CHAPTER 2 HEMA.pdf

MEGAKARYOPOIESIS MEGAKARYOPOIESIS ❑ maturation of megakaryocyte. ❑ THROMBOPOIESIS is defined as the genesis of platelets. ❑ The stem cell for platelets is the hemocytoblast. ❑ Megakaryocytes are the largest cells in the bone marrow and are polyploid, possessing multiple chromosome...

MEGAKARYOPOIESIS MEGAKARYOPOIESIS ❑ maturation of megakaryocyte. ❑ THROMBOPOIESIS is defined as the genesis of platelets. ❑ The stem cell for platelets is the hemocytoblast. ❑ Megakaryocytes are the largest cells in the bone marrow and are polyploid, possessing multiple chromosome copies. ❑ Megakaryocyte progenitors arise from the common myeloid progenitor under the influence of the transcription gene product, GATA-1, regulated by cofactor FOG1. STAGES IN THROMBOCYTE DEVELOPMENT ❑ MEGAKARYOBLAST ❑ Earliest recognizable stage of maturation ❑ THROMBOPOIETIN – a special hormone responsible for the commitment of the megakaryoblast to differentiate further into mature stages ❑ The cell is large, irregularly shaped with a single or several round or oval nuclei and with a blue, non-granular cytoplasm. ❑ Nucleoli are usually present. ❑ Centrally located nucleus with fine delicate chromatin strands ❑ High nuclear to cytoplasmic ratio STAGES IN THROMBOCYTE DEVELOPMENT ❑ PROMEGAKARYOCYTE ❑ Presence of bluish granules in the cytoplasm adjacent to the nucleus. ❑ The nucleus in this second stage of maturation has usually divided one or more times (2N) and the cell has increased in size. ❑ Granules are termed dense, alpha and lysosomal and are dispersed through the cytoplasm ❑ This cell differs from the megakaryoblast in that there are bluish granules in the cytoplasm adjacent to the nucleus. STAGES IN THROMBOCYTE DEVELOPMENT ❑ MEGAKARYOCYTE WITHOUT PLATELETS ❑ Abundant light blue to pink cytoplasm with numerous purple-red or pink granules; nucleus has 8, 16 or 32 overlapping lobes; no nucleolus ❑ Megakaryocyte nuclei may have from 2 to 32 lobes and, in unusual cases, may have up to 64 lobes. ❑ Megakaryocytes develop copious cytoplasm, which differentiates into platelets. ❑ Approximately 5 days – matures in the BM ❑ Formation of Demarcating Membrane System (DMS) ❑ DMS functions as the future membrane system of the metamegakaryocyte’s new offspring, the platelet ❑ Capable of endomitosis, In endomitosis, the nucleus is duplicated but there is no cell division, resulting in a polyploid cell ❑ MEGAKARYOCYTE WITH SHEDDING PLATELETS (MK 4) ❑ Have demarcated granular clumps of platelets streaming from the margins ❑ Capable of endomitosis ❑ 9-12 days – life span of cytoplasmic fragments of megakaryocyte ❑ Final stage of cell line maturation ❑ MEGAKARYOCYTE PRODUCING PLATELETS (METAMEGAKARYOCYTE) ❑ Capable of endomitosis ❑ PLATELETS/THROMBOCYTES ❑ Small fragments of megakaryocytes ❑ Formation is regulated by thrombopoietin  Blue-staining outer region, purple granules  Granules contain serotonin, Ca2+, enzymes, ADP, and platelet-derived growth factor (PDGF) PLATELETS ❑ Platelets are nonnucleated blood cells that circulate at 150 to 400 x 109/L, with average platelet counts slightly higher in women than in men and slightly lower in both sexes when over 65 years old. ❑ NORMAL VALUE: 150,000 TO 400,000 platelets/cu.mm ❑ 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. 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 PLATELET STRUCTURE ❑ Platelets, or thrombocytes, are small discoid cells (0.5 to 3.0 μm) that are synthesized in the bone marrow and stimulated by the hormone thrombopoietin ❑ Platelets have an average diameter of 2 to 4 μm, with younger platelets being larger than older ones. (Turgeon) ❑ Although circulating, resting platelets are biconvex, the platelets in blood collected using the anticoagulant ethylenediaminetetraacetic acid (EDTA, lavender closure tubes) tend to “round up.” (Rodak) ❑ On a Wright-stained wedge-preparation blood film, platelets appear circular to irregular, lavender, and granular.  Dense blue to purple (Romanowsky stain: light blue cytoplasm with evenly dispersed, fine red-purple granules)  Because of their small size, it is difficult to examine the internal structure of platelets using light microscopy ❑ Platelets, although anucleate, are strikingly complex and are metabolically active. Their ultrastructure has been studied using scanning and transmission electron microscopy, flow cytometry, and molecular sequencing. (Rodak) ❑ 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.  These large cells (80 to 150 μm) 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. ❑ There is no reserve of platelets in the bone marrow: 80% are in circulation and 20% are in the red pulp of the spleen.  In the blood, the platelet surface is even, and they flow smoothly through veins, arteries, and capillaries.  In contrast to leukocytes, which tend to roll along the vascular endothelium, platelets cluster with the erythrocytes near the center of the blood vessel.  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). ❑ Life span: 9 – 12 days (4 – 9 days)  The normal peripheral blood platelet count is 150 to 400 x 109/L.  The platelet count decreases with increasing age, such that after 65 years of age, platelet counts of 122 to 350 x 109/L and 140 to 379 x 109/L are seen in men and women, respectively.  