Foam cells are a distinctive type of cell characterized by their “foamy” appearance. This look comes from an excessive accumulation of lipids, primarily cholesterol, within their cytoplasm. These cells are a feature in various health conditions, signaling an imbalance in lipid metabolism or a chronic inflammatory process.
Cellular Identity and Appearance
Foam cells primarily develop from macrophages, a type of white blood cell involved in immune system defense. Other cell types, such as smooth muscle cells, can also transform into foam cells.
Their “foamy” appearance is due to numerous small lipid droplets accumulating inside the cell. These droplets are predominantly composed of cholesterol esters. Oxidized low-density lipoprotein (oxLDL) is the main lipid macrophages take up to become foam cells.
How Foam Cells Form
The formation of foam cells begins when macrophages internalize large amounts of modified lipids. Oxidized low-density lipoprotein (oxLDL) is taken up by specific proteins on the macrophage surface called scavenger receptors. Unlike standard LDL receptors, scavenger receptors do not reduce their activity when there is already a high level of cholesterol inside the cell, allowing continuous, unregulated lipid uptake.
Once internalized, these lipids, especially cholesterol esters, build up within the cell’s cytoplasm, overwhelming its normal processing and efflux mechanisms. This results in the visible lipid droplets and the characteristic foamy appearance. Chronic inflammation often contributes to this process, encouraging lipid modification and uptake.
Foam Cells in Atherosclerosis
Foam cells play a significant role in atherosclerosis, a condition where plaque builds up inside the arteries. They are a hallmark of the earliest stages, forming fatty streaks within the arterial wall. These cells contribute to the progression of atherosclerotic plaques.
As foam cells accumulate, they also promote local inflammation. Over time, many of these foam cells die, releasing their stored lipids and cellular debris into the arterial wall. This forms a lipid-rich, necrotic core within the plaque, which can make the plaque unstable. An unstable plaque is susceptible to rupture, leading to blood clots that block blood flow and cause heart attacks or strokes.
Broader Disease Connections
While their role in atherosclerosis is well-documented, foam cells are also observed in a range of other health conditions. For example, they are found in xanthomas, which are visible lipid deposits that can appear under the skin or in tendons. Their presence in such conditions underscores a common theme of dysregulated lipid metabolism leading to cellular lipid accumulation.
Foam cells are also noted in certain genetic lipid storage disorders, such as Niemann-Pick disease. They can also be a feature of chronic inflammatory conditions like rheumatoid arthritis and are found within the granulomas of infectious diseases such as tuberculosis. In these diverse conditions, foam cell formation often points to an underlying issue with lipid handling or a persistent inflammatory response.