Medical School · Year 2 · Hematology Oncology · includes a quiz and discussion video

Lecture 01: Hematopoiesis and Blood Components

Unit 2.9: Hematology and Oncology

Learning Objectives

By the end of this lecture, students will be able to:

1. Describe the process of hematopoiesis and its regulation
2. Explain the structure and function of red blood cells
3. Describe white blood cell types and their functions
4. Explain platelet structure and function
5. Describe plasma components and their roles
6. Explain the complete blood count and its interpretation

Lecture Outline

I. Overview of Hematopoiesis

Hematopoiesis refers to the formation and development of blood cells from multipotent stem cells through a series of differentiation steps. This essential physiological process occurs in different anatomical locations throughout human development, beginning in the yolk sac during early embryogenesis, transitioning to the liver and spleen during fetal life, and ultimately establishing permanent residence in the bone marrow by birth. In adults, hematopoiesis is confined to the bone marrow of the axial skeleton, pelvis, and proximal portions of long bones, where the microenvironment provides optimal conditions for stem cell maintenance and differentiation. Extramedullary hematopoiesis, occurring in the liver or spleen, is considered pathological in adults and typically indicates bone marrow failure or infiltration.

Hematopoietic stem cells represent the foundation of blood cell production and possess two remarkable properties that distinguish them from other cell types. Self-renewal allows these cells to divide and produce daughter cells that retain full stem cell potential, thereby maintaining the stem cell pool throughout an individual's lifetime. Multipotency enables differentiation into all blood cell lineages, including erythrocytes, leukocytes, and platelets. These stem cells are characterized by specific surface markers including CD34 positivity, CD38 negativity, and lineage marker negativity, which allow their identification and isolation for transplantation purposes. Most hematopoietic stem cells reside in a quiescent state within specialized bone marrow niches, becoming activated only when needed to replenish blood cell populations.

The hematopoietic hierarchy describes the progressive differentiation pathway from primitive stem cells to mature blood cells through intermediate progenitor stages. Long-term and short-term hematopoietic stem cells sit atop this hierarchy, giving rise to multipotent progenitors that become increasingly lineage-restricted. The common myeloid progenitor generates granulocytes, monocytes, erythrocytes, and megakaryocytes, while the common lymphoid progenitor produces B cells, T cells, and natural killer cells. Oligopotent progenitors such as the granulocyte-monocyte progenitor and megakaryocyte-erythrocyte progenitor further restrict differentiation potential before producing lineage-committed cells that mature into functional blood cells.

The bone marrow microenvironment, also called the niche, provides essential signals that regulate hematopoietic stem cell behavior and blood cell production. Osteoblasts lining the endosteal surface help maintain stem cell quiescence and prevent premature differentiation or exhaustion of the stem cell pool. Endothelial cells forming the sinusoidal vasculature support differentiation and allow mature cells to enter circulation. Stromal cells and adipocytes secrete growth factors and cytokines that direct lineage commitment and support progenitor expansion. The extracellular matrix provides structural support and sequesters growth factors, creating concentration gradients that guide cell migration and differentiation.

<image>Panel A: Hierarchical diagram showing hematopoietic stem cell differentiation into common myeloid and lymphoid progenitors with subsequent lineage branching. Panel B: Cross-sectional illustration of bone marrow niche showing osteoblasts, endothelial sinusoids, stromal cells, and adipocytes surrounding hematopoietic cells. Panel C: Schematic of stem cell surface markers CD34, CD38, and lineage markers used for identification. Panel D: Comparison of hematopoietic sites during fetal development versus adult life showing yolk sac, liver, spleen, and bone marrow.</image>

II. Hematopoietic Growth Factors

Cytokines and growth factors provide essential signals that regulate the survival, proliferation, and differentiation of hematopoietic progenitors. Erythropoietin drives red blood cell production, thrombopoietin stimulates platelet formation, and granulocyte colony-stimulating factor promotes neutrophil development. Granulocyte-macrophage colony-stimulating factor supports multiple myeloid lineages, while macrophage colony-stimulating factor specifically directs monocyte and macrophage development. Interleukin-3 exhibits multilineage activity affecting multiple progenitor populations, and stem cell factor acts on early progenitors to maintain their survival and proliferative capacity.

