Medical School · Year 1 · Histology · includes a quiz and discussion video

Lecture 6: Blood and Hematopoiesis

Unit 1.2: Histology and Basic Tissues

Learning Objectives

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

1. Identify the components of blood and their functions
2. Describe the morphology and function of red blood cells
3. Classify white blood cells and identify them on peripheral blood smear
4. Describe platelet structure and function
5. Explain the process of erythropoiesis and leukopoiesis
6. Identify hematopoietic cells at different stages of maturation

Blood as Connective Tissue

Blood is a specialized fluid connective tissue that circulates throughout the body, serving as the transport medium for gases, nutrients, wastes, hormones, and immune cells. Unlike other connective tissues with solid or gel-like matrices, blood has a liquid extracellular matrix—plasma—in which the cellular elements are suspended.

Blood can be separated into its components by centrifugation. Plasma constitutes approximately 55% of blood volume and appears as a straw-colored fluid above the cellular elements. Plasma is 90% water by weight, with the remainder consisting of dissolved proteins (albumin for osmotic pressure, globulins including antibodies, and fibrinogen for clotting), electrolytes (sodium, potassium, chloride, bicarbonate), nutrients (glucose, amino acids, lipids), metabolic wastes (urea, creatinine), dissolved gases, and hormones.

The formed elements of blood—cells and cell fragments—constitute approximately 45% of blood volume and comprise three main categories. Red blood cells (erythrocytes) are the most numerous and transport respiratory gases. White blood cells (leukocytes) are nucleated cells that perform immune functions. Platelets (thrombocytes) are cell fragments essential for hemostasis.

The hematocrit measures the percentage of blood volume occupied by red blood cells specifically. Normal values differ by sex: males typically range from 40-54%, while females range from 36-48%. This difference reflects the influence of testosterone on erythropoiesis. Hematocrit is clinically important in diagnosing anemia (decreased hematocrit) and polycythemia (increased hematocrit).

When blood is centrifuged, a thin layer called the buffy coat appears between the plasma and the packed red cells. This layer, typically less than 1% of blood volume, contains the white blood cells and platelets.

<image>Panel A: Centrifuged blood tube showing top portion of straw-yellow translucent plasma at approximately 55% volume with contents list including water 90%, proteins, electrolytes, nutrients, wastes, hormones, and dissolved gases. Panel B: Very thin whitish-gray buffy coat layer at less than 1% volume containing WBCs and platelets with magnified inset showing distinct cell types. Panel C: Bottom portion of dark red-maroon packed red blood cells at approximately 45% volume representing the hematocrit measurement. Panel D: Normal hematocrit range comparison showing male 40-54% versus female 36-48% with clinical laboratory style demarcations and percentage labels.</image>

Red Blood Cells (Erythrocytes)

Red blood cells are the most abundant cells in blood, numbering approximately 4-6 million per microliter (higher in males, lower in females). Their primary function is oxygen transport from the lungs to tissues and carbon dioxide transport from tissues to the lungs.

Structure

The mature red blood cell has a distinctive biconcave disc shape, measuring approximately 7-8 micrometers in diameter and about 2.5 micrometers thick at the periphery, thinning to less than 1 micrometer in the center. This biconcave shape optimizes the surface area-to-volume ratio for gas exchange—the total membrane surface area of all RBCs in the body is approximately 3,000 square meters. The shape also increases flexibility, allowing RBCs to deform as they pass through capillaries as narrow as 3 micrometers.

Mature mammalian red blood cells are anucleate—they extrude their nuclei during development. They also lack mitochondria, ribosomes, and other organelles. Without mitochondria, RBCs cannot perform oxidative phosphorylation and rely entirely on glycolysis for ATP production. The absence of nuclei and protein-synthesizing machinery means RBCs cannot repair or replace their components, contributing to their limited lifespan.

On a stained peripheral blood smear, red blood cells appear as round cells staining pink to salmon (eosinophilic) due to their hemoglobin content. The biconcave shape creates a characteristic central pallor—the thin center stains lighter than the thicker periphery. Central pallor typically occupies about one-third of the cell diameter; increased central pallor suggests hypochromia (reduced hemoglobin content), while decreased pallor suggests spherocytosis.

