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Skeleton System

Skeleton System

Objectives

  • Identify the functions of the skeletal system.

  • Describe the features of a long bone.

  • Compare and contrast compact and spongy bone.

  • Identify the roles of different cellular components in bone growth and ossification.

  • Describe the process of bone growth, development, and repair.

  • Describe the impact of aging, exercise, and lifestyle on bone remodeling.

  • Identify tooth structure, types of teeth, and dental succession.

The Skeletal System

  • Components:

    • Bones (Skeleton): Framework of the body.

    • Joints: Allow for movement between bones.

    • Cartilages: Connect bones at joints and provide flexibility.

    • Ligaments: Connect bone to bone.

  • Divisions of the Skeletal System:

    • Axial Skeleton: Comprised of the skull, vertebral column, and rib cage.

    • Appendicular Skeleton: Includes the limbs and girdles (pelvic and pectoral).

Functions of Bones

  • Support.

    • Provides a hard framework that supports the body and cradles soft organs.

  • Protection.

    • Fused bones, such as those in the skull, vertebrae, and rib cage, offer vital protection to underlying organs.

  • Leverage.

    • Skeletal muscles use bones as levers for movement, enhancing mechanical efficiency.

  • Mineral Storage.

    • Major storage for minerals, particularly calcium and phosphate essential for bodily functions.

  • Storage of Lipids.

    • Yellow marrow serves as a lipid storage site.

  • Blood Cell Formation (Hematopoiesis).

    • Red blood cells are formed within the marrow cavities of certain bones.

Bone Markings

  • Definition: Surface features of bones that indicate various functions or points of attachment.

  • Sites of attachment for:

    • Muscles

    • Tendons

    • Ligaments

  • Categories of Bone Markings:

    • Projections and Processes.

    • Features that grow out from the bone surface.

    • Depressions or Cavities.

    • Indentations that serve various functional purposes.

Bone Classification - Shapes

  • Long Bones:

    • Characterized by a shaft (diaphysis) and two ends (epiphyses).

    • Mainly composed of compact bone, with possible spongy interior.

    • Examples: Femur, tibia, fibula, humerus, phalanges.

  • Short Bones:

    • Cube-like, predominantly composed of spongy bone with a thin compact bone surface layer.

    • Examples: Carpals and tarsals.

  • Flat Bones:

    • Thin, flattened structures often slightly curved, with two layers of compact bone surrounding a spongy bone layer.

    • Examples: Sternum, ribs, skull, scapula.

  • Irregular Bones:

    • Bones that do not fit the previous classifications.

    • Examples: Vertebrae, pelvis.

  • Suture Bones:

    • Small, irregular bones found between flat bones, particularly in the skull.

  • Sesamoid Bones:

    • Small and flat bones that develop inside tendons, usually near joints of certain limbs (e.g., knees, hands, feet).

Compact & Spongy Bone

  • Bone Tissue Types:

    • Compact Bone:

    • Homogeneous in structure, dense and forms the outer layer of bones.

    • Spongy Bone (Cancellous, Diploe, Trabecular):

    • Composed of fine, needle-like pieces of bone with many open spaces inherent to its structure.

Gross Anatomy of a Long Bone

  • Diaphysis:

    • Shaft of the bone, composed mainly of compact bone.

  • Epiphysis:

    • Ends of the bone; primarily spongy bone covered with a thin layer of compact bone.

    • Articulates with other bones at joints.

  • Metaphysis:

    • The region at which the diaphysis and epiphysis meet, acts as the growth plate during development.

Structures of a Long Bone

  • Arteries:

    • Provide nutrients to bone cells.

  • Periosteum:

    • Outer covering of the diaphysis, consists of fibrous connective tissue.

    • Sharpey's fibers:

    • Connect periosteum to the underlying bone and associated ligaments and tendons.

  • Endosteum:

    • Lines the marrow cavity, vital for bone growth and repair.

Functions of Periosteum

  • Isolates bone from surrounding tissues.

  • Provides a route for circulatory and nervous supply.

  • Participates in bone growth and repair.

  • Establishes a strong attachment site for ligaments and tendons.

Articular Cartilage

  • Covers the external surface of the epiphyses.

  • Composed of hyaline cartilage, which decreases friction at joint surfaces.

Medullary Cavity

  • Located within the shaft (diaphysis).

  • Contains yellow marrow (primarily fat) and red marrow (for blood cell formation) in adults.

Microscopic Anatomy of Compact Bone

  • Components:

    • Lamellae: Layers of bone matrix.

    • Osteon (Haversian system): The basic functional unit of compact bone; osteocytes are arranged in concentric circles around a central canal containing blood vessels.

    • Lacuna: Cavities containing osteocytes.

    • Canaliculus: Tiny canals that connect lacunae, facilitating nutrient and waste exchange.

    • Central (Haversian) Canal: Houses blood vessels running vertically through the bone.

    • Sharpey's Fibers: Connect periosteum with underlying bone tissue.

  • Blood Vessel Pathways: Include perforating (Volkmann's) canals that are perpendicular to the central canals and provide blood flow to the bone and marrow.

Compact Bone vs. Spongy Bone

  • Compact Bone:

    • Dense and structured; contains osteons.

  • Spongy Bone:

    • Lacks osteons; the matrix forms an open network of trabeculae without blood vessels.

Bone Marrow Composition

  • Red Marrow:

    • Fills the spaces between trabeculae of spongy bone; responsible for forming red blood cells and supplying nutrients to