Lecture 23 BIO 203: Lecture on Bone Physiology, Hematopoiesis, and Hemostasis (copy)

Final Exam Logistics and Course Deadlines

• Exam Schedule: The final exam (Exam Three) is scheduled for May 18 at $02:15\,PM$.

• Location: The exam will take place in the Javits Lecture Center on the Stony Brook Main Campus.

• Attendance and Arrival: Students are strongly encouraged to arrive on campus well in advance of the start time, especially those commuting, to allow time to decompress. No extensions on time will be granted; all students must end at the same time regardless of arrival.

• Student Capacity: There are approximately $500$ students in BIO 203; as such, they will be spread out across several reserved rooms in Javits. Specific classroom details will be posted on Brightspace.

• SASE Accommodations: Students with SASE-approved accommodations must take the exam at the SASE testing center rather than Javits. Appointments must be made well in advance.

• Exam Scope:

  • Technically, the final is not cumulative but covers Modules 9 through 12 (Endocrine system through the Immune system).

  • However, the material is interconnected. Concepts from earlier modules (e.g., basic biological principles) are foundational and will be referenced throughout later material.

Fundamental Concepts of Bone as Living Tissue

• Definition: Bone is defined as a living tissue because it contains living cells embedded within a mineralized matrix.

• Anatomical Classification:

  • Trabecular Bone: Also known as "spongy bone" (though the instructor prefers "trabecular"). It consists of mineralized ribbons or struts called trabeculae. It is found at the ends (epiphyses) of long bones (e.g., femur, humerus) and provides a strong but lightweight architecture.

  • Compact Bone: Denser bone that forms the diaphysis or the shaft of the bone, providing structural strength to the skeleton.

• Adaptive Changes: Because bone is living tissue, it undergoes reorganizational changes in response to physical stress. For example, taking up jogging results in the remodeling of trabeculae to better accommodate new mechanical forces.

• Bone as an Organ: A single bone (like the femur) is classified as an organ because it is composed of multiple distinct tissue types working together, including:

  • Bone tissue.

  • Cartilage (at articulating surfaces).

  • Endothelial cells (lining blood vessels).

  • Yellow bone marrow (adipose/lipid storage).

  • Red bone marrow (site of hematopoiesis).

• Articulating Surfaces: These are the ends of the bone where cartilage provides a smooth surface, allowing the bone to function as a joint.

• Evidence of Life: Historical specimens from the 1860s demonstrate that a femur broken while an individual was alive could repair itself through cellular mechanisms, even if misaligned. Conversely, a bone broken post-mortem shows no repair, as the constituent cells were no longer living.

Specialized Bone Cell Types

• Chondrocytes: Cartilage cells found at the articulating surfaces and the epiphyseal plates of growing bones. They are responsible for laying down the cartilage matrix.

• Osteoblasts: Bone-building cells that lay down new bone tissue. They secrete key proteins like collagen and proteoglycans, as well as calcium and phosphate which mineralize into the bone matrix.

• Osteoclasts: Cells responsible for the resorption (breakdown) of bone. They dissolve the mineralized matrix to release calcium back into the blood.

• Osteocytes: Mature, inactive osteoblasts that reside within the matrix of mature bone that has ceased growing.

Comparison of Cartilage and Bone

• Commonalities: Both are types of connective tissue characterized by cells dispersed within an extracellular matrix (ECM).

• Cartilage Specifics:

  • Cells: Chondrocytes.

  • ECM Composition: Proteins such as collagen and proteoglycans (proteins modified with carbohydrates).

  • Mineralization: Cartilage is NOT mineralized, allowing it to remain flexible (e.g., the external ear can bend and bounce back).

• Bone Specifics:

  • Cells: Osteocytes.

  • ECM Composition: Collagen (different isoform than cartilage) and proteoglycans.

  • Mineralization: Characterized by hydroxyapatite, a calcium-based mineral resulting from the precipitation of calcium and phosphate. Hydroxyapatite binds tightly to collagen, imparting rigid structural properties.

Mechanisms of Linear Bone Growth

• Linear Growth Period: Occurs after birth through the teenage years. It is driven primarily by Growth Hormone (GH).

• The Epiphyseal Plate (Growth Plate):

  • Located between the epiphysis and diaphysis. It is a site consisting originally of cartilage.

  • Chondrocyte Proliferation: Cartilage cells divide in linear rows, adding mass and secreting ECM, which pushes the ends of the bone further apart.

  • Apoptosis and Replacement: Older chondrocytes swell and die (apoptosis). Osteoblasts follow behind, replacing the disintegrating cartilage with mineralized bone.

  • Closure: In late adolescence (roughly ages $17$ to $20$), chondrocyte division slows and eventually stops. Osteoblast activity eventually fills the entire plate with bone, leaving an "epiphyseal scar." At this point, linear growth is no longer possible.

• Growth Pathologies:

  • Gigantism: Caused by an anterior pituitary tumor in childhood that overproduces GH, leading to extreme height (e.g., Robert Wadlow reaching $8\text{ feet }11\text{ inches}$).

  • Acromegaly: Caused by a GH-producing tumor in adulthood after epiphyseal plates have closed. It results in enlarged internal organs and thickened facial features and hands (e.g., Andre the Giant).

  • Treatment: Modern treatment requires neurosurgery to remove the anterior pituitary tumor.

Bone Remodeling and Calcium Homeostasis

• The Remodeling Balance: Bone is constantly being deposited (osteoblasts) and resorbed (osteoclasts) in a homeostatic balance.

• Osteoclast Mechanism:

  • Osteoclasts are large, multinucleate cells formed by the fusion of macrophages.

