Cellular layers

Epithelium Cellular layer interfacing with external environment which con-... 

FIG. 2 Mechanisms of drug transfer in the cellular layers that line different compartments in the body. These mechanisms regulate drug absorption, distribution, and elimination. The figure illustrates these mechanisms in the intestinal wall. (1) Passive transcellular diffusion across the lipid bilayers, (2) paracellular passive diffusion, (3) efflux by P-glycoprotein, (4) metabolism during drug absorption, (5) active transport, and (6) transcytosis [251]. 

The development of the neurofibrillary pathology is not random. It follows a stereotyped pattern with regard to affected nerve cell types, cellular layers and brain regions, with little inter-individual variation. This has been used to define six neuropathological stages of Alzheimer s disease [28]. The very first nerve cells in the brain to develop... 

Figure 11.1 Ultrastructure of the human lung alveolar barrier. The tissue specimen is obtained via lung resection surgery. (A) Section through a septal wall of an alveolus. The wall is lined by a thin cellular layer formed by alveolar epithelial type I cells (ATI). Connective tissues (ct) separate ATI cells from the capillary endothelium (en) within which an erythrocyte (er) and granulocyte (gc) can be seen. The minimal distance between the alveolar airspace (ai) and erythrocyte is about 800-900 nm. The endothelial nucleus is denoted as n. (B) Details of the lung alveolar epithelial and endothelial barriers. Numerous caveolae (arrows) are seen in the apical and basal plasma membranes of an ATI cell as well as endothelial cell (en) membranes. Caveolae may partake transport of some solutes (e.g., albumin). (C) ATII cells (ATII) are often localised in the comers of alveoli where septal walls branch off. (D) ATII cells are characterised by numerous multilamellar bodies (mlb) which contain components of surfactant. A mitochondrion is denoted as mi.

The epithelium, the most superficial cellular layer of the cornea of the eye, is chemically less resistant than the keratinized epidermis of the skin. However, during ocular accidents, we know that it takes a few seconds for the first lesions to appear. This delay is bound to multiple factors, winking reflex, protective and diluting effect of the lachrymal liquid, effect of sweeping of the palpebral movements. After a short period, a kinetic of diffusion will set up in a variable way according to the nature of the corrosive. 

In the phasmid Eurycantha calcarata, a CSP has been isolated from the cellular layer underlying the cuticle (Marchese el ah, 2000). In the cockroach Periplaneta americana, the CSP plO is expressed in legs and antennae (Nomura et al., 1992 Kitabayashi et al., 1998 Picimbon et al., 2001), and other CSPs have been detected in tissues including legs, brain and cerci (Picimbon and Leal, 1999 unpublished). 

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. 

Structure Cellular layers Noncellular layers Function... 

The mouth and esophagus are composed of two layers, the mucosa and submucosa (Figure 3.5). The mucosa is lined on its outer surface by a stratified squamous epithelium with layers of polyhedral cells of the intermediate layers and low columnar cells of the basal layer. Below the cellular layer is the lamina propria containing loose connective tissue with blood vessels and small aggregates of lymphocytes. Smooth muscle within the mucosa (muscularis mucosal layer) is seen as small bundles. The submucosa... 

The described principle of equal force (stress) and added deformations (strains) equally applies to parallel layers of any kind, provided that their structure is isotropic. However, if any of the layers in the array is incompressible and softer than the rest, then it will expand laterally upon the force application. This is a familiar experience. When a sandwich or a layered cake is compressed, the filling sometimes leaks out from the sides, as shovm schematically in Figure 10.10. For such a situation. Equations (10.7) or (10.8) will not be an appropriate model. However, because the cellular layers retain their cross-sectional area, and because the free p>art of the expanded filling does not transmit any stress (theoretically), the stress-strain relationship of the array can still be calculated by accounting for the exuded material. 

The applicability of the method has been recently demonstrated by Barrett et al. (2005). They also showed that it can be used to monitor the moisture exchange between the filling and a model bread. In such a case, the deviation from the model s predictions will reveal that the mechanical properties of the layers have been altered. (The situation would be different, of course, if the liquid or semi-liquid filling was being absorbed into the neighboring cellular layers). 

An important factor in the distribution of drugs is the permeability of the capillaries. Capillaries are the microscopically small blood vessels across the very thin walls of which metabolites and gases are exchanged between blood and tissues. Capillaries have a cellular layer - the endothelium, supported by a basal membrane consisting of proteins and proteoglycans (Figure microcirculation). 

Adhesiveness prevents detachment from the surface of the epithelium, and causes firm attachment to the underlying cellular layer. 

Summing up, NM behavior in the in vitro testing conditions is very complex and NMs can undergo several processes and modifications that can affect their biological and chemical activity as well as transport down to the cellular layer and the cellular uptake, as summarized in Fig. 1. Understanding of these processes is crucial to allow for a proper and reliable assessment of the final toxic effect induced by the studied NMs. 

Piriform cortex (Fig. 15 C-D 17 A-F), also referred to as pyriform or prepyriform cortex, is a phylogenetically old, paleocortical structure. PC is located along the entire length of the lateral olfactory tract at the ventrolateral convexity of the base of the cortex. PC is thicker and more elaborate caudally than it is rostrally. PC is allocortical, having only two-three cellular layers and is thinner and less complex than the neocortex which has six layers. Haberly and Price (1978b) divided the piriform cortex into 3 layers that are further subdivided on the basis of cytoarchitecture and afferent connections. 

I. Some smaller cells in layer Ila lack basal dendrites (Haberly and Price, 1978b) and are reminiscent of dentate granule cells in the hippocampus. Layer III is a thicker but less densely packed cellular layer containing larger pyramidal cells whose dendrites also extend throughout layer I. Layer III also contains large multipolar cells whose dendrites do not enter layer I and other intrinsic neuronal types. Layer III exhibits a superficial... 

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