Blood capillaries fenestrated

Capillaries are the exchange vessels of the body. They have structural variations to allow different levels of metabolic exchange (of exogenous and endogenous substances) between blood and the surrounding tissues. The structure of the walls varies depending on their resident tissue. There are three major types of blood capillaries continuous fenestrated and sinusoidal (discontinuous) [1] ... 

Figure 5.1 Schematic illustration of the structure of the wall of different classes of blood capillaries. (1) Continuous capillary (as found in the general circulation). The endothelium is continuous with tight junctions between adjacent endothelial cells. The subendothehal basement membrane is also continuous. (2) Fenestrated capillary (as found in exocrine glands and the pancreas). The endothelium exhibits a series of fenestrae which are sealed by a membranous diaphragm. The subendothehal basement membrane is continuous. (3) Discontinuous (sinusoidal) capillary (as found in the liver, spleen and bone marrow). The overlying endothelium contains numerous gaps of varying size. The subendothehal basement is either absent (hver) or present as a fragmented interrupted structure (spleen, bone marrow)...

Estrogens increase capillary fenestration, causing edema. The resulting decrease in blood volume results in increased secretion of aldosterone and ADH. resulting in the retention of sodium and water. 

Particle size. Particles greater than 7 pm are larger than blood capillaries ( 6 pm) and become entrapped in the capillary beds of the lungs (which may have fatal effects). The majority of particles that pass the lung capillary bed accumulate in the elements of the RES (spleen, liver and bone marrow). The degree of splenic uptake increases with particle size. Removal of particles > 200 nm is due to a non-phagocytic process (physical filtration) in the spleen and phagocytosis (by Kupffer cells) by the liver. Particles < 200 nm decreases splenic uptake and the particles are cleared by the liver and bone marrow. Colloidal particles not cleared by the RES can potentially exit the blood circulation via the sinusoidal fenestration of the liver and bone marrow. 

Like the walls of other capillaries, the glomerular capillary wall consists of a single layer of endothelial cells. However, these cells are specialized in that they are fenestrated. The presence of large pores in these capillaries makes them 100 times more permeable than the typical capillary. These pores are too small, however, to permit the passage of blood cells through them. 

The blood-brain barrier is markedly different from peripheral capillaries Peripheral capillaries are fenestrated with openings up to 50 nm wide. In contrast, cerebral endothelial cells are closely connected by tight junctions and zonulae occludentes, resulting in extremely high transendothelial resistances of up to 1500-2000 12 cm2 [16] (Figure 17.1). The capillaries are surrounded by a basal membrane enclosing intermittently pericytes, which have been postulated to be involved in host defense. The outer surface of the basement membrane is covered by astrocytic foot processes. Most likely, secretion of soluble growth factors by astrocytes plays an important role in endothelial cell differentiation. 

Polymeric micelles. When water-soluble polymers are conjugated with lipophilic, poorly water-soluble polymers, the resulting copolymers are amphiphilic and can be used to constitute spherical micelles.54 The sizes of the polymeric micelles range between 10 and 100 nm, which is ideal for preferential extravasation at the fenestrated capillary blood vessels. The polymeric micelles have a hydrophobic core consisting of the... 

There is an increased blood flow in brain tumors (ref. 589), and the blood-brain barrier is leaky in and around 9L tumors because the blood vessels associated with these (ref. 568), and other (ref. 565-567), tumors are fenestrated. This well-known leakiness of tumor capillaries, which in the case of brain tumors includes breaches in the blood-brain barrier (ref. 566,568 cf. Section 12.2), would allow extravasation of small particulate matter (cf. ref. 590-594) or LCM. Once in the tumor area, LCM remain there because of an affinity for tumor cell surface components (cf. ref. 531 see also Chapter 14). At least 4 different types of experimental tumors in rats (C6 glioma, 9L gliosarcoma, Novikoff hepatoma, and Walker-256 carcinosarcoma), as well as several spontaneous tumors in dogs (ref. 570), do interact with LCM in a preferential manner (cf. Chapters 12 and 13), suggesting that LCM affinity may be for tumor cells in general (ref. 531). 

The brain capillary endothelium comprises the lumenal and ablumenal membranes of capillaries, which are separated by approximately 300 ran of endothelial cytoplasm (Figure 13.2). The structural differences between brain capillary endothelium and non-brain capillary endothelium are associated with the endothelial tight junctions. The non-brain capillaries have fenestrations (openings) between the endothelial cells through which solutes can move readily via passive diffusion. In brain capillaries, the endothelium has epithelial-like tight junctions which preclude movement via paracellular diffusion pathways. There is also minimal pinocytosis across brain capillary endothelim, which further limits transport of moieties from blood to brain. 

---