Figure 1.

Role of NGP and cytoskeleton in platelet formation. Megakaryocytes (MKs) are differentiated from hematopoietic stem cells (HSCs), and their maturation is marked by the polyploidized nucleus and the formation of an invaginated membrane system. Mature MKs extend cytoplasmic branches, named proplatelets, into sinusoidal microvessels in the bone marrow and shed platelets. In this process, microtubules are fundamental for proplatelet protrusion and elongation. Actin polymerization plays an important role in the dynamic bending and branching of proplatelets. Actomyosin provides mechanical force for proplatelet segmental contraction and intermediate narrowing, by which the size of platelets is limited. The NGP known as Sema7A has been shown to regulate platelet formation by inhibiting MKs differentiation. This is the only known NGP with this capability.