MR

CH6 - BONE PT 1 - bone tissue & structure

Functions and Roles of the Skeletal System

  • Movement: bones form joints; muscles attach to bones via tendons; muscles span joints; contraction of muscle cells enables movement at joints.
  • Protection: bony casings protect critical organs (lungs in the rib cage, brain in the cranium, pelvic organs in the pelvis).
  • Blood cell production: bones are sites of hematopoiesis, especially in the ends of long bones and in flat bones when we are young.
    • Blood stem cells reside in red bone marrow and can undergo mitosis to replace themselves.
    • These stem cells differentiate into white blood cells (immunity), red blood cells (oxygen transport), and platelets (clotting).
    • Red bone marrow is the site of this production; in long bones, red marrow is in the epiphyses; flat bones also host red marrow.
  • Mineral storage: bones store minerals; calcium is the major mineral stored in bone and is also crucial for:
    • Blood clotting, nerve conduction, and muscle contraction.
    • The body can absorb calcium from bone when needed; when calcium is consumed, it is put back into bone, creating a reservoir.
    • Calcium phosphate and other minerals are stored as well; the mineral salts contribute to bone hardness.
    • Conceptual metaphor: skeleton as a salt block of calcium with a constant mineral supply.
  • Fat storage: yellow bone marrow stores fat in the medullary cavity of long bones.
  • Additional considerations: bones contribute to overall body support, leverage for movement, and serve as a mineral reservoir for several physiological processes.

Bone Types (Overview and Examples)

  • Long bones: longer than wide; e.g., humerus. Have a shaft (diaphysis) and ends (epiphyses).
  • Short bones: roughly as wide as they are long; e.g., carpal bones.
  • Flat bones: two layers of compact bone with a middle layer of spongy bone (diploe); e.g., skull bones, sternum.
  • Irregular bones: varied shapes, e.g., vertebrae.
  • Sesamoid bones: develop within tendons; e.g., patella.
  • Wormian bones: extra bones that form within skull sutures.
  • Note: the lecture focuses on two main types for anatomy: long bones and flat bones.

Key Terms to Know (from the Transcript)

  • Epiphysis (proximal and distal): ends of a long bone.
  • Diaphysis: the shaft of a long bone.
  • Medullary cavity: hollow center of the diaphysis containing yellow marrow.
  • Endosteum: a cellular layer lining the medullary cavity; can enlarge or shrink the cavity.
  • Periosteum: outer covering of bone with two layers:
    • Inner cellular layer (osteoblasts/osteoclasts involved in growth and remodeling).
    • Outer fibrous layer (dense irregular connective tissue).
  • Sharpey fibers: collagen fibers that extend from periosteum into the bone to anchor the periosteum and prevent peeling away.
  • Articular cartilage: hyaline cartilage covering the ends of long bones.
  • Haversian canal (central canal): part of an osteon containing blood vessels.
  • Volkmann canals (perforating canals): horizontal channels that connect Haversian canals and bring blood to the osteons.
  • Osteon: concentric lamellae around a central Haversian canal; osteocytes reside in lacunae connected by canaliculi.
  • Lacunae: small chambers containing osteocytes.
  • Canaliculi: tiny channels that connect osteocytes and allow nutrient/waste exchange.
  • Osteoblasts: bone-forming cells that secrete osteoid (the extracellular matrix).
  • Osteocytes: mature bone cells that reside in lacunae and maintain bone tissue.
  • Osteoprogenitor cells: bone stem cells derived from mesenchyme; can divide and differentiate into osteoblasts.
  • Osteoclasts: large bone-resorbing cells.
  • Osteoid: unmineralized bone matrix secreted by osteoblasts.
  • Mesenchyme: embryonic connective tissue capable of forming bone via osteoprogenitor cells.
  • Diploe: the spongy bone layer between two layers of compact bone in flat bones.

Anatomy of a Long Bone (Structure and Components)

  • Long bone organization:
    • Epiphyses: the ends of the bone; proximal epiphysis is closer to the trunk; distal epiphysis is farther from the trunk.
    • Diaphysis: the shaft (growth upon).
    • Medullary cavity: hollow center in the diaphysis; contains yellow marrow (fat).
    • Epiphyses are composed of spongy (trabecular) bone where red marrow resides.
  • Periosteum and endosteum:
    • Periosteum encases the bone; two layers:
    • Inner cellular layer with osteoblasts and osteoclasts (bone growth/remodeling).
    • Outer fibrous layer of dense irregular connective tissue.
    • Sharpey fibers: collagen fibers anchoring the periosteum to the underlying bone.
    • Periosteum is highly innervated, contributing to pain sensation in fractures.
    • Endosteum lines the medullary cavity and contains cells that can enlarge or shrink the cavity; important for remodeling.
  • Outer and inner bone structure:
    • Compact bone (cortex): dense, lamellar organization; supports the bone structurally; contains osteons.
    • Spongy bone (trabecular bone): lattice of trabeculae with spaces filled by red marrow; no central blood vessels running through (no true osteons).
  • Articular cartilage:
    • Hyaline cartilage covering the ends of long bones (articular surfaces) to reduce friction and absorb shock.
  • Blood supply and vascular architecture:
    • Haversian canals (central canals) run longitudinally through osteons to supply blood.
    • Volkmann canals (perforating canals) run perpendicular to connect adjacent osteons and deliver blood laterally.
  • The osteon components:
    • Concentric lamellae form the rings around the Haversian canal.
    • Osteocytes sit in lacunae between lamellae.
    • Canaliculi connect lacunae to each other and to the Haversian canal, enabling nutrient/waste exchange.
  • Functional implications of remodeling:
    • Osteoblasts on the bone surface secrete osteoid to form new bone; osteocytes maintain the matrix.
    • Osteoclasts resorb bone, enabling remodeling and shaping.
    • The balance between osteoblast activity and osteoclast activity maintains bone homeostasis; alterations in activity can thicken or thin bones, adapt to weight changes, or respond to injury.

