Skeletal System: Bone Tissue

Skeletal System: Osseous Tissue (Bone)

Components of the Skeletal System

• Cartilage:
  • Forms the embryonic skeleton (hyaline cartilage).
  • Ossifies into bone.
  • Covers the ends of long bones in adults for joint surfaces.
• Ligaments:
  • Connect bones, holding them together.
  • Dense connective tissue cords.

Functions of the Skeleton

• Support:
  • Provides a framework for soft tissues.
• Protection:
  • Flat bones (skull, rib cage) protect vital organs (brain, heart, lungs).
• Movement:
  • Muscles attach to bones, using them as levers and pulleys.
• Electrolyte and Acid-Base Balance:
  • Bones store calcium and phosphate ions.
  • Release and reincorporate ions to maintain electrolyte balance.
  • Buffer pH changes by releasing or reincorporating salts.
• Blood Formation:
  • Bone marrow produces red and white blood cells.

Embryonic Tissue Layers

• Ectoderm: Outer layer.
• Mesoderm: Middle layer.
  • Bone, blood, and muscle are derived from the mesoderm.
  • Bone contains bone marrow.
• Endoderm: Inner layer.

Osseous Tissue (Bone Tissue)

• Classification: Connective tissue.
• Composition:
  • Starts as soft organic material (matrix).
  • Hardens through mineralization (calcium and phosphate).
• Components:
  • Osseous tissue.
  • Nervous tissue (nerves present).
  • Bone marrow.
  • Blood supply.
  • Cartilage (joint surfaces).
  • Periosteum (outer connective tissue sheath).
• Dynamic Remodeling: Constantly building and breaking down bone.

Bone Classes

• Flat Bones:
  • Protectors of internal organs.
  • Examples: Scapula (shoulder blade), sternum (breastbone).
• Short Bones:
  • Gliders, allowing limited movement.
  • Examples: Carpal bones (wrist), tarsal bones (ankle), talus (large tarsal bone).
• Long Bones:
  • Act as levers for movement.
  • Typical of upper and lower appendages.
  • Examples: Femur, bones of the leg, bones of the foot and toes.
• Irregular Bones:
  • Do not fit into other categories.
  • Examples: Sphenoid (cranial bone), vertebrae.

Anatomy of a Long Bone

• Diaphysis:
  • Shaft, the long cylindrical middle region.
• Epiphyses:
  • Enlarged ends, leading to joint surfaces.
• Articular Cartilage:
  • Covers joint surfaces, allowing ease of movement.
• Periosteum:
  • Connective tissue sheath covering the shaft.
• Marrow Cavity:
  • Hollow inner part of the shaft, filled with yellow bone marrow.
• Endosteum:
  • Lining of the marrow cavity.
• Epiphyseal Plate (Growth Plate):
  • Region on the epiphyses where long bone growth occurs.
  • Fuses as the epiphyseal line after reaching full height, stopping bone growth.

Anatomy of Flat Bones

• Layers: Outer and inner layers of compact bone with a middle layer of spongy bone.
• Spongy Bone:
  • Contains trabeculae (lattice network) with spaces in between.
  • Strong but light tissue.
• Compact Bone:
  • More substantial.
• Function: Protect from blows.

Bone Cells

• Osteogenic Cells:
  • Derived from embryonic fibroblasts.
  • Develop into osteoblasts.
• Osteoblasts:
  • Form bone by producing the soft organic matrix.
  • Have abundant rough endoplasmic reticulum for fiber production.
• Osteocytes:
  • Mature cells trapped in the hardened matrix.
  • Maintain and reabsorb matrix.
  • Respond to strain sensors (stress responsive).
• Osteoclasts:
  • Bone-dissolving cells.
  • Large, multinucleated cells formed from fused stem cells.
  • Have a ruffled border for bone dissolving.

Bone Remodeling

• Balance: Balance between osteoclast (bone dissolving) and osteoblast (bone making) activity.
• Stress Response: Bones respond to stress, building more bone and becoming stronger.
• Orthodontics: Utilizes bone remodeling by applying strain to teeth to shift them.
  • Bone dissolves on the stress side and rebuilds on the lagging side.
• Exotic Examples:
  • Foot binding: Deforms feet.
  • Neck rings: Elongates neck, resulting in the elongation of the transverse processes of the cervical vertebrae.

Bone Matrix

• Organic Matter (⅓):
  • Collagen (strength).
  • Glycosaminoglycans (GAGs).
  • Proteoglycans.
  • Glycoproteins.
• Inorganic Matter (⅔):
  • Calcium and phosphate (mostly hydroxyapatite).
  • Calcium carbonate (10%).
  • Other minerals.
• Properties:
  • Mineral part resists compression.
  • Collagen provides flexibility, preventing fractures.

Osteon: Structural Unit of Compact Bone

• Components:
  • Central Canal: Contains blood supply and nerves.
  • Lamellae: Multiple layers of tissue in ring conformations.
  • Perforating Canals: Branched pathways for blood and nerve entry/exit.

Spongy Bone

• Location: Heads (epiphyses) of long bones.
• Trabeculae: Lattice network.
• Spaces: Filled with red bone marrow.
• Stress Response: Trabeculae increase at points of stress.

Bone Marrow

• Varieties:
  • Red Bone Marrow: Hemopoietic, produces red and white blood cells and platelets.
    • Found in axial skeleton (skull, vertebrae, thoracic cage) and heads of humerus and femur.
  • Yellow Bone Marrow: Inactive, fatty form.
    • Transitions from red marrow in remaining bones of limbs post-childhood.
    • Can revert to red marrow with significant blood loss.
  • Gelatinous Bone Marrow: Nonfunctional, jelly-like, develops in later life.