Molecular Components

Hemoglobin constitutes approximately 97% of the dry weight of red blood cells, accounting for their red color and oxygen-carrying capacity. Each hemoglobin molecule contains four globin chains, each bound to a heme group containing an iron atom that reversibly binds oxygen. Normal adult hemoglobin (HbA) contains two alpha chains and two beta chains.

The red cell membrane contains a complex cytoskeletal network that maintains the cell's shape and flexibility. Spectrin proteins form a meshwork just beneath the membrane, linked to the lipid bilayer through ankyrin and band 3 protein. Defects in these structural proteins cause hereditary spherocytosis and elliptocytosis, conditions where RBCs lose their normal shape and are prematurely destroyed.

Function and Life Span

Red blood cells transport oxygen bound to hemoglobin (as oxyhemoglobin) from the lungs to peripheral tissues, where low oxygen partial pressure causes release. They transport carbon dioxide from tissues to lungs partly bound to hemoglobin (as carbaminohemoglobin) and partly as bicarbonate ion (after conversion by carbonic anhydrase). Hemoglobin also participates in pH buffering.

The average red blood cell circulates for approximately 120 days before being removed from circulation. Aged or damaged RBCs are recognized and phagocytosed by macrophages, primarily in the spleen (also liver and bone marrow). The hemoglobin is broken down: globin chains are recycled as amino acids, iron is reclaimed and transported by transferrin for reuse, and the porphyrin ring is converted to bilirubin, which is conjugated in the liver and excreted in bile. Given the approximately 25 trillion red blood cells in circulation, about 200 billion new RBCs must be produced daily to replace those removed—a rate of over 2 million per second.

<image>Panel A: Red blood cell side view showing biconcave disc shape with labeled dimensions of 7-8 micrometer diameter, 2.5 micrometer thickness at edges, and approximately 1 micrometer at center, plus top view showing circular outline with visible paler central region. Panel B: Peripheral blood smear with Wright stain showing multiple salmon-pink RBCs with central pallor occupying one-third diameter and occasional rouleaux stacking like coins. Panel C: Central pallor comparison inset showing normal versus increased pallor in hypochromic cell with thin peripheral rim versus decreased pallor in uniformly dense spherocyte without clearing. Panel D: RBC membrane schematic cross-section showing lipid bilayer, purple spectrin meshwork, green band 3, orange glycophorin, blue ankyrin connecting spectrin to band 3, and small red hemoglobin circles filling cytoplasm.</image>

White Blood Cells (Leukocytes)

White blood cells are nucleated cells responsible for immune defense. Unlike red blood cells, which function entirely within the vasculature, white blood cells use the blood primarily as a transport medium to reach tissues where they perform their functions. The total white blood cell count in healthy adults ranges from 4,500 to 11,000 cells per microliter, far fewer than red blood cells.

White blood cells are classified into two major groups based on the presence or absence of visible cytoplasmic granules. Granulocytes contain prominent specific granules that distinguish the three types: neutrophils, eosinophils, and basophils, named for their staining characteristics. Agranulocytes lack prominent specific granules and include lymphocytes and monocytes (though they may contain some azurophilic granules).

The differential white blood cell count expresses the percentage of each cell type. In normal adults, neutrophils predominate (50-70%), followed by lymphocytes (20-40%), monocytes (2-8%), eosinophils (1-4%), and basophils (<1%). A useful mnemonic for this order is "Never Let Monkeys Eat Bananas" or "Neutrophils, Lymphocytes, Monocytes, Eosinophils, Basophils." The absolute count (cells per microliter) is often more clinically meaningful than percentages.

<image>Panel A: Pie chart showing neutrophils as largest segment at 50-70% in pale pink-tan and lymphocytes as second largest at 20-40% in blue with percentage ranges and absolute counts per microliter labeled. Panel B: Smaller pie segments showing monocytes at 2-8% in purple, eosinophils at 1-4% in red-orange, and basophils at less than 1% in dark blue-purple barely visible. Panel C: Wright-stained oil immersion images of each cell type in circular insets connected by lines to their respective pie chart segments showing distinctive morphology. Panel D: Mnemonic table showing Never Let Monkeys Eat Bananas corresponding to Neutrophils, Lymphocytes, Monocytes, Eosinophils, and Basophils in order of abundance.</image>

Granulocytes

Neutrophils

Neutrophils are the most abundant white blood cells in peripheral blood and serve as the first responders in acute bacterial infections. They are also called polymorphonuclear leukocytes (PMNs) or simply "polys" due to their characteristic nuclear shape.

