Smooth muscle cell-reticulin lamellar units of 13.2 microns thickness composing the aortic intima

Smooth muscle cells of the aortic intima are generally of two forms, spindle and stellate. Spindle cells are typically axial in their orientation, while stellate cells lie parallel to the luminal surface; cell processes do not characteristically extend in the radial direction through the intimal thickness. Evidence is given here to suggest that these cells are clustered in layers of about 13.2 microns thickness which are separated by condensations of reticulin fibers. These layered clusters may extend as much as 1 cm to 2 cm in the axial direction. The numbers of layers appear to increase during growth and maturation to a stable value of about 12 by age 30-40. With further aging and growth of fibrous plaques, the layers seem to become thicker and to merge, obliterating their boundaries, to become 15 to 35 microns or more in average thickness. This expansion and merging of lamellar units precedes atheronecrosis and appears to represent an important precursor of the necrotic core. The greatest growth of fibrous plaques, at ages 40 to 60, takes place after the stabilization of cell numbers at ages 30 to 40, and is almost as likely to happen in the least cellular as in the most cellular places. Hence, these data suggest that smooth muscle cell numbers are not important determinants of the locations nor of the growth rates of fibrous plaques in the lateral thoracic aorta.