Pre Existing Interconnecting Arterioles and Collateral Circulation

Collateral vessels develop from pre-existing interconnecting arteri-oles, but not all species are endowed with a sufficient number to react

Table 1

Comparison of Characteristics of Pig and Dog Models in Angiogenesis Studies

Model Pre-exist- Infarct type Coronary Innate ing inter- with coro- system collateral connecting nary ligation circulation arterioles

Dog Numerous

Transmural

Nontrans-mural

Response Preference to ameroid in angiogen-constrictor esis studies

3-coronary Very limited system endocardial

2-coronary Numerous system epicardial

Small capillary size vessels Large muscular arteries

High

Low rapidly to critical coronary stenosis. Morphological studies of coronary vessels in different species have shown that while rats, rabbits, and pigs have anatomical end arteries with no arteriolar connections, dogs and cats are well endowed with these vessels and guinea pigs hearts exhibit a truly abundant arteriolar network (2). The normal human heart has fewer interconnecting arterioles than a dog heart (3,4). Pigs have a very limited innate collateral circulation, with only sparse endocardial connections; however, dogs have numerous, generally epicardial, innate anastomoses, which are thought to have greater potential for development than those of pigs (5). This difference has resulted in a preference for the pig model for angiogenesis studies.

The canine ameroid model is characterized by the development of large, muscular collateral arteries with limited new capillary growth and is the ideal model to study neoarteriogenesis (6). In contrast, neovas-cularization in response to ischemia in the pig ameroid model consists mainly of small vessels about the size of capillaries that lack an arterial coat. These vessels mostly develop around areas of focal necrosis (7), although they can be found throughout the ischemic territory. The low pressure in the collateral system is perhaps one of the reasons why these vessels remain thin-walled but do not leak. Another explanation could be that porcine myocardium does not have an efficient smooth muscle-recruiting mechanism such as the angiopoietin-tie-2 system (8).

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