Neutrophils measure 12-15 micrometers in diameter. Their defining feature is a segmented nucleus with 2-5 lobes connected by thin chromatin strands. The lobes are usually not in the same plane, giving different appearances depending on the cell's orientation. The chromatin is densely packed (heterochromatic), staining dark purple. The cytoplasm is pale pink and contains fine granules that stain neither strongly acidic nor basic—hence "neutro-philic."

Neutrophils contain three types of granules. Primary (azurophilic) granules are actually lysosomes present in all cells of the myeloid lineage, containing myeloperoxidase, defensins, and various hydrolytic enzymes. Secondary (specific) granules are unique to neutrophils and contain lactoferrin (bacteriostatic by binding iron), collagenase, and other antimicrobial substances. Tertiary granules contain gelatinase and other enzymes facilitating migration through tissues.

Functionally, neutrophils are professional phagocytes that engulf and destroy bacteria and other pathogens. They are attracted to sites of infection by chemotactic factors, exit the bloodstream through diapedesis, migrate through tissue, and phagocytose microorganisms. Killing occurs through both oxygen-dependent mechanisms (respiratory burst producing reactive oxygen species, myeloperoxidase-halide system) and oxygen-independent mechanisms (defensins, lysozyme, lactoferrin). Neutrophils are short-lived cells that die within 1-2 days in tissue after phagocytosing pathogens; the accumulation of dead neutrophils forms pus.

In females, approximately 3% of neutrophils display a small drumstick-shaped appendage on one nuclear lobe, representing the inactivated X chromosome (Barr body). Band cells are immature neutrophils with unsegmented horseshoe-shaped nuclei; their increased presence in blood (a "left shift") indicates accelerated release from bone marrow, typically in response to infection.

<image>Panel A: Two neutrophils at 1000x oil immersion in Wright-stained smear showing characteristic multilobed nuclei with 3 and 5 lobes connected by thin chromatin strands and densely clumped dark purple chromatin with pale pink dusty cytoplasm containing fine granules. Panel B: Band cell comparison showing characteristic horseshoe-shaped unsegmented nucleus plus female neutrophil inset with small drumstick Barr body appendage projecting from nuclear lobe. Panel C: Schematic diagram of three granule types color-coded showing large purple primary azurophilic granules with myeloperoxidase, smaller pink secondary specific granules with lactoferrin, and small colorless tertiary granules with gelatinase. Panel D: Electron microscopy inset showing granule ultrastructure at 10-15 micrometer cell diameter scale demonstrating detailed internal organization.</image>

Eosinophils

Eosinophils are specialized for defense against parasites and for modulating allergic reactions. They constitute 1-4% of circulating white blood cells.

Eosinophils measure 12-17 micrometers in diameter. Their nucleus typically has two lobes connected by a thin chromatin strand, sometimes described as resembling eyeglasses or spectacles. The defining feature is their large, prominent cytoplasmic granules that stain bright red-orange with the acidic dye eosin—hence "eosino-philic." These granules are uniform in size and nearly fill the cytoplasm, giving the cell a striking appearance easily distinguishable from other leukocytes.

The specific granules of eosinophils contain major basic protein and eosinophil cationic protein, which are toxic to parasites (particularly helminths). In electron microscopy, these granules display a distinctive crystalloid core within the matrix. Eosinophil granules also contain histaminase and other enzymes that degrade mediators released by mast cells and basophils, thereby modulating allergic inflammation.

Eosinophils are found in high numbers in tissues exposed to the external environment, particularly the gastrointestinal tract, respiratory tract, and skin, even when blood eosinophil counts are normal. Blood eosinophilia (increased eosinophil count) is associated with parasitic infections, allergic conditions (asthma, allergic rhinitis, atopic dermatitis), certain drug reactions, and some malignancies. The mnemonic "NAACP" helps recall causes: Neoplasm, Allergy, Asthma, Collagen-vascular disease, and Parasites.

