5. Platelets The platelets of mammals are small, anucleated ‘cells’ present in the peripheral blood at concentrations of approximately 150–400 × 109/L. The term ‘thrombocyte’ has also been used to identify these cells, but is more appropriately applied to the nucleated haemostatic cells of birds and reptiles. Platelets are formed by fragmentation of the cytoplasm of large, multinucleated cells (megakaryocytes) present in haematopoietic tissue. In the peripheral blood, platelets have an important role in effecting haemostasis. An inadequate number or impaired function of platelets may result in haemorrhagic diathesis and subsequent blood loss. This chapter describes the general aspects of mammalian platelet structure and function, and the physiological and pathological mechanisms that may affect platelet concentration and function. Platelet structure and composition When viewed by light microscopy, platelets are typically irregularly disc-shaped, 2–4 µm in diameter and stain a neutral to pale basophilic colour with Romanowsky stains. One or more punctate basophilic granules may be observed. The morphology of platelets is similar for most species, although some variation in the shape and size may occur. The morphology of the platelets from monotremes has been studied, with the short-beaked echidna reported to have large, spindle-shaped platelets (Hawkey, 1975) and filamentous platelets (Canfield, 1998), whereas the platelets of the platypus have been described as round to ovoid in shape (Canfield and Whittington, 1983). The author has observed occasional filamentous or crescent-shaped platelets, concomitant with disc-shaped platelets, in a wide range of Australian mammals (Plate 56). Large platelets, referred to as macro-or mega-platelets (Plate 57), may be occasionally observed in clinically healthy animals and are also observed in increased numbers when platelet production is increased. Platelets may also be morphologically characterised by their volume. The mean platelet volume (MPV) may be measured by some automated haematology analysers and used to quantify the proportion of ‘large’ platelets. The MPV was determined to be 7.2 ± 0.35 fL for red-legged pademelons (Agar and Spencer, 1993b) and 4.0–6.4 fL (n = 12) for Parma wallabies (Clark et al., unpublished data). When examined by electron microscopy, platelets are delineated by an outer membrane and contain several cytoplasmic organelles, including filaments, microtubules, alpha-granules, ‘dense bodies’, lysosomes, mitochondria and glycogen granules (Jain, 1986; Tablin, 2000). The platelet membrane is composed of a glycocalyx, plasma membrane and, in some species, invaginations referred to as the ‘open canalicular system’ (OCS). A dense tubular system of membrane may be evident throughout the cytoplasm. Filaments and microtubules are usually present at the periphery and give the platelet its contractile ability. Glycogen, evident as small, moderately electron-dense granules, is distributed throughout the cytoplasm. Moderately electron-dense alpha-granules and electron-dense ‘dense bodies’, mitochondria, and lysosomes may be observed throughout the cytoplasm (Plate 58). The ultrastructure of platelets from the platypus has been described (Canfield and Whittington, 1983). They are round to elongated and have organelles that include microtubules, glycogen granules, alpha-granules and small mitochondria. Platelets contain a large number of substances required for haemostasis (Jain, 1986; Gentry, 2000a; Tablin, 2000). The platelet membrane contains a range of glycoproteins and phospholipids and notably provides a source of arachidonic acid. Alpha-granules contain coagulation factors, growth factors, glycoproteins and platelet-specific proteins, such as platelet factor 4 and beta-thromboglobulin. Dense granules contain nucleotide triphosphates (such as ATP), histamine, serotonin, calcium and magnesium ions and glycoproteins (such as GP IIb–IIIa). Glycogen granules provide a source of energy, although platelets are capable of active uptake of glucose. The dense tubular system provides a reservoir of calcium and the lysosomes contain acid hydrolases. There have been very few studies undertaken to determine the substances present in the platelets of Australian native mammals. Platelets from the quokka were found to have a low concentration of histamine (Lynch and Turner, 1975). The lipid compositions of platelets from the killer whale (Patterson et al., 1998) and southern elephant seal (Fayolle et al., 2000) have been reported. Production of platelets The platelets of mammals are produced from the cytoplasm of megakaryocytes. Megakaryocytes are large (50–200 µm in diameter) multinucleated cells, with ovoid nuclei composed of fine to reticular and generally inconspicuous nucleoli, and moderate to large amounts of granular, basophilic to amphophilic cytoplasm. In most mammals, megakaryocytes are predominantly located within the bone marrow, but may also be observed in the spleen, particularly in murids (Long and Williams, 1982; Tanaka et al., 1988). Megakaryocytes have been observed in the bone marrow of the quokka (Lewis et al., 1968) and the koala (Spencer and Canfield, 1995). Interestingly, megakaryocytes have been observed in the spleen, but not the bone marrow, of a platypus (Tanaka et al., 1988). The differentiation of megakaryocytes from haematopoietic stem cells and the production of platelets is regulated by a complex interaction of many regulatory factors, most notably thrombopoietin (Norol et al., 1998; Kaushansky, 1999; Shivdasani, 2001). The developmental stages of megakaryocytes are further discussed in Chapter 6. Platelets are formed by the fragmentation of the cytoplasm of megakaryocytes. Once released into the peripheral blood, the average lifespan of a platelet is 5–7 days in the dog and 5 days in the rat (Jain, 1993). There have not been any published studies that have determined the lifespan of platelets from any species of Australian native mammal. Function of platelets The biology of platelets is complex (Gentry, 2000b
INTRODUCTION
CHARACTERISTICS OF PLATELETS
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