Haematology

Question 1

What is a platelet?

Answer 1

Platelets are small, colourless disc-shaped cell bodies approx 2-3 micrometres in diameter.
normal value is 150-400x20 to the power 9/L of blood.

Question 2

How are platelets produced?

Answer 2

They are produced in the bone marrow and are derived from megakaryocytes. They under go non-mitotic nuclear replication with increasing cytoplasm. As the cell enlarges platelets bud off.
They are stored in the spleen for ~ 36 hours before being released into circulation.

Question 3

What is the average life span of a platelet?

Answer 3

7-10 days

Removed from the blood stream by the reticuloendothelium system

Question 4

Can you describe the structure of the platelet?

Answer 4

They are small disc shaped cell bodies that are anucleic so cannot undergo protein synthesis.
They have a glycocalyx external layer and many invaginations within the membranes.
The glycocalyx layer has an outer layer of glycoproteins for platelet adhesion.
Within the cytoplasm there are:
- structural proteins
- dense tubular system
- microtubules
- many organelles - including a granules, dense granules, lysosomes and mitochondria.

Question 5

What is the platelets function?

Answer 5

The primary function is formation of platelet plug in response to endothelial damage,

Question 6

How do platelets form a platelet plug?

Answer 6

This is a process of 3 steps:

1. Adhesion
2. Activation
3. Aggregation

Adhesion:

• blood vessel damage = exposed subendothelium that binds to vWF - binds to GP Ib receptor.
• this exposes the GP IIb/IIIa receptor - binds to vWF and fibronectin - causing change in shape of platelet.

Activation
- release of platelet granules that contain enzymes that facillitate platelet aggregation/adhesion + vasoconstriction.

Aggregation

• Released ADP and thromboxane A2
• ADP platelet-platelet adhesion liberating further A2 + ADP
• ADP activates fibrinogen receptor - IIb/IIIa

Question 7

How are platelets stored?

Answer 7

Platelet concentrates are either single concentrates or pooled as platelet concentrates obtained from 4-6 units.
It is stored at 20-26 degrees C at constant agitation to prevent platelets binding to the bag.
The bag is made of polyolefin - better aeration of platelets.