Blood and Body Fluids (BLF) 103 - Histology II

Course Information

• Academic Year: 2024-2025
• Year: 1
• Semester: 1
• Module: Blood and Body Fluids (BLF) 103

Histology of Blood Cells II

• Instructor: Safinaz Salah Eldin
• Department: Histology
• Institution: Cairo University

Objectives

• Recognize different blood elements
• Describe the histological characteristics of RBCs, leukocytes (granular & non-granular), and platelets at light and electron microscope levels
• Correlate normal RBC structure with function and abnormalities with clinical conditions (like hemolytic anemia)
• Differentiate between RBCs and WBCs
• Recall the normal count of RBCs, leukocytes, and platelets
• Correlate platelet ultrastructure with function
• Differentiate WBC types in blood smears
• Identify abnormal variations in RBC and platelet counts and their causes
• Interpret alterations in blood cell counts in relation to clinical conditions
• Define the total and differential leukocyte count
• Describe the histological organization and locations of bone marrow
• Identify types of bone marrow
• Recognize the histological structure of cells involved in hemopoiesis (blood cell formation)

Introduction

• Non-granular leukocytes
• Comparison between RBCs and WBCs
• Platelets
• Bone marrow
• Hemopoiesis

Non-granular Leukocytes

• Lymphocytes (small and large)
• Monocytes
  • Percentage: 3-8%, size: 13-20µm
  • Life span: 3 days in blood, 3 months in connective tissue
  • Light microscope (LM) characteristics: large, eccentric, kidney-shaped nucleus, pale with one or two nucleoli, non-clear pale basophilic cytoplasm (frosted glass appearance due to lysosomes - azurophilic granules)
  • Electron microscope (EM) characteristics: euchromatic nucleus with a clear nucleolus, pseudopodia, well-developed mitochondria, rough endoplasmic reticulum (rER), Golgi apparatus (GA), and numerous primary and secondary lysosomes
  • Function: highly phagocytic cells, remain in the blood for 3 days, then enter connective tissue and become macrophages. Antigen-presenting cells. precursors of other phagocytes (e.g., dust cells, Kupffer cells, osteoclasts, microglia)
  • Monocytosis: elevated monocyte count (>8%) due to chronic infections (TB, syphilis, glandular fever), and monocytic leukemia
  • Monocytopenia: decreased monocyte count (<3%) as in pancytopenia (low levels of all blood cells)

Lymphocytes

• Percentage: 20-30% of total WBCs

• Size: 6-8µm (small) or 10-15µm (large)

• LM characteristics: small lymphocytes - large, round, dark nucleus; small rim of pale cytoplasm; large lymphocytes - indented nucleus, more abundant, deeply basophilic cytoplasm

• EM characteristics: small - heterochromatic nucleus, numerous free ribosomes, two centrioles, small azurophilic granules; large - euchromatic nucleus, prominent nucleolus, more mitochondria, rough ER, Golgi apparatus (GA), lysosomes

• Function: actively motile, circulate between blood and lymphatic organs

• Types of lymphocytes: T-lymphocytes, B-lymphocytes, natural killer cells

  • T-lymphocytes:

  • Percentage: 60-80%, life span in years

  • Maturation site: bone marrow, migrate to the thymus for differentiation and receptor acquisition (thymic education)

  • Surface markers: T-cell receptors (TCRs), e.g., CD4, CD8

  • Function: cell-mediated immunity

  • Cytotoxic T-cells (CD8+): produce perforins to kill virally infected, transplanted, or neoplastic cells

  • Helper T-cells (CD4+): activate B cells in the humoral immune response

  • Regulatory T-cells (suppressor): maintain tolerance to self-antigens and suppress excessive immune responses

  • Memory T-cells: provide rapid response to re-exposure to the same antigen (secondary immune response)

  • B-lymphocytes:

  • Percentage: 20-30% of total lymphocytes

  • Maturation site: bone marrow where they acquire receptors

  • Surface markers: receptors for IgM and IgD

  • Function: humoral immunity (Antibody production)

  • Activated by T-helper cells to produce plasma cells or become memory cells for rapid response to future exposures

