Structure
- Discs ~2\(\mu\)m diameter
- No nucleus. Have mitochondria, endoplasmic reticulum, and golgi body
- Cell membrane with receptors (e.g. for Thrombin, ADP), integrins (bind denuded collagen), GPIIb/IIIa
- Contractile proteins (actin + myosin)
- Granules (contain ADP, calcium, serotonin, factor V, fibrinogen, calcium)
- Arachadonic acid \(\to\) thromboxane A2 when platelet activated
Function: primary haemostasis
- Platelet adheres to denuded collagen directly or via vWF
- Activated by collagen binding, thomboxane A2, thrombin, or ADP \(\to\) degranulation, shape change from disk to irregular spiny sphere, contraction \(\to\) vessel spasm, activate more platelets
- Aggregation by crosslinking: plt(\xrigharrow{GPIIb/IIIa}\)fibrinogen\(\xleftarrow{GPIIb/IIIa}\)plt
- This forms a platelet plug
Function: interaction with coagulation cascade
- Activate platelet expresses phosphatidylsterine
- With calcium, FXa, and FVa \(\to\) forms prothrombin activator complex \(\to\) activates II \(\to\) generates thrombin spark
- Degranulation provides factor V and Ca to fuel thrombin burst, and fibrinogen to fuel propogation phase
Function: initiate vessel healing
- Release platelet-derived growth factor
- Causes fibroblasts to migrate to site, proliferate, initiate vessel healing
Formation
- Pluripotent stem cell \(\xrightarrow{mitosis}\) myeloid progenitor \(\xrightarrow{mitosis}\) megakaryoblast \(\xrightarrow{mitosis}\) promegakaryocyte \(\xrightarrow{mitosis}\) megakaryocyte
- Megakaryocyte: large cell with huge nucleus containing 32 copies of each chromosome
- Platelets bud from megakaryocyte; each produces ~5000 platelets
- Liver and kidney produce TPO \(\to \ \uparrow\)megakaryocyte differentiation, \(\uparrow\) platelet budding