Ultrastructural morphology of CD41^hi and CD41^lo cells. Transmission electron microscopy of CD41^hi cells (A, low magnification; B, high magnification) shows the key features of thrombocytes, including the characteristic surface-connected canalicular system and a cytoplasm packed with numerous small granules. Ultrastructural examination of CD41^lo cells (C, low magnification; D, high magnification) shows an immature phenotype. Cells were generally homogeneous in appearance showing an irregular shape with small protrusions. Ribosomes and mitochondria were observed in the rim of cytoplasm surrounding the nucleus. However, these cells lacked the numerous granules observed in the more mature CD41^hi thrombocytes. The nuclei were more irregular in shape and a prominent band of heterochromatin at the margin of the nucleus was observed in the majority of cells. The heterochromatin appears more fragmented through the nucleus and less marked at the margin. Scale: A,C: bar = 2 μm; B,D: bar = 1 μm.