The Circulatory System

Cystocyte

Spherule cell

Adipohemocyte

FIGURE 17.6. Different types ofhemocytes. [After R. F. Chapman, 1971, The Insects: Structure and Function. By permission of Elsevier/North-Holland, Inc., and the author.]

Spherule cell

Adipohemocyte

FIGURE 17.6. Different types ofhemocytes. [After R. F. Chapman, 1971, The Insects: Structure and Function. By permission of Elsevier/North-Holland, Inc., and the author.]

Though several types of hemocytes have been recognized, which differ in size, stain-ability, function, and cytology (including fine structure) (Figure 17.6), their classification and relationships have proven difficult. This difficulty stems partly from the natural structural variability and multifunctional nature of some hemocytes both within and among species, and partly from the differences in methodology and criteria used to distinguish different hemocytes (Arnold, 1974; Crossley, 1975; Jones, 1977). Notwithstanding these difficulties, it is apparent that three types of hemocytes are common to almost all insects, though one or more additional types may also occur in a given species. In this account, the scheme of Arnold (1974) is followed.

The three types common to most insects are prohemocytes, plasmatocytes, and granular hemocytes (granulocytes). Prohemocytes (stem cells) are small (10 |am or less in diameter), spherical, or ellipsoidal cells whose nucleus fills almost the entire cytoplasm. They are frequently seen undergoing mitosis and are assumed to be the primary source of new hemocytes and the type from which other forms differentiate. Plasmatocytes (phagocytes) are cells of variable shape and size, with a centrally placed, spherical nucleus surrounded by well vacuolated cytoplasm. In the cytoplasm are a well-developed Golgi complex and endoplasmic reticulum, as well as many lysosomes. The cells are capable of amoeboid movement and are phagocytic. Granulocytes are usually round or disc-shaped, with a relatively small nucleus surrounded by cytoplasm filled with prominent granules. In some species they are amoeboid and phagocytic which, together with the occurrence of intermediate forms, suggests that they may be derived from plasmatocytes. More often, they are non-motile and appear to be involved in intermediary metabolism.

Other types of hemocytes include adipohemocytes, oenocytoids, spherule cells, and cystocytes. As their name indicates, the adipohemocytes are cells whose cytoplasm normally contains droplets of lipid. In addition to lipid droplets, the cytoplasm may have non-lipid

526 vacuoles and granules that contain carbohydrate material. The cells, which occasionally are phagocytic, are considered by some authors to be a form of granulocyte. Oenocytoids are spherical or ovoid cells with one, occasionally two, relatively small, eccentric nuclei. They are almost never phagocytic, as the absence of lysosomes, small Golgi complexes, and poorly developed endoplasmic reticulum attest. The oenocytoids are fragile cells that easily lyse, leading some authors to suggest that they may be a type of cystocyte. Spherule cells are readily identifiable cells whose central nucleus is often obscured by the mass of dense spherical inclusions occupying most of the cytoplasm. Though reported from a range of orders, they are especially common in higher Diptera and Lepidoptera, and a variety of functions have been proposed for them, including phagocytosis, uptake and transport of materials, synthesis of some blood proteins, and a role in bacterial immunity. Antibody-binding studies support the view that spherule cells may be a form of granulocyte (Gardiner and Strand, 1999). Cystocytes (coagulocytes) are spherical cells in whose small central nucleus the chromatin is so arranged as to give the nucleus a "cartwheel-like" appearance. The cytoplasm contains granules that, when liberated from these fragile cells, cause the surrounding plasma to precipitate. Thus, the cells, which are again a specialized kind of granulocyte, play a major role in hemolymph coagulation.

4.2.2. Functions

The major functions of hemocytes are endocytosis, nodule formation, encapsulation, and coagulation. For the first three of these a key element is the ability to distinguish between foreign (including altered self) and self. Hemocytes probably also have a variety of metabolic and homeostatic functions.

Endocytosis. This is a process whereby a cell plasma membrane folds around a substance that is thus ingested by the cell without rupture of the membrane. It includes pinocytosis ("cell drinking") in which the material ingested is in solution and phagocytosis where the material engulfed is particulate. Once it reaches the interior of the cell, the membrane-bound vesicle containing the substance fuses with enzyme-containing lysosomes and degradation of the substance ensues. In insects the primary phagocytic cells are plasmatocytes, though, as noted above, other types of hemocytes may also engulf material. Like other endocytotic cells, hemocytes are apparently selective with regard to what they ingest though the basis of this selectivity is not well known.

Endocytosis, especially phagocytosis, is important in both metamorphosis and defense against disease, as well as in routine cleaning up of dead or damaged cells. In some insects (including Lepidoptera and higher Diptera), during metamorphosis, phagocytic hemocytes invade many larval tissues and bring about their rapid histolysis. However, invasion does not occur (at least in muscle which has been well studied) until autolysis has begun in tissue cells. Autolysis is believed to be under hormonal control and presumably makes tissue appear foreign to the hemocytes. In other insects (including Coleoptera and mosquitoes) the hemocytes never invade larval tissues during metamorphosis, and the tissues disappear strictly by autolysis. A variety of microorganisms (viruses, bacteria, fungi, rickettsias, and protozoa) are known to be phagocytosed by hemocytes, and in most instances insects have excellent resistance to infection as a result. However, for some viruses, phagocytosis is not followed by digestion and therefore does not prevent infection. Rather, the virus uses this process as a means of entering a cell prior to replication. Furthermore, habitual protozoan and fungal parasites often escape phagocytosis, presumably because they are not recognized as foreign by the hemocytes.

Beekeeping for Beginners

Beekeeping for Beginners

The information in this book is useful to anyone wanting to start beekeeping as a hobby or a business. It was written for beginners. Those who have never looked into beekeeping, may not understand the meaning of the terminology used by people in the industry. We have tried to overcome the problem by giving explanations. We want you to be able to use this book as a guide in to beekeeping.

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