Membrane structure of the cells of the lamprey notochord

Thin-section and freeze-fracture electron microscopy have been used to dete rmine the types of junctions that are present in the notochord of adult lam preys and thereby propose ways in which the functional properties of the no tochord are integrated. The notochord consists of a fibrous sheath, which s urrounds a single layer of small 'epithelial' cells and a central core of l arge vacuolated cells. The epithelial and vacuolated notochordal cells are all linked by numerous desmosomes, which are the predominant type of interc ellular junction in the notochord. Furthermore, the notochordal epithelium is linked to the inner surface of the notochordal sheath by many hemidesmos omes, which are associated with patches of basal lamina-like material. The above linkages, produced by desmosomes and hemidesmosomes, together with th e tensile strength of the notochordal sheath, must he crucial for maintaini ng the structural integrity of the notochord during the sinuous swimming mo vements exhibited by lampreys. The communicating (gap) junctions, that are present between vacuolated cells, would facilitate metabolic coupling betwe en these cells. Zonulae occludentes ('tight junctions') were not found betw een notochordal epithelial cells, indicating that there is no effective par acellular barrier to the diffusion of macromolecules, ions and small polar solutes at the outermost layer of the notochordal cells.