Morphology of greater rhea (Rhea americana) blood cells using light and scanning electron microscopy

Abstract

The greater rhea (Rhea americana) is the largest bird native to Brazil and is classified as near threatened with extinction. Hematology is an essential tool for assessing the health status of birds. However, detailed information on the blood cell morphology of this species is still scarce. In this context, this study aimed to describe the morphology of greater rhea blood cells using light microscopy and scanning electron microscopy (SEM). We used 48 healthy adult specimens of both sexes kept in captivity in the Brazilian semiarid. Blood smears were prepared and stained using the Giemsa method for light microscopy analysis, and additional samples were processed for SEM. Elliptical nucleated erythrocytes, heterophiles, eosinophils, basophils, monocytes, lymphocytes, and thrombocytes were identified in light microscopy. Their morphological characteristics were generally compatible with those described for other birds and ratites. The high frequency of heterophiles and the predominantly elliptical shape of thrombocytes, a characteristic described as species-specific, stood out. SEM enabled detailed visualization of the surface architecture of blood cells, revealing irregularities, roughness, granulations, and cytoplasmic projections that are not observable by light microscopy. Greater rhea blood presents all major cell types, including red blood cells, monocytes, lymphocytes, eosinophils, heterophils, basophils, and thrombocytes. Although SEM is not applicable in routine clinical practice, its findings broaden the morphofunctional understanding of blood cells and provide important insights for comparative, physiological, and conservation studies involving other bird species.