Ultrastructural histochemical and autoradiographic studies on the developing chick notochord

Summary The author examines by means of ultrastructural, histochemical and autoradiographic techniques the development of the notochord of the chick embryo in the period between Hamburger-Hamilton's Stages 9 and 26. The interest in this research is that, during this period, the notochord is transformed from an apparently undifferentiated organ into an organ with secretory activity, and at the same time becomes an inductive organ in the development of the axial skeleton.The study shows that, starting from Stage 10, the notochord cells become hypertrophic and acquire the characteristics of secretory elements, with the production of chondroitin sulphate (A and/or C) and probably of collagen microfibrils. Such substances not only cluster densely around the notochord, thus contributing to the formation of the perichordal sheath, but they also diffuse outwards, becoming distributed among the sclerotomic cells; the latter, on the other hand, start to differentiate into cartilaginous cells only after the halo deriving from the notochord has enlarged and diffused among them.It is also pointed out that there is a close temporal connection between the high degree of secretory activity of the notochord and the start of its inductive activity on the sclerotomes.