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CAMBRIAN PERIOD

542-488 million years ago

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chordate blood vessels

THE CIRCULATORY SYSTEM

The contractile ventral blood vessel of cephalochordates is homologous to the vertebrate heart. Ancestral cephalochordates evolved a pattern of blood circulation which is retained in all vertebrates (at least as embryos) which included a dorsal aorta and a hepatic portal system. Blood vessels possess a simple lining.

The vertebrate heart is a modified blood vessel. This raises the obvious question: at what point should a contractile ventral blood vessel be called a heart? While hemichordates, urochordates, and pognophorans possess definitive, albeit primitive hearts, the existence of a true heart in lancelets is less clear. While some classify the rhythmically contracting ventral blood vessel in lancets as the branchial artery, others consider it as a 1 chambered heart which may be homologous to the truncus arteriosus or to the sinus venosus (with a conus arteriosus) of vertebrate hearts (Prosser, 1973; Weichert, 1970; Willey, p. 47). The growing body of genetic evidence suggests that this structure is homologous to the vertebrate heart. Although the Amphioxus heart has no separate chambers, valves, endocardium, or epicardium, it does express amphiNk2-tin, a homolog of vertebrate NK2 and Drosophila tinman genes which are expressed in the developing heart. Vertebrates and Amphioxus also express members of the BMP family, TGF , GATA, MEF, and FGF in the developing heart (Holland, 2003).

Venous valves are known from Amphioxus and valves may also exist in arteries in Amphioxus and sharks (Dutta, 367; Webster, 1974, p. 67; Hoar, 1983).

In Amphioxus, blood vessels develop which all craniates will retain, at least as embryos. In all vertebrate embryos, a primordial circulatory system develops in which blood travels from the heart cranially through a ventral aorta and, after passing through an aortic arch, proceeds caudally through the dorsal aorta to the yolk sac and then back to the heart. (Torrey).In Amphioxus and all higher chordates, a pair of anterior cardinal veins develops to drain blood from the head and all vertebrate embryos retain these primitive vessels. (Weichert, 1970, p.560). In Amphioxus, there is a large subintestinal vein which transports nutrients from the gut cranially. In Amphioxus, the hepatic portal vein forms a plexus of vessels in the hepatic cecum which rejoin to form the hepatic vein and subsequently the portion of the endostylar artery called the sinus venosus. (Ruppert, from Harrison, 1997, p. 445-52; Willey 49,54). The hepatic portal system, which brings blood directly from the GI tract to the liver, has been little modified since the early chordates. In both lancelets and hagfish, the hepatic portal vein is contractile (Guenther, 154). In Amphioxus, blood vessels reach the myosepta but not the muscle myotomes themselves and skeletal muscle receives little vascular supply (Ruppert, from Harrison, 1997, p. 458; Willey ).
The endothelial lining of hagfish blood vessels resemble the vessels of Amphioxus and the lymphatic vessels of mammals. Hagfish vessel linings seem intermediate between those of Amphioxus and those of higher vertebrates in their endothelial nature, the presence of open junctions, and the lack of fenestrae. The capillary endothelium of sharks also shares characteristics found in mammalian lymphatics rather than mammalian capillaries. Comparative anatomy suggests that blood vessels have evolved through several stages, beginning with an interrupted basement membrane lining only to a second stage represented by a lining made of separated endothelial cells and then finally an endothelial lining with many open junctions to an endothelial lining in which cells are joined by closed junctions. This progression is observed in comparing mammalian lymphatic vessels as the smallest vessels form larger and larger vessels and also when comparing the progression of vessels from primitive chordates through mammals. These changes seem to have occurred in response to the need for higher blood pressures in more complex chordates (Casley-Smith, 1975).