Arthropod immune system: I. Comparative light and electron microscopic accounts of immunocytes and other hemocytes of Blattella germanica (Dictyoptera: Blattellidae).

Abstract

Light microscopy (LM) and scanning (SEM) and transmission electron microscopy (TEM) disclose seven types of hemocytes in adults of the German cockroach, Blattella germanica (Dictyoptera: Blattellidae)-namely, prohemocytes (PR), granulocytes (GR), plasmatocytes (PL), spherulocytes (SP), adipohemocytes (AD), coagulocytes (CO), and oenocytoids (OE). Their shape, size, surface appearance, and internal ultrastructure are characterized here. The shapes and relative sizes of hemocytes noted in vitro have been compared with those flowing through the wing veins (in vivo). The GRs constitute more than 90% of the hemocyte population and have roles in the defense system such as encapsulation and phagocytosis. They are characterized by numerous cytoplasmic granules and marginal microtubular bundles, which are commonly noted in the blood cells of both vertebrates and invertebrates, but have been rarely reported in insect hemocytes. The PRs are characterized by being spherical and small, having a relatively large nucleus and a small area of cytoplasm. The polymorphic PLs have some micropapillae on the cell surface and fewer granules than the GRs. The SPs are characterized by an oblate shape, raspberrylike appearance in SEM, and many spherules in the cytoplasm. The ADs are spherical and have large, refringent fat droplets. The COs possess a distinctive cartwheellike appearance when observed by LM but resemble GRs in TEM preparations. The OEs are opaque and often have an eccentric nucleus. Of the seven types of hemocytes, all, except the COs, can be recognized both in vivo and in vitro by LM; the COs can be recognized only in vitro. SEM does not allow the COs to be distinguished from the GRs, except in samples in which coagulation has occurred. All hemocyte types can be recognized by TEM. Thus, all three types of microscopy allow the various types of hemocytes to be recognized; unequivocal discrimination among the cells is best achieved by combining all the three kinds of microscopy.