Model of ontogenic regulation of MK morphogenesis. In most cells, most P-TEFb is sequestered in an inactive state within the 7SK small nuclear ribonucleoprotein (snRNP) complex. In adult megakaryopoiesis (left arrow), downregulation of LARP7 and proteolysis of MePCE destabilize 7SK snRNA, leading to unopposed P-TEFb activation. This mode of P-TEFb activation promotes upregulation of MK morphogenesis factors and repression of erythroid markers. In fetal megakaryopoiesis (right arrow), IGF2BP3 stabilizes 7SK snRNA despite the downregulation of LARP7 and proteolysis of MePCE. Persistence of 7SK allows for inhibition of P-TEFb, dampening both the upregulation of MK morphogenesis factors and lineage consolidation via erythroid repression. Professional illustration by Somersault 18:24.