Absence of  Dense granules in blood platelets


The bleeding tendency is caused by a defect of the blood Platelets.  Platelets are cell fragments in the blood that play an essential role in blood clotting and wound repair.  Platelets are the body's first line of defence in preventing excessive blood loss. They are made in the bone marrow and circulate in the bloodstream in large quantities. They act directly to plug small leaks in vessels by clumping (aggregating) and obstructing blood loss through breaks in blood vessels.

Above left: Enhanced picture of a ‘plug’ forming.  The blue shapes are ‘activated’ blood platelets that have changed shape and become spiky or ‘stellate’. The red shapes are red blood cells,  The yellow strands are Fibrin threads which form a ‘net’.

Above middle: Enhanced picture of an activated platelet above a disk shaped un-activated platelet.

Above right: Electron micrograph of activated ‘stellate’ platelets adhering to one another.

Enhanced picture of an activated platelet above a disk shaped un-activated platelet. Electron micrograph of activated ‘stellate’ platelets adhering to each other. Enhanced picture of a ‘plug’ forming.  The blue shapes are ‘activated’ blood platelets that have changed shape and become spiky or ‘stellate’. The red shapes are red blood cells,  The yellow strands are Fibrin threads which form a ‘net’.

More about blood clotting and Hermansky-Pudlak Syndrome


When an injury such as a cut occurs normal platelets normally release the contents of their dense granules (ADP and ATP).  ADP attracts surrounding platelets and they become ‘activated’ and change their shape to become spiky (Stellate).  They become ‘sticky’ and build up on the wall of a damaged blood vessel.  In HPS there is a lack of ADP due to the absence of dense granules so that platelets are not attracted to each other.and do not change shape and become activated.


The ATP contained in the dense granules of normal platelets is necessary for the production of threads of fibrin in the blood.  Fibrin acts like a tangled fishing net that red blood cells and platelets get trapped in. Together they form a blood clot, or ‘plug’ at the site.  In HPS the lack of dense granules (and ATP) means that the fibrin threads do not form a strong net.  Plugs are slow to form and when they eventually form they are unstable and can come apart easily.

Electron micrograph of normal platelets with dense granules showing as black spots.      Picture of choc chip cookie to illustrate the analogy with dense granules in normal platelets.                     Electron micrograph of HPS platelets lacking dense granules.     Picture of a plain butter cookie illustrating the analogy of no choc chips equals no dense granules in HPS platlets.


                                    


Normal platelets release chemicals into the bloodstream  when they come into contact with a damaged part of a blood vessel.   The chemicals  trigger a chain of events (called the Coagulation Cascade) that in turn causes the platelets to stick together and plug leaking blood vessels at the site of a cut or injury. The chemicals (ADP, ATP, serotonin, calcium, phosphate) are normally stored in ‘dense granules’  inside the platelets themselves  (these are sometimes also called  ‘dense bodies’ or ‘delta granules’). However, the platelets of people  with HPS do not have dense granules and so in turn lack the necessary chemicals. Without them, HPS platelets do not work causing poor clotting or a tendency to bleed.


Dense granules are little sacks inside the platelets that make them look like a "choc chip cookie" when seen through a very powerful microscope (Electron Microscope or EM).  In HPS the dense granules are missing and the platelets look like a "butter cookie" instead of a "choc chip cookie".


Von Willebrand’s Factor (vWF) antigen activity

Approximately 30 percent of people who have HPS also have a decrease in both platelet and plasma von Willebrand's Factor (vWF) antigen activity - another blood component responsible for normal clotting. There is not a clear ‘statistical’ relationship between the level of vWF antigen and the severity of bleeding in patients with HPS.  Some HPS patients may have reduced vWF activity levels and have severe bleeding whilst others may have moderate bleeding.  Conversely, some HPS patients may have normal vWF activity but have severe bleeding.

Remember !


It is possible for a person with albinism to have a bleeding disorder caused by a separate medical condition that is unrelated to either albinism or HPS.


People who have either albinism or HPS are as prone to unrelated conditions and diseases as any other person.


When a person is tested for HPS within the UK National Health System (NHS) their blood is typically routinely tested for the most common bleeding (coagulation) disorders. This is an important safeguard because some conditions can result in a pattern of symptoms similar to the bleeding disorder of HPS.

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What causes the bleeding disorder