Platelets

Figure21. Diagram of the human platelet

Blood platelets (thrombocytes) are nonnucleated, disk-like cell fragments 2-4 μm in diameter. They range in number from 180-320xl0⁹/L of blood. Platelets derive from large cells in the bone marrow, called megakaryocytes.

In the formation of platelets, small bits of cytoplasm are separated from the peripheral regions of the megakaryocyte.

Human platelets have a life span of about 10 days. In stained blood smears, platelets often appear in clumps. Each platelet has a peripheral light blue-stained transparent zone, the hyalomere and a central zone containing purple granules, called the granulomere.

Platelet functions:

1. Primary aggregation

Discontinuities in the endothelium, produced by blood vessel lesions, are followed by absorption of plasma proteins on the subjacent collagen. Platelets immediately aggregate on this damaged tissue, forming a platelet plug.

2. Secondary aggregation.

Platelets in the plug release the content of their alfa and delta granules.

3. Blood coagulation

During platelet aggregation, factors from the blood plasma, damaged blood vessels and platelets promote the sequential interaction (cascade) of approximately 13 plasma proteins, giving rise to a polymer, fibrin that forms a three-dimensional network of fibers trapping red cells, leukocytes and platelets to form a blood clot or thrombus.

4. Clot retraction.

5. Clot removal.

Дата добавления: 2015-07-25