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Structure and Function of Leukocytes in the Family Macropodidae

Hulme-Moir, Karen Lisa (2007) Structure and Function of Leukocytes in the Family Macropodidae. PhD thesis, Murdoch University.

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Leukocytes play a central role in protecting the body against infectious organisms and their research is essential for understanding the mechanisms of immunity. By studying leukocytes across a range of species, insights are provided into differing strategies employed to ensure resistance to disease. Surprisingly, the structure and function of marsupial leukocytes has received very limited study. Marsupials represent a major evolutionary pathway with distinct differences in reproduction and development from placental mammals. These differences in the life history of marsupials place unique challenges on the immune system, and differences in leukocyte structure and function could be reasonably expected. In this thesis, studies were undertaken to examine the cytochemical, ultrastructural and functional features of leukocytes from species of marsupials, belonging to the family Macropodidae (kangaroos and wallabies). The aim of these studies was to elucidate the characteristics of macropodid leukocytes and to compare and contrast these features with the known characteristics of other mammalian leukocytes.

Leukocytes from two species of macropodid, the tammar wallaby (Macropus eugenii) and the western grey kangaroo (Macropus fuliginosis), formed the basis of this study with additional material provided from quokka (Setonix brachyurus), woylie (Bettongia pencillata) and red kangaroo (Macropus rufus). Staining characteristics of cells were examined following reaction with Sudan black B, peroxidase, chloroacetate esterase, naphthyl butyrate esterase, alkaline phosphatase and periodic acid-Schiff. Peroxidase and Sudan Black B reactions were similar to domestic animal species but chloroacetate esterase and naphthyl butyrate esterase were unreliable as markers for macropodid neutrophils and monocytes, respectively. Significant variation in staining for alkaline phosphatase was seen between species of macropodid. Tammar wallabies and quokka demonstrated strong neutrophil alkaline phosphatase activity whereas western grey kangaroos, red kangaroos and woylies contained no activity within their leukocytes.
Peroxidase and alkaline phosphatase cytochemistry were also assessed at the ultrastructural level with transmission electron microscopy. This allowed the identification of distinct granule populations within macropodid neutrophils. Two subcellular compartments containing alkaline phosphatase activity were identified within tammar wallaby neutrophils. These were considered equivalent to secretory vesicles and a subpopulation of specific granules. Tubular vesicles containing alkaline phosphatase were also identified within the eosinophils of tammar wallabies. These structures were a novel finding having not been reported previously in the eosinophils of other animal species.

In addition to cytochemistry, the general ultrastructure of leukocytes from tammar wallabies and western grey kangaroos were reported. Results were similar to previous reports for other marsupial species. The current body of knowledge was extended by the first detailed description of the ultrastructure of basophils in a marsupial.

To assess functional aspects of macropdid neutrophils, flow cytometric assays were performed examining oxidative burst responses and phagocytosis. Reactive oxygen species were generated by neutrophils from tammar wallabies and western grey kangaroos in response to phorbol 12-myristate 13-acetate but not N-formyl-Met-Leu-Phe or opsonised bacteria. Phagocytosis of opsonised bacteria was also measured in neutrophils from tammar wallabies, which was poor in contrast to ovine neutrophils. However, flow cytometric studies were limited by sample preparation. Further optimisation of isolation methods for tammar wallaby leukocytes should be undertaken before dogmatic conclusions are drawn.
Overall, the results of this thesis demonstrate that, in the areas examined, the general characteristics of leukocyte structure and function of mammals are present in macropodids. However differences were identified both within and outside of the macropodid group. These differences have important ramifications for the use of ‘model’ species in the study of leukocyte biology in marsupials. The results also provide potentially useful tools for the clinical diagnosis of haematological disease in macropodids and may be of interest to those studying comparative and evolutionary aspects of leukocyte structure and function.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): School of Veterinary and Biomedical Sciences
Supervisor(s): Clark, Phillip
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