Attempts are being made to arrest atherosclerosis in symptomatic adults by interfering with several recognized risk factors. Because the obstructive fibrous plaques of atherosclerosis in adults are suspected to result from progression of fatty streaks that developed in the arteries during childhood, the question has now been raised whether symptomatic disease in adulthood might not be prevented more easily by stopping the evolution or progression of fatty streaks in children. We propose to study whether fatty streaks are indeed the necessary precursors of fibrous plaques, and whether quantity or type of lipid in fatty streaks determines their conversion to fibrous plaques. We will seek to identify arterial developmental factors responsible for initiating fibrous plaque formation just after arterial segments. By determining the age at which fatty streaks first acquire initial features of irreversibility, we thus intend to establish how early preventive measures should be begun. We will have available for study the coronary arteries and the aortas of infants (beginning with full-term birth), children, adolescents, and young adults (up to and including 24 years of age) dying in New Orleans. We will approach our objectives by using correlative electron microscopy, cytochemistry, and chemical lipid analysis. By morphological means we will determine the types of cells, their lipid content, and lysosomal activity, and, by chemical means, the total lipid content and lipid composition of fatty streaks of different arterial segments and of different age groups. We will also use morphological means to determine different cell types and their lysosomal activity in the nonatherosclerotic intimal thickening of different arterial segments. Through those methods we will attempt to identify those factors that are associated with the formation of fibrous plaques.