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)

This quiz focuses on the histological characteristics of blood cells, including RBCs, leukocytes, and platelets. You will explore their structures under light and electron microscopes, while also correlating these features with clinical conditions such as hemolytic anemia. Test your knowledge on identifying blood cell types and understanding their counts in relation to various health conditions.