The walls of veins

Veins

The walls of veins are thinner than the walls of arteries, while their diameter is larger. In contrast to arteries, the layering in the wall of veins is not very distinct. The tunica intima is very thin. Only the largest veins contain an appreciable amount of subendothelial connective tissue. Internal and external elastic laminae are absent or very thin. The tunica media appears thinner than the tunica adventitia, and the two layers tend to blend into each other. The appearance of the wall of veins also depends on their location. The walls of veins in the lower parts of the body are typically thicker than those of the upper parts of the body, and the walls of veins which are embedded in tissues that may provide some structural support are thinner than the walls of unsupported veins.
Venous vessels originate from the capillary network which coalesce into the smallest venous vessels the

Venules.

They are larger than capillaries. Small venules are surrounded by pericytes. A few smooth muscle cells may surround larger venules. The venules merge to form

Small to medium-sized veins which contain bands of smooth muscle in the tunica media. The tunica adventitia is well developed. In some veins (e.g. the veins of the pampiniform plexus in the spermatic cord) the tunica adventitia contains longitudinally oriented bundles of smooth muscle. Aside from most veins in the head and neck, small to medium-sized veins are also characterised by the presence of valves. The valves are formed by loose, pocket-shaped folds of the tunica intima, which extend into the lumen of the vein. The opening of the pocket will point into the direction of blood flow towards the heart. One to three (usually two) pockets form the valve. Blood flowing towards heart will pass the pockets. If the flow reverses, blood will fill the pockets which will occlude the lumen of the vein and prevent the return of blood into the part of the vein preceding the valve. The ability of the valves to prevent backflow depends to some extent on the state of contraction (tone) of the smooth muscle in the wall of the vein.

The largest veins of the abdomen and thorax

do contain some subendothelial connective tissue in the tunica intima, but both it and the tunica media are still comparatively thin. Collagen and elastic fibres are present in the tunica media. The tunica adventitia is very wide, and it usually contains bundles of longitudinalsmooth muscle. The transition from the tunica adventitia to the surrounding connective tissue is gradual. Valves are absent.
Vasa vasorum are more frequent in the walls of large veins than in that of the corresponding arteries - probably because of the lower oxygen tension in the blood contained within them.

 
Suitable Slides

sections of veins - H&E, van Gieson or elastin (in combination with other stains)
Like arteries, veins and venules are present in many sections. The basic features of their structure are however more difficult to identify because of the thin walls of veins - in particular in small veins. It is best to resort to sections of large veins, which together with large muscular arteries often have their "own section" in teaching collections.

Vein, human - H&E The tunica intima is very narrow and the internal elastic lamina is difficult to identify - even in elastin stained sections. A few elastic fibres below the endothelium form only a very thin and incomplete internal elastic lamina. Smooth muscle is present in the tunica media, but it is organised less regular than in the artery. The tunica media is, again as compared to the artery, very thin and there is no sharp border between the tunica media and the tunica adventitia. The tunica adventitia of the largest veins contains coarse collagen fibres, elastic fibres and longitudinal bundles of smooth muscle. Small and medium sized veins will not contain smooth muscle in the tunica adventitia. Draw part of the wall of the vein, label the tunics and indicate the presence of smooth muscle, fibres and their types.

 
Suitable Slides

sections containing small to medium sized veins - H&E, van Gieson or trichrome
Unless a specifically prepared slide is available, I would recommend looking at skin slides, in which I have found quite a few nice valves in the veins located at the border between dermis and hypodermis.

Vein Valve - H&E Unless the section has been specifically prepared to illustrate valves, you will have to search for a while - probably through several sections. If a vein with valves is present in the section, it should be easy to identify. Valves are only found in small to medium-sized veins. You should see one or two bands of tissue in the lumen of the vein. Each band is formed by two apposing layers of tunica intima. The bands may share their origin from the inner aspect of the wall of the vein or they may have separate origins. Folding of the tissue bands forming the valves is variable. Draw the vein and valve and label the tunics which you can identify. Try to indicate the position of the section in the schematic drawing of the valve. You can observe the function of vein valves. Put your finger firmly on one of the veins which are visible on the back of your hand. Use another finger to stroke over the vein in the direction of the wrist. The vein will empty and not backfill (closed valves!) until you release the first finger.