Proposed model of DMS formation and expansion. DMS formation initiates at focal areas on the cell surface. During subsequent MK maturation, a distinct pre-DMS structure is formed in the perinuclear region, through a PM invagination process that resembles cleavage furrow formation. Further expansion requires vesicular membrane delivery from multiple Golgi complexes that are targeted to the developing DMS and possibly via direct lipid transfer through ER-DMS tethering.