Platelets are produced predominantly by
bone marrow megakaryocytes as a result of budding of the cytoplasmic membrane (www.bmj.com/./809). The precusor
of the megakaryocyte-the megakaryoblast arises by a process of differentiation
from the haemopoetic stem. The magakarycyte matures by edomitotic synchronic
replication, enlarging the cytoplasmic volume as the number of nuclear lobes
increase in multiples of two. Very early on invagination of plasma membrane are
seen, called the demarcation membrane, which evolves through the
development of
the magakarycyte into a highly branched network. At a variable stage in
development, most commonly at the eight nucleus stage, the cytoplasm becomes
granular. Platelets form by fragmentation of magakarycyte cytoplasm,
approximately each magakarycyte giving rise to 1000-5000 platelets (Hoffbrand
et al; 2006).
On average they are 1.5-3um in diameter (Lewis et al; 2008) while (Hoffbrand
et al; 2006) says they are 3.0-0.5um in diameter with a mean volume of
7-11 fl. They do not contain a nucleus and are bounded by a typical lipid
bilayer membrane. Beneath the outer band lies the marginal of microtubules,
which contains the shape of the platelet and depolymerise when aggregation
begins. The central cytoplasm is dominated by the three types of platelet
granules: the dense (δ) granules, a- Granules and lysosomal granules (Lewis
at al 2008). The more frequent specific granules contains a heparin
antagonist PF4, platelet derived growth factor (PDGF), β Thromoboglobulin,
fibrinogen, von willebrand factor and other clotting factors. Dense (δ)
granules are less common and contains adenosine diphosphate (ADP),
adenosine triphosphate (ATP) ,
5-hydropxytrptamine (5-HT), Calcium (Hoffbrand
et al, 2006) while (Lewis et al; 2008) included serotonin, pyrophosphate,P5 Lectin
(CD62), transforming growth factor beta
(I), catecholamine, guanosine diphosphate/guanosine triphosphate (GDP/GTP).
Lysosomes contain hydrolytic enzymes and peroxisomes contain catalase
(hoffbrand et al; 2006).
Once release from the bone marrow, young
platelets are trapped in the spleen for 36 hours before entering the
circulation, where they have a primary
haemostatic role. The platelet membrane has integral glycoproteins essential in the site of
interaction with the plasma initial events of adhesion and aggregation leading
to formation of the platelet plug during haemostatis. (www.bmj.com./../809. The platelet membrane is the site of interaction with the plasma
environment and with the damaged vessel wall. It consists of
phospholipids cholesterol, glycolipids
and at least nine
glycoprotein named glycoprotein I glycoprotein IX. After activation the membrane also expresses binding sites for several
conagulation proteins such as factor
XI and factor VIII (Lewis et al;
2008).