Differentiative ability of the tibial periosteum for the embryonic chick.

The claim that the hypertrophic cartilage of a transversely divided 9-day chick tibia is able to induce the differentiation of osteoblasts when placed against the fibrous periosteum of a second, intact, 9-day tibia in vitro, is refuted. Tibia from 7- and 9-day-old chick embryos were transversely sectioned though either the diaphysis or epiphysis and these diaphyseal and epiphyseal grafts were then placed in close association with the fibrous periostea of intact, host tibiae of the same or different ages, at either the diaphyseal or epiphyseal level. All of the sixteen possible combinations of paired tibiae were established in organ culture for 7 days on three types of culture media and two types of atmospheres. In response to the close contact established when diaphyseal grafts were used, the fibrous layer of the periosteum underwent hyperplasia to form fibroblasts at the contact site. Contact of the graft with the periosteum as not sufficient to allow osteoprogenitor cells to accumulate, to differentiate into osteoblasts, or to deposit osteoid. We concluded that the outer, fibrous layer of the periosteum is fibroblastic and not osteblastic.