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Tissue Layer

Indentation testing is a widely used technique for nondestructive mechanical analysis of articular cartilage. Although cartilage shows an inhomogeneous, layered structure with anisotropic mechanical properties, most theoretical indentation models assume material homogeneity and isotropy. In the present study, quantitative polarized light microscopy (PLM) measurements from canine cartilage were utilized to characterize thickness and structure of the superficial, collageneous tissue layer as well as to reveal its relation to experimental indentation measurements. In addition to experimental analyses, a layered, transversely isotropic finite element (FE) model was developed and the effect of superficial (tangential) tissue layer with high elastic modulus in the direction parallel to articular surface on the indentation response was studied. The experimental indentation stiffness was positively correlated with the relative thickness of the superficial cartilage layer. Also the optical retardation, which reflects the degree of parallel organization of collagen fibrils as well as collagen content, was related to indentation stiffness. FE results indicated effective stiffening of articular cartilage under indentation due to high transverse modulus of the superficial layer. The present results suggest that indentation testing is an efficient technique for the characterization of the superficial degeneration of articular cartilage.

tissue layer

In this paper we examine the dynamics of an initially stable bubble due to ultrasonic forcing by an acoustic wave. A tissue layer is modelled as a density interface acted upon by surface tension to mimic membrane effects. The effect of a rigid backing to the thin tissue layer is investigated. We are interested in ultrasound contrast agent type bubbles which have immediate biomedical applications such as the delivery of drugs and the instigation of sonoporation. We use the axisymmetric boundary integral technique detailed in Curtiss et al. (J. Comput. Phys., 2013, submitted) to model the interaction between a single bubble and the tissue layer. We have identified a new peeling mechanism whereby the re-expansion of a toroidal bubble can peel away tissue from a rigid backing. We explore the problem over a large range of parameters including tissue layer depth, interfacial tension and ultrasonic forcing.

In animal development, cellular diversity is generated within tissues which in turn are derived from germ layers. Similar to the germ layers in animals, plants establish three distinct tissue layers early in development which each give rise to a distinct set of cell types. To investigate the role of tissue-layer-specific cues in generating plant cellular diversity we studied the spatial regulation of an epidermal cell type, trichomes (hairs), by the two genes, GLABRA1 (GL1) and TRIPTYCHON (TRY). Ubiquitous expression of the positive regulator GL1 in the absence of the negative regulator TRY leads to ectopic trichome formation not only on additional organs but also in subepidermal tissue layers. Trichomes in inner tissue layers can differentiate the same morphology and show a spacing pattern comparable to trichomes in the epidermis. This clearly shows that cell type specification takes place downstream of tissue-specific cues. We propose a model of how the tissue and organ specificity of trichome induction is regulated in normal development.

The mucosa, or mucous membrane layer, is the innermost tunic of the wall. It lines the lumen of the digestive tract. The mucosa consists of epithelium, an underlying loose connective tissue layer called lamina propria, and a thin layer of smooth muscle called the muscularis mucosa. In certain regions, the mucosa develops folds that increase the surface area. Certain cells in the mucosa secrete mucus, digestive enzymes, and hormones. Ducts from other glands pass through the mucosa to the lumen. In the mouth and anus, where thickness for protection against abrasion is needed, the epithelium is stratified squamous tissue. The stomach and intestines have a thin simple columnar epithelial layer for secretion and absorption.

The smooth muscle responsible for movements of the digestive tract is arranged in two layers, an inner circular layer and an outer longitudinal layer. The myenteric plexus is between the two muscle layers.

The dermis is a connective tissue layer sandwiched between the epidermis and subcutaneous tissue. The dermis is a fibrous structure composed of collagen, elastic tissue, and other extracellular components that includes vasculature, nerve endings, hair follicles, and glands. The role of the dermis is to support and protect the skin and deeper layers, assist in thermoregulation, and aid in sensation. Fibroblasts are the primary cells within the dermis, but histiocytes, mast cells, and adipocytes also play important roles in maintaining the normal structure and function of the dermis.

The dermis is a connective tissue layer of mesenchymal origin located deep to the epidermis and superficial to the subcutaneous fat layer.[1] The composition of the dermis is mainly fibrous, consisting of both collagen and elastic fibers. Between the fibrous components lies an amorphous extracellular "ground substance" containing glycosaminoglycans, such as hyaluronic acid, proteoglycans, and glycoproteins.

