Hematology Introduction

Organization of blood and blood forming organs

What is hematology?

• Hematology is the study of blood which is composed of plasma (~55%), and the formed elements which are:– The erythrocytes (RBCs) (~45%)

• Contain hemoglobin • Function in the transport of O2 and CO2

– The Leukocytes (WBCs) and platlets (thrombocytes) (~1%)

• Leukocytes are involved in the body’s defense against the invasion of foreign antigens.

• Platlets are involved in hemostasis which forms a barrier to limit blood loss at an injured site.

What is hematology?

• Hematology is primarily a study of the formed cellular elements.

• Alterations in the formed elements in the blood are usually a result of disease rather than being the primary cause of disease. – In fact, variations in the formed elements in the blood

are often the first sign that disease is occurring in the body.

– The changes caused by disease may be detected by lab tests that measure deviations of the blood constituents from the normal values. These lab test may include:

What is hematology?

• RBC count• WBC count• Platlet count• Hematocrit (packed cell volume)• Mean corpuscular volume (MCV)• Mean corpuscular hemoglobin concentration (MCHC)

• Under normal conditions the production, release, and survival of blood cells is a highly regulated process. Quantitative and/or qualitative hematologic abnormalities may result when there is an imbalance between cell production, release, and/or survival.

What is hematology?

– Age, sex, and geographic location are involved in physiologic changes in normal values of the formed cellular elements

– Pathologic changes in the values of the formed cellular elements occur with disease or injury.

– Normal values for a group are determined by calculating the mean for healthy individuals of the group and reporting the normal range as the mean +/- 2 standard deviations

What is hematology?

• Hematopoiesis is a term describing the formation and development of blood cells.– Cells of the blood are constantly being lost or destroyed.

Thus, to maintain homeostasis, the system must have the capacity for self renewal. This system involves:

• Proliferation of progeny stem cells• Differentiation and maturation of the stem cells into the

functional cellular elements.• In normal adults, the proliferation, differentiation, and

maturation of the hematopoietic cells (RBCs, WBCs, and platlets) is limited to the bone marrow and the widespread lymphatic system and only mature cells are released into the peripheral blood.

Marrow-derived Blood cells

• Neutrophils: inflammation, anti-bacterial • Monocytes: inflammation, anti-fungal• Eosinophils: anti-parasitic, hypersensitivity • Basophils: IgE-mediated hypersensitivity• Erythrocytes: hemoglobin• Platelets: coagulation

Blood cell replacement

• Neutrophils: ~ 10 hours

• Platelets: ~10 days

• Erythrocytes: ~ 110 days in circulation

• Marrow replaces 1011 blood cells/day during health

• Increase replacement rate when needed

Marrow status during changes in blood cell numbers?

• RBCs: polycythemia, anemia

• Neutrophils: neutrophilia, neutropenia

• PLTs: thrombocytosis, thrombocytopenia

• Leukemias

Embryonic-fetal-adult sites of hematopoiesis

• Yolk sac

• Mesonephros

• Liver*

• Spleen*

• Marrow

*EMH sites in adult

Hematopoiesis in adults

• Restricted to marrow of axial skeleton and epiphyses of long bone (mammals)

• Inactive sites fill with adipocytes that reserve this space in event of increased hematopoiesis

• Additional sites if needed (EMH): spleen, liver

Marrow collection sites

• Iliac crest

• Long bones

• Ribs

• Sternum

Sinusoidal marrow structureExtravascular site of hematopoiesis, BMB

Blood cell exit from marrow

• Retraction of reticular cell processes and transendothelial migration to blood

• Loss of receptors to adhesion proteins– Fibronectin: erythrocytes– Hemonectin: neutrophils

• Close proximity to sinus lumen (megakaryocyte, RBC)

“Inappropriate” release of

precursors (NRBCs) into blood

• Marrow damage (e.g. Pb, hypoxia)

• Altered marrow structure by infiltrative lesion (myelophthisis): fibrosis, neoplasm

• Proliferation of abnormal precursors (hematopoietic neoplasm = leukemia)

Hematopoiesis & Growth Factors

Apoptosis : programmed cell death

Necrosis : forced cell death

Regulation of Hematopoiesis (cont.)

