A study on the blood vascular system of the lamprey gill filament.

The blood vessels in the gill filaments of lamprey, Lampetra japonica, were studied with an electron microscope. Each gill filament is supplied by an afferent filament artery (afa) flanked by a pair of filament veins (fv), along the outer border of the axial plate, and an efferent filament artery (efa) along the inner, free border. Between these arteries are interposed the cavernous body and the blood lacunae. The marginal channel arises from the cavernous body, which leads to the afa, runs along the free border of the secondary lamella collecting oxygenated blood from the lamellar blood lacunae, and finally joins efa. This channel is considered to play an important role in the gill circulation as a bypass between afa and efa. Pillar cells were found only in the blood lacunae of the axial plate and the secondary lamellae. The basal lamina is generally absent on the vascular wall, but only a small amount of dense material could be seen on the basal surface of endothelial cells of afa and fv: this is regarded as an anchoring device between the cell and connective tissue fibrils. Fenestrations are generally absent in the endothelial layer but numerous gaps were seen in the wall of fv. Arterio-venus anastomoses were found between afa or the cavernous body and fv, and between efa or the collecting arteries of the efferent branchial artery and the peribranchial venous sinus. The anastomosing channel consists of the inner endothelial layer and the outer smooth muscle layer, which is thicker in the intermediate portion of the channel. The endothelial cells called "AVA cells" here are characterized by (1) dense cytoplasms, (2) apical microvilli and (3) basal cytoplasmic processes closely associated with smooth muscle cells.