  • They utilize the enzyme carbonic anhydrase to produce hydrogen ions through the reaction:       $H_2O + CO_2 \rightleftharpoons H_2CO_3 \rightleftharpoons H^+ + HCO_3^-$

  • A hydrogen ion pump moves $H^+$ into the extracellular space as hydrochloric acid ($HCl$), which, along with enzymes, dissolves the hydroxyapatite matrix.

• Osteoporosis: A condition occurring when osteoclast activity exceeds osteoblast activity, leading to thinning, fragile bones.

• Calcium Balance Requirements:

  • The body requires dietary calcium, which is absorbed in the small intestine via Vitamin D.

  • Normal extracellular fluid (ECF) calcium level is approximately $2.5\,millimolar$.

  • Storage: $99\%$ of the body's calcium is stored in the bone matrix.

  • Regulation: If dietary intake or Vitamin D is insufficient, Parathyroid Hormone (PTH) and Vitamin D stimulate bone resorption to mobilize calcium for critical cellular functions (e.g., second messages, muscle contraction, vesicle fusion).

Composition of Blood as Connective Tissue

• Classification: Like bone and cartilage, blood is a connective tissue because it consists of cells (formed elements) dispersed in an extracellular matrix (plasma).

• Comparison of ECM: In blood, the ECM is liquid (mostly water), whereas in bone, it is mineralized collagen.

• Centrifuged Blood Components:

  • Red Blood Cells (Erythrocytes): Roughly $42\%$ of packed volume.

  • Buffy Coat: A thin layer containing white blood cells (Leukocytes) and platelets.

  • Plasma: The fluid portion containing water, trace elements (vitamins/minerals), amino acids, lipids, gases ($O_2$, $CO_2$), and many proteins.

• Plasma Proteins:

  • Albumin: The most abundant plasma protein. These large globular proteins are responsible for creating Colloid Osmotic Pressure.

Hematopoiesis and Cell Lineages

• Definition: Hematopoiesis is the production of all blood cells.

• Location: Occurs in the red bone marrow. In adults, this is limited to the ends of long bones and certain flat bones.

• Stem Cells:

  • Pluripotent Stem Cells: Undifferentiated cells in the marrow that can become many (but not all) types of blood cells.

• Lineages:

  • Lymphocyte Lineage: Gives rise to T cells, B cells, and Natural Killer cells.

  • Myeloid Lineage: Gives rise to various paths leading to Monocytes (macrophages/dendritic cells), Neutrophils, Eosinophils, Basophils, Megakaryocytes, and Erythrocytes (RBCs).

• Bone Marrow Environment: The marrow is highly vascularized with Venous Sinuses. These are veins with discontinuous endothelial linings that allow extensive exchange between the marrow stroma (where cells develop) and the circulation.

Platelets (Thrombocytes)

• Origin: Platelets are not true cells; they are fragments of large, multinucleate cells called Megakaryocytes located in the red bone marrow.

• Structure: They lack a nucleus (making protein transcription impossible) but contain mitochondria (for ATP generation), smooth ER, and vesicles.

• Size: Significantly smaller than red blood cells.

• Activation: They transition from an "inactive" (smooth) state to an "active" (spiky) state. This shape change is energy-dependent (using ATP) and involves protein polymerization.

Mechanisms of Hemostasis (Blood Clotting)

• Hemostasis: The process of stopping bleeding after vessel injury. It involves four main steps:

  1. Vasoconstriction: Immediate narrowing of the vessel to limit blood flow.

  2. Platelet Plug Formation: Platelets adhere to the site and aggregate.

  3. Coagulation: Formation of a stable fibrin mesh.

  4. Restriction: Limiting the clot to only the site of injury.

• Platelet Plug Formation:

  • Damage to the vessel wall exposes Collagen (which acts as a ligand).

  • Receptors on platelets bind to collagen, activating the platelets.

  • Activated platelets change shape and release signaling molecules (cytokines) to recruit more platelets, forming a loose "platelet plug."

• The Coagulation Cascade (Extrinsic Pathway):

  • This is the primary physiological pathway triggered by vessel injury.

  • Tissue Factor (TF): A transmembrane protein on smooth muscle cells/fibroblasts exposed by injury.

  • Activation Sequence:

    1. Inactivation Factor VII in circulation binds to exposed Tissue Factor.

    2. This complex activates Factor VII (a serine protease).

    3. Active VII cleaves inactive Factor X into Active Factor X.

    4. Active X cleaves Prothrombin into active Thrombin.

    5. Thrombin cleaves Fibrinogen into Fibrin.

  • Fibrin Polymerization: Fibrin molecules form a sticky, stringy polymer (resembling silly string) that cross-links and traps RBCs, WBCs, and platelets to form a stable clot.

• Signal Amplification: Each step in the enzymatic cascade acts as an amplifier, allowing a small initial stimulus (vessel damage) to produce a massive, rapid deposition of fibrin.

Clot Dissolution (Fibrinolysis)

• Repair Phase: Once the clot stops blood loss, tissue repair begins.

• Tissue Plasminogen Activator (TPA):

  • TPA is a protein that activates Plasmin.

  • Plasmin is the enzyme that cleaves the fibrin polymer, breaking the clot into fragments.

• Therapeutic Use: TPA can be administered clinically within a specific time window to dissolve pathological clots (e.g., in heart attacks or strokes) and save lives.

Questions & Discussion

• Technical Interruption: During the transition from calcium balance to blood smears, the Zoom record/connection crashed briefly for the instructor and students. The lecture resumed with a review of the blood smear slide.

• Clarification on Lineages: The instructor clarified that while Neutrophils and Lymphocytes are both White Blood Cells, Neutrophils are part of the innate immune system and develop through a different lineage than the lymphocytes (T cells/B cells/NK cells) of the adaptive immune system.