Erythropoietin serves as the primary regulator of red blood cell production and exemplifies the feedback mechanisms controlling hematopoiesis. The kidneys produce approximately ninety percent of circulating erythropoietin, with the liver contributing the remaining ten percent. Tissue hypoxia activates the hypoxia-inducible factor pathway, which upregulates erythropoietin gene transcription and increases hormone production. Erythropoietin binds to receptors on colony-forming unit-erythroid cells and proerythroblasts, promoting their survival, proliferation, and terminal differentiation into mature red blood cells. Recombinant erythropoietin has become an important therapeutic agent for treating anemia associated with chronic kidney disease, cancer chemotherapy, and other conditions.

Colony-stimulating factors have revolutionized supportive care in hematology and oncology through their ability to accelerate blood cell recovery. Granulocyte colony-stimulating factor, available as filgrastim and pegfilgrastim, effectively treats chemotherapy-induced neutropenia and mobilizes stem cells from bone marrow into peripheral blood for collection and transplantation. Granulocyte-macrophage colony-stimulating factor, marketed as sargramostim, supports myeloid recovery following bone marrow transplantation and treats certain neutropenic conditions. Thrombopoietin receptor agonists such as romiplostim and eltrombopag stimulate platelet production in immune thrombocytopenia and aplastic anemia. These agents represent targeted interventions that harness natural regulatory mechanisms to address specific cytopenias.

Hematopoiesis is tightly regulated through positive and negative feedback mechanisms that maintain blood cell counts within normal ranges. Hypoxia triggers increased erythropoietin production, which stimulates red cell production until oxygen delivery normalizes and suppresses further hormone release. Stromal signals including stem cell factor and FLT3 ligand maintain progenitor populations and support their response to lineage-specific factors. Transcription factors such as GATA-1, PU.1, and C/EBP-alpha function as master regulators that control lineage commitment by activating lineage-specific gene programs while repressing alternative fates. Dysregulation of these transcriptional networks underlies many hematologic malignancies.

<image>Panel A: Diagram of cytokine receptors and their downstream signaling pathways including JAK-STAT activation. Panel B: Illustration of the hypoxia-inducible factor pathway linking tissue oxygen levels to erythropoietin production. Panel C: Chart showing clinical applications of recombinant growth factors including filgrastim, epoetin, and thrombopoietin agonists. Panel D: Negative feedback loop demonstrating how increased red cell mass suppresses erythropoietin production.</image>

III. Red Blood Cells

Erythropoiesis describes the developmental process by which committed progenitors differentiate into mature red blood cells through a series of morphologically distinct stages. Proerythroblasts are large nucleated cells with basophilic cytoplasm reflecting high RNA content needed for hemoglobin synthesis. As maturation proceeds through basophilic, polychromatic, and orthochromatic erythroblast stages, cells accumulate hemoglobin while progressively condensing and eventually extruding their nuclei. Reticulocytes are anucleate cells that retain residual RNA and organelles, spending one to two days maturing in circulation before becoming fully mature red blood cells. This entire process takes approximately one week and produces cells optimized for oxygen transport.

Mature red blood cells possess a distinctive biconcave disc shape measuring seven to eight micrometers in diameter that maximizes surface area for gas exchange. Unlike most cells, mammalian erythrocytes lack nuclei and mitochondria, relying entirely on anaerobic glycolysis for ATP production. The plasma membrane consists of a lipid bilayer anchored to an underlying spectrin-based cytoskeleton that confers remarkable deformability, allowing cells to navigate through capillaries smaller than their own diameter. Red blood cells circulate for approximately 120 days before being removed by splenic macrophages that recognize senescent changes in the membrane. This exceptional lifespan combined with a production rate of two million cells per second maintains the red cell mass.

