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 (My xine glutinosa, Paramyxine atami, Eptatretus stoutii) and in the adult s of three species of lampreys (Geotria australis, Lampetra japonica, Petromyzon marinus). However, it was found neither in the ammocoetes o f any of the species examined, nor in the adults of the lamprey Lampet ra fluviatilis. The complex arrangement of dermal capillaries describe d 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 m ost of the usual complement of organelles found in the endothelia of h igher vertebrates. The pericapillary space contains proteoglycans, gly coproteins and tubular microfibrils (diam. 11-15 nm) and, in hagfishes , also irregularly arranged collagen fibrils (diam. 40-120nm). The thi ckness 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 al l three species of hagfishes. In comparison with the endothelial cells of lampreys, those of hagfishes have a better developed basement memb rane, possess small desmosomes in the flange-like intercellular contac t regions, and contain numerous plasmalemmal smooth tubules, rather th an 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 Myx ine glutinosa and Paramyxine atami, which burrow and may therefore hav e 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 th e epidermis, where mucous cells are very abundant. The particularly we ll-developed dermal capillary network in Geotria australis may facilit ate the transport of oxygen through the unusually thick dermis of thei r migrating adults as they move around barriers on their upstream migr ation, and must thus presumably have to employ cutaneous respiration.