**Core Concept**
The growth and progression of atheromatous plaques involve the release of various growth factors, including platelet-derived growth factor (PDGF), fibroblast growth factor (FGF), and tumor necrosis factor-alpha (TNF-α), which stimulate the production of extracellular matrix components, leading to plaque expansion.

**Why the Correct Answer is Right**
The cells most likely to release these growth factors in the plaques are macrophages, which are present in high numbers within the atheromatous lesions. Macrophages play a crucial role in the development and progression of atherosclerosis by releasing various pro-inflammatory and pro-atherogenic factors, including PDGF, FGF, and TNF-α, which stimulate the proliferation and migration of smooth muscle cells, leading to the accumulation of extracellular matrix and the expansion of the plaque. The release of these growth factors by macrophages promotes the transformation of the plaque into a more unstable and rupture-prone phenotype.

**Why Each Wrong Option is Incorrect**
**Option A:** Smooth muscle cells are involved in the proliferation and migration within the plaque, but they are not the primary source of growth factors that lead to increased extracellular matrix production.
**Option B:** T lymphocytes play a role in the immune response within the plaque, but they are not the primary source of growth factors that lead to increased extracellular matrix production.
**Option C:** Endothelial cells line the inner surface of blood vessels, but they are not the primary source of growth factors that lead to increased extracellular matrix production within the plaque.

**Clinical Pearl / High-Yield Fact**
Macrophage-derived growth factors, such as PDGF and FGF, play a critical role in the development and progression of atherosclerosis by promoting the accumulation of extracellular matrix and the expansion of the plaque.

**Correct Answer: C. Macrophages.