Hemoglobin is the oxygen-carrying protein that constitutes approximately ninety-five percent of red blood cell dry weight. Each molecule consists of four globin chains, each containing a heme group with a central iron atom in the ferrous state capable of reversibly binding oxygen. Adult hemoglobin A comprises two alpha and two beta chains, constituting ninety-seven percent of adult hemoglobin. Hemoglobin A2, composed of two alpha and two delta chains, accounts for two to three percent, while fetal hemoglobin containing gamma chains instead of beta chains normally falls below one percent in adults. Oxygen binding exhibits cooperativity, producing a sigmoidal dissociation curve that facilitates efficient oxygen loading in the lungs and unloading in tissues.

Red blood cell metabolism supports the unique requirements of oxygen transport and membrane integrity despite the absence of mitochondria. Glycolysis generates ATP needed to maintain ion gradients, membrane flexibility, and cell shape through energy-dependent processes. The Rapoport-Luebering shunt produces 2,3-diphosphoglycerate, which binds hemoglobin and reduces oxygen affinity, promoting oxygen release to tissues. The hexose monophosphate shunt generates NADPH required for reducing glutathione, which protects against oxidative damage to hemoglobin and membrane lipids. Methemoglobin reductase maintains hemoglobin iron in the functional ferrous state, preventing accumulation of non-functional methemoglobin.

<image>Panel A: Sequential stages of erythroid maturation from proerythroblast through reticulocyte to mature red blood cell showing nuclear condensation and hemoglobin accumulation. Panel B: Detailed cross-section of red blood cell membrane showing spectrin-actin cytoskeleton anchored to integral membrane proteins. Panel C: Three-dimensional structure of hemoglobin tetramer with four globin chains and heme groups highlighted. Panel D: Oxygen-hemoglobin dissociation curve demonstrating sigmoidal shape with factors causing rightward and leftward shifts.</image>

IV. White Blood Cells

Granulocytes comprise neutrophils, eosinophils, and basophils, named for their prominent cytoplasmic granules that contain antimicrobial and inflammatory mediators. Neutrophils represent the most abundant white blood cell type, constituting fifty to seventy percent of circulating leukocytes, and serve as the primary defense against bacterial and fungal infections through phagocytosis and release of toxic granule contents. Eosinophils specialize in combating parasitic infections and participate in allergic inflammation through release of major basic protein and other cytotoxic mediators. Basophils, the rarest granulocyte, contain histamine and other vasoactive substances that contribute to allergic reactions and immune regulation.

Neutrophils possess distinctive morphology and functional characteristics that enable their role as first responders to infection. Their nucleus contains three to five lobes connected by thin chromatin strands, giving rise to the term polymorphonuclear leukocyte. Primary or azurophilic granules contain myeloperoxidase, defensins, and proteases, while secondary or specific granules hold lactoferrin, collagenase, and membrane receptors. Neutrophils circulate for only hours to days before migrating into tissues or undergoing apoptosis. Band forms represent slightly immature neutrophils with unsegmented nuclei; their increased presence, termed a left shift, indicates active bone marrow response to infection or inflammation.

Agranulocytes include monocytes and lymphocytes, which lack the prominent granules characteristic of granulocytes. Monocytes are large cells with kidney-shaped nuclei that circulate briefly before migrating into tissues where they differentiate into macrophages, dendritic cells, or osteoclasts depending on local signals. Tissue macrophages perform phagocytosis, antigen presentation, and cytokine secretion, living for months to years in their tissue environments. Lymphocytes are smaller cells with round nuclei and scant cytoplasm that mediate adaptive immune responses through T cell-mediated cellular immunity and B cell-derived antibody production.

The normal white blood cell differential reflects the relative proportions and absolute counts of each leukocyte type in peripheral blood. Neutrophils normally range from 2,500 to 7,500 per microliter, representing fifty to seventy percent of total white cells. Lymphocytes typically number 1,000 to 4,000 per microliter, accounting for twenty to forty percent. Monocytes comprise two to eight percent with absolute counts of 100 to 800 per microliter. Eosinophils represent one to four percent at 50 to 500 per microliter, while basophils rarely exceed one percent or 100 per microliter. Absolute counts provide more clinically useful information than percentages when evaluating leukocyte abnormalities.

