List of abbreviations
of micros-
specialist terms
explained in
English +

Every attempt was made to provide correct information and labelling, however any liability for eventual errors or incompleteness is rejected!

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Dr. med.
H. Jastrow

of use
Overview arterioles (Arteriolae):
Pages with explanations are linked to the text below the images if available! (Labelling is in German)
metarteriole + blood cells
metarteriole + plasma cells 
in connective tissue (monkey)
elastica interna of an
arteriole (rat)
longitudinal section of 
a wall of an arteriole (rat)
wall of a larger
arteriole (rat)
metaterieole, human
Arteriols (small pulse vessels; Terminologia histologica: Arteriolae) are smaller vessels transporting blood away from the heart. They are not as large as arteries which they connect via metarteriols to capillaries. Metarteriols are the smallest arteriols that only show a single layer of smooth muscle cells. While loosing these smooth muscle cells and the internal elastic membrane they finally become capillaries.
Regarded from interior to exterior the wall of an arteriole has the following layers:
1. Tunica intima consists of an endothelium and an internal elastic membrane.
The endothelium has a monolayer of squamous endothelial cells. The squamous character of the endothelial cells can only be seen under the high pressure physiologically present in living individuals. Since preparations are usually NOT fixed and embedded under pressure endothelial cells artificially appear cuboid or even columnar in most preparations. In contrast to arteries that have a Stratum subendotheliale the latter is lacking in arteriols. This means the endothelial cells directly contact the internal elastic membrane.
The internal elastic membrane (Membrana elastica interna) is a thick membrane consisting of elastic fibres. It comprises the border to the media.
2. Tunica media The media mainly shows smooth muscle cells and some elastic- and collagen fibres.
3. Tunica externa (adventitia) The adventitia is a smooth connective tissue rich in collagen fibres with fibrocytes and free connective tissue cells. It is fixing the arteriole to surrounding tissues and shows some non-myelinated nerve fibres. In contrast to arteries, no nutrifying own vessels (Vasa vasorum) and no external elastic membrane is seen in arteriols.
Metarterioles (precapillary arterioles; Terminologia histologica: Arteriolae precapillares, Metarteriolae) are the smallest arterioles with a length usually below 100 µm and diameters of only 8 to 10 µmThey have a flat (under usual blood pressure) to cuboidal (normal in fixed tissue)endothelium of non-fenestrated endothelial cells comprising the intimaMetarterioles continue from arterioles and start when the latter begin to loose smooth muscle cells resulting in a no longer continuous media. This means metarterioles show a single layer of non-continuous smooth muscle cells in their media. The latter encircle the non-fenestrated endothelial cells and may in case of contraction even entirely stop the blood flow. As in arterioles the latter show on some places small finger-like processes that directly contact endothelial cells in small areas. Otherwise there is a small basement membrane below the endothelium with its flat endothelial cells. Metarterioles no longer show an internal elastic membrane. Since the media is no loger completely filled with smooth muscle cells collagen and very few elastic fibrils appear here which continue into a very thin, hardly detectable adventitia which can not be clearly distinguished from surrounding loose connective tissue.
Tiny precapillary sphincters (Terminologia histologica: Sphincter precapillares) are formed directly before the origins of capillaries from arterioles on some places. Here some smooth muscle cells encircle the lumen completely to reduce or stop the blood flow into capillaries. As described above the metarterioles themselves serve as precapillary sphincters but their contraction lasts for a longer time. Metarterioles continue into capillaries which are characterized by a complte loss of smooth muscle cells or their replacement by pericytes, respectively.

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--> Electron microscopic atlas Overview
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Three images were kindly provided by Prof. H. Wartenberg; other images, page & copyright H. Jastrow