Morphological comparison of major arteries in the ridgeback prawn, Sicyonia ingentis

The structure of the major vessels conducting hemolymph away from the heart of the ridgeback prawn, Sicyonia ingentis, was examined using light and electron microscopy. Three varieties of vessel wall morphology were observed. In most vessels the wall is composed of the following four layers, proceeding from lumen to exterior: (1) thick basal lamina, which selectively stains with Verhoffapos;s Van Giesonapos;s stain for elastin, (2) a continuous layer of cells rich in microtubules, (3) a loose connective tissue layer with highly branched cells widely separated by a fibrillar matrix, which also stains for elastin, and (4) a thin basal lamina covering the outer surface of the vessel. The dorsal abdominal artery shows the second type of vessel wall construction and differs from the previous vessels in the following ways: (1) the inner basal lamina is thinner, (2) a layer of striated muscle replaces the unspecialized endothelial layer, and (3) the connective tissue layer includes two well-organized bands of elastin-like material. The hematopoietic vessels that branch off the ophthalmic arteries display the third type of vessel wall morphology. The lumen of the vessel is lined by a very thin basal lamina, which completely encircles the endothelial cells. The rest of the wall is composed of hemocytes, presumably in various stages of maturation, embedded in a matrix of fibroblast-like cells and collagen fibrils. The morphology of the vessel wall is the same as that previously described for the tubules of the hematopoietic nodules. The functional significance of these different vessel walls is discussed.