Primary Hemostasis

Platelets: An Introduction

Platelets were recognized in 1882 by Bizzozero as a cell structure different from red and white cells. However, it was not until 1970 that platelets' relationship to hemostasis and thrombosis became so important.9 Every cubic millimeter of blood contains one-fourth of 1 billion platelets, resulting in approximately a trillion platelets in the blood of an average woman. Each platelet makes 14,000 trips through the bloodstream in its life span of 7 to 10 days.7

Platelet Development

Platelets, or thrombocytes, are small discoid cells (0.5 to 3.0 pm) that are synthesized in the bone marrow and stimulated by the hormone thrombopoietin. They are developed through a pluripotent stem cell that has been influenced by colony-stimulating factors (CSF) produced by macrophages, fibroblasts, T-lymphocytes, and stimulated endothelial cells. The parent cells of platelets are called megakaryocytes (Fig. 15.2). These large cells (80 to 150 pm) are found in the bone marrow. Megakaryocytes do not undergo complete cellular division but undergo a process called endomitosis or endoreduplication creating a cell with a multilobed nucleus. Each megakaryocyte produces about 2000 platelets. Thrombopoietin is responsible for stimulating maturation and platelet release. This hormone is generated primarily by the kidney and partly by the spleen and liver.10 There is no reserve of platelets in the bone marrow: 80% are in circulation and 20% are in the red pulp of the spleen. Platelets have no nucleus but do have granules: alpha granules, and dense granules. These granules are secreted during the platelet release reaction and contain many biochemically active components such as serotonin, ADP, and ATP. They are destroyed by the reticuloendothelial system (RE).

Platelet development occurs in the following sequence:

1. Megakaryoblasts are the most immature cell (10 to 15 pm) with a high nuclear to cytoplas-mic ratio and two to six nucleoli.

2. Promegakaryocyte is a large cell of 80 pm with dense alpha and lysosomal granules.

3. Basophilic megakaryocyte shows evidence of cytoplasmic fragments containing membranes, cytotubules, and several glycoprotein receptors.

4. The megakaryocyte is composed of cytoplasmic fragments that are released by a process called the budding of platelets.

Platelet Structure and Biochemistry

Platelets have a complex structure comprised of four zones: the peripheral zone, the sol gel zone, the

Figure 15.2 Megakaryocyte, the platelet parent cell.

Open canalicular system

Dense tubular system

Dense granules

ADP EPI Thrombin PAF

Collagen Thromboxane

Dense tubular system

Dense granules

Open canalicular system

Figure 15.3 Schematic diagram of platelet morphology.

organelle zone, and the membrane system (Table 15.1). Figure 15.3 is a diagram of platelet morphology.

Platelet Function and Kinetics

Platelets play an important role in both the formation of a primary plug as well as the coagulation cascade. The formation of a plug at the site of a cut vessel serves as the initial mechanical barrier. The lumen of the vessel is lined with endothelial cells; a break in this will initiate a series of reactions.

There are four phases to platelet function:

• REACTION 1 (ADHESION): Platelets adhere to collagen and undergo shape change from

Table 15.1

The Four Functional

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