DDDS 601 Biological Foundations of Dentistry - Peripheral Blood and Hemopoiesis

Introduction to Peripheral Blood and Hemopoiesis

Blood is a specialized form of connective tissue characterized by:
- Cells: Referred to as formed elements.
- "Ground Substance": The blood plasma.
- "Fibers": These are absent in fluid blood and present only during clot formation.
Primary functions of blood include:
- Transport: Of respiratory gases ( $O_2$ , $CO_2$ ), nutrients, waste products, and hormones.
- Regulation: Control of body temperature, acid/base balance ( $pH$ ), and osmotic pressure.
- Protection and Defense: Through mechanisms like blood clotting and immunity.

Blood Composition and Physical Characteristics

Total blood volume accounts for approximately $8\%$ of total body weight.
Other fluids and tissues make up the remaining $92\%$
Whole blood is divided into two primary components by volume:
- Blood Plasma ( $55\%$ \n - Formed Elements ( $45\%$
Plasma Composition (by weight):
- Proteins ( $7\%$ ):
- Albumins ( $54\%$ )
- Globulins ( $38\%$ )
- Fibrinogen ( $7\%$ )
- Other proteins ( $1\%$ )
- Water ( $91.5\%$ )
- Other Solutes ( $1.5\%$ ):
- Electrolytes, Nutrients, Gases, Regulatory substances, Vitamins, and Waste products.
Formed Elements Concentration (per $\mu \text{L}$ ):
- Platelets: $150,000 \text{--} 400,000/\mu \text{L}$
- White blood cells (Leukocytes): $5,000 \text{--} 10,000/\mu \text{L}$
- Red blood cells (Erythrocytes): $4.8 \text{--} 5.4 \times 10^6/\mu \text{L}$
Hematocrit: The percentage of Red Blood Cells (RBCs) in whole blood volume.
- Normal adult male: $40 \text{--} 50\%$
- Normal adult female: $35 \text{--} 45\%$

Erythrocytes (Red Blood Cells)

Morphology:
- Diameter: $6.5\,\mu \text{m}$ to $8\,\mu \text{m}$ , with a mean diameter of $7.5\,\mu \text{m}$ .
- Structure: Anucleate biconcave disc.
- Flexibility: Extremely deformable (exceptions occur in specific clinical diseases).
- Lifespan: Approximately $120\,\text{days}$ .
Function:
- Transport of respiratory gases which bind to hemoglobin ( $Hb$ ).
- The biconcave disc shape increases the surface area for gas binding.
Blood Group Systems:
- Agglutinogens (antigens) are embedded in the RBC plasma membrane: $A$ , $B$ , $AB$ , and $O$ .
- Agglutinins (antibodies) in the plasma target missing antigens.
Hemoglobin ( $Hb$ ):
- $HbA$ is the most prevalent form in adults.
- Structure consists of four polypeptide chains: two alpha ( $\alpha$ ) and two beta ( $\beta$ ) chains.
- Each chain contains an iron-containing heme group ( $Fe^{2+}$ center).

Leukocytes (White Blood Cells)

Leukocytes are categorized based on the presence or absence of specific cytoplasmic granules.
All leukocytes perform their functions outside of the blood vessels in the connective tissue.

Granulocytes

Neutrophils ( $60 \text{--} 70\%$ ):
- Size: $12\,\mu \text{m}$ to $15\,\mu \text{m}$ .
- Nucleus: Segmented in mature form (Polymorphonuclear leukocytes/PMNs). Cells with > 5 lobes are hypersegmented (old cells).
- Barr Body: An inactive $X$ chromosome appearing as a "drumstick" appendage specifically in females.
- Granules:
- Specific granules: Most abundant, stain light blue to violet, smallest.
- Azurophilic granules: Primary lysosomes containing enzymes for killing microorganisms; larger than specific granules.
- Tertiary granules: Include phosphasomes (phosphatases) and metalloproteinase granules (aiding movement through tissue).
- Function: First cells to leave circulation to arrive at sites of injury; perform phagocytosis. Significant component of pus.
Eosinophils ( $2 \text{--} 4\%$ ):
- Size: $12\,\mu \text{m}$ to $15\,\mu \text{m}$ .
- Nucleus: Bilobed.
- Specific Granules: Large, stain red to orange, contain a crystalline core (internum) surrounded by a matrix (externum).
- Crystalline core: Contains Major Basic Protein ( $MBP$ ), which makes up $50\%$ of the protein and kills parasitic worms.
- Matrix: Contains eosinophil cationic protein, eosinophil peroxidase, and eosinophil-derived neurotoxin.
- Function: Monitor connective tissue beneath epithelia (lung, gut, uterus, vagina) for parasites; phagocytose Ag-Ab complexes to modulate inflammation.
Basophils ( $0.5 \text{--} 1.0\%$ ):
- Size: $12\,\mu \text{m}$ to $15\,\mu \text{m}$ .
- Nucleus: Lobated, but usually masked by granules.
- Specific Granules: Large and stain dark blue to purple.
- Contents: Hydrolytic enzymes, heparin, histamine, and leukotrienes.
- Function: Similar to connective tissue mast cells, causing vascular dilation and increased permeability.

