Pulmonary platelet production: a physical analogue of mitosis?

Cellular reduplication is normally achieved by mitosis. In mammals, a system unique to cell biology has evolved that does not utilise this reproductive process. Platelets are derived from the cytoplasm of megakaryocytes but they are not produced by mitosis. At present, both the site and mechanism of platelet production are still debated. This article describes a production mechanism that retains the natural simplicity of binary division. It is predominantly physical. Megakaryocytes leave the bone marrow as whole cells or large cytoplasmic fragments. The cytoplasm of these large circulating cells undergoes a sequence of binary divisions at the dichotomous branches of the pulmonary microcirculation. The lungs trap the majority of megakaryocyte naked nuclei that are removed subsequently by phagocytosis. After the organisation of a microtubular cytoskeleton, the much smaller cytoplasmic fragments leave the pulmonary bed as circulating platelets.