Epithelial layer

PDE1C2 and PDE4A are expressed. PDE1C2 is found in the cilia of the epithelium, where it colocalizes with adenylyl cyclase. PDE4A is found throughout the epithelial layer, but not in cilia. Therefore, as in the kidney mesangial cells, different PDEs must be working on different cyclic nucleotide pools. More recently, substantial data has been developed for compartmenta-tion of cAMP and PDEs in cardiac myocytes. 

The cornea is the first structure of the eye to be in contact with incident light. It is composed of five distinct layers lying parallel to its surface the outer epithelium, which is continuous with the epithelial layers of the conjunctiva the epithelial basal lamina the keratocyte-containing stroma, which is a collagen structure arranged so that it is transparent Descemet s membrane and, finally, the endothelium adjacent to the aqueous humour. 

Morphologically, the lens is arranged so that dedifferentiated or disorientated lens fibres are not eliminated but are pushed to the core (Spector, 1984,). Damage to the cell-fibre membranes or to the proteins they contain is irreversible. The lens is therefore dependent on the peripheral cells of the epithelial layer for protection against insult. 

Drosophila Ddc is expressed primarily in the CNS and the hypoderm, the epithelial layer of the fly that secretes the cuticle. In the CNS, Ddc is expressed in a small subset of neurons that produce either dopamine or serotonin (Budnik and White, 1988 Valles and White, 1988). In the hypoderm, Ddc expression leads to synthesis of dopamine, which is further metabolized into quinones that have a vital function in the cross-linking, hardening, and pigmentation of the fly cuticle (Wright, 1987). The developmental profile of DDC activity in these two tissues is quite different (Hirsh, 1986). DDC is first detected during late embryo-... 

Figure 2 Comparison of intestinal epithelial cells in culture and in situ. (A) Human colon Caco-2 cells grown in culture for 16 days on a semiporous filter. (B) Epithelial layer of rat jejunum. AP, apical or luminal membrane B, basal or abluminal membrane BM, basement membrane G, goblet cell LS, lateral space mv, microvilli Nu, nucleus TJ, tight junction. Bars equal 10 pm. 

Alterations of permeability of plasma membranes, epithelial layers and histo-haematic barriers... 

The sites of action and effects of ozone and other photochemical oxidants are described in Chapters 8 and 9. Recent work with primates has suggested that ozone is absorbed along the entire respiratory tract, penetrates more into the peripheral nonciliated airways, and causes more lesions in the respiratory bronchioles and alveolar ducts as the inhaled ozone concentration increases from 0.2 to 0.8 ppm. The most common and most severe tissue damage was observed in the respiratory bronchioles. The ciliated cells in the terminal bronchioles and the Type 1 cells in the epithelial layer of the proximal alveoli of rats were the... 

The dynamic properties of the mucus fluid, serous fluid, and epithelial layers of the respiratory tract are important for the transport, absorption, and desorption of reactive gases. The cilia beat at a fairly constant frequency within the stationary serous layer and cause the outer mucus layer to move up the respiratory tract. Clearance of deposited particles and absorbed gases in the ciliated tracheobronchial tree depends partly on the movement of this mucus layer. 

The composition of the extracellular compartment is kept constant by the cooperation of the cellular transport systems with various transport systems which are located in epithelial layers connecting the organism with its environment (Table 1). Examples are the epithelia of the intestine, the kidney and of various glands. It is their function to establish a relatively constant concentration of the respective solutes... 

Cells in the external epithelial layers are always surrounded by a protective covering of some kind. 

The analysis of transfer mechanisms of drugs across the intestinal epithelial layer has passed a long way since the theory of lipid pore membrane [118] in which the total pore area of the intestinal membranes was calculated (and found to be low compared with the total surface of the mucosal aspect of the gut), through the Fickian diffusion calculations of the transport of unionized moieties of drug molecules (the Henderson-Hasselbach equation), which led to the conclusion that acidic drugs are absorbed in the stomach [119,120]. 

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