Population dynamics of arterial cells during atherogenesis: VIII. Separation of the roles of injury and growth stimulation in early aortic atherogenesis in swine originating in pre-existing intimal smooth muscle cell masses

Abstract

This is a study of focal masses of smooth muscle cells that are found normally in the intima of large and medium sized arteries from intrauterine life to old age in man and most experimental animals. These intimal cellular masses are of special interest because they appear to be sites of predilection for atherogenesis. The experimental animals used in this study were young swine; intimal cellular masses were those in the abdominal aorta; arterial cell population changes were studied from approximately the 8th to the 16th weeks of life and comparisons were made between normolipidemic and hyperlipidemic swine; methods included overall cell counts and isotopic techniques designed to trace cells through multiple divisions. Fifteen swine were given [ 3H]thymidine on approximately the 60th day of life. Five were sacrificed on the 75th day as a baseline group; five were fed a hyperlipidemic diet from the 75th to the 135th day of life and the remaining five were continued on a conventional low-cholesterol mash diet until the 135th day, when both groups were sacrificed. Total smooth muscle cells in intimal cellular masses of the abdominal aorta averaged 1.59 million in the 75-day-old baseline group; 2.06 million in the 135-day-old mash-fed group; and 4.73 million in the 135-day-old hyperlipidemic group. This tripling in the number of cells in intimal cellular masses of hyperlipidemic swine (as compared to no significant increase in the corresponding value in mash-fed swine) during the initial 60 days on the diet occurred prior to development of overt gross lesions and probably represents the earliest phase of atherogenesis. In the isotopic portion of the study, we attempted to trace the behavior of the original cell population of the intimal cellular masses (re smooth muscle cell births and deaths) for the 60 days in the two dietary groups and to identify differences that accounted for the 2- to 3-fold greater increase of cells in the hyperlipidemic diet group. In regard to cell loss (probably by death) in the mash-fed group, 39% of the cells present at the onset were lost (without surviving progeny) over the 60 days as compared with 40% in the hyperlipidemic group—hence no significant difference. In regard to percentage of original cells surviving but not dividing in 60 days in the mash-fed group, the value was 30% and in the hyperlipidemic no non-dividers were detected; thus more cells were recruited from non-dividers into the active dividing population in the hyperlipidemic group. As regards the 60-day dividing population in the mash dict group, 31% of the original cells divided one or more times with an average of 3.66 live progeny per dividing cell; in the hyperlipidemic group 60% divided one or more times with an average of 5.27 live progeny per divided cell. Thus, the hyperlipidemic diet produced a mitogenic effect on the smooth muscle cells of the intimal cellular masses manifested by significantly larger numbers of cells in the 60-day dividing population and by significantly more divisions per dividing cell (and hence more progeny) than in the mash-fed group. Smooth muscle cell losses were negligible in the media of both groups; numbers of cells were increased considerably in both by growth as expected and no differences were observed in this respect between the two groups. The cell losses in the intimal cellular masses, though apparently not related to the diet at this stage of atherogenesis, probably accounted for localization of the diet-related changes to the intimal cellular masses in the hyperlipidemic swine.