Continuous endothelium

In addition molecules have been identified on continuous endothelium that facilitate binding of albumin to endothelium, increase capillary permeability, and theoretically can increase paracellular transport or transcytosis of albumin-bound molecules. An example of this is the gp60 albumin-binding glycoprotein molecule identified on continuous endothelium in multiple tissues (heart, lung, skeletal muscle, adipose, peritoneum, and intestinal smooth muscle) but absent from continuous endothelium in cerebral cortex, and sinusoidal or fenestrated endothelium in liver, adrenal, pancreas, and intestinal lamina propria [21,22],... 

Schnitzer JE. gp60 is an albumin-binding glycoprotein expressed by continuous endothelium involved in albumin transcytosis. Am J Physiol 1992 262(1 Pt 2) H246-54. 

With other tissues, like heart muscle and adipose tissue, the capillaries are of the unfenestrated type. They possess a continuous endothelium with individual... 

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. 

The outermost layer, the epicardium, is the thin membrane on the external surface of the heart. The innermost layer, the endocardium, consists of a thin delicate layer of cells lining the chambers of the heart and the valve leaflets. The endocardium is continuous with the endothelium, which lines the blood vessels. 

Another vasoactive substance produced by the endothelium is thromboxane A2 (TxA2). Normally, small amounts of TxA2 are released continuously however, increased synthesis appears to be associated with some cardiac diseases. Synthesized from arachidonic acid, a plasma membrane phospholipid, TxA2 is a potent vasoconstrictor. Furthermore, this substance stimulates platelet aggregation, suggesting that it plays a role in thrombotic events such as myocardial infarction (heart attack). Nonsteroidal anti-inflammatory drugs such as aspirin and ibuprofen block formation of TxA2 and reduce formation of blood clots. 

Another metabolite of arachidonic acid is prostacyclin (PGI2). As with TxA2, PGI2 is produced continuously. Synthesized by vascular smooth muscle and endothelial cells, with the endothelium as the predominant source, PGI2 mediates effects that are opposite to those of TxA2. Prostacyclin causes vasodilation and inhibits platelet aggregation and, as a result, makes an important contribution to the antithrombogenic nature of the vascular wall. 

On the neutrophil, the major selectin expressed is L-selectin. This molecule is constitutively expressed on mature neutrophils but may be expressed at low levels (50% of adult) in neonates. Stimulation of endothelial cells with thrombin, histamine, IL-1 and some other agents induces neutrophils (and other leukocytes) to leave the circulation and adhere to the endothelium. They do this by rolling onto the surface of the endothelium, to which they attach via P-selectin translocated from storage sites in Weibel-Palade bodies to the surface of the endothelium upon activation. The expression of P-selectin is short-lived and is replaced on the endothelial surface by E-selectin (whose expression is also regulated by some cytokines), which continues the endothelial-leukocyte interaction. 

Particulates can either cross into the lymphatics at the spaces in the tracheobronchial wall where epithelial cells directly overlay lymphoid tissue or pass through the endothelium of thin capillary walls in the air spaces. The transfer is a portion of a clearance mechanism that assists the lung in maintaining its normal function of gas exchange. Absorbtion and transport mechanisms of a variety of materials that enter the lymphatics continue to be studied. It was shown early in this century that water, dyes, proteins, bacteria, lipids, and particulates enter the lymphatic system relatively easily. The rates of transport and quantity vary with the size and chemistry of the material. Classic studies by Kihara (1924 1950) and Nishikawa (1941) dem-... 

Corneal endothelium effects The effect of continued administration of brinzolamide and dorzolamide on the corneal endothelium has not been fully evaluated. 

One important mechanism of serotonin elimination is the (re-) uptake, e.g. by platelets. Furthermore, serotonin is metabolized by monoaminox-idase to 5-hydroxyindoleacetaldehyde and, subsequently, by an aldehyde dehydrogenase to 5-hydroxyindolacetic acid. The vascular effects of serotonin are complex. The direct interaction with vascular smooth muscle induces a vasoconstriction, whereas the stimulation of 5-HT-receptors on the endothelium induces the release of vasorelaxant factors with a dilatation as a result. An intravenous application of serotonin increases the pressure in the pulmonary circulation. A continuous infusion results... 

Prior to the 3rd week, embryonic nutrition occurs by diffusion of maternal blood. At that point, development of the primitive blood and blood vessels begins in the extraembryonic mesoderm of the yolk sac. These cells differentiate into angioblasts that form cords and clusters, which in turn canalize. Cells in the peripheiy become flat and form the endothelium, whereas the inner cells give rise to the primitive blood cells. By fusion and continuous budding, the extraembryonic vessels that have contact with maternal circulation establish contact with vessels arising from the embryo proper. The mesenchymal cells surrounding the primitive endothelial cells differentiate to form the muscular and connective tissues of the vessel wall. The primitive heart... 

A passive flux of water continually flows across the endothelial layer toward the stroma, which has a tendency to swell. An active pump mechanism pulls an aqueous flux in the opposite direction which controls corneal turgescence [13]. Corneal deturgescence is an ATP-dependent process of the endothelial cells and as such any disruption of the endothelium may result in corneal oedema, thereby affecting corneal transparency. The specific distribution of different proteoglycans across the cornea has recently been implicated in water gradients across the cornea. This water gradient serves to diminish dehydration of the front of the cornea, which is exposed to the atmosphere. 

The barrier to paracellular diffusion potentially isolates the brain from many essential polar nutrients such as glucose and amino acids that are required for metabolism and, therefore, the BBB endothelium must contain a number of specific solute carriers (transporters) to supply the CNS with its requirements for these substances. The formation of tight junctions essentially confers on the BBB the properties of a continuous cell membrane, both in terms of the diffusional characteristics imposed by the lipid bilayer, and the directionality and properties of the specific transport proteins, and solute carriers (SLC) that are present in the cell membrane. Examples of BBB solute carriers (SLC transporters) are listed in Table 27.2. 

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