and networks that can be created from fibrous protein structures. 

All of these structures have an epithelial lining that lies at the interface as well as extracellular matrix including basement membranes and loose connective tissue that supports the cellular layers (Table 3.2). These tissues are similar in their general structure they all have an inner cellular layer, supportive connective tissue, and an outer cellular layer. It is important to be familiar with the structure of these tissues to be able to analyze how external and internal mechanical forces are transduced at both the macroscopic and microscopic level into and out of cells. The effect of mechanical loading on these tissues is complex, but as discussed above, with increased frictional forces on the epidermis, the surface layer of skin actually increases the thickness of the epidermis. 

On visual inspection, blood vessels appear to be fairly homogeneous and distinct from surrounding connective tissue. The inhomogeneity of the vascular wall is realized when one examines the tissue under a low-power microscope, where one can easily identify two distinct structures the media and adventitia. For this reason the assumption of vessel wall homogeneity is applied cautiously. Such an assumption may be valid only within distinct macroscopic structures. However, few investigators have incorporated macroscopic inhomogeneity into studies of vascular mechanics [ 1 ]. 

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