<image>Panel A: Comparison of granulocyte morphology showing multilobed neutrophil nucleus, bilobed eosinophil with red-orange granules, and basophil with dark purple granules. Panel B: Neutrophil granule contents diagram showing primary granules with myeloperoxidase and secondary granules with lactoferrin. Panel C: Monocyte-to-macrophage differentiation pathway in different tissue environments. Panel D: Lymphocyte subtypes including T cells, B cells, and natural killer cells with their characteristic surface markers.</image>

V. Platelets

Thrombopoiesis is the process by which megakaryocytes produce platelets through a unique mechanism involving cytoplasmic fragmentation rather than cell division. Megakaryoblasts are committed progenitors that undergo repeated rounds of DNA replication without cell division, a process called endomitosis, becoming polyploid cells with sixteen to thirty-two times the normal DNA content. Mature megakaryocytes extend long cytoplasmic projections called proplatelets into bone marrow sinusoids, where shear forces and internal cytoskeletal processes release individual platelets into circulation. Each megakaryocyte produces one to three thousand platelets, and thrombopoietin synthesized by the liver regulates this process in response to circulating platelet mass.

Platelets are small anucleate cell fragments measuring two to four micrometers in diameter with complex internal structure supporting their hemostatic functions. Dense or delta granules contain adenosine diphosphate, serotonin, and calcium that amplify platelet activation when released. Alpha granules store von Willebrand factor, fibrinogen, platelet factor 4, and growth factors essential for coagulation and wound healing. Surface glycoprotein receptors mediate platelet adhesion and aggregation, with GPIb-IX-V binding von Willebrand factor for initial adhesion and GPIIb/IIIa binding fibrinogen for platelet-platelet aggregation. The open canalicular system provides channels for granule content release and membrane expansion during activation.

Platelet function in hemostasis involves sequential processes of adhesion, activation, and aggregation that form the primary hemostatic plug. Adhesion occurs when von Willebrand factor bridges exposed subendothelial collagen to platelet GPIb receptors, anchoring platelets at sites of vascular injury. Activation triggers dramatic shape change from disc to sphere with extending pseudopods, along with granule release that recruits and activates additional platelets. Aggregation results from fibrinogen binding to activated GPIIb/IIIa receptors on adjacent platelets, creating a platelet plug that provides the surface for coagulation factor assembly. Activated platelets also expose phosphatidylserine, which serves as a procoagulant surface accelerating thrombin generation.

Normal platelet parameters include a count of 150,000 to 400,000 per microliter, reflecting the balance between production and consumption. Mean platelet volume ranges from seven to eleven femtoliters, with larger platelets generally being younger and more reactive. Platelet distribution width indicates variation in platelet size within a sample. Platelets circulate for eight to ten days before removal by splenic and hepatic macrophages, with approximately one-third of the platelet mass sequestered in the spleen at any given time. Splenomegaly can significantly increase this splenic pool, causing apparent thrombocytopenia despite normal total body platelet mass.

<image>Panel A: Megakaryocyte extending proplatelet processes into bone marrow sinusoid with platelets budding from the tips. Panel B: Cross-sectional ultrastructure of platelet showing alpha granules, dense granules, open canalicular system, and dense tubular system. Panel C: Sequential steps of platelet adhesion via von Willebrand factor, activation with shape change, and aggregation via fibrinogen bridges. Panel D: Diagram showing platelet surface glycoproteins GPIb-IX-V and GPIIb/IIIa with their respective ligands.</image>

VI. Plasma

Plasma is the liquid component of blood, comprising approximately fifty-five percent of total blood volume and serving as the medium for transporting cells, nutrients, hormones, and waste products. Water constitutes approximately ninety-two percent of plasma volume, providing the solvent in which all other components are dissolved or suspended. Proteins account for approximately seven percent of plasma, while the remaining one percent includes electrolytes, glucose, lipids, hormones, and metabolic waste products. Plasma can be separated from cellular components by centrifugation of anticoagulated blood, yielding a straw-colored fluid essential for numerous diagnostic tests and therapeutic products.