Erythropoietin

• Site of synthesis: Renal peritubular cells

• Stimulus: Decreased arterial O2 up-regulates EPO gene expression.

• Action: Prevents apoptosis (promotes survival) of erythroid precursors

• Paraneoplastic? Renal disease?

Erythropoiesis

• Nuclear condensation, then loss

• Decreasing cell and nuclear size

• Loss of mitotic activity

• Cytoplasm less basophilic, more eosinophilic

Reticulocytes(NMB stain)

• Stage after loss of nucleus• Final Hb synthesis from ribosomes• Released into blood and undergo maturation in

spleen (except horses!) • Clinical application

Erythrocyte Maturation

0%10%20%30%40%50%60%70%80%90%

100%

80 60 40 20 0

Maturation (nuclear area)

HgbRNA

Colony stimulating factors

(GM-CSF, G-CSF)

• Inflammatory cytokine

• Inhibits apoptosis

• Enhance function of mature cells

Granulopoiesis

• Nuclear shape change

• Nuclear condensation w/o pyknosis

• Cytoplasm: loss of basophilia, appearance then loss of granules

• Loss of mitotic activity

Proliferation

pool

Maturation

pool*

*Storage pool = segmenters within maturation pool

Monocytopoiesis

monoblast promonocyte monocyte

• Short marrow transit time of monocytes (~3 days)

• No storage pool in marrow

• Marker of marrow recovery

Monocytes become tissue macrophages

• Pulmonary macrophages• Intestinal macrophages• Langerhans cells (skin)• Osteoclasts (bone)• Kupffer cells (liver)• Microglial cells (CNS)• Peritoneal, splenic, lymph node, etc.

macrophages

Thrombopoiesis

• Megakaryocytes: nuclear replication (endomitosis) without cell division. Favors larger cytoplasmic volume?

• Cytoplasmic fragmentation producing large “proplatelets”

• Proplatelets fragment into platelets

Thrombopoietin• Site of synthesis: Liver

• Produced at constant rate and binds to platelets and megakaryocytes.

• Increased “free” TPO occurs with fewer platelets, stimulating megakaryocyte proliferation.

Stages of Lymphopoiesis

• Hematopoietic stem cells (HPSCs): All the blood cell lineages.

• No CD3 on T cells.• No CD19 on B cells.• CD34+• Embryos : Liver and the bone marrow.• Growth factors CSFs induces differentiation.• HSCs→ progenitor cell → Mature cells.

Lymphoid cells development

• Occurs in the thymus gland.• Lymphoid progenitor, IL-7, Thymocyte, mature T

cell.• Lymphoid progenitor, Pre-B cell, B cell.• NK cells.• SCID→IL-2, IL-7 and IL-7 receptors are missing.• Stem Cell Transplantation.• Gene therapy.

Lymphoid cell types

• B cells: Ab production, BCR, MHC II, CD19, APC, Plasma cell.

• T cells: Same morphology, CD4+, CD8+, TCR, viruses and other intracellular infections.

• NK cells: Same morphology, No Ag receptor, innate, lyse virally infected cells and tumor cells.

Blood cell kinetics

An example: Regulation of Hematopoiesis

Hematopoietic Stem Cells

(HSCs) Totipotency Self-renewal

Myeloïde progenitor Lymphoid

progenitor

CFU-GMCFU-BasoCFU-EoBFU-MegBFU-E

Plaquettes NeutrophilesEosinophiles BasophilesErythrocytes

Monocytes

Macrophages

Thymocyte

LT4 LT8LB

Multiplication and Differentiation

Engagement

Bone Marrow

Regulation of Hematopoiesis