The ultrastructure of the blood vessels of Branchiostoma lanceolatum (Pallas) (Cephalochordata)

Summary The ultrastructure of the blood vessels of Branchiostoma has been studied using selected characteristic vessels as examples. It is shown that the vessels are a part of the original blastocoelic cavity and are delimited either by the basal laminae of adjacent epithelia or by connective tissue developed in the blastocoelic space. A brief account of the kinds of connective tissue is given. The observed contractility of some vessels depends on two types of contractile filaments situated in the basal part of the surrounding coelomic epithelia. Amoebocytelike cells are present in the blood. They may sometimes lie in contact with the wall of the vessels or with each other, but never form a typical endothelium with junctional complexes and a basal lamina of its own. Actually, there is no endothelium in any part of the vascular system. It is suggested that the term endothelium should be reserved for a closed cellular lining (with junctions) on the luminal side of the vessel wall, standing on a basal lamina of its own and forming a barrier for the exchange between blood and surrounding tissue. It is concluded that the principal structure of the vascular system of Branchiostoma is different from that of vertebrates, but the same as that of other coelomate invertebrates. The blood vessels in these animals are typically delimited directly by a basal lamina secreted by epithelia (epidermal, coelomic or intestinal) lying peripheral to this lamina, and a true endothelium is not present (with a few questionable exceptions).Abbreviations ac atrial cavity - ace atrial epithelium - ao aorta - ap atrial plexus - ax axon bundle - bc blood cell - bl basal lamina - bl ₁ basal lamina of intestinal epithelium - bl ₂ basal lamina of visceral coelomic epithelium - bl ₃ basal lamina of parietal coelomic epithelium - bl ₄ basal lamina of atrial epithelium - bll basement lamella - cf contractile filaments - co coelomic cavity - coe coelomic epithelium - coe _p parietal coelomic epithelium - coe _v visceral coelomic epithelium - ct dense connective tissue - dv longitudinal dorsal vessel - ep epidermis - epe epipharyngeal groove epithelium - epg epipharyngeal groove - fb fibroblast (?) - fi collagen fiber - fl fibril layer - go gonad - hd hemidesmosome - ie intestinal epithelium - in intestine proper - ip intestinal plexus - iv afferent intestinal vessel - ld liver diverticulum - lu vascular lumen - me myocoelic epithelium - ml muscle lamella - mp myoseptal plexus - ms myoseptum - my myomer - myc myocoelic cavity - nc notochord - ns notochordal sheath - ph pharynx - suc subchordal coelom - sv subintestinal vessel - svv segmental ventral vessel - vv longitudinal ventral vessel Supported by a grant from the Danish Natural Science Research Council