Plasma proteins perform diverse functions essential for maintaining physiological homeostasis and providing immune defense. Albumin is the most abundant plasma protein at 3.5 to 5.0 grams per deciliter, maintaining oncotic pressure that prevents fluid loss from the vascular compartment and serving as a carrier for drugs, bilirubin, fatty acids, and hormones. Immunoglobulins or antibodies provide humoral immunity against pathogens and constitute the second largest protein fraction. Fibrinogen and other coagulation factors enable blood clotting, complement proteins participate in innate immunity, and lipoproteins transport cholesterol and triglycerides throughout the body.

Albumin deserves special attention given its clinical importance as both a diagnostic marker and therapeutic agent. Synthesized exclusively by the liver, albumin has a half-life of approximately twenty days, making it a useful indicator of chronic liver synthetic function and nutritional status. Hypoalbuminemia causes decreased oncotic pressure and contributes to edema formation in conditions such as cirrhosis, nephrotic syndrome, and severe malnutrition. Beyond maintaining oncotic pressure, albumin binds and transports numerous endogenous and exogenous compounds, affecting the pharmacokinetics of many medications. Intravenous albumin administration is used therapeutically for volume expansion and in specific conditions such as spontaneous bacterial peritonitis.

The distinction between plasma and serum is clinically important and reflects whether blood samples are collected with or without anticoagulants. Plasma is obtained from anticoagulated blood and contains all coagulation factors including fibrinogen. Serum is the liquid remaining after blood has clotted and therefore lacks fibrinogen and other factors consumed during coagulation. Plasma samples are required for coagulation testing since the factors being measured must be preserved. Serum is preferred for most chemistry and serological tests because fibrinogen can interfere with certain assays. Understanding this distinction ensures appropriate sample collection for specific laboratory evaluations.

<image>Panel A: Pie chart showing plasma composition with water, proteins, and other solutes in their relative proportions. Panel B: Serum protein electrophoresis pattern showing albumin peak and globulin fractions with labeled regions. Panel C: Diagram illustrating albumin's multiple functions as oncotic pressure maintainer and carrier protein for various ligands. Panel D: Comparison of plasma versus serum collection tubes and their different components.</image>

VII. Complete Blood Count (CBC)

The complete blood count provides comprehensive quantitative assessment of circulating blood cells and is among the most frequently ordered laboratory tests in clinical medicine. Red blood cell parameters include the count itself, hemoglobin concentration, and hematocrit, which together assess oxygen-carrying capacity. Normal values differ by sex, with men having higher values than women due to testosterone's stimulatory effect on erythropoiesis. For adult men, normal hemoglobin ranges from 14 to 18 grams per deciliter with hematocrit of 40 to 54 percent, while women normally have hemoglobin of 12 to 16 grams per deciliter and hematocrit of 37 to 47 percent. These parameters represent the starting point for evaluating anemia or polycythemia.

Red blood cell indices provide information about cell size and hemoglobin content that guides differential diagnosis of anemia. Mean corpuscular volume measures average red cell size in femtoliters, with normal values of 80 to 100 femtoliters, and classifies anemias as microcytic, normocytic, or macrocytic. Mean corpuscular hemoglobin indicates the average hemoglobin mass per red cell at 27 to 33 picograms, while mean corpuscular hemoglobin concentration represents hemoglobin concentration within cells at 32 to 36 grams per deciliter. Red cell distribution width quantifies variation in red cell size, with elevated values indicating anisocytosis that may suggest iron deficiency, mixed deficiencies, or reticulocytosis.

Mean corpuscular volume provides the foundation for morphologic classification of anemia and directs subsequent diagnostic evaluation. Microcytic anemia with MCV below 80 femtoliters suggests iron deficiency, thalassemia, anemia of chronic disease, or sideroblastic anemia. Normocytic anemia with MCV between 80 and 100 femtoliters includes acute blood loss, hemolysis, early nutritional deficiencies, chronic disease, and bone marrow failure. Macrocytic anemia with MCV exceeding 100 femtoliters most commonly results from vitamin B12 or folate deficiency, liver disease, hypothyroidism, or myelodysplastic syndromes. This classification system efficiently narrows the differential diagnosis and guides appropriate testing.

