Arteriole

Arterioles are precapillary arteries (resistance vessels) with a diameter of less than 0.5 mm. This figure shows a section across the wall of an arteriole from the tunica adventitia of a human urethra. The endothelium is continuous 2 . Smooth circular muscles form the underlying tunica media 3 . Bundles of unmyelinated axons 6 are located in the tunica externa. Contact between two neighboring endothelial cells .

1 Nucleus of an endothelial cell

2 Cytoplasmic process from an endothelial cell

3 Smooth muscle cell with filaments and cell organelles

4 Golgi apparatus of a smooth muscle cell

5 Nucleus of a perivascular fibrocyte

6 Unmyelinated axon

7 Connective tissue of the tunica externa

8 Arteriole lumen

Electron microscopy; magnification: × 8000

Arteriole

Arterioles precede the capillaries. They are therefore of ten called precapillary arteries. In the hierarchy of the arterial system, arterioles are the smallest members. Along with the capillaries and venules, they form the terminal circulator y system (microcirculation ).  The tunica intima of the arterioles consists of endothelium 1 with an underlying layer of connective tissue 2 . An internal elastic membrane is usually not present, except occasionally in a fragmented form. The tunica me dia shows only one layer of muscle cells 3 . In this micrograph, the myocytes of the tunica me dia 3 are mostly cut tangentially or across their axis. This indicates that the myocytes are arrange d in a spiral form. Collagen fibrils in different proportions occur between the muscle cells. The tunica adventitia 4 is a narrow layer. It consists of connective tissue cells and collagen f ibers.

1 Endothelium

2 Subendothelial connective tissue

3 Myocytes

4 Tunica adventitia

Electron microscopy; magnification: × 3000

References

Kuehnel, W.(2003). Color Atlas of Cytology, Histology, and Microscopic Anatomy. 4^th edition . Institute of Anatomy Universitätzu Luebeck Luebeck, Germany . Thieme Stuttgart · New York .