Blood and Hematopoiesis

BIO 408

Histology

Dr. D. L. Daley

Blood

n Formed elements

n RBCs or erythrocytes

n WBCs or leukocytes

n Platelets

n Plasma

n Extracellular matrix of blood

n Centrifugation of whole blood

n Formed elements on the bottom - red precipitate - 44% (hematocrit)

n Buffy coat - 1% - WBCs & platelets

n Top 55% - supernatant - plasma

Hematocrit & Smear

Plasma

n 90% - Water

n 9% - Proteins

n 1% - inorganic salts, ions, nitrogenous compounds, nutrients & gases

n During coagulation organic & inorganic components leave the plasma & become integrated into the clot

Remaining fluid is straw colored and is called serum

Erythrocytes or RBCs

n Biconcave-shaped disk

n 7.5mm in diameter & 2.0mm thick

n Shape provides RBCs with large surface area

n No nuclei or organelles

n Contain hemoglobin

n Salmon pink color with Wrights & Giemsa stains

Erythrocytes or RBCs

n Males - 5 x 10⁶ RBCs per mm³

ⁿ Females - 4.5 x 10⁶ RBCs per mm³

n Average life span of 120 days

n When they reach that age they display on their surface a group of oligosaccharides

n RBCs bearing these sugar groups are destroyed by macrophages of the spleen, bone marrow and liver

Hemoglobin

n A large tetrameric protein composed of four polypeptide chains, each of which is covalently bound to an iron-containing heme

Erythrocyte Membrane

n Glycophorin A is the most common protein of the cell membrane

n Some glycophorin B, C and D

n Ion channels

n Anion transporter 3 protein - anchoring site for ankyrin

n Band 4.1 protein acts as anchoring site for glycophorins

n Thus the above two anchor the cytoskeleton

n A hexagonal lattice composed chiefly of spectrin tetramers, actin and adducin - maintains the biconcave shape

Erythrocyte Membrane

n Extracellular surface has specific inherited carbohydrate chains that act as antigens and determine the blood group - for transfusion

n A and B antigens are most notable

n A, B, AB and O groups

n Rh group - complex of more than two dozen antigens

n Rh antigens C, D and E are very common - in 85% of Americans (Rh positive)

Leukocytes

n White Blood cells (WBCs)

n 6500 - 10,000 per mm³ of blood

n Typically leave the blood stream by diapedesis

n Types

n Granulocytes

n Neutrophils, eosinophils & basophils

n Agranulocytes

n Lymphocytes & monocytes

Neutrophils

n PMNs - polymorphonuclear leukocytes or neutrophils

n Most common type of WBC - 60 to 70% of total WBC population

n Multilobed nucleus - in females the nucleus has a characteristic small appendage, the “drumstick” - inactive second X chromosome - called Barr body - not always evident in all cells

n First WBC on the scene to clean up an infection

Neutrophils

n Three types of granules

n Small, specific granules - .1mm

n Enzymes & pharmacological agents for killing microbes

n Larger azurophilic granules - .5mm

n Lysosomes

n New discovered tertiary granules

n Gelatinase & cathepsins & glycoproteins destined for the cell membrane

Neutrophil Functions

n Interact with chemotactic agents to migrate to sites invaded by microorganisms

n They enter postcapillary venules in regions of inflammation

n They also adhere to various selectin molecules of endothelial cells of the venules by their selectin receptors

Neutrophil Functions

n This interaction (with selectin molecules) causes the neutrophil to roll slowly along the vessels endothelial lining

n They use interleukin-1 & TNF to induce endothelial cells to express intercellular adhesion molecule type 1 (ICAM-1) - integrin molecules of neutrophils bind to these

Neutrophil Functions

n With binding of ICAM-1 - neutrophils stop migrating & cross the endothelium into the connective tissue compartment

n Once in - they destroy microorganisms by phagocytosis and the release of hydrolytic enzymes and/or respiratory burst (superoxide 0₂^- is formed)

n Assist in the initiation of the inflammatory response by manufacturing and releasing leukotrienes - initiate inflammation

Eosinophils

n Less than 4% of total WBC population

n 10-14 mm in diameter

n Bilobed nucleus

n Granules

n Specific granules -oblong -1mm to 1.5 mm in length and <1.0 mm in width - stain deep pink

n Nonspecific granules - lysosomes - similar to those in neutrophils

Eosinophil Functions

n Bind histamine, leukotrienes and eosinophil chemotactic factor

n Causes migration to sites of inflammation, allergic reaction or parasitic worm infection

n Degranulate - eosinophil cationic protein on the surface of parasitic worms - kills worm by causing holes to from in them - helps entry of superoxides and hydrogen perioxide

n Also release substances to enhance inflammation

n Engulf antigen- antibody complexes

Basophils

n Similar to Mast cells in function - different origin

n Less than 1% of WBCs

n 8-10 mm in diameter - s shaped nucleus

n Nucleus masked by large nonspecific granules found in the cytoplasm

n Several surface receptor including IgE receptors

Basophil Granules

n Specific granules - stain dark blue to black

n About 0.5 mm in diameter - usually pressed against the periphery of the cell

n Get rough perimeter

n Contain heparin, histamine, eosinophil chemotactic factor, neutrophil chemotactic factor and peroxidase

n Nonspecific granules - lysosomes

Basophil Functions

n In response to some antigens in certain individuals - plasma cells produce IgE

n Fc portion attaches to the FceRI receptors of basophils and mast cells

n Next time the same antigen arrives - they bind to the IgE molecules - degranulate

n Leukotrienes produced

n Histamines released - dilation, smooth muscle contraction in bronchial tree & leakiness in blood vessels

