The structure of vitamin D was identified in 1900. In the following decades till the end of the century, numerous studies elucidated its metabolism and its effects on calcium-phosphate homeostasis and its correlation to skeletal mineral metabolism. Indeed, some authors have claimed that the bone is an “intracrine organ” for vitamin D metabolism, based on its direct effects on the bone cells and on the fact that the bone cells themselves can convert 25D into 1,25D by means of the 25-hydroxyvitamin D 1-alpha-hydroxylase (CYP27B1), thereby participating in the autocrine and paracrine loops of vitamin D metabolism.