Final answer:
Fibroblasts use a receptor-mediated mechanism to uptake abnormal levels of cholesterol. This mechanism involves binding proteins in the plasma membrane and the formation of coated vesicles to bring the substances into the cell.
Step-by-step explanation:
According to the information provided, fibroblasts use a receptor-mediated mechanism to uptake abnormal levels of cholesterol. In receptor-mediated endocytosis, binding proteins in the plasma membrane attach to the particles that need to be taken up, causing the membrane to invaginate and form a coated vesicle that brings the substances into the cell. Failure of receptor-mediated endocytosis can result in diseases such as familial hypercholesterolemia, where the LDL receptors are defective or missing entirely, leading to high levels of cholesterol in the blood.