Emperipolesis of hematopoietic cells within megakaryocytes in bone marrow of the rat

Emperipolesis of hematopoietic cells within megakaryocytes was found in rats. The incidence was less than 0.3% in young rats (2 to 12 months old) but increased to 2-5% among the aging rats (18 to 24 months old). The incidence increased markedly in hyperplastic bone marrow secondary to chronic suppurative or neoplastic lesions. Mature neutrophils appeared to be the most common marrow cell engulfed by megakaryocytes. By light microscopy, engulfed cells were separated from megakaryocyte cytoplasm by a narrow pericellular space. By electron microscopy, marrow cells engulfed by megakaryocytes were located in the open canalicular system. Cell membranes of both engulfed cells and megakaryocytes were intact, and there was no fusion of cell membranes or phagosome formation in the megakaryocytes.