Light and electron-microscope studies of the dermal capillaries in 3 species of hagfishes and 3 species of lampreys

A dermal capillary network was found in three species of hagfishes (Myxine glutinosa, Paramyxine atami, Eptatretus stoutii) and in the adults of three species of lampreys (Geotria australis, Lampetra japonica, Petromyzon marinus). However, it was found neither in the ammocoetes of any of the species examined, nor in the adults of the lamprey Lampetra fluviatilis. The complex arrangement of dermal capillaries described for adult L. fluviatilis by Czopek and Sawa (1971) was apparently an artefact. The dermal capillaries of hagfishes and lampreys are lined by a continuous endothelium, comprising flattened cells that possess most of the usual complement of organelles found in the endothelia of higher vertebrates. The pericapillary space contains proteoglycans, glycoproteins and tubular microfibrils (diam. 11-15 nm) and, in hagfishes, also irregularly arranged collagen fibrils (diam. 40-120nm). The thickness of the endothelium varied amongst species, ranging from c. 100 nm in G. australis and P. marinus to over 500 nm in L. japonica and all three species of hagfishes. In comparison with the endothelial cells of lampreys, those of hagfishes have a better developed basement membrane, possess small desmosomes in the flange-like intercellular contact regions, and contain numerous plasmalemmal smooth tubules, rather than the smooth vesicles that are typically present in endothelial cells. In addition, the hagfish dermal capillaries have pericytes. Amongst hagfishes, the dermal capillary network is Ear better developed in Myxine glutinosa and Paramyxine atami, which burrow and may therefore have to rely to some degree on cutaneous respiration, than in Eptatretus stoutii which does not burrow. However, a dermal capillary network is probably important for all species of hagfishes since it would enable the precursors required for synthesizing mucus to be transported to the epidermis, where mucous cells are very abundant. The particularly well-developed dermal capillary network in Geotria australis may facilitate the transport of oxygen through the unusually thick dermis of their migrating adults as they move around barriers on their upstream migration, and must thus presumably have to employ cutaneous respiration.