An ultrastructural study on the platelets from peripheral blood samples from 20 healthy adult Arabian oryx (Oryx leucocoryx) was performed. Characteristic was the extreme polymorphism of both the platelets, as well as their alpha granules. They vary in size from 100 to 800 nm in diameter and their numbers typically are less than those reported for humans and other animal species. Also, the alpha granules in contrast to those of humans and animals, such as the Arabian tahr, do not have nucleoids.Typically, the oryx platelets exhibit only 1-2 electron-dense bodies per cell and they lack an open canalicular system. Of particular interest is the presence of Type I multivesicular bodies, which have previously only been described in human megakaryocytes and are hypothesized as being intermediate development stages of alpha and dense granules. K E Y W O R D S oryx, platelets, ultrastructure | 245 AL-AMRI et AL.

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An ultrastructural study on the lymphocytes from peripheral blood samples from 20 healthy adult Arabian oryx (Oryx leucocoryx) was undertaken. Small lymphocytes ranged in size from 2–3.5 µm and exhibited the typical repertoire of organelles found in small lymphocytes of other animals but showed no evidence of azurophilic granules. Medium‐sized lymphocytes were 5–6.5 µm in diameter and occasionally demonstrated azurophilic granules. Microvilli were a common finding of lymphocytes. Of particular interest was the presence of multivesicular bodies, which have previously only been described in human lymphocytes. Both small‐ and medium‐sized lymphocytes of the Arabian oryx were smaller than those reported for other animals. Small lymphocytes exhibited short, thick microvilli, whereas medium‐sized lymphocytes had long thin microvilli, a single nucleolus and occasionally azurophilic granules and multivesicular bodies.