Occurrence of heparin in the invertebrate Styela plicata (Tunicata) is restricted to cell layers facing the outside environment - An ancient role in defense?

Heparin is an intracellular product of vertebrate mast cell currently used as exogenous anticoagulant. Despite of the potent biological activities of exogenous heparin, its physiological function has not been clearly establis hed yet. Here, a heparin with similar structure and anticoagulant propertie s to the mammalian counterpart was shown to occur as the intracellular prod uct of test cells, a cell monolayer that surrounds egg of the invertebrate Styela plicata (Chordata-Tunicata). As in the case of mammalian mast cells, heparin from the ascidian test cells is removed from the intracellular gra nules after incubation with compound 48/80, Following fertilization, the te st cells surrounding the developing larva still retain heparin as metachrom atic granulation. In the adult invertebrate, heparin occurs as intracellula r granules at the apical tip of epithelial cells surrounding the lumen of b oth intestine and pharynx, in close contact with the external environment. This is the first description of the presence of heparin in cytoplasmic gra nules of epithelial-like cells around the lumen of sites exposed to externa l agents. This arrangement may reflect the participation of heparin in defe nse mechanisms in this invertebrate.