'Chordoid tissue': biological significance and relevance of the term.

The significance and relevance of the term 'chordoid tissue' are discussed on the ground of some ultrastructural features of the notochordal cells, observed in 15 chick embryos from stage 5HH to hatching. The characteristics of the cytoplasmic organelles and of the nucleus, the membrane alterations and in particular the vacuolization show the notochordal cells undergo a process of necrobiosis rather than of apoptosis. The degeneration of the notochord, that not casually starts in the notochordal core, seems to be due to a progressive metabolic isolation, related to the lack of blood vessels and to the formation of a thick perichordal sheath. These findings suggest that the notochordal tissue shows the same cytologic features of any tissue degenerating owing to a metabolic damage. The term 'chordoid tissue', in the meaning of a tissue with peculiar structure and function, does not seem hence to be relevant, also on the ground of the fact that in the species in which a nucleous pulposus arises from notochord remnants, the notochordal cells show the same ultrastructural characteristics detectable in chick embryos.