A Basic Understanding of Platelets

Platelets are a component of whole blood. They are the smallest of blood cells and can only be seen under a microscope. They are shaped like small plates in their non-active form. Platelets are produced in bone marrow, primarily responsible for hemostasis (blood clotting). A blood vessel will send out a signal when it becomes damaged and when platelets receive that signal, they’ll respond by traveling to the area and transforming into their “active” formation. To make contact with the broken blood vessel, platelets grow long tentacles and then resemble a spider or an octopus. Platelets are also responsible for the construction of new connective tissue and revascularization. Inside platelets, there are a number of intracellular structures present containing glycogen, lysosomes, and two types of granules: 1) alpha granules contain clotting factors, growth factors, and other proteins, and 2) dense granules contain factors that promote platelet aggregation (ADP, calcium, serotonin). Thrombin is required in order for platelets to become active. Once this occurs, platelets morph into different shapes and develop branches that spread over injured tissue. This process is referred to as aggregation. Eventually the granules contained within the platelets release growth factors, which stimulate the inflammatory cascade and healing (Everts P, 2006). Below is a partial list of key nutritive, regenerative components found in platelets:

  • Epidermal Growth Factor (EGF)
  • Fibroblast Growth Factor (FGF)
  • Platelet Derived Growth Factor (PDGF)
  • Transforming Growth Factor-Alpha (TGF-a)
  • Transforming Growth Factor-Beta (TGF-b)
  • Insulin-like Growth Factors-1,2
  • Vascular Endothelial Growth Factor (VEGF)
  • Connective Tissue Growth Factor (CTGF)
  • Anti-Inflammatory Factors
  • Bacteriostatic Agents
  • Small Molecules:
    • ATP
    • ADP
    • Serontonin
    • Histamine
  • Excellular Matrix Molecules:  Fibronectin
  • Keratinocyte Growth Factor

Image from a light microscope (500 ×) from a Giemsa-stained peripheral blood smear showing platelets (navy blue dots) surrounded by red blood cells (pink and circular).

Source: Wikipedia.

Nurden, Alan T., Paquita Nurden, Mikel Sanchez, Isabel Andia, and Eduardo Anitua. “Platelets and wound healing.” Frontiers in bioscience: a journal and virtual library 13 (2008): 3532-3548.

Clemetson, K. J. (2012). Platelets and primary haemostasis. Thrombosis research, 129(3), 220-224.

Platelets are sourced and processed from either patients or donors.  Patient sourced and processed platelets are commonly called autologous serum (click here to learn more about autologous serum).  Donor platelets are collected and processed for therapeutic applications one of two ways. One, they are pooled from multiple donors to make therapeutic doses by using a centrifuge to separate platelet-rich-plasma from multiple units/donors of whole blood. Two, platelets are also obtained by a process called apheresis, or plateletpheresis. In this process, blood is drawn from single donors into apheresis instruments which separate platelets from other blood components, returning those unused components back to individual patients. Regardless, before or following multiple transfusions–platelets have long been “pooled” safely for use in patients based on the specific collection or treatment regimen.

Complications from the transmission of blood products are very rare. The safety of the blood supply in the United States and blood products continues to improve and is at its highest levels ever. Increasingly, extensive safeguards designed to protect patients from unsuitable blood and blood products have and continue to be mandated by the FDA. In addition to significant redundancy in safety measures throughout the transfusion chain, the following are examples of mandated safety measures currently in place:

  • Donor eligibility. Donors are extensively pre-screened by blood collection centers prior to any donation on over 60 risk factors. Donors are then deferred or rejected if certain risk factors are acknowledged.
  • Optimal product processing, handling and storage. Continuous training and supervision of personnel responsible for collection, processing, handling and storage of blood products help further insure high quality standards and product safety.
  • Skin preparation and initial diversion. Improved donor arm disinfection techniques and SOPs have also lowered the risk of bacterial transfer at the puncture site. And when combined with diversion (discarding the first 30-40ml of whole blood from the collection bag) the risk of bacterial infection at puncture site has been reduced by 77%.
  • Bacterial detection methods. Donated platelets are subjected to stringent methods for detecting bacteria in platelet products prior to transfusion greatly reducing the risk of sepsis among others.
  • Tests for infectious diseases. The FDA mandates that all donated blood products be tested for anti-trypanosoma cruzi, hepatitis B & C, HIV 1/2, HTLV I/II, syphilis, West Nile virus and increasingly Chagas disease. Confirmatory tests are performed in duplicate if test results are positive in order to rule out false positives. Once testing is completed, those units of blood free of infection are made available for sale and transfusions when needed. Those in which infection is detected are discarded, and the donor is notified as well as prohibited from future blood donation.
  • Pathogen reduction methods. Pathogen reduction is a pro-active approach being adopted in the industry to further reduce the risk of transfusion transmitted infections and is proving effective for most known and emerging pathogens. The goal of pathogen inactivation is to reduce transmissible pathogens (bacteria, viruses and protozoa) without compromising therapeutic efficacy of the blood product or introducing secondary risks.