Identification of unstimulated constitutive immunocytes, by enzyme histochemistry, in the coenenchyme of the octocoral Swiftia exserta.

Most animals rely on circulating hemocytes as cellular effectors of immunity. These cells traditionally have been characterized by morphology, function, and cellular contents. Morphological descriptions use granule differences and cell shapes; functional descriptions rely on phagocytic ability and oxygen transport; and cellular content descriptions include cytochemical features and key enzymes. Key enzymes used to identify phagocytes in tissues include hydrolytic enzymes, peroxidase, and--in invertebrates--phenoloxidase. Cnidaria such as Swiftia exserta lack a circulatory system, thereby complicating the identification of immune effector cells. As a first step in identifying immunocytes, this study focused on basic enzymes used during phagocytosis and encapsulation; both processes have been reported in octocorals such as S. exserta. Earlier work suggested that there are two populations of phagocytic cells: a constitutive population and an induced population following a trauma-associated challenge. To identify the constitutive immune effector cells in S. exserta in a nonactivated state, we used cryosections of unstimulated animals and the following enzymes to serve as identifying proxies due to their roles in phagocytosis and encapsulation: (1) acid phosphatase, (2) alkaline phosphatase, (3) non-specific esterase, (4) β-glucuronidase, (5) peroxidase, and (6) phenoloxidase. Our results indicate that in unstimulated animals, two distinct cell populations could function as immunocytes. These cell types were differentiated by their enzyme reactivity and their location within the mesoglea of S. exserta, and have been described as either "oblong granular cells" or "granular amoebocytes."