Intercellular connective tissue fibrils in the notochordal epithelium of the early chick embryo

Chick embryos of the White Leghorn strain were incubated from one‐half to four days and sacrificed by immersion in toto . in Karnovsky's fixative. The cervical region of each embryo was excised and prepared for electron microscopy. Intercellular materials, which appear indistinguishable from the early connective tissue components of the embryonic tissue space, appear within dilated interstices of the notochordal epithelium beginning at approximately Hamburger‐Hamilton stage 14. At progressively older stages this intercellular material appears to organize and aggregate to form fibrils. By stage 22 widely expanded intercellular spaces contain boundary (basement) membrane material, amorphous substance and microfibrils of various sizes. Moreover, larger, more mature fibrils that exhibit collagen‐like periodic striations are present. The existence of connective tissue materials in cellular interstices deep within the central portion of the notochord strongly suggests that these substances are derived from notochordal epithelial cells. Furthermore, the similarity of the intercellular connective tissue components to those deposited on the notochordal surface suggests that these latter materials are likewise a secretory product of the notochord. The significance of the perinotochordal connective tissue (notochord sheath) with respect to the recent emphasis on the role of surface‐associated substances to cytodifferentiation is discussed.