Structure of the conus arteriosus and ventral aorta in the sea lamprey, Petromyzon marinus, and the Atlantic hagfish, Myxine glutinosa: microfibrils, a major component

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Abstract
Light and electron microscopic observations of the conus arteriosus (CA) and ventral aorta (VA) of the sea lamprey, Petromyzon marinus, and the Atlantic hagfish, Myxine glutinosa, reveal the presence of a unique form of fibrous extracellular matrix within their media and an unusual type of connective tissue forming the lateral subendothelial distensions found only in the lamprey CA. The extracellular matrix of the media in lamprey and hagfish CA and VA is characterized by large concentrations of tubular, elastic fibrelike microfibrils 11–17 nm in diameter. No elastic fibrelike elastin component could be found. Collagen fibrils, matrix granules, and intergranular filaments make up the remainder of the matrix. The loose connective tissue composing the lateral distensions of the larval and adult lamprey CA contains ultrastructural features similar to those of lamprey mucocartilage. It is speculated that since lampreys and hagfish are living representatives of the most primitive group of vertebrates, the microfibrils present in their major arteries may represent a primitive form of elastic fibre in which elastin has not yet developed. The functional significance of the extracellular microfibrils in the CA and VA of lamprey and hagfish and the occurrence of lateral distensions in the lamprey CA are discussed.

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