White blood cell and platelet parameters complete the standard CBC analysis. Total white blood cell count normally ranges from 4,500 to 11,000 per microliter, with automated differentials providing absolute counts and percentages for neutrophils, lymphocytes, monocytes, eosinophils, and basophils. The absolute neutrophil count, calculated from total white count and neutrophil percentage, defines neutropenia when below 1,500 per microliter. Platelet count normally ranges from 150,000 to 400,000 per microliter, with thrombocytopenia below and thrombocytosis above these values. Mean platelet volume may provide additional clinical information in certain thrombocytopenic conditions.

<image>Panel A: Sample CBC report with normal values highlighted and abnormal results flagged for clinical attention. Panel B: Flow diagram showing MCV-based classification of anemia into microcytic, normocytic, and macrocytic categories with differential diagnoses. Panel C: Illustration of how automated hematology analyzers measure cell size, hemoglobin content, and cell counts using impedance and light scatter. Panel D: Age-related variations in normal CBC values from newborns through adults.</image>

VIII. Peripheral Blood Smear

Red blood cell morphology assessment on peripheral blood smear reveals abnormalities that provide specific diagnostic clues beyond automated indices. Microcytes are small red cells seen in iron deficiency and thalassemia, while macrocytes are enlarged cells characteristic of megaloblastic anemia and liver disease. Spherocytes lack central pallor and appear as small dense spheres in hereditary spherocytosis and autoimmune hemolytic anemia. Schistocytes are fragmented red cells with sharp edges indicative of microangiopathic hemolytic anemia seen in thrombotic thrombocytopenic purpura, hemolytic uremic syndrome, and disseminated intravascular coagulation. Target cells display a central hemoglobin spot within the area of central pallor, occurring in liver disease, thalassemia, and hemoglobin C disease.

Additional red blood cell findings provide important diagnostic information when recognized on smear review. Howell-Jolly bodies are nuclear remnants appearing as single dense purple inclusions, normally removed by the spleen and therefore indicating functional asplenia when present. Basophilic stippling represents aggregated ribosomes appearing as fine blue dots throughout the cytoplasm, seen in lead poisoning, thalassemia, and sideroblastic anemia. Pappenheimer bodies are iron-containing granules that stain with standard stains, appearing in sideroblastic anemia and post-splenectomy. Heinz bodies require supravital staining to detect and represent denatured hemoglobin in G6PD deficiency and unstable hemoglobin disorders. Rouleaux formation describes red cells stacked like coins, occurring with elevated plasma proteins in multiple myeloma and inflammatory states.

White blood cell abnormalities visible on peripheral smear assist in diagnosing infections, hematologic malignancies, and other conditions. Left shift describes increased band forms and more immature myeloid cells, indicating active bone marrow response to infection or inflammation. Toxic granulation appears as prominent dark azurophilic granules in neutrophils during severe infection or inflammation. Dohle bodies are pale blue cytoplasmic inclusions in neutrophils associated with infection, burns, and pregnancy. Hypersegmented neutrophils with more than five nuclear lobes characterize megaloblastic anemia. Auer rods are crystallized azurophilic granules pathognomonic for acute myeloid leukemia. Atypical lymphocytes with abundant blue cytoplasm and irregular shapes indicate viral infections, particularly Epstein-Barr virus infectious mononucleosis.

Platelet abnormalities visible on smear include size variations and distribution changes that provide diagnostic information. Giant platelets approaching or exceeding red cell size suggest disorders such as MYH9-related conditions or active regeneration in immune thrombocytopenia. Platelet clumping causes artifactually low automated platelet counts, often due to EDTA-induced antibodies, and should prompt repeat testing with citrate anticoagulation. Gray platelets lacking normal granular appearance indicate alpha granule deficiency. Assessment of platelet adequacy on smear, with normal appearing as approximately one platelet per ten to twenty red cells, helps verify automated counts and detect pseudothrombocytopenia.

