Bone is a mineralized connective tissue that is a major component of the skeletal system. Bone cells form bone, which is composed of a matrix of collagen and calcium phosphate minerals. There are three main types of bone cells in the body. Osteoclasts are large cells that break down bones to absorb and assimilate. Osteoblasts regulate bone mineralization and produce osteoid-like (organic material of the bone matrix) that mineralizes to form bone. Osteoblasts mature to form bone cells. Bone cells help bone formation and help maintain calcium balance. From transporting oxygen throughout the body to fighting infection, blood cells are vital to life. The three main types of cells in the blood are red blood cells, white blood cells and platelets. Red blood cells determine blood type and are also responsible for delivering oxygen to cells. White blood cells are cells of the immune system that destroy pathogens and provide immunity. Platelets help to clotting and prevent excessive blood loss due to ruptured or damaged blood vessels. Blood cells are produced by the bone marrow.