This count represents only two thirds of total body platelets; the remaining one third is sequestered within the spleen.  Sequestered platelets are immediately available in times of demand— for example, in acute inflammation or after an injury, after major surgery, or during plateletpheresis. PLATELET ULTRASTRUCTURE ❑ Peripheral Zone  consists of the platelet’s outer membrane and related structures  is associated with platelet adhesion and aggregation.  Glycocalyx  surface coat  composed of plasma proteins and carbohydrate molecules that are related to the coagulation, complement, and fibrinolytic systems  with glycoproteins (GP) for platelet adhesion and aggregation (Ia, Ib, Ic, IIa, IIb, III, IV, & V)  the GP receptors of the glycocalyx mediate the membrane contact reactions of platelet aherence, change of cellular shape, internal contraction, and aggregation  with coagulation factors which adhere in this layer (I, V, VIII, X, XI, XII & XIII)  ❑ Plasma Membrane  under the glycocalyx  serves as the physical & chemical barrier between the intracellular & extracellular constituents of the platelet  site of receptors for clotting factors & aggregating agents ❑ Submembrane Area  underlies the plasma membrane  prevents contact between organelles & internal side of platelet cell wall  contributes to regulation of the normal platelet discoid shape  links the membrane & the inner cell body ❑ Sol – Gel Zone  constitutes the matrix or muscle & skeletal portion of the platelet  serves as a stable gel component to regulate the arrangement of the internal organelles & microtubular system within the resting platelet body  influences communication of the organelles with the platelet’s external surroundings  underlies the submembrane filaments ❑ 3 Major parts of the Sol-Gel Zone  Microtubules  for cytoskeletal support of platelet (microtubular cytoskeleton – maintains the discoid shape)  Microfilaments  for platelet contraction & pseudopod formation  Thrombosthenin (a contractile protein) or actomyosin (clot retraction)  both a myosin and an actin protein ❑ Organelle Zone  consists of granules, dense bodies, lysosomes, peroxisomes & mitochondria  serves as the metabolic center to influence platelet function in response to exogenous stimuli such as hypercoagulation, viruses & foreign bodies  constitutes the major portion of the platelet cytoplasms ❑ Alpha granules  larger than dense granules and contains:  PF4 (Platelet factor 4)  binds heparin and therefore neutralizes heparin’s coagulation inhibitory effect)  ßTG (beta - thromboglobulin)  PDGF (platelet-derived growth factor)  stimulates smooth muscle cell growth and proliferation  TGF-beta (transforming growth factor-ß)  Chemotactic factor  Permeability factor  Fibrinogen  Albumin  Plasminogen activator inhibitor  Factor V, vWF, Factor VIII  Thrombospondin  Fibronectin  PS (Protein S) ❑Dense granules  secretes the following:  ADP  promotes primary aggregation of platelets  ATP  Calcium  Magnesium  Epinephrine  Phosphate  Serotonin (5 – HT) ❑ Lysosomes  platelet vesicles that contain acid hydrolases that are released at the site of the forming thrombus or platelet plug to digest cellular debris and foreign materials  contributes to clot lysis ❑ Mitochondria  ATP synthesis for platelet metabolism PLATELET MEMBRANE SYSTEM ❑ Surface – Connecting Canalicular system (SCCS)  serves as delivery routes for substances ingested or extruded by the platelet  twists sponge-like throughout the platelet, enabling the platelet to store additional quantities of the same hemostatic proteins found on the glycocalyx  allows for enhanced interaction of the platelet with its environment, increasing access to the platelet interior as well as increasing egress of platelet release products  is the route for endocytosis and for secretion of alpha granule contents upon platelet activation  Dense Tubular System (DTS)  contains specific peroxidase activity that is important for platelet prostaglandin synthesis  the site of prostaglandin and thromboxane synthesis and sequestration of calcium  bears a number of enzymes that support platelet activation including phospholipase A2, cyclooxygenase, and thromboxane synthetase, which support the production of thromboxane A2, and phospholipase C, which supports production of the signaling molecules inositol triphosphate (IP3) and diacylglycerol (DAG)  site of arachidonic acid metabolism  parallel and closely aligned to the SCCS  a condensed remnant of the rough endoplasmic reticulum PLATELET FUNCTION ❑ Hemostasis  stop of blood flow  serotonin (vasoconstrictor)  platelet-plug formation or hemostatic plug stabilization ❑ Blood coagulation  solidifying of blood fluid  chemicals found in alpha & dense granules  Example: ADP  its membrane has phospholipids that activate intrinsic system of blood clotting ❑ Clot retraction  platelet produces contractile protein that is capable of clot retraction  thrombosthenin or actomyosin (contractile protein)  when clotted blood is retracted, serum is released ❑ Vascular integrity  secretes growth factor that promotes growth & multiplication of vascular endothelial cells, vascular smooth cells & fibroblasts  platelets, vitamin C & adrenocorticosteroids are important for maintaining vascular integrity