Agranulocytes

Lymphocytes ( $20 \text{--} 25\%$ ):
- Size: $6\,\mu \text{m}$ to $18\,\mu \text{m}$ ; most are small ( $6\,\mu \text{m} \text{--} 8\,\mu \text{m}$ , about RBC size).
- Nucleus: Round, dark-staining, with a high nuclear/cytoplasmic ratio.
- Types:
- $T$ -cells ( $\approx 80\%$ ): Cellular immunity.
- $B$ -cells ( $\approx 15\%$ ): Mature into plasma cells for antibody production.
- Null ( $NK$ ) cells ( $\approx 5\%$ ): Lack $T$ or $B$ surface antigens; natural killer cells.
- Notes: Only leukocyte that returns to blood from connective tissue; variable lifespan (days to years).
Monocytes ( $3 \text{--} 8\%$ ):
- Size: $12\,\mu \text{m}$ to $20\,\mu \text{m}$ (large cells).
- Nucleus: Indented to kidney-shaped; lower nuclear/cytoplasmic ratio than lymphocytes.
- Cytoplasm: Gray-blue with light purple nucleus.
- Function: Precursor to the mononuclear phagocyte system, including macrophages and osteoclasts. Facilitates antigen presentation.

Thrombocytes (Platelets)

Characterisitics:
- Tiny membrane-limited cytoplasmic fragments derived from megakaryocytes in the bone marrow.
- Size: $2\,\mu \text{m}$ to $4\,\mu \text{m}$ .
- Lifespan: Less than $14\,\text{days}$ .
- Regions: Granulomere (central, dark) and Hyalomere (peripheral, light blue).
Internal Components:
- Delta ( $\delta$ ) granules ( $250 \text{--} 300\,\text{nm}$ ): Contain $Ca^{2+}$ ions, $ATP$ , $ADP$ , and serotonin.
- Alpha ( $\alpha$ ) granules ( $300 \text{--} 500\,\text{nm}$ ): Contain fibrinogen and Platelet-Derived Growth Factor ( $PDGF$ ).
- Lambda ( $\lambda$ ) granules ( $175 \text{--} 250\,\text{nm}$ ): Contain lysosomal enzymes.
- Specialized structures: Open canalicular system for molecule liberation; marginal bundle of microtubules to maintain shape.
Hemostatic Function:
- Primary Aggregation: Forms a platelet plug at vascular lesions.
- Secondary Aggregation: Increase in plug size via release of adhesive glycoprotein and $ADP$ .
- Coagulation: A 13-protein cascade producing fibrin to trap blood cells and form a thrombus (clot).
- Clot processes: Retraction from the vessel lumen followed by removal once the vessel wall restores.

Hemopoiesis

Overview: The process of blood cell formation. Requires continual renewal due to the finite lifespan of formed elements.
Sites: Prenatal (liver, spleen, bone marrow, thymus); Postnatal (red bone marrow).
Stem Cells:
- Pluripotential stem cells give rise to Hematopoietic Stem Cells ( $HSC$ ) and Mesenchymal Stem Cells ( $MSC$ ).
- $HSC$ differentiate into Lymphoid and Myeloid Multipotential Hematopoietic Stem Cells ( $MHSC$ ).
Key Growth Factors:
- Erythropoietin (Hematopoietin).
- Granulocyte/Macrophage Colony Stimulating Factor ( $GM\text{-}CSF$ ).
- Thrombopoietin.
- Interleukins (produced by lymphocytes).

Erythropoiesis (RBC Formation)

Proerythroblast: Large cell with nucleolus.
Basophilic Erythroblast: Polyribosomes synthesize hemoglobin; condensed nucleus, no nucleolus.
Polychromatophilic Erythroblast: Cytoplasm has mixed polyribosomes and hemoglobin.
Orthochromatophilic Erythroblast: Packed with hemoglobin; expels nucleus.
Reticulocyte: Residual polyribosomes visible via cresyl blue staining.
Mature Erythrocyte: Released into peripheral blood.

Granulopoiesis (WBC Formation)

Myeloblast: No cytoplasmic granules.
Promyelocyte: First secretion of azurophilic granules.
Myelocyte: Initial secretion of specific granules (neutrophilic, eosinophilic, or basophilic).
Metamyelocyte: Abundant specific granules, few azurophilic granules.
Band/Stab Cell: Immature stage specifically designated for neutrophils.
Mature Neutrophil, Eosinophil, or Basophil.

Thrombopoiesis (Platelet Formation)

Megakaryoblast: $15 \text{--} 50\,\mu \text{m}$ ; large polyploid nucleus; basophilic cytoplasm.
Megakaryocyte: Giant cells ( $35 \text{--} 150\,\mu \text{m}$ ) with lobulated nuclei. Demarcation membranes form plasma membrane invaginations to define areas where platelets are shed.
Platelets: Shed from megakaryocytes into bone marrow sinusoids.

Bone Marrow Microenvironment

Hemopoietic compartments are organized around sinusoids lined with endothelium and adventitial cells.
Erthyroblastic islets are localized pockets of RBC development.
Transendothelial Migration:
- Formed elements pass from the marrow into sinusoids.
- In peripheral tissues, leukocytes leave blood via a multi-step process: Selectins and receptors facilitate rolling, while integrins and the immunoglobulin superfamily facilitate adhesion and passage into connective tissue.

Questions & Discussion

Question: Which of the following properties do mast cells and basophils have in common?
- A. Found in blood
- B. Develop from lymphocytes
- C. Secrete heparin and histamine
- D. Decrease during time of infection
- E. Migrate to line the inside of blood vessels
Answer: C. Secrete heparin and histamine. Basophils are often described as functionally similar to mast cells found in connective tissue, specifically regarding the contents and secretion of their specific granules.