MLAB 1227: C OAGULATION K ERI B ROPHY -M ARTINEZ The Platelets

A LL A BOUT P LATELETS … Second major component of the hemostatic system

P LATELETS What is a platelet? Small 2-3 µm Disc-shaped Anuclear Reddish-purple granules Fragments of megakaryocyte cytoplasm Surface has a sponge-like look, openings are channels that extend deep within cell

P LATELETS Life span 7-10 days Reference Range x 10 9 /L

P LATELET : S IDE NOTE S EEN IN CONDITIONS WITH INCREASED NEED AND / OR DESTRUCTION May Hegglin anomaly, Bernard- Soulier syndrome, pregnancy, Giant platelets Micromegakaryocytes= Dwarf Megs

A NATOMY OF A P LATELET Divided into 4 zones Peripheral Structural Organelle Membrane

P ERIPHERAL ZONE : Responsible for platelet adhesion and aggregation Glycocalyx: Fluffy surface coat Contains glycoprotein receptors: GPIb binds von Willebrand’s factor needed for platelet adhesion to collagen GPIIb/IIIa binds fibrinogen needed for aggregation Bind ADP and thrombin, promoting aggregation Factors I, V, VIII on surface, involved in 2 o hemostasis Membrane: Exposed on platelet activation Layer called PF3 (platelet factor) surface for interaction of plasma coagulation factors Initiation of formation of thromboxane A 2. This stimulates aggregation and vasoconstriction

S TRUCTURAL / SO - GEL ZONE Responsible for platelet retraction/contraction functions and platelet shape ◦ Microtubules/Microfilaments ◦ Cytoskeleton ◦ Binding proteins ◦ Actin ◦ Myosin

O RGANELLE ZONE  Responsible for storage and platelet release functions ◦ Granules  Dense bodies, alpha granules, lysosomal granules and microperoxisomes ◦ Mitochondria ◦ Glycogen

M EMBRANE Z ONE Responsible for secretion of granule contents and storage of calcium Two systems Surface-connected open canalicular system (OCS) Dense tubular system(DTS)

A NATOMY OF A P LATELET research.org/1/function_morpho.htm#

P RODUCTION OF P LATELETS Made in Bone marrow Need dictates the amount of platelets produced. Stimulus for production is the platelet mass in circulating blood ~ 80 % and megakaryocyte mass in bone marrow Originate from CFU-GEMM to form CFU-Meg Cytokines and growth factors such as IL-3 and GM-CSF influences progenitor stages

P LATELET D EVELOPMENT Megakaryoblast µm Increased nuclear: cytoplasmic ratio Promegakaryocyte 80 µm Dense alpha and lysosomal granules Basophilic megakaryocyte Megakaryocyte

P RODUCTION OF P LATELETS Precursor Cell= Megakaryocyte Produces about 2000 platelets Platelets are released via sinuses of bone marrow 2/3 pb 1/3 sequestered in spleen

P RODUCTION OF P LATELETS Thrombopoietin (TPO) Regulates platelet development Influences all stages of megakaryocyte production Produced in the liver, kidney and spleen

P RODUCTION OF P LATELETS How does TPO work? Maintains a constant number of platelets in peripheral blood by binding Mpl (platelet receptor). Bound TPO can not stimulate proliferation of bone marrow progenitor cells The higher the platelet count, the more TPO is bound and stimulation of bone marrow is decreased.

McKenzie, Shirlyn B., and J. Lynne. Williams. "Chapter 29." Clinical Laboratory Hematology. Boston: Pearson, Print. Platelet Research Laboratories. Platelet Function. Retrieved from research.org/1/function_hemo.htm. R EFERENCES