Basophils

Basophils are the rarest white blood cells, constituting less than 1% of circulating leukocytes. They are structurally and functionally similar to tissue mast cells, though they arise from a different developmental lineage.

Basophils measure 14-16 micrometers in diameter and have a bilobed or S-shaped nucleus. However, the nucleus is frequently obscured by the prominent cytoplasmic granules, which are so large and numerous that they overlay the nuclear profile. These granules stain dark purple-blue with basic dyes—hence "baso-philic." The intense basophilia often makes the cell appear as a mass of dark granules with barely visible nuclear material.

Basophil granules contain histamine, heparin, and leukotrienes—mediators of immediate hypersensitivity. Like mast cells, basophils express high-affinity IgE receptors (FcεRI) on their surface. When IgE bound to these receptors is cross-linked by multivalent antigen, basophils degranulate, releasing their contents. This underlies allergic responses including anaphylaxis. Basophils have a limited role compared to mast cells in allergic tissue reactions because they must be recruited from blood, whereas mast cells are already resident in tissues.

<image>Panel A: Neutrophil at 1000x showing 4-lobed nucleus with dense dark purple chromatin and pale pink cytoplasm with fine barely visible dusty granules demonstrating neutral staining affinity. Panel B: Eosinophil showing bilobed eyeglass-shaped nucleus with large uniform bright red-orange refractile granules nearly filling cytoplasm plus electron microscopy inset of crystalloid core within specific granule. Panel C: Basophil showing large dark purple-blue granules dominating and obscuring underlying bilobed nucleus with only hints visible between darker more variable-sized granules than eosinophils. Panel D: Comparison table listing nucleus shape, granule staining, granule contents, and primary function for each cell type with color key showing eosin as acidic red dye and methylene blue as basic blue dye explaining naming convention.</image>

Agranulocytes

Lymphocytes

Lymphocytes are the principal cells of adaptive immunity, responsible for recognizing specific antigens and mounting targeted immune responses. They constitute 20-40% of circulating white blood cells and are the predominant cells in lymphoid tissues.

Lymphocytes in peripheral blood are heterogeneous in size. Small lymphocytes, the most common form in blood, measure 6-8 micrometers—only slightly larger than red blood cells. Medium lymphocytes measure 10-12 micrometers, and large lymphocytes 12-15 micrometers. Regardless of size, lymphocytes share a characteristic appearance: a large, round nucleus with dense chromatin occupying most of the cell volume, surrounded by a thin rim of basophilic (blue) cytoplasm. The nucleus-to-cytoplasm ratio is high. Occasional azurophilic granules may be present, particularly in larger lymphocytes.

Although morphologically similar, lymphocytes include functionally distinct populations that cannot be distinguished by light microscopy alone. T lymphocytes (thymus-derived) mediate cell-mediated immunity, including direct killing of virus-infected cells (cytotoxic T cells) and regulation of other immune cells (helper T cells). B lymphocytes (bone marrow-derived) are responsible for antibody production; when activated, they differentiate into plasma cells. Natural killer (NK) cells are larger lymphocytes with more abundant cytoplasm and granules that kill virus-infected and tumor cells without prior sensitization. Identifying these subtypes requires immunophenotyping with antibodies against specific surface markers (CD markers).

Lymphocytes are unique among blood cells in their ability to leave the blood, circulate through tissues and lymph, and return to blood—a process called lymphocyte recirculation that allows immune surveillance throughout the body. Unlike other white blood cells, lymphocytes can survive for years, maintaining immunological memory.

Monocytes

Monocytes are the largest normal cells in peripheral blood, measuring 12-20 micrometers in diameter. They are precursors to tissue macrophages and constitute 2-8% of circulating white blood cells.

The nucleus of a monocyte is characteristically kidney-shaped, horseshoe-shaped, or deeply indented rather than rounded or lobed. The chromatin is less condensed than in lymphocytes, giving a more dispersed, lacy appearance. The cytoplasm is abundant and has a distinctive pale blue-gray color with fine azurophilic granules (lysosomes), often described as having a "ground glass" appearance. Small vacuoles may be present.

