Amyloid beta monomers are soluble and contain short regions of beta sheet and polyproline II helix secondary structures in solution,[4] though they are largely alpha helical in membranes;[5] however, at sufficiently high concentration, they undergo a dramatic conformational change to form a beta sheet-rich tertiary structure that aggregates to form amyloid fibrils.[6] These fibrils deposit outside neurons in dense formations known as senile plaques or neuritic plaques, in less dense aggregates as diffuse plaques, and sometimes in the walls of small blood vessels in the brain in a process called amyloid angiopathy or congophilic angiopathy.