Polarized light micrograph of cholesterol crystals, the main component of all biological membranes (it determines their fluidity and permeability) and gallstones. It is the major precursor for the synthesis of vitamin D and hormones cortisol, cortisone, aldosterone, progesterone, estrogen, and testosterone. High levels of cholesterol transported within low-density lipoprotein (or "bad cholesterol") in the circulatory system are strongly associated with atherosclerosis, myocardial infarction, cardiovascular disease, and stroke. It plays an important role in the immune system and in brain synapses. Cell degeneration in Alzheimer's disease is linked to cholesterol levels in the surface membrane. Field of view size: 2.97 x 4.45 mm (0.117 x 0.175 in)