ABSTRACT
Embryologically bone forms in two separate ways. The first is by intramembranous osteogenesis in which bone forms directly from the differentiation of mesenchymal stem cells through osteoprogenitor intermediates into osteoblasts. The second is endochondral osteogenesis in which bone forms exactly the same as in intramembranous ossification but only after cartilage develops, involutes, calcifies, and ultimately is resorbed. Some hormones are endocrine in function, having effect on several different organs including bone. Some are paracrine and autocrine with local affects directly on bone. Cytokines is a rapidly expanding, diverse group of bone-cell modulating factors which currently include interferons, tumor necrosing factors, interleukins, colony stimulating factors, monokines, lymphokines, and neuropeptides. Most of these substrates operate as autocrines or paracrines. The release of these reactive substrates gives credence to the theory that bone cell metabolism is master-controlled by the nervous system as well as the endocrines.
