The circulating cells in hemolymph are called hemocytes. Insects lack erythrocytes, and hemocytes cannot be directly equated with vertebrate leukocytes. Some fraction of hemocytes remains sessile and attached to the surfaces of tissues, and in some species (mosquitoes, for example) such cells may account for a majority of the hemocytes. Several different morphological types of hemocytes can be identified in each insect species. Some commonly observed hemocyte types are illustrated in Fig. 1. Prohemocytes are small, round cells that may be precursors from which some other cell types develop. Granular hemocytes contain conspicuous cytoplasmic granules that can be discharged as part of a defensive response

FIGURE 1 Examples of hemocyte types from a lepidopteran, Manduca sexta. (A) Plasmatocytes. The plasmatocyte shown on the left has just begun to spread, whereas the one on the right has spread extensively. (B) Granulocytes. (C) Oenocytoids. (D) Spherulocytes.

to invading parasites. Plasmatocytes usually contain few granules and are characterized by their ability to change from round or spindle-shaped cells in suspension to extensively flattened, ameboid cells after attaching to a substrate. Spherule cells contain very large cytoplasmic granules, which may contain mucopolysaccharides. Oenocytoids are large cells that synthesize prophenoloxidase.

Plasmatocytes and granular hemocytes are usually the two most abundant hemocyte types, although their proportions can vary between species and within a species at different developmental stages. These two hemocyte types participate in immune responses, including: (1) phagocytosis of small organisms such as bacteria; (2) nodule formation, in which multiple hemocytes aggregate to trap microorganisms; and (3) encapsulation, in which hemocytes attach to the surface of a larger parasite and form a multilayered hemocyte capsule, in which the parasite is killed. Nodules and capsules often become melanized through the action of phenoloxidase. Hemocytes, especially plasmatocytes, also aggregate in a type of coagulation response, sealing wounds to prevent hemolymph loss. Another function of hemocytes is in synthesis of the extracellular matrix that covers tissues exposed to the hemolymph. Granular hemocytes appear to be the primary cell type involved in this aspect of hemocyte function.

See Also the Following Articles

Circulatory System • Fat Body • Immunology • Vitellogenesis • Water and Ion Balance, Hormonal Control of

Further Reading

Gillespie, J. P., Kanost, M. R., and Trenczek, T. (1997). Biological mediators of insect immunity. Annu. Rev. Entomol. 42, 611—643. Gupta, A. P. (1985). Cellular elements in the hemolymph. In "Comprehensive Insect Physiology, Biochemistry, and Pharmacology" (G. A. Kerkut and L. I. Gilbert, eds.), Vol. 3. Pergamon, New York. Haunerland, N. H. (1996). Insect storage proteins: Gene families and receptors. Insect Biochem. Mol. Biol. 26, 755—765. Kanost, M. R., Kawooya, J. K., Law, J. H., Ryan, R. O., Van Heusden, M. C., and Ziegler, R. (1990). Insect haemolymph proteins. Adv. Insect Physiol 22, 299-396. Lackie, A. M. (1988). Haemocyte behaviour. Adv. Insect Physiol. 21, 85-178.

Mullins, D. E. (1985). Chemistry and physiology of the hemolymph. In "Comprehensive Insect Physiology, Biochemistry, and Pharmacology" (G. A. Kerkut and L. I. Gilbert, eds.), Vol. 3. Pergamon, New York. Ryan, R. O., and van der Horst, D. J. (2000). Lipid transport biochemistry and its role in energy production. Annu. Rev Entomol. 45, 233-260. Telfer, W. H., and Kunkel, J. G. (1991). The function and evolution of insect storage hexamers. Annu. Rev. Entomol. 36, 205-228. Wyatt, G. R. (1961). The biochemistry of insect hemolymph. Annu. Rev. Entomol. 6, 75-102.

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