Fatty streak

Fatty streak, though composed of macrophage white blood cells, not fat, is the term generally given to the earliest stages of atheroma, as viewed at autopsy, looking at the inner surface of arteries, without magnification. It is not visible by current technologies in living humans, even by IVUS, the imaging technology with the highest spatial resolution for visualizing artery walls in vivo.

The fatty streak is the first grossly visible lesion in the development of atherosclerosis. It appears as an irregular off white to yellow-white discoloration near the luminal surface of the artery. Actually the streaks are not fat, but small collections of monocyte-derived macrophages located beneath the inner, endothelial layer of arteries. The fatty streak mainly consists of foamy appearing macrophage cells, sometimes with some additional T lymphocytes, aggregated platelets, localized smooth muscle cells, etc.

The macrophage cells, under a microscope, have a foamy-like appearance because of large collections of membrane bound vesicles within their cytoplasm. Since cholesterol within cells resides primarily within the cell membranes, the large accumulation of membranes results in an elevated local content of membrane bound cholesterol and other fats.

En-mass the foamy macrophages usually have an off white to yellow-white color and were named because they were thought to "look like" (gross, non-microscopic perspecitive) "streaks of fat" against the otherwise quite red/pink muscular tissue background forming the wall of arteries.