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I know for microvasculature

New microvasculature (arterioles, venules) are always more permeable as they lack pericytes.

and then I have this sentence about new blood vessels (not specific)

New vessels contain initially only proliferative endothelium, the basement membrane has not formed yet, and pericytes are absent.

However, I think pericytes are only around microvasculature.

I think the second statement is correct for microvasculature too.

Why are new vessels more permeable in microasculature?

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Is there something I'm missing to answer your question? Perhaps I don't understand the question then. Otherwise, please accept to help our questions/accepted answers ratio. –  Atl LED Feb 28 at 18:54

1 Answer 1

In adult/pediatric humans, no they don't. A good figure to get everyone on the same page was made by Cleaver & Melton:

Comparison of the composition large and small human blood vessels

It is important to note that:

In fact, endothelial diversity is reflected by vessel size-specific, organ-specific and even age-specific differences. (1)

Developing blood vessels in the fetus can be a bit of a different story, and highlights the fact the one should loose the "inert plumbing" mentality of the previous century. Modern research tells us that the epithelium is a very dynamic region that participates heavily with signalling.

When large vessels are growing the fetus, mesenchymal stem cells and neural crest cells are the normal precursors to pericytes, but depending on exposure the line between pericyte and smooth-muscle cell can be blurred (2). I'm assuming it's beyond the scope of the answer to go into the signalling differences.

A good review of endothelium generation, and the surrounding tissue, can be found here.

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But think about angiogenesis. There are forming new new vessels in microvasculature which do not have pericytes. I think that should make them more permeable. –  Masi Mar 14 at 6:56
    
Actually they are going to have at least mesenchymal stem cells recruited before the vessel scaffolding is extended. You might have the order backwards. I don't know the permeability difference between full differentiated pericytes and stem cells, but it may not be drastic. I will see if I can find some lit on the subject, but I'm worried there won't be any. –  Atl LED Mar 14 at 15:13

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