Microscopic anatomy of the thin-walled vessels leaving the heart of the lobster Homarus americanus: anterior median artery

Abstract. The anterior median artery is an unpaired vessel that leaves the anterior end of the lobster ( Homarus americanus ) heart and supplies hemolymph to the brain, the antennae, and the eyestalks. This vessel has a trilaminar organization, consisting of a tunica interna with elastic fibrils, a tunica intermedia represented by a bilayered cell mass, and a tunica externa with collagen fibrils. The exposed ends of the medial cells in the tunica intermedia exhibit small, diffuse bundles of microfilaments that are penetrated by microtubules. These bundles have a circumferential or a slightly oblique orientation relative to the lumen of the vessel. The precise role of the microfilaments is unresolved. If the irregularly shaped bundles are static structures, they might contribute to the non-linear elasticity of the artery. Alternatively, if they generate force, a coordinated contraction of the medial cells might reduce the luminal diameter of the artery and, thus, retard hemolymph flow. Microfilaments of the medial cells anchor to subplasmalemmal filamentous mats, some of which are integral to intermediate junctions and some of which are associated with unbounded cell membranes (hemi-intermediate junctions). Contraction of the microfilament-bearing cells would have to occur without the benefit of nervous innervation or the participation of communicating (gap) junctions. If cell contractility is confirmed, a reclassification of the anterior median artery, from capacitance vessel to resistance vessel, is in order, and the bilayered cell mass in the tunica intermedia would likely qualify as the first unstriated muscle found in crustaceans.