Monocytes circulate in blood for only 1-3 days before migrating into tissues, where they differentiate into macrophages. The specific phenotype of the macrophage depends on the tissue environment, producing specialized forms: Kupffer cells in the liver, alveolar macrophages in the lungs, microglial cells in the central nervous system, osteoclasts in bone, and histiocytes in connective tissue generally. All these cells retain the fundamental macrophage functions of phagocytosis, antigen presentation, and cytokine secretion.

Monocytes and macrophages are key cells of the innate immune system, providing defense against pathogens through phagocytosis, processing and presenting antigens to lymphocytes (bridging innate and adaptive immunity), secreting cytokines that regulate inflammation and recruit other immune cells, and contributing to wound healing and tissue remodeling.

<image>Panel A: Small lymphocyte at 6-8 micrometers shown slightly larger than adjacent RBCs with large round nucleus occupying nearly entire cell and thin blue cytoplasm rim demonstrating high nuclear-to-cytoplasmic ratio. Panel B: Medium lymphocyte at 10-12 micrometers and large lymphocyte at 12-15 micrometers showing size range with more visible cytoplasm and occasional granules but all having dense clumped nuclear chromatin. Panel C: Two monocytes showing nuclear shape variation with kidney-shaped and deeply indented horseshoe nuclei, abundant pale blue-gray ground-glass cytoplasm with fine granules, more dispersed lacy chromatin, and visible small vacuoles. Panel D: Comparison table of size, nucleus shape, chromatin pattern, cytoplasm amount and color, and function for lymphocytes versus monocytes with arrows highlighting key distinguishing features.</image>

Platelets (Thrombocytes)

Platelets are small, anucleate cell fragments essential for hemostasis—the prevention and cessation of bleeding. Despite lacking nuclei, they are metabolically active and contain a variety of organelles and granules.

Structure

Platelets are the smallest formed elements in blood, measuring 2-4 micrometers in diameter. They are fragments of cytoplasm shed from large bone marrow cells called megakaryocytes—a single megakaryocyte produces several thousand platelets. On blood smears, platelets appear as small, irregularly shaped, light purple bodies, often in clusters (platelet clumping is an artifact of handling but also reflects their natural tendency to aggregate).

Under higher magnification, platelets show two zones. The central granulomere appears more darkly staining and contains granules and organelles. The peripheral hyalomere is paler and contains the circumferential band of microtubules that maintains the discoid shape and the actin-myosin system responsible for platelet contraction during clot formation.

Granules

Platelets contain several types of granules whose contents are released upon activation. Alpha granules are the most numerous and contain fibrinogen (for fibrin clot formation), von Willebrand factor (for platelet adhesion), platelet-derived growth factor (for wound healing), and various coagulation factors. Dense granules (delta granules) are fewer but contain small molecules crucial for amplifying platelet activation: ADP and ATP, calcium ions, and serotonin. Lysosomes contain hydrolytic enzymes that contribute to clot remodeling.

Function

Platelets are central to primary hemostasis. When blood vessel injury exposes subendothelial collagen, platelets adhere to the damaged site via von Willebrand factor. Adhesion activates the platelets, which change shape from discoid to spherical with extending pseudopodia, and degranulate, releasing their granule contents. Released ADP and thromboxane A2 recruit and activate additional platelets (aggregation), forming a platelet plug that temporarily seals the wound. The platelet surface provides a phospholipid platform for assembly of coagulation factor complexes, linking primary hemostasis to the coagulation cascade (secondary hemostasis). Platelet-derived growth factors subsequently contribute to wound healing.

The normal platelet count ranges from 150,000 to 400,000 per microliter. Platelets circulate for 8-10 days before being removed by splenic and hepatic macrophages. Approximately one-third of platelets are sequestered in the spleen at any time.

