The branchial circulation and the gill epithelia in the Atlantic hagfish, Myxine glutinosa L.

The vessels of the branchial circulation of the Atlantic hagfish, Myxine glutinosa, and their relationship with the gill epithelia have been studied by light and electron microscopy. The inner surface of the pouch wall (containing the interconnected radial arteries) and the afferent and efferent unbranched portions (cavernous tissues) of the radially oriented gill folds are covered by a multilayered epithelium. The lamellar portion that is characterized by pillar cells is lined by a thin bilayered epithelium. The thin-walled sinusoid system is part of an arterio-venous circulation. This is demonstrated by the presence of arterio-venous anastomoses and by the connection to the peribranchial sinus. The sinusoid system has a close spatial relationship to the multilayered epithelium. The multilayered epithelium consists of pavement cells, cells of the medial layer and basal cells. Granulated cells are often found in the basal half of the epithelium. The pavement cells are characterized by large vesicles in close apposition to the apical plasma membrane. Ionocytes, which display a cytoplasmic tubular system that is continuous with the intercellular space, a high number of mitochondria, and small apical vesicles, are present. The occurence of the ionocytes in the afferent multilayered epithelium as well as the bilayered lamellar epithelium, the morphology of the ionocyte, and the absence of accessory cells is reminiscent of the freshwater teleost gill, and in part the elasmobranch gill, and is discussed in relation to osmo- and ion regulation.