The Notochord

Summary

The notochord of hagfishes is composed of a central rod of epithelial cells which are surrounded by a complex sheath; there are no traces of any vertebral structures. The notochord epithelial cells are interconnected by abundant desmosomes which are intracellularly associated with a well-developed system of intermediate filaments (diameter 10–12 nm). The vast majority of the notochord epithelial cells is characterized by a 60 μm wide vacuole containing heteromorphic material which reacts positively with several histochemical tests for carbohydrates. The peripheral notochord cells are rich in RER and other cell organelles, suggesting active protein synthesis. In these cells the initial steps of vacuole formation can be found. The notochord sheath consists of two main layers, the inner and the outer sheath, which are separated by a distinct elastica externa, a layer of electron dense extracellular material. The acellular PAS-positive and 50 μm thick inner sheath is composed of densely packed 8–11 nm tubular microfibrils, the chemical composition of which has not yet been determined. The outer 20–35 μm thick sheath consists of typical 30–50 nm thick cross-striated collagen fibrils, tubular microfibrils, elastic-like fibres, proteoglycans, fibroblasts and single blood capillaries.