<image>Panel A: Megakaryocyte in bone marrow as very large 50-100 micrometer cell with multilobed polyploid nucleus and extensive granular cytoplasm with long proplatelet processes extending and releasing platelets into sinusoidal capillary cross-section. Panel B: Individual platelets at 1000x in Wright-stained smear as small 2-4 micrometer irregular purple fragments showing distinction between darker central granulomere and paler peripheral hyalomere with cluster demonstrating aggregation tendency. Panel C: Platelet ultrastructure schematic showing discoid shape, blue circumferential microtubule band in hyalomere, open canalicular system, dense tubular calcium storage system, large pink alpha granules, small black dense granules, lysosomes, mitochondria, and glycogen. Panel D: Activation shape change inset from discoid to spherical with extended pseudopodia plus table of alpha granule contents including fibrinogen, vWF, and PDGF and dense granule contents including ADP, ATP, calcium, and serotonin.</image>

Hematopoiesis

Hematopoiesis is the formation and development of blood cells. In adults, hematopoiesis occurs almost exclusively in the red bone marrow, though the site changes during development.

Sites of Hematopoiesis by Age

During the first two months of embryonic development, blood cells form in the yolk sac (primitive or yolk sac hematopoiesis). From approximately the second through seventh month of fetal development, the liver becomes the primary site of blood cell production, with the spleen also contributing. Beginning around the fifth month of gestation, bone marrow hematopoiesis commences and gradually assumes predominance. By birth, the bone marrow is the primary hematopoietic organ.

In infants and young children, active (red) marrow occupies essentially all bone marrow cavities. With age, red marrow progressively retreats to the axial skeleton (vertebrae, ribs, sternum, pelvis) and the proximal ends of the femur and humerus, while peripheral bones convert to yellow (fatty) marrow. In adults, approximately half of bone marrow is hematopoietically active. Under stress (severe anemia, for example), yellow marrow can reconvert to red marrow, and even the liver and spleen can resume hematopoiesis (extramedullary hematopoiesis).

Hematopoietic Stem Cells

All blood cells derive from hematopoietic stem cells (HSCs), which reside in specialized bone marrow niches. HSCs possess two defining properties: pluripotency (the ability to give rise to all blood cell types) and self-renewal (the ability to divide and produce daughter cells that retain stem cell properties). These characteristics make HSCs the basis for bone marrow transplantation.

HSCs are rare cells—approximately 1 in 10,000 to 1 in 100,000 marrow cells. They can be identified by surface markers (notably CD34+ in humans) and by functional assays demonstrating their ability to reconstitute all blood cell lineages.

Hematopoietic Lineages

HSCs give rise to two major lineages through progressively restricted progenitors. The myeloid lineage produces erythrocytes (red blood cells), megakaryocytes (which produce platelets), granulocytes (neutrophils, eosinophils, basophils), and monocytes. The lymphoid lineage produces T lymphocytes, B lymphocytes, and natural killer cells. Various cytokines and growth factors regulate commitment to particular lineages, proliferation, and differentiation. Key growth factors include erythropoietin (EPO) for red cells, thrombopoietin (TPO) for megakaryocytes and platelets, granulocyte colony-stimulating factor (G-CSF) for neutrophils, and multiple interleukins for lymphocyte development.

<image>Panel A: Hematopoietic stem cell at apex branching into common myeloid progenitor and common lymphoid progenitor with red erythroid lineage leading to megakaryocytes, platelets, and red blood cells regulated by EPO and TPO. Panel B: Myeloid branches from CMP showing blue granulocytic lineage producing neutrophils, eosinophils, and basophils regulated by G-CSF and GM-CSF plus green monocyte-macrophage lineage differentiating into Kupffer cells, alveolar macrophages, osteoclasts, and microglia. Panel C: Lymphoid branches from CLP showing purple lineage producing T lymphocytes via thymus, B lymphocytes leading to plasma cells, and NK cells regulated by IL-3 and IL-7. Panel D: Mature cells drawn as Wright-stained blood smear appearance with color-coding key showing red for erythroid, blue for granulocytic, green for monocyte-macrophage, yellow for megakaryocyte-platelet, and purple for lymphoid lineages.</image>

Erythropoiesis

Erythropoiesis is the process by which red blood cells develop from committed progenitors in the bone marrow. The process takes approximately 7 days and involves progressive cell division, hemoglobin accumulation, and ultimately nuclear extrusion.

Stages of Development

The proerythroblast is the earliest recognizable erythroid precursor. It is a large cell (14-19 μm) with a round nucleus containing fine chromatin and prominent nucleoli. The cytoplasm is deeply basophilic (blue) due to abundant ribosomes. The proerythroblast divides to produce basophilic erythroblasts.

