SPP1 and Alzheimer disease: As an endocrine organ, the skeleton gradually secretes bone-derived factors such as glycoprotein sclerostin (SOST), osteocalcin (OCN), osteopontin (OPN), small glial cells, and hematopoietic stem cells, which regulate the physiological characteristics of AD in the brain through the blood–brain barrier and improve the metabolic process of AD.