<image>Panel A: Gallery of red blood cell morphology abnormalities including spherocytes, schistocytes, target cells, sickle cells, and tear-drop cells with brief descriptions. Panel B: White blood cell abnormalities showing toxic granulation, Dohle bodies, hypersegmented neutrophils, and Auer rods in blast cells. Panel C: Comparison of normal platelets, giant platelets, and platelet clumping artifacts. Panel D: Systematic approach to peripheral smear examination under low and high power magnification.</image>

IX. Bone Marrow Examination

Bone marrow examination is indicated when peripheral blood findings cannot explain cytopenias, when hematologic malignancy is suspected, for staging certain cancers, and for diagnosing infiltrative or storage diseases. Unexplained cytopenias require bone marrow evaluation to distinguish production problems from peripheral destruction and to identify underlying causes such as myelodysplasia, aplastic anemia, or malignancy. Suspected leukemia, lymphoma, or multiple myeloma generally requires bone marrow confirmation for diagnosis and classification. Staging for non-Hodgkin lymphoma and certain solid tumors may include bone marrow biopsy to detect occult involvement. Fever of unknown origin may prompt examination to identify infections, granulomatous diseases, or malignancies.

Bone marrow aspiration and biopsy are complementary procedures typically performed together to provide comprehensive assessment. Aspiration yields liquid marrow that is spread on slides for morphologic examination and can be processed for flow cytometry immunophenotyping, cytogenetic analysis, and molecular studies. Biopsy obtains a core of bone containing marrow that is processed for histologic sections showing architectural relationships and cellularity. The posterior iliac crest is the preferred site in adults due to accessible bone with abundant marrow, though sternal aspiration represents an alternative when the pelvis cannot be accessed. Both procedures require local anesthesia and can typically be performed safely as outpatient procedures.

Normal bone marrow findings provide the reference against which abnormalities are measured. Cellularity, the ratio of hematopoietic cells to fat, normally approximates 100 minus the patient's age in percentage terms, declining naturally with aging. The myeloid to erythroid ratio normally ranges from 2:1 to 4:1, reflecting the shorter lifespan of granulocytes compared to red cells. Megakaryocytes should be present in normal numbers with typical lobulated nuclei. Iron stores, assessed by Prussian blue staining, should be visible in macrophages as storage iron. Blast cells should constitute less than five percent of nucleated cells, with higher percentages defining myelodysplastic syndromes or acute leukemia.

Special studies performed on bone marrow samples provide diagnostic and prognostic information beyond routine morphology. Flow cytometry identifies cell surface markers that characterize leukemias and lymphomas, distinguishing B-cell from T-cell neoplasms and identifying aberrant marker patterns. Conventional cytogenetics reveals chromosomal abnormalities through metaphase karyotyping, detecting translocations, deletions, and numerical changes important for diagnosis and prognosis. Fluorescence in situ hybridization detects specific genetic changes when cells are not dividing or when targeting known abnormalities. Molecular testing using polymerase chain reaction and next-generation sequencing identifies gene mutations, fusion transcripts, and other genetic alterations increasingly important for treatment selection.

<image>Panel A: Illustration of bone marrow aspiration and biopsy technique at the posterior iliac crest showing needle placement and angle. Panel B: Normal bone marrow aspirate smear showing hematopoietic precursors at various stages of maturation. Panel C: Bone marrow biopsy section demonstrating normal cellularity with trilineage hematopoiesis and appropriate myeloid to erythroid ratio. Panel D: Examples of special stains including Prussian blue for iron and reticulin for fibrosis.</image>

X. Clinical Correlations

Anemia classification by pathophysiologic mechanism provides a framework for understanding and diagnosing decreased red blood cell mass. Decreased production results from bone marrow failure, nutritional deficiencies, chronic disease, or infiltrative processes that impair erythropoiesis despite adequate raw materials. Increased destruction or hemolysis shortens red cell survival through intrinsic defects affecting the membrane, enzymes, or hemoglobin, or through extrinsic factors including antibodies, mechanical trauma, or infections. Blood loss causes anemia through acute hemorrhage or chronic occult bleeding that depletes iron stores over time. Combining this mechanistic approach with morphologic classification based on MCV efficiently directs diagnostic evaluation.

