The Histology Guide

Arteries

There are three main types of arteries:

  • Elastic arteries
  • Muscular arteries
  • Arterioles

Take a look at this cross-section through an elastic artery, and identify the three main layers - tunica intima, tunica media and tunica adventitia.

Elastic arteries:

These arteries that receive blood directly from the heart - the aorta and the pulmonary artery.:

These need to be elastic because:
They are relatively thin compared to their diameter.
When the heart contracts, and ejects blood into these arteries, the walls need to stretch to accommodate the blood surge, storing energy. The arterial hydrostatic pressure that results from ventricular contraction is the 'systolic blood pressure' (systole is greek for contract).

Between heart contractions, the elastic walls recoil, to maintain blood pressure, continuing to move blood even when ventricles are relaxed. The arterial hydrostatic pressure between contractions is the 'diastolic blood pressure' (diastole is greek for dilatation).The walls of these arteries have lots of elastin.

Tunica adventitia - has small 'vasa vasorum' as the large arteries need their own blood supply. Tunica media is broad and elastic with concentric fenestrated sheets of elastin, and collagen and only relatively few smooth muscle fibres.

Tunica intima is made up of an epithelium, which is a single layer of flattened epithelial cells, together with a supporting layer of elastin rich collagen. This layer also has fibroblasts and 'myointimal cells' that accumulate lipid with ageing, and the intima layer thickens, one of the first signs of atherosclerosis.

Muscular artery

These arteries distribute blood to various parts of the body. These include arteries such as the femoral and coronary arteries. The walls of these arteries have lots of smooth muscle, which means that they are able to contract or relax (dilate) to change the amount of blood delivered, as needed.

Comparing these arteries to the elastic arteries, the sheet of elastin is now much reduced, and found at the border between the tunica intima and tunica media in a layer called the internal elastic layer (IEL) which can be seen very clearly. Less well defined is the external elastic layer (EEL), between the tunica media and tunica adventitia. There is a well defined circular layer of smooth muscle in the tunica media.

The tunica intima has an endothelium of flattened endothelial cells. The tunica media is primarily a layer of smooth muscle, with some elastin an collagen. muscle layer, and is sandwiched between the IEL and EEL. The Tunica Adventitia is very broad, and mostly contains collagen and elastin.

Arterioles:

This shows a diagram of an arteriole. The internal elastic lamina layer is still present. The T.M. (tunica media layer) has no more than six concentric rings of smooth muscle, and the tunica adventia (T.A.) layer is approximately the same size as the T.M.

This is a photo of some stained smaller arterioles. The TA layer is difficult to see, but concentric rings of muscle can be seen in the T.M. layer.

Can you find the arteriole?

Larger arterioles have a lumen less than 100 to 300 µm in diameter. Arterioles are small arteries that deliver blood to capillaries. Arterioles control blood flow through capillary beds by contracting or dilating the the size of the lumen, and therefore the tunica media layer contains concentric rings of smooth muscle to do this. This compartment is important in determining your blood pressure as the narrow diameter of these blood vessels resists blood flow, and the back pressure helps to stretch the walls of the arteries during heart contractions.

The tunica intima is very thin, and mostly consists of a single layer of squamous epithelium.The tunica media consists almost entirely of a single layer up to six layers of smooth muscle cells, and there is no EEL. The Tunica adventitia is about the same size as the tunica media layer, merges in with surrounding tissue.

TEM of arteriole

This is an EM of a a very small arteriole. There is only one layer of smooth muscle (M), but there is still an internal elastic layer (IEL).