The basophilic erythroblast is smaller than the proerythroblast with a round nucleus and intensely basophilic cytoplasm. Hemoglobin synthesis begins but is not yet visible. Nuclear chromatin begins to condense.

The polychromatophilic (polychromatic) erythroblast marks the transition phase when both basophilic ribosomes and eosinophilic hemoglobin contribute to cytoplasmic staining, producing a gray or lilac color—hence "polychromatic" (many colors). The nucleus is smaller with increasingly condensed chromatin.

The orthochromatic erythroblast (normoblast) has accumulated sufficient hemoglobin that the cytoplasm appears predominantly pink (eosinophilic), resembling a mature RBC. The nucleus has become small and pyknotic (very dense, dark), preparing for extrusion. This is the last stage capable of cell division.

The reticulocyte is formed when the orthochromatic erythroblast extrudes its nucleus (enucleation). The resulting cell is nearly mature but retains residual RNA (ribosomes, messenger RNA) for a few more days. With supravital stains (brilliant cresyl blue, new methylene blue), this RNA appears as a blue reticulum, hence the name reticulocyte. Reticulocytes are slightly larger than mature RBCs and may show a faint blue tinge (polychromasia) on Wright stain. They complete their maturation partly in the marrow and partly in circulation, remaining identifiable for 1-2 days after release into blood.

The erythrocyte is the fully mature red blood cell, having lost all RNA and taken on the characteristic biconcave shape.

Regulation

Erythropoietin (EPO) is the principal regulator of erythropoiesis. It is produced primarily by peritubular interstitial cells in the kidney in response to hypoxia (low tissue oxygen). EPO acts on erythroid progenitors in the marrow to prevent their apoptosis and promote proliferation and maturation. Recombinant EPO is used clinically to treat anemia, particularly in chronic kidney disease where endogenous production is impaired.

Effective erythropoiesis requires adequate supplies of iron (for hemoglobin synthesis), vitamin B12 and folate (for DNA synthesis during cell division), and various other nutrients.

<image>Panel A: Proerythroblast as large 14-19 micrometer round cell with large nucleus containing fine chromatin and 1-2 nucleoli with deeply basophilic dark blue cytoplasm followed by basophilic erythroblast slightly smaller with early chromatin condensation and intensely blue cytoplasm. Panel B: Polychromatophilic erythroblast medium-sized with smaller clumped chromatin nucleus and gray-blue to lilac cytoplasm as pink hemoglobin mixes with blue ribosomes followed by orthochromatic erythroblast with small pyknotic dark dense nucleus and predominantly pink-salmon cytoplasm. Panel C: Reticulocyte as anucleate cell slightly larger than mature RBC with faint blue-gray polychromasia plus brilliant cresyl blue inset revealing blue reticular network of residual RNA followed by mature biconcave erythrocyte with central pallor. Panel D: Progressive changes graph showing decreasing cell size, increasing hemoglobin, decreasing nuclear size to zero, and decreasing basophilia with key points noting cell division through orthochromatic stage, enucleation, and EPO action at early stages.</image>

Leukopoiesis

Leukopoiesis encompasses the development of all white blood cells. Granulopoiesis (the formation of neutrophils, eosinophils, and basophils) follows a defined sequence in the marrow, while lymphopoiesis involves both marrow and peripheral lymphoid organs.

Granulopoiesis

The development of neutrophils illustrates the stages of granulopoiesis; eosinophils and basophils follow a parallel sequence but develop their distinct granules.

The myeloblast is the earliest recognizable granulocyte precursor—a large cell with a round to oval nucleus containing fine chromatin and prominent nucleoli. The cytoplasm is basophilic without visible granules.

The promyelocyte develops primary (azurophilic) granules, which appear as prominent purple granules in the cytoplasm. The nucleus remains large and round with nucleoli present. Primary granules are actually lysosomes and will be present in all subsequent stages, though they become less prominent as specific granules accumulate.