Leukocyte disorders encompass both quantitative abnormalities of too many or too few cells and qualitative defects affecting function. Neutrophilia typically results from infection, inflammation, corticosteroid therapy, or myeloproliferative disorders, while neutropenia may be drug-induced, autoimmune, or due to bone marrow failure. Lymphocytosis occurs with viral infections such as infectious mononucleosis, pertussis, or chronic lymphocytic leukemia. Eosinophilia suggests allergic conditions, parasitic infections, or certain malignancies. Recognizing whether changes are reactive or clonal guides further workup, with features such as circulating blasts, unexplained cytopenias, or persistent unexplained abnormalities warranting hematologic evaluation.

Platelet disorders include thrombocytopenia from decreased production, increased destruction, or sequestration, as well as thrombocytosis that may be reactive or clonal. Thrombocytopenia due to bone marrow failure differs fundamentally from immune destruction, which is characterized by rapid platelet consumption with large circulating platelets. Reactive thrombocytosis commonly accompanies iron deficiency, infection, inflammation, and malignancy, while clonal thrombocytosis occurs in myeloproliferative neoplasms. Qualitative platelet defects from drugs such as aspirin, uremia, or inherited conditions may cause bleeding despite normal platelet counts.

Pancytopenia, the simultaneous reduction of all three cell lines, represents a particularly concerning finding requiring prompt evaluation. Bone marrow failure from aplastic anemia represents one major category, characterized by hypocellular marrow with fatty replacement. Bone marrow infiltration by leukemia, lymphoma, metastatic cancer, or fibrosis displaces normal hematopoietic tissue. Nutritional deficiency, particularly severe vitamin B12 or folate deficiency, causes ineffective hematopoiesis with paradoxically hypercellular marrow. Hypersplenism sequesters cells in an enlarged spleen, reducing circulating counts despite normal or increased production. The peripheral smear, bone marrow examination, and additional targeted testing distinguish these causes.

<image>Panel A: Algorithm for evaluating anemia based on MCV and reticulocyte count, branching to specific diagnoses. Panel B: Differential diagnosis of pancytopenia organized by bone marrow cellularity findings. Panel C: Comparison of reactive versus clonal leukocytosis features including clinical context, cell maturation, and clonality testing. Panel D: Approach to thrombocytopenia distinguishing decreased production from increased destruction based on clinical features and bone marrow findings.</image>

Summary

• Hematopoiesis occurs in bone marrow in adults, where hematopoietic stem cells maintain self-renewal capacity and differentiate into all blood cell lineages through progressively restricted progenitors
• Growth factors including erythropoietin for red cells, thrombopoietin for platelets, and G-CSF for neutrophils regulate lineage-specific production through receptor-mediated signaling pathways
• Red blood cells are biconcave discs containing hemoglobin for oxygen transport, lacking nuclei and mitochondria, and surviving approximately 120 days in circulation
• White blood cells include neutrophils as the most common type providing bacterial defense, lymphocytes mediating adaptive immunity, and monocytes that become tissue macrophages
• Platelets are anucleate fragments from megakaryocytes that mediate primary hemostasis through adhesion to injured vessels and aggregation via fibrinogen bridges
• Plasma contains albumin maintaining oncotic pressure, immunoglobulins for immune defense, and coagulation factors for hemostasis
• The complete blood count provides hemoglobin, hematocrit, red cell indices including MCV for morphologic classification, white cell differential, and platelet count
• Peripheral blood smear reveals morphologic abnormalities including schistocytes in microangiopathic hemolysis, spherocytes in hereditary spherocytosis, and blasts in acute leukemia
• Bone marrow examination with aspiration for morphology and flow cytometry, plus biopsy for architecture, evaluates unexplained cytopenias and suspected malignancies

Key Terms

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