Monocytes

n Largest blood cells - 12-15 mm in diameter

n Large acentric kidney shaped nucleus

n Stay in circulation only a few days - then migrate through the endothelium of venules and capillaries into connective tissue - then they differentiate into macrophages

Function of Monocytes

n Avid phagocytes

n Produce cytokines that activate the inflammatory response as well as proliferation & maturation of other cells

n Certain macrophages phagocytose antigens & present antigenic portions, the epitomes in conjunction with class II human leukocyte antigen also called histocompatability complex antigen (MHC II)

n Thus antigen-presenting cells

Function of Monocytes

n May fuse with each other to form foreign-body giant cells to phagocytose the large foreign particle

Lymphocytes

n 20 -25% of the total circulating leukocyte population

n Round cells a little large that RBCs

n 8-10 mm in blood smears

n Three types

n B lymphocytes

n T lymphocytes

n Null cells

Functions of Lymphocytes

n B cells are responsible for antibody mediated immunity

n Become plasma cells when stimulated by appropriate antigen - secrete antibodies

n T cells are responsible for cell mediated immunity

n Null cells - two populations

n Circulating stem cells

n Natural killer cells - can kill some foreign & virally altered cells without the influence of T cells

Platelets

n Small non-nucleated fragments derived from megakaryocytes in the bone marrow

n Peripheral clear region - hyalomere

n Tubular systems - surface opening - for rapid uptake of activating molecules & dense tubular system - probably grabs Ca⁺⁺ - reduce “stickiness”

n Central darker region - granulomere

n Granules

n a granules - factors involved in aggregation, vessel repair & coagulation of the blood

n g granules - platelet aggregation, adhesion & vasoconstriction

n l granules - enzymes that aid in clotting

Bone Marrow

n Medullary cavity of long bones and the interstices between trabeculae of spongy bone contain the marrow

n Responsible for the formation of blood cells (hemopoiesis) - functions from the 5th month of prenatal life until death

n Marrow of newborns - red marrow - many RBCs forming

n By 20 years of age, the diaphyses of long bones house yellow marrow (abundance of fat cells) & little or no hemopoiesis

Hemopoiesis

n Prenatal

n Mesoblastic phase - starts 2 weeks after conception in mesoderm of yolk sac - see mesenchymal cells from clusters called blood islands

n Peripheral cells - become the vessel walls

n remaining cells become nucleated erythrocytes

n Hepatic phase - starts 6th week of gestation

n Nucleated erythrocytes

n Leukocytes appear by 8th week

Prenatal

n Splenic phase - during second trimester - continues along with the hepatic pahse until end of gestation

n Hemopoiesis in the the bone marrow begins by the end of the second trimester - myeloid phase

Postnatal Hemopoiesis

n Occurs exclusively in the bone marrow

n Daily more than 10¹¹ blood cells are produced in the marrow to replaced cells that leave the blood stream, die or are destroyed

n Stem cells undergo multiple divisions and differentiate

Stem Cells

n Pluripotential hemopoietic stem cells (PHSCs)

n All blood cells arise from these - about 0.1% of the nucleated cell population of bone marrow

n Can divide & give rise to more PHSCs & two types of multipotential hemopoietic stem cells (MHSCs)

n Colony-forming unit - spleen (CFU-S) - gives rise to myeloid line (erythrocyte, granulocytes, monocytes and paltelets)

n Colony-forming unit - lymphocyte (CFU-Ly) - gives rise to lymphoid cell line (B cells and T cells)

n Both PHSCs & MHSCs resemble lymphocytes

Progenitor and Precursor Cells

n Progenitor cells

n Resemble small lymphocytes & are unipotential - committed to forming a single cell line (e.g. eosinophils)

n These cells are controlled by specific hemopoietic factors

n Precursor cells

n Arise from progenitor cells & are incapable of self renewal

n Undergo cell division & differentiation - eventually giving rise to a clone of mature cells

n As differentiation proceeds the successive cells become smaller - begin to look more like adult cells

Hemopoietic Growth Facors

n Also called colony stimulating factors

n Hemopoiesis is controlled by numerous growth factors

n Each factor works on specific stem cells, progenitor and precursor cells

n The factor induce rapid cell division & differentiation

Erythropoiesis

n Formation of RBC

n Under the control of several cytokines

n Steel factor, interleukin 3 and 9, granulocyte-monocyte colony stimulating factor (GM-CSF) and erythropoietin from the kidneys)

Granulocytopoiesis

n Formation of granulocytes

n Neutrophils, eosinophils and basophils

n Under the control of several cytokines

n G-CSF, GM-CSF, IL-1, IL-6, TNF-a and IL-5

Monocytopoiesis

n Monocytes share their bipotential cells with neutrophils

n CFU-GM divide & give rise to CFU-G and CFU-M (monoblasts)

n The monoblasts divide & differentiate into monocytes

Platelet Formation

n Under the control of thrombopoietin - which induces the development and proliferation of very large cells called megakaryocytes

n The unipotential platelet progenitor, CFU-Meg gives rise to megakaryoblasts - these undergo endomitosis & become polyploidy (as much as 64N)

n These differentiate into megakaryocytes

n Megakaryocytes - cytoplasmic processes fragment along narrow invaginations of the cell membrane called demarcation channels into pro-platelets - then released as platelets

Lymphopoiesis

n Multipotential stem cell CFU-Ly divides in the marrow to form two progenitor cells

n CFU-LyB - give rise to B cells

n CFU-LyT - give rise to T cells

n Both B and T cells proceed to lymphoid organs such as spleen and lymph nodes

n Here they form clones of immunocompetent B and T cells