The myelocyte marks the appearance of specific (secondary) granules—neutrophilic, eosinophilic, or basophilic depending on the cell's lineage commitment. The nucleus is round to slightly indented, and nucleoli are no longer visible. This is the last stage capable of cell division.

The metamyelocyte has a kidney-shaped or bean-shaped nucleus and a full complement of granules. The cell is post-mitotic and continues maturation.

The band cell has a horseshoe-shaped or curved nucleus that has not yet segmented into distinct lobes. Band cells are released into blood during high demand (infections), creating a "left shift" in the differential count.

The segmented neutrophil (or "seg") is the mature form with a fully segmented, multilobed nucleus connected by thin chromatin strands.

Granulocyte colony-stimulating factor (G-CSF) is the primary growth factor regulating neutrophil production. Recombinant G-CSF is used clinically to stimulate neutrophil production in neutropenic patients.

Clinical Correlations

Disorders of blood cells are common and clinically important, reflecting either deficient production, excessive destruction, or malignant proliferation.

Anemia refers to a reduction in the oxygen-carrying capacity of blood, whether due to decreased red cell number, decreased hemoglobin content, or abnormal hemoglobin. Iron deficiency anemia, the most common form, produces small (microcytic), pale (hypochromic) red cells because iron is insufficient for normal hemoglobin synthesis. Vitamin B12 or folate deficiency impairs DNA synthesis, producing larger than normal (macrocytic) red cells because cells grow but cannot divide normally. Hemolytic anemias result from premature red cell destruction, whether from intrinsic defects (hereditary spherocytosis, sickle cell disease) or extrinsic factors (autoimmune destruction, mechanical damage).

Leukocytosis (elevated WBC count) and leukopenia (decreased WBC count) may affect specific cell types. Neutrophilia (elevated neutrophils) typically indicates bacterial infection; a "left shift" (increased band forms) confirms active bone marrow response. Lymphocytosis (elevated lymphocytes) is typical of viral infections. Eosinophilia suggests parasitic infection, allergic disorders, or certain other conditions (recall "NAACP": neoplasm, allergy, asthma, collagen-vascular disease, parasites). Neutropenia increases susceptibility to bacterial infection and is a common complication of chemotherapy.

Thrombocytopenia (low platelet count) increases bleeding risk. Causes include decreased production (bone marrow failure, chemotherapy), increased destruction (immune thrombocytopenia, disseminated intravascular coagulation), or sequestration (hypersplenism). Clinical manifestations include petechiae (pinpoint hemorrhages), purpura, mucosal bleeding, and increased bleeding with trauma or surgery.

Leukemias are malignant proliferations of white blood cell precursors. They are classified by the cell lineage affected (myeloid vs. lymphoid) and by clinical course (acute vs. chronic). Acute leukemias are characterized by accumulation of immature cells (blasts) that fail to mature, while chronic leukemias involve proliferation of more mature but still abnormal cells.

Summary

Blood is a specialized fluid connective tissue comprising plasma (55%) and formed elements (45%). Red blood cells, white blood cells, and platelets are suspended in plasma and perform distinct functions.

Red blood cells are anucleate, biconcave discs optimized for oxygen transport. They contain hemoglobin, circulate for approximately 120 days, and are removed by splenic macrophages. Their production (erythropoiesis) is regulated by erythropoietin from the kidney.

White blood cells are divided into granulocytes (neutrophils, eosinophils, basophils) and agranulocytes (lymphocytes, monocytes). Neutrophils are the most numerous and serve as first responders to bacterial infection. Eosinophils combat parasites and modulate allergic reactions. Basophils participate in immediate hypersensitivity. Lymphocytes mediate adaptive immunity. Monocytes are precursors to tissue macrophages.

Platelets are small, anucleate fragments from megakaryocytes that are essential for primary hemostasis. They form the initial platelet plug and provide a surface for coagulation factor activation.

Hematopoiesis occurs in the bone marrow from pluripotent stem cells that give rise to myeloid and lymphoid lineages. Erythropoiesis proceeds through recognizable stages from proerythroblast to mature erythrocyte, with progressive hemoglobin accumulation and ultimate nuclear extrusion. Granulopoiesis progresses from myeloblast through promyelocyte, myelocyte, metamyelocyte, and band cell to segmented granulocyte.

Key Terms

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