On the fine structure of arc-capillaries: true a.v. anastomoses or sphincter capillaries?

The arc-capillaries represent vessels of the terminal bed: they are not true arterio-venous anastomoses. They exhibit a typical capillary structure of their wall. The existence of smooth muscle cells and modified smooth muscle cells, such as epithelioid cells, cannot be demonstrated. Likewise, a specific activation by nerves does not exist. The arc-capillaries differ from the net-capillaries neither in the structure of their wall nor in diameter. At the origin of the arc-capillaries from the final arteriole there are no smooth muscle cells, only pericytes (adventitial cells). The arc-capillaries branch off where the final arteriole no longer possesses smooth muscle cells. The electron microscopic studies showed that arc-capillaries do not represent sphincter capillaries.