Introduction

Platelets are small anucleated cell fragments, which are essential components of primary haemostasis. They are derived from the cytoplasmic fragmentation of a giant precursor cell, the megakaryocyte (MK). MKs arise and mature in the bone marrow, along with the other blood cell precursors, e.g. granulocytes and erythroblasts. 150 to 400 × 109/l platelets usually circulate within normal human blood with an average lifespan of approximately 10 days. Therefore one would predict that about 15 × 109 – 40 × 109 platelets have to be produced each day to maintain normal levels. Appropriate platelet production is not only dependent upon a normal rate of thrombopoiesis but also upon the delivery of platelets of the correct size and functionality with normal subcellular organization. Therefore, the qualitative as well as the quantitative aspects of thrombopoiesis will be discussed in this chapter.

Cellular aspect of platelet production

MK localization

Normal human MKs, are mainly located in the bone marrow (Fig. 2.1, see colour plate). They are often gathered into small groups usually up to three, which consist of different cells of various size, ploidy and maturation stages. They are usually located in close proximity to a vascular sinusoid: This is important as MKs and endothelial cells can communicate by many potential pathways: MKs contain mitogenic factors for endothelial cells, e.g. vascular endothelial growth factor (VEGF), whereas endothelial cells express receptors and adhesion molecules which can potentially retain MK close to the bloodstream where future platelets could be delivered.