Defence Mechanisms and Stem Cells in Holothuria polii and Sipunculus nudus

Abstract

Holothuria polii (Phylum Echinodermata) and Sipunculus nudus (Phylum Sipuncula) have been used as model systems for studying invertebrate immunity. In the coelomic fluid of Holothuria polii, coelomocytes (i.e. amoebocytes, Type I, Type II, Type III spherule cells) and stem cells are present. After an injection of formalin-fixed sheep erythrocytes into the coelomic cavity, structural modifications in the animal’s, such as the coelomic cavity, stone canal and periesophageal ring were observed. Cellular events observed within the coelomic cavity include the recruitment of a large number of coelomocytes from the haemopoietic areas, phagocytosis carried out by amoebocytes and the formation of brown masses. The stone canal is not an immunocompetent organ but it is involved in the production and activity of the amoebocytes. The periesophageal ring is an organ which produces spherule cells. Haemerythrocytes, urn cell complexes, brown bodies, Type I and Type II granulocytes and the lamine of connective tissue are all distinguishable in the fluid coelomic of Sipunculus nudus. Clusters of Type II granulocytes at various differentiation stages are also present. The consequences of a cutaneous wound in Holuthuria polii and Spinuculus nudus and the cells involved in healing were studied. Type I granulocytes in Sipunculus nudus are capable of extracellular digestion and they are immunoreactive to antibodies directed against IL-4, IL-10 and EGF. The results obtained regarding the healing of the wound, the presence and location of stem cells are comparable to the results obtained from vertebrates.