Atherosclerotic lesions in the coronary arteries of hyperlipidemic swine. Part 1. Cell increases, divisions, losses and cells of origin in first 90 days on diet.

The intima of the proximal portion of the coronary arteries of young swine is normally thickened by accumulations of cells about 90% of which are smooth muscle cells (SMC) and about 10% are of probable monocyte origin. Extracellular components such as collagen and elastic tissue are also present but we have chosen to emphasize their cellular nature by calling the regions of thickened intima, intimal cell masses (ICM). We have previously shown that atherosclerotic lesions produced in the coronary arteries of swine by 90 days of feeding a hyperlipidemic (HL) diet arise almost exclusively in the normally occurring ICM. We are reporting here a study of the pathogenesis of these lesions following killing at 0, 14, 49 and 90 HL diet days with comparisons between ICM in control mash-fed swine and ICM-lesions in the HL swine. We found that in the ICM: lipid accumulation was present by 14 days and increased thereafter; the lipid was mostly in SMC but percentage wise the monocyte-macrophages were involved as much or more, cell division activity was increased 3-4-fold by 49 days, cell numbers in ICM were similar in HL and control swine at 49 days but were about 6-fold greater in the HL swine at 90 days, (now in ICM-lesions), at 90 days, circa 90% of the cells appeared to be of SMC and circa 10% of monocyte origin both in the ICM-lesions of the HL swine and in the normal ICM of the controls. The data suggest but do not prove that early lipid accumulation precedes increased cell divisions especially among the SMC component and this in turn precedes increased numbers of cells in the ICM. Although SMC constitute the major cell component of the ICM-lesion at 90 days, the monocyte-macrophage-like cells also increase in number as a result of the HL diet and constitute a small but definite minor component. One possible explanation for the increased cell division activity is that one of the lipid constituents is acting as mitogen; another possibility is that the effect of a well known mitogen such as platelet-derived growth factor is enhanced by the lipid; another is that the monocytes are being stimulated to produce monocyte-derived growth factor. In any event in the very early stage of atherogenesis in the coronary arteries in these experiments excessive proliferation of resident SMC in the ICM appears to be the predominant feature.