Tunica intima
Transverse section through a small artery and vein of the mucous membrane of the epiglottis of a child. (Tunica intima is at "e".)
Details
Part of	Wall of blood vessels
Identifiers
Latin	tunica intima
MeSH	D017539
TA98	A12.0.00.018
TA2	3922
TH	H3.09.02.0.01003
FMA	55589
Anatomical terminology [edit on Wikidata]

The tunica intima (Neo-Latin "inner coat"), or intima for short, is the innermost tunica (layer) of an artery or vein. It is made up of one layer of endothelial cells (and macrophages in areas of disturbed blood flow),^[1][2] and is supported by an internal elastic lamina. The endothelial cells are in direct contact with the blood flow.

The three layers of a blood vessel are an inner layer (the tunica intima), a middle layer (the tunica media), and an outer layer (the tunica externa).

In dissection, the inner coat (tunica intima) can be separated from the middle (tunica media) by a little maceration, or it may be stripped off in small pieces; but, because of its friability, it cannot be separated as a complete membrane. It is a fine, transparent, colorless structure which is highly elastic, and, after death, is commonly corrugated into longitudinal wrinkles.

The structure of the tunica intima depends on the blood vessel type.^[3]

Elastic arteries – A single layer of endothelial and a supporting layer of elastin-rich collagen. The layer also contains fibroblasts, immune cells and smooth muscle cells.^[1]

Muscular arteries – Endothelial cells

Arterioles – A single layer of endothelial cells

Veins – Endothelial cells^[3]

The inner coat consists of:

1. A layer of pavement endothelium, the cells of which are polygonal, oval, or fusiform, and have very distinct round or oval nuclei. This endothelium is brought into view most distinctly by staining with silver nitrate.
2. A subendothelial layer, consisting of delicate connective tissue with branched cells lying in the interspaces of the tissue; in arteries of less than 2 mm in diameter the subendothelial layer consists of a single stratum of stellate cells, and the connective tissue is only largely developed in vessels of a considerable size.^{[citation needed]}
3. An elastic or fenestrated layer, which consists of a membrane containing a network of elastic fibers, having principally a longitudinal direction, and in which, under the microscope, small elongated apertures or perforations may be seen, giving it a fenestrated appearance. It was therefore called by Henle the fenestrated membrane. This membrane forms the chief thickness of the inner coat, and can be separated into several layers, some of which present the appearance of a network of longitudinal elastic fibers, and others a more membranous character, marked by pale lines having a longitudinal direction. In minute arteries the fenestrated membrane is a very thin layer; but in the larger arteries, and especially in the aorta, it has a considerable thickness.

Endothelium had been seen to be simply the boundary between the blood in the lumen and the walls of the vessels. However, endothelium has been shown to release local chemicals called endothelins which are powerful vasoconstrictors.^[4] Endothelins help to regulate capillary exchange and alter blood flow by their constriction of the smooth muscle in the walls. Vasoconstriction increases blood pressure, and its overexpression can contribute to hypertension and cardiovascular disease.^[5]

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  Vein
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  Microphotography of arterial wall with calcified (violet colour) atherosclerotic plaque (H&E stain)

This article incorporates text in the public domain from page 498 of the 20th edition of Gray's Anatomy (1918)

• Histology image: 66_02 at the University of Oklahoma Health Sciences Center – "Aorta"
• Anatomy photo: Circulatory/vessels/vessels7/vessels2 - Comparative Organology at University of California, Davis — "Bird, vessels (LM, High)"
• Image at About.com