Key result
Histology reveals 2 distinct outer arterial layers, a compact tunica externa and loose adventitia.
Why the study?
The structure and layering of the arterial tunica adventitia beyond the tunica media in different arteries and species was not clearly defined.
Histological analysis of animal arteries suggests differentiating the outer vessel wall into a compact 'tunica externa' and a loose 'tunica adventitia', which varies by artery type and species.
Answers are grounded in this paper and cite their sources.
Challenges single adventitia model in select arteries; leaves open whether revised nomenclature aids human vascular research or surgery.
Tunica adventitia or tunica externa is the outer layer of the blood vessel wall. It consists of connective tissue with vasa and nervi vasorum and plays a key role in vascular health. The aim of our study was to compare the wall layers beyond tunica media in arteries of different type and location. The following arteries of pig, dog and cat were processed histologically and analysed by light microscopy: aorta ascendens, arcus aortae, aorta thoracica, aorta abdominalis, arteria (a.) femoralis, a. tibialis cranialis, a. carotis communis, a. lingualis, a. basilaris, a. cerebralis media, a. testicularis and aa. jejunales. We found two layers of connective tissue outside the media: (1) a compact layer with many elastic fibres in muscular and few in elastic arteries and (2) an outer layer of loose connective tissue. The compact layer was missing in aorta ascendens, arcus aortae and intracranial vessels. Adventitial stripping removed only the loose connective tissue layer. In spite of the still present compact layer, stripped arteries were very flimsy. We suggest using the term 'tunica externa' for the compact connective tissue layer and 'tunica adventitia' for the outermost loose connective tissue layer as in other organs. The presence of the tunica externa differs between species, arteries and arterial side, as well as the removability of tunica adventitia and tunica externa by anatomical dissection.
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Witter et al. (2016) studied this question. Histological analysis of arterial wall layers was evaluated on Structure of the arterial tunica externa/adventitia. Histological analysis of animal arteries revealed two connective tissue layers outside the media: a compact layer (tunica externa) and an outer loose layer (tunica adventitia).
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