● Leaks occur frequently in the smaller vessels and a platelet plug
is often sufficient to seal them: blood coagulation is not necessary.
- ● Platelets (also called Thrombocytes) are cell fragments ~1 - 4m in diameter formed from Megakaryocytes (see previously) in the
- bone marrow or blood especially as they squeeze through the smaller capillaries.
● Normally blood contains ~150,000 - 300,000 platelets/mm3
have no nucleus and therefore like RBCs cannot replicate
contain contractile proteins
including actin and myosin
(as found in muscle) and thrombosthenin
which allow the platelets
to rapidly release intracellular granules
- ● Platelets also contain remnants of the ER and Golgi apparatus in order to: produce enzymes store Ca2+ produce mitochondria and the enzymes necessary to produce ATP
- produce prostaglandins
- produce two important proteins (namely Fibrin-stabilizing factor and a growth factor which stimulates endothelial cells, vascular smooth muscle cells and fibroblasts to multiply and grow...all of which are necessary for repair)
● On the external face of the platelet
cell membrane are glycoproteins
which prevent them from adhering to the normal undamaged endothelial surface but does adhere to damaged endothelium and even more so to exposed collagen fibers from the underlying connective tissue.
also contain high concentrations of phospholipids
that are involved in blood clotting.
have a short ½-life
(~8 - 12 days) and are removed by macrophages in the spleen (phagocytosis).
● Contact between platelets
and damaged endothelium or exposed collagen stimulates the platelets
to swell and assume irregular shapes and radiating pseudopodia
: contractile proteins cause the release of intracellular granules containing important factors. The platelets
become "sticky" and adhere to collagen
and a protein called von Willlebrand factor
from blood. In addition they secrete large amounts of ADP
(blocked by "Plavix") and TXA2
which activate additional nearby platelets
which themselves then adhere to the original ones in a + feedback loop: this is the platelet plug
● The plug
is loose at first and this is often adequate to plug the leak but subsequently during blood clotting the fibrin fibers
formed further strengthen it.