Objectives: 

We have recently observed that in some animals the media of the carotid artery is composed of two distinct layers. The inner 2/3 has a circumferential orientation of components whereas the outer 1/3 has an axial orientation. The aims of this study are to establish common features in the animals in which this 'extra' medial layer is seen and to characterise the differences between the two layers in structure and function.

Materials: 

We have studied carotid arteries from the following mammalian orders. Artiodactyla, carnivora, cetacea, erinaceomorpha, lagomorpha, perissodactyla, pilosa, primates, rodentia and soricomorpha. Conventional staining and transmission electron microscopy confirmed the morphology and thickness of the layers. Immunohistochemical staining was performed to characterise the cells and to identify their phenotype.

Results: 

Conventional staining revealed the presence of the two-layered media in the artiodactyla, the perissodactyla and the cetacea but not in any of the other orders. Immunohistochemistry showed that the distribution of fibrillin 1, NAV 1.8, tenascin C, fibronectin and collagen I & III were the same in the two layers. Alpha actin, desmin and collagen IV were seen only in the inner layer, suggesting a contractile phenotype for the cells therein and providing no clear indication of the function of the cells in the outer layer.

Conclusions: 

The inner medial layer had a circumferential orientation of components and contractile vascular smooth muscle cells; whereas the components of the outer layer were orientated axially with fibroblast-like cells of unknown function and a denser distribution of scleroprotein. All the animals in which the outer layer was observed are located downstream from a particular point on the mammalian supertree, corresponding to the superorder cetartiodactyla, suggesting that this structure evolved over a relatively short period of time (approximately 80 million years ago) for as yet unknown reasons.