The dermis is divided into two layers: the papillary dermis and the reticular dermis. The papillary dermis is the superficial layer, lying deep to the epidermis. The papillary dermis is composed of loose connective tissue that is highly vascular. The reticular layer is the deep layer, forming a thick layer of dense connective tissue that constitutes the bulk of the dermis.

The dermis houses blood vessels, nerve endings, hair follicles, and glands. There are many cell types found within the connective tissue of the dermis, including fibroblasts, macrophages, adipocytes, mast cells, Schwann cells, and stem cells.[5] Fibroblasts are the principal cell of the dermis. Mast cells are typically found surrounding dermal capillaries.

The structure of the dermis provides a connective tissue framework for strength, flexibility, and protection of the deeper anatomical structures. Collagen and extracellular components like hyaluronic acid fortify the skin and facilitate an anchor for the epidermis via hemidesmosomes and other adhesive basement membrane zone (BMZ) components.[6] Oxytalan fibers may also play a role in anchoring the epidermis. Elastic tissue also helps support the skin and provide flexibility. The blood vessels in the dermis are crucial for maintenance of the epidermis and epidermal appendages. Nutrients via blood support the epidermis, hair follicles, and sweat glands. The vascular network further permits the dermis to host an inflammatory response via recruitment of neutrophils, lymphocytes, and other inflammatory cells. The dermal blood supply also plays a role in temperature regulation discussed below.

Vasoactive dermal vessels regulate body temperature. Specialized structures called glomus bodies also take part in thermoregulation through AV shunt formation.[7] Glomus bodies are complexes of glomus cells, vessels, and smooth muscle cells that predominate in the digits, palms, and soles.[8] Although often within the dermis, eccrine sweat glands are ectoderm-derived epidermal appendages that invaginate into the deeper tissue of the dermis and subcutaneous layer.[9]

The dermis contains many cell types. Fibroblasts, the principal cell of the dermis, handle the synthesis of collagen, elastic and reticular fibers, and extracellular matrix material. Histiocytes are tissue macrophages present within the connective tissue that assist the immune system. Mast cells are inflammatory cells located in the perivascular areas of the dermis. Mast cells secrete vasoactive and proinflammatory mediators important in inflammatory reactions, collagen remodeling, and wound healing.[11] Dermal adipocytes are a distinct cell population from the subcutaneous adipose tissue. Dermal adipocytes not only provide insulation and energy storage but also assist in hair follicle regeneration and wound healing.[12][13]

The dermis is examined using a standard skin biopsy. The tissue sample should first be fixated with formalin to preserve tissue structure. After fixation, the specimen is dehydrated with an alcohol (e.g., ethanol) to remove water. The alcohol agent is then cleared using xylol. After, the tissue sample is embedded in paraffin. After hardening of the paraffin medium, a microtome slices the specimen. The tissue specimen may be stained according to hematoxylin and eosin (H&E) staining protocols.

Immunofluorescence of tissue samples is an important diagnostic tool in autoimmune blistering diseases such as bullous pemphigoid and dermatitis herpetiformis. For example, linear deposition of immunoglobulin G (IgG) and complement (C3) along the dermoepidermal junction is characteristic of bullous pemphigoid. Granular deposits of IgA in the dermal papillae is characteristic of dermatitis herpetiformis.[14]

Light microscopic analysis of H&E-stained samples delineates the epidermis, dermis, and subcutaneous adipose. The epidermis is easily visualized due to the presence of basophilic keratinocytes. Scanning across the tissue sample, one can appreciate the alternating dermal papillae and rete ridges. Dermal papillae are the protrusions of dermal connective tissue into the epidermal layer. Rete ridges are the extensions of epidermis into the dermal layer. This undulating pattern is more apparent in thick skin of the hands and palms. There is no clear line of distinction between the papillary and reticular dermis. Collagen patterns are mostly horizontal throughout. The superficial papillary dermis possesses thinner elastic fibers compared to the thicker elastic fibers of the reticular dermis. The papillary dermis is composed of loose connective tissue (LCT) and is highly vascular. The reticular dermis shows thick collagen bundles and forms the bulk of the dermal layer. 041b061a72

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