Three types of blood cells have been identified in Drosophila: lamellocytes, plas-matocytes, and crystal cells. Lamellocytes are usually observed in Drosophila only if the organism has certain mutations or is encapsulating foreign material within the organism (Rizki and Rizki 1992). During the encapsulation response, lamellocytes differentiate in the lymph glands and then migrate to the site of encapsulation, where they flatten and spread to create a barrier around the encapsulated material. This immune response is dependent on the lamellocytes. Researchers have identified a lineage of Drosophila that naturally lacks lamellocytes, and the larvae are unable to encapsulate parasitic wasp eggs (Eslin and Doury 2006). Plasmatocytes comprise the vast majority (~95%) of hemocytes in both larvae and adults. These hemocytes recognize and phagocytose invading microorganisms. They appear early in embryonic development and multiply in the larval stages. Plasmatocytes circulate throughout the animal in order to detect pathogens and activate the immune response when necessary. At the end of the third larval instar, the larval lymph glands release additional blood cells into the hemolymph. Many embryonic and larval hemocytes persist through metamorphosis to eventually become a population of sessile cells clustered around the dorsal blood vessel of the adult fly (Holz et al. 2003). Despite the change in localization, adult plasmatocytes carry out the same macrophage-like functions as larval hemocytes; it has been shown that both larval and adult blood cells are capable of effectively phagocytosing a variety of pathogens in less than thirty minutes (Fig. 1; Elrod-Erickson et al. 2000). Crystal cells are a small proportion of hemocytes, and most (~95%) do not circulate (Carton et al. 1986). These cells release

Fig. 1 Drosophila hemocytes act as macrophages. FITC-conjugated E. coli particles are phago-cytosed by sessile hemocytes along the adult dorsal blood vessel (A) and by circulating hemocytes in larvae (B). Nuclei in (B) are stained with Hoechst 33258

reactive oxygen molecules and are involved in melanization (Rizki and Rizki 1959), which is not covered in this chapter. This chapter discusses encapsulation and phagocytosis.

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