Coelomocytes: Biology and Possible Immune Functions in Invertebrates with Special Remarks on Nematodes

Hindawi Publishing Corporation International Journal of Zoology Volume 2009, Article ID 218197, 13 pages doi:10.1155/2009/218197 Review Article Coelomocytes: Biology and Possible Immune Functions in Invertebrates with Special Remarks on Nematodes Qudsia Tahseen Nematode Research Laboratory, Department of Zoology, Aligarh Muslim University, Aligarh 202002, India Correspondence should be addressed to Qudsia Tahseen, Received 4 June 2008; Revised 22 September 2008; Accepted 6 November 2008 Recommended by Gregory Demas All metazoans are exposed to a wide range of microbes and have evolved complex immune defenses used to repel infectious agents. Coelomocytes play a key role in the defense reactions of most invertebrates. They are involved in important immune functions, such as phagocytosis, encapsulation, graft rejection, and inflammation, as well as the synthesis and secretion of several humoral factors especially in annelids and echinoderms. Coelomocytes in nematodes are variable in shapes from round, ovoid, cuboidal, and spindle-shaped to stellate or branched cells that are found usually at fixed positions in the pseudocoelom. Their number usually varies from 2 to 6. The model nematode, C. elegans lacks an adaptive immune system and the coelomocytes are capable of endocytosis, but their involvement in phagocytosis of bacteria seems unlikely. The aim of this review is to evaluate current knowledge on coelomocytes of invertebrates with special reference to nematodes. The morphology and structure of these coelomocytes are discussed along with their origin. Their relative positions and diversity in different nematode groups have also been discussed and illustrated. Copyright © 2009 Qudsia Tahseen. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. 1. Introduction Invertebrate organisms have developed a variety of defense reactions to fight invading foreign agents. The invertebrates possess nonadaptive, innate, nonclonal, nonanticipatory immune responses contrary to vertebrate responses which are induced, adaptive, acquired, clonal, and anticipatory [1]. Invertebrates do not possess the immunoglobulins found in higher animals, although proteins containing immunoglobulin-like domains have been identified [2]. The common defense mechanisms used by most invertebrates to protect themselves against infectious agents are the synthesis and secretion of antibacterial and antifungal proteins, agglutination and nodule formation, encapsulation of foreign objects, and phagocytosis. During defense reactions, invariably the foreign organisms are found to be encapsulated and melanized and enzymes (i.e., phenol oxidase) play a vital role in defense reactions. Among the lower forms, protists are the prototypes of macrophages; sponges distinguish between self and nonself; cnidarians have the phagocytes and cnidoblasts while some species are also provided with C3-like molecules. Careful study of the phylogeny of the immune system has revealed the evolution of three important components, namely, the macrophage, lymphatic, and hematopoietic systems [3]. The most ancient is the macrophage system (largely found in invertebrates) that arises in the coelomic cavity as mesenchymal amoeboid cells or coelomocytes for recognition of self from nonself and for ingestion of foreign particles. The lymphatic system in higher animals develops from the endoderm of pharyngeal pouches, while the hematopoietic system originates from the splanchnic mesoderm of the yolk sac as hematogenic tissue, containing hemangioblasts. Coelomocytes n. pl. (Gr. koilos, hollow; kytos, container) are cells that tend to be obscure but are apparently omnipresent in most coelomates. The nonmuscle macrophage-like cells inhabit the body cavity or the coelomic spaces of many invertebrates. In annelids, the cells found in coelom are categorized as coelomocytes, chloragogen cells (eleocytes), and haemocytes [4]. The molluscs such as gastropods have haemocytes in the body cavity [5]. The coelomic cells are also referred to as haemocytes in most arthropods [6]. In echinoderms, the coelomocytes occupy perivisceral coelomic cavities, the water-vascular system, and the haemal system besides the connective tissue and tissues of various organs [7, 8]. Nematodes possess mesenchymatous 2 coelomocytes in their pseudocoelom, adjacent to the gonads or other internal organs in the anterior or posterior body regions. These cells were assumed to be phagocytic to purify the body fluid and, therefore, attributed different functions by different workers and assigned different names such as amoebocytes, elaeocytes, athrocytes, and phagocytes. Due to their small size and relatively lesser number, these cells were largely ignored in nematodes particularly in the early developmental stages [9]. The ability of nematodes to osmoregulate varies considerably; free-living forms which are exposed to wide variations in osmotic pressure are extremely efficient osmoregulators; the parasitic forms, on the other hand, may have relatively limited osmoregulatory capacity. In animal parasitic nematodes, the stellate pseudocoelomocytes have been suggested to maintain the pseudocoelomic fluid, either as phagocytic cells removing bacteria and other pathogens or collecting certain xenobiotics molecules and effectively removing them from the coelomic fluid or as further playing a role in haem metabolism [10–12]. Although the function of nematode pseudocoelomocytes (=coelomocytes) was not precisely known for a long time, yet the coelomocytes were frequently studied. Coelomocytes were first observed and reported in the form of four stellate cells in Parascaris equorum in anterior third region of the body by Bojanus [13]. Later Bugnion [14] and von Linstow [15] regarded them as blood corpuscles of nematodes. Jägerskiöld [16–18], Nassonov [19, 20], and Shipley [21] also observed and referred to such cells in their studies. Rauther [22] described them to be fixed in position and attached to the body wall by fine processes. They were also reported to be present in oxyurids and ascarids by Martini [23, 24] and Höeppli [25]. Stefanski [26] found them existing between the base of pharynx and anterior end of ovary or testis in Rhabditella axei while B.-G. Chitwood and M. B. Chitwood [27] reported two binucleated “X” bodies in Cephalobellus papilliger. B. G. Chitwood and M. B. Chitwood [28], Weinstein [11, 29], Douvres et al. [30], Peregrine [12], Boghen and Davey [31], Ishikawa [32], Poinar and Jansson [33], A. F. Bird and J. Bird [34] described coelomocytes in free-living and animal parasitic nematodes. 2. Evaluation of Morphology of Coelomocytes 2.1. Shape. The pseudocoelomic body cavity of the rotifer Asplanchna spp. contains free cells (coelomocytes) that form a highly dynamic, three-dimensional polygonal network of filo (...truncated)