Anatomy of Flat Bones (Structure and Diploe)

  • Flat bones have two layers of compact bone with a middle diploe of spongy bone.
  • Diploe houses red marrow in many flat bones, enabling hematopoiesis similarly to cancellous bone.
  • The sternum and the bones of the skull are examples:
    • Sternum: compact bone on both sides with diploe in between; can yield bone marrow for biopsy.
    • Ilium (part of the pelvis): another common site for bone marrow harvesting.
  • Endosteum lines the internal surfaces of hollow spaces; osteoprogenitor cells reside here as well.
  • Spongy bone in flat bones: trabeculae with osteocytes in lacunae; canaliculi connect within trabeculae; no true Haversian canals inside the trabeculae because the bone is bathed in marrow blood.

Spongy vs. Compact Bone (Structure and Blood Supply)

  • Compact bone:
    • Dense, organized into osteons with a central Haversian canal.
    • Contains osteocytes in lacunae, connected by canaliculi within concentric lamellae.
    • Provides strength and rigidity; resists bending.
  • Spongy bone:
    • Made of trabeculae (thin beams) forming a lattice; spaces between trabeculae contain red marrow.
    • Lacks osteons and a central Haversian canal.
    • Blood supply comes from surrounding red bone marrow; canaliculi still connect osteocytes to neighboring lacunae.
    • In long bones, spongy bone is mainly in the epiphyses; in flat bones, it forms the diploe.

Bone Tissue and Cellular Lineage (Osteoblasts, Osteocytes, Osteoclasts)

  • Osteoprogenitor cells:
    • Bone stem cells that arise from mesenchyme; capable of mitosis.
    • Differentiate into osteoblasts.
  • Osteoblasts:
    • Secrete osteoid (the organic matrix) which mineralizes with calcium salts to form bone.
    • Do not divide after differentiating (become osteocytes).
  • Osteocytes:
    • Mature bone cells residing in lacunae within the bone matrix.
    • Communicate with other cells via canaliculi.
  • Osteoclasts:
    • Large cells that resorb bone; important for remodeling and release of minerals.
  • Osteoid and mineralization:
    • The osteoid laid down by osteoblasts becomes mineralized with calcium phosphate to form hard bone.
    • The calcium salts give rigidity; collagen provides some flexibility.
  • Remodeling and homeostasis:
    • Constant remodeling involves osteoblast-mediated new bone formation and osteoclast-mediated resorption.
    • This remodeling adapts bone to mechanical demands (e.g., changes in body weight, immobilization).
  • Embryology and development:
    • Mesenchyme-derived cells form osteoprogenitor cells, which then form osteoblasts and osteocytes.
    • Osteoclasts derive from monocytes/macrophage lineage (not from osteoprogenitors).

Matrix Composition and Mechanical Properties

  • Matrix components:
    • Osteoid: organic matrix secreted by osteoblasts; largely collagen fibers.
    • Mineral salts: calcium phosphate salts (e.g., hydroxyapatite) deposited in the matrix; give hardness.
  • Hydroxyapatite: the mineral component of bone mineral, commonly represented as
    Ca{10}(PO4)6(OH)2
  • Functional implications:
    • Calcium salts provide hardness and compressive strength.
    • Collagen fibers provide flexibility and a degree of toughness; without collagen, bones would be brittle.
  • Nutritional considerations:
    • Vitamin D is required to absorb calcium effectively; deficiency can lead to soft bones in children (rickets) and deformities.
    • Adequate calcium intake is essential for maintaining bone mineralization.

Clinical and Practical Implications

  • Bone marrow transplantation:
    • Some transplants rely on stem cells from red bone marrow to reconstitute blood cell production (RBCs, WBCs, platelets).
  • Fracture pain:
    • Periosteum has rich innervation; damage to bone or periosteum during fracture causes significant pain.
  • Nutritional and developmental health:
    • Adequate calcium and vitamin D intake supports bone mineralization and structural integrity.
    • Weight changes influence bone remodeling: gain of 50–100 pounds can trigger thickening to support increased load; loss can lead to bone thinning if disuse occurs.
  • Diagnostic and surgical relevance:
    • Flat bones like the sternum or ilium are common sites for bone marrow sampling or harvesting.
    • Understanding the distinct anatomy of cortical vs. trabecular bone informs imaging and treatment strategies.

Quick Reference: Visual and Structural Landmarks

  • Long bone landmarks:
    • Proximal epiphysis, distal epiphysis, diaphysis (shaft), medullary cavity, endosteum, periosteum, articular cartilage.
    • Haversian (central) canal, osteon, lamellae, osteocytes in lacunae, canaliculi, Volkmann canals.
  • Flat bone landmarks:
    • Outer and inner compact bone layers, middle diploe (spongy bone), red marrow within diploe.
  • Microstructure:
    • Osteoprogenitor cells → osteoblasts → osteocytes; osteoclasts resorb bone; osteoblasts secrete osteoid which mineralizes.
    • Spongy bone contains trabeculae and red marrow; lacks true osteons and Haversian canals.
  • Terminology recap:
    • Periosteum with Sharpey fibers; endosteum; epiphyses; diaphysis; articular cartilage; diploe; trabeculae; osteons; lacunae; canaliculi; Haversian canals; Volkmann canals.