Bone Formation

• Osteogenesis: Synthesis of new bone.
• Ossification: Tissue previously not bone becomes bone.
• Types:
  • Intramembranous Ossification:
    • Flat bone development.
    • Derived from mesenchyme.
    • Mesenchyme condenses with blood infiltration.
    • Osteogenic cells produce osteoblasts, forming soft organic matrix.
    • Periosteum forms.
    • Trabeculae form in middle spongy layer.
    • Compact bone forms on outer and inner layers.
  • Endochondral Ossification:
    • Long bone development.
    • Fetal skeleton starts as hyaline cartilage covered with perichondrium.
    • Bone forms first in the center (primary ossification center).
    • Bony collar develops.
    • Blood is introduced, primary ossification center is dissolved, becoming primary marrow cavity.
    • Secondary ossification centers develop on the ends.
    • Epiphyses and diaphysis are formed.

Long Bone Growth

• Occurs at the epiphyseal plate.
• Epiphyseal plate fuses to become the epiphyseal line, stopping growth.
• Growth at the epiphyseal plate driven by hyperplasia.
• Muscle growth driven by hypertrophy (increase in cell size).

Bone Remodeling Through Life

• Interstitial Growth: Increase in length.
• Appositional Growth: Increase in width.
• Imbalance: Can lead to disease (e.g., Paget's disease).
  • Abnormal bone buildup or dissolving.

Bone Deposition vs. Resorption

• Bone Deposition (Bone Making):
  • Osteoblasts produce the bone matrix, which then mineralizes.
• Bone Resorption (Bone Dissolving):
  • Osteoclasts dissolve bone. Large, multinucleated cells with ruffled border.
  • Osteoclasts pump hydrogen ions (H+) out through pumps into resorption bay, drawing chloride ions (Cl-).
  • $H+ + Cl- \rightarrow HCl$ (Hydrochloric acid breaks down bone).

Bone Contribution to Electrolyte and pH Balance

• Reservoir: Bone is a reservoir for phosphate and calcium salts.
• Phosphate:
  • Component of nucleic acids (DNA, RNA).
  • ATP (adenosine triphosphate).
  • Phospholipids (plasma membrane).
  • pH buffers.
• Calcium:
  • Stimulus for nerve and muscle contraction.
  • Blood clotting.
  • Exocytosis.
• Imbalances:
  • Phosphate imbalances are usually managed by the kidneys.
  • Calcium imbalances can be consequential.
    • Hypocalcemia: Low blood calcium levels.
    • Hypercalcemia: High blood calcium levels.

Hormones Regulating Blood Calcium Levels

• Calcitriol (Activated Vitamin D):
  • Sunshine hormone (produced with UV exposure).
  • Targets skeletal, urinary, and digestive systems.
  • Stimulates bone resorption.
  • Limits calcium excretion by the kidneys.
  • Enhances calcium absorption from the gut.
  • Overall, increases blood calcium levels.
  • Deficiency: Rickets (childhood), osteomalacia (adults).
• Calcitonin
  • Secreted when blood calcium levels are too high.
  • Acts to "tone down" blood levels.
  • Inhibits osteoclast activity.
  • Enhances osteoblast activity.
  • Important early in life; little effect in adults.
  • Negative feedback loop (corrects the imbalance).
• Parathyroid Hormone (PTH):
  • Becomes active when blood calcium levels fall.
  • Goal is to raise blood calcium levels.
  • Inhibits osteoblast activity.
  • Stimulates osteoclast activity.
  • Stimulates calcitriol synthesis.
  • Negative feedback loop.

Summary of Hormonal Effects

• Calcium in Blood: Measure of importance.
• Options:
  • Calcium free in the blood.
  • Calcium conserved within bone mineral.
  • Absorption from diet vs. excretion.
• Calcitriol: Increases blood calcium levels.
• Parathyroid Hormone: Increases blood
• Calcitonin Decreases blood calcium levels.

Bone Fractures

• Causes:
  • Stress (accidents, falls, sports injuries).
  • Pathology (bone disease, osteoporosis, Paget's disease, cancer).
• Types:
  • Open vs. Closed: Bone has injured surrounding soft tissues and penetrated the skin vs not.
  • Displaced: Linear bone is out of alignment.
  • Greenstick: Slivers off of the periosteum and shaft of the long bone. Common in children.
  • Comminuted: Bone shatters.
  • Based on Break Orientation:
    • Transverse.
    • Oblique.
    • Spiral (lifting or turning).
  • Callus and Pott's fractures: Distal forearm or leg, sports injuries.
  • Avulsions: Piece of bone at the insertion point (e.g., calcaneus, volar plate of digit, superior patella).

Bone Fracture Healing

• Hematoma Formation: Bleeding into the site of injury, forming a blood clot.
• Soft Callus Formation: Fibrocartilage forms, with invasion of new blood vessels and turns into spongy bone at a point
• Regeneration of Compact Bone, typically takes 8-12 weeks,

Osteoporosis

• Loss of both matrix (inorganic) and organic parts of bone.
• Increases risk of pathological fractures.
• Postmenopausal white women are most susceptible.
• Treatment:
  • Estrogen replacement.
  • Parathyroid hormone treatment.
• Prevention:
  • Exercise.
  • Calcium and vitamin D in diet.