**4. Dense granules**

6 Blood Cell – An Overview of Studies in Hematology

proteins in this region (1).

systems actively exchange molecules (1).

This is the zone where granule's, peroxisome's, lysosome's and mitochondria's are localized. There are enzymes, adenine nucleotids, calcium, serotonin and many other

There is a distinguishing feature of platelets that their plasma membrane contains wide spread invaginations that forms a network inside platelet. Finally with pore openings the inner network is directly in contact with outer zone. This system is named as "open canallicular system" (OCS) and with this system an extensive amount of surface area stays as potential in silent state. With this system also platelet gains a large area for molecular trafficking. A second canal system is composed from endoplasmic reticulum networks and named as "Dense Tubular System" (DTS). Here in DTS many enzymes and calcium ions that are important for activation are located. DTS is not directly connected to outer membrane (1, 14) but has close connections with OCS. These two

The granulles have diameters ranging between 200 to 500 nm and they are found as spherical or oval structures (15). There are 3 types of granules in platelets, Alfa Granules, Dense granules, lysosomes. Alpha granules are most prominent in terms of material content and majority. These granules include inflammatory molecules, cytokines, cell-activating molecules, proteins, Growth Factors, adhesion molecules,

integrins and other proteins These granules are filled by megakaryocytes (3).

It is widely accepted that these granules come from the budding of trans golgi apparatus

These are 200-400 nm diameter granules widespread in the cytoplasm (16) which gives the granular appearance in Romanoski stained smear preparations, each platelet contains around 50-80 of these granules. The content of granules is very diverse; a brief list is given in

When platelets are activated these alpha granules fuse with each other, OCS and plasma membrane. The secretion of alpha granules is mediated by some proteins (such as SNARE)

The secretions effect platelet and cells in the environment (such as endothelial, leukocytes)

A rare syndrome named as Gray Thrombocyte Syndrome (GTS) is both involved with the quantity and quality of platelets which cases susceptibility for bleeding. In GTS the proteins synthesized by megakaryocytes are abnormal and don't enter platelets as they do in normal individuals and additionally the endocytotic mechanisms don't work properly. As a result

the secretions spread to bone marrow and a fibrosis forms (miyelofibrozis)(22, 23).

3. Organel Zone:

4. Membrane Zone

**3. Alpha granules** 

table 1 (14, 18, 19, 20, 21).

and membrane lipids (19).

for migration, adhesion and proliferation(14).

organel of megakaryocytes (16, 17).

These are smaller granules with 150 nm diameter (24), because of the calcium and phosphate content there image seems dense under electron microscopic (EM) observation (21, 25). Each platelet contains 3-8 of these granules (14). The components of dense granules are briefly given in Table 2 (10, 14, 19, 20).

Ca Mg P pyrophosphate Nucleotides ATP, GTP, ADP, GDP Membrane proteins CD63 (granulophysin) LAMP 2 Serotonin GPIb, GPIIb/IIIa P-Selectin Histamine Epinephrine

**Table 2.** Some main components of dense granules.

In activated platelet these granules fuse with plasma membrane and expel their ingredients to their environment which causes other platelets to aggregate and a local vasoconstriction (especially by serotonin) in the involved vessels. Also the ADP content is a very important participant for homeostasis (14).

The importance of the components of dense granules for homeostasis is recognized when the diseases of the deficiency of dense granules was defined as Hermansky-Pudlak

Syndrome (26, 27, 28) and Chediak Higashi Syndrome. In both syndromes stoppage of bleeding is defective based on the impairment in dense granules (14).

PF3

 TGF CD63 Cathepsin

(35).

Acid phosphatase Glucose-6 phosphatase

N-Acetyl-galactosominidase ATP = adenosine triphosphate

lysosomal membrane proteins (LAMP-1, LAMP-2)

**Table 3.** Some main components of platelet lysosomes

**6. Autologous platelet rich plasma (PRP)** 

to obtain high concentrations of these growth factors (34).

technique to accelerate the wound healing (37).

The application of growth factors in medical practice is one of the areas where basic clinical research has focused its attention but there are many problems associated with their local administration. For example, recombinant human growth factors are not cost effective, they have limited shelf life, and ineffectively delivered to target cells and in addition, to get efficient therapy, large doses are needed. The use of autologous platelets concentrates for tissue regeneration and wound healing has now become an alternative easy and cheap way

The autologous blood collected from a patient just before surgery can be prepared as platelet concentrates, platelet-rich plasma (PRP) and platelet gel for the treatment the patient specifically needs (35). These forms are prepared by gradient density centrifugation techniques to obtain high (x5) concentration of platelets (36). This autologous concentration includes a large amount of growth factors, especially PRP is an easy and inexpensive

This quite new field is open for research, there are a lot of techniques still under development stage such as platelet gels can be obtained by adding thrombin to autologous platelet-rich plasma. The initiation of fibrin polymerization and the release of platelets factors and cytokines can be achieved by the specific activators such as thrombin, glass, freeze-thaw cycle to platelet-rich plasma depending on what is required during the surgery

In spite of the distinct features of platelet-rich plasma (PRP) and its use by different fields of

medicine, no adverse reactions were documented until now(38, 39, 40, 41).

Arabinosidase

acid hydrolases cathepsins

Platelets 9
