ABSTRACT
The close relationships between notochord and cartilage, the formation of notochord tumors (chordomas) with features of notochord and cartilage, the transformation of notochord cells into chondroblasts by de- and redifferentiation or by metaplasia, and the transformation series notochord → chordoid → chondroid → cartilage, all demonstrate shared developmental features of this class of cells and tissues.
Notochord is an unusual tissue with large vacuolated cells connected by desmosomes and surrounded by an expanded basement membrane as a notochord sheath. Notochord and cartilage cells both synthesize and deposit type II collagen, the collagen type that has always been referred in the literature of developmental biology as “cartilage-type collagen.” Given that no invertebrate cartilage contains type II collagen and that the notochord is a more ancient vertebrate tissue than is cartilage, it may be that type II collagen originated in the chordate or vertebrate notochord and was co-opted by cartilage-forming cells. This would make type II collagen notochord-type collagen and not cartilage-type collagen in a phylogenetic context and the deposition of type II collagen into the notochord sheath a precursor of the deposition of type II collagen into the ECM of vertebrate cartilage.
