Maturation and amplification of the thrombocytic cells is achieved by a sequence of cells which are termed megakaryocyte progenitors, promegakaryoblasts and megakaryoblasts and finally promegakaryocytes and megakaryocytes. Amplification in these cells follows distinct mechanisms. Cellular multiplication occurs only at the progenitor stage. In promegakaryoblasts and megakaryoblasts a second amplification process does not yield any increase in cell number but rather consists of a series of endoduplications resulting in a polyploid cell whose nuclear and cytoplasmic volume are roughly proportional to ploidy level. Finally arrest of DNA synthesis and endoduplications marks the transition to the megakaryocyte stage. The present paper will briefly review the physiology of these cells.