The hemocytes of Polyandrocarpa misakiensis, with particular reference to immunocytes.

Polyandrocarpa misakiensis is a polystyelid compound ascidian, common along the coasts of the temperate regions of Japan, which can reproduce asexually through continuous budding from parental zooids. The hemolymph of this species contains the following main cell-types: undifferentiated cells, phagocytes, including both hyaline amebocytes and macrophage-like cells, microgranular leukocytes, macrogranular leukocytes, morula cells and pigment cells. Immunocytes are represented by phagocytes and morula cells. Phagocytes are capable of constitutive macropinocytosis, can release lectins and ingest macrogranular leukocytes in order to provide nutrients required for growth of developing buds. Like other compound ascidians, Polyandrocarpa morula cells (MCs) represent one of the most abundant circulating hemocyte types. They share the presence of DOPA-oxidase activity inside their vacuoles, ascribable to the presence of phenoloxidase, a key enzyme in invertebrate immune responses, which is released in extracellular compartment upon the recognition of foreign cells or molecules thus inducing a cytotoxic response. This enzyme can be considered, in all respects, a differentiation marker of morula cells as no other cell types show similar enzyme activity. Recently, we were able to clone a cDNA sequence of 1985 bp, representing most of the PO transcript. It shows homology with arthropod hemocyanins and, through in situ hybridization, we demonstrated that MC are the only cells expressing the corresponding mRNA.