• Natural killer cells:

  • Percentage: 5-10%
  • Maturation site: develop from T and B cell precursors but do not mature in the thymus
  • Surface markers: CD16, no T or B cell receptors
  • Function: innate early immunity (no T-helper cell stimulation), similar to cytotoxic T cells, secrete antiviral interferon

• Lymphocytosis: increased lymphocyte count (>30%) due to physiological conditions in children, chronic infections (TB, whooping cough), or leukemia

• Lymphocytopenia: decreased lymphocyte count (<20%) as in pancytopenia

Platelets

• Structure: small oval cytoplasmic fragments (not cells), originate from megakaryocytes
• Size: 2-4µm, shape: oval; has 2 zones on LM: outer pale basophilic (Hyalomere), and inner dark granular zone
• Function: platelet aggregation (platelet plug formation), maintain shape (via microtubules), aid clot retraction (using actin microfilaments), contain serotonin, ATP, ADP, Ca++, and clotting factors and growth factors (stored in granules)
• Platelet granules: alpha, delta (dense), lambda (lysosomes)
• Membranous channels: Open canalicular system (invaginations involved in releasing serotonin during vasoconstriction of injured vessels). Dense tubular system (stores Ca++)
• Thrombocytopenia (purpura): low platelet count (<50,000/mm³) due to decreased platelet production (bone marrow depression) or increased platelet breakdown (autoimmune disease). Symptoms: prolonged bleeding time, excessive bleeding in response to minor trauma.

Bone Marrow

• Active Red Bone Marrow
• Present in most bones during childhood
• In adults, located in mostly flat, short, and irregular bones
• Yellow Bone Marrow
• Present in shafts of long bones in adults.
• A storage site that can be transformed to active red bone marrow as needed
• Structure of Bone Marrow: Stroma (reticular cells, reticular fibers, fibroblasts, mesenchymal cells, osteogenic cells, fat cells), and blood sinusoids (irregular blood capillaries with wide lumen, lined by endothelial cells, blood cells leave via pores)
• Free cells: include the different stages of blood cell development, and the ratio of immature neutrophils to immature erythrocyte cells being 5:1, and a much shorter lifespan than erythrocytes
• Stem Cells: pluripotential, multipotential (myeloid and lymphoid), and unipotent (restricted) progenitor cells (give rise to mature blood cells)

Hemopoiesis

• Formation of blood cells
• Occurs in bone marrow and lymphoid organs (e.g., thymus).

Reticulocytes

• Small (9µm), immature RBCs slightly larger than mature RBCs
• Cytoplasm: acidophilic (Hb) with remnants of ribosomes and polysomes that form reticulum
• In peripheral blood: less than 1%
• Stain: Brilliant Cresyl Blue (supravital stain)
• Increased reticulocyte percentage suggests increased RBC production in response to hemorrhage or RBC destruction

Megakaryocytes

• Very large cells (50–70µm)
• Single, multilobed nuclei
• Basophilic cytoplasm
• Invaginations in the cell membrane form demarcation channels, dividing cytoplasm into fragments (platelets) that shed
• Extend pseudopodia (platelet ribbons) from which platelets detach

Summary

• Monocytes (LM, EM, function)
• Lymphocytes (LM, EM, function, T cells, B cells, natural killer cells)
• Platelets (LM, EM, function)
• Megakaryocyte
• Bone Marrow (Types, Structure)
• Hemopoiesis (Stages of blood cell formation)
• Reticulocytes

Interactive Questions (Examples)

• Compare RBCs and WBCs (types, content, number, size, lifespan, origin, shape, function, motility)
• Identify the described cell type in a peripheral blood smear (given characteristics)
• Predict platelet adaptations for their function

Interactive Questions (Examples)

• Interactive question regarding comparison of RBCs and WBCs
• Interactive question regarding cell types and their description in a blood smear
• Interactive question predicting adaptation of platelet for their normal function

Clinical Note

• Bone Marrow Aspirate/biopsy (used for diagnosing aplastic anemia and leukemia)
• Bone Marrow Transplantation (donation of stem cells)