Variability in the Presence of Elastic Fibre‐like Structures in the Ventral Aorta of Agnathans (Hagfishes and Lampreys)

This paper investigates whether elastic fibre‐like structures are present in the ventral aorta of hagfishes and lampreys. Fibres, which are morphologically similar to the elastic fibres of gnathostomatous (jawed) vertebrates, are shown to be present in the ventral aorta of the hagfish Paramyxine atami , and also, but to a lesser extent, the dorsal aorta of this species and the ventral aorta of another hagfish, Eptatretus stouti. The ‘elastic’ tissue formed irregular sheet‐like aggregates, comprising well‐defined amorphous material surrounded by tubular microfibrils, whose diameters ranged from 10 to 15 nm. While this tissue was most abundant in the same region of the aortae as that occupied by the elastica interna in the blood vessels of gnathostomes, it was also found deeper in the wall of these blood vessels. Although tubular microfibrils were found in the ventral aorta of the hagfish Myxine glutinosa and the lamprey Geotria australis , these were never associated with well–defined, amorphous material. This parallels the results of previous studies on Myxine glutinosa and another species of lamprey, Petromyzon marinus. Thus, the elastic fibre–like tissues found in the ventral aortae of P. atami and E. stouti provide the first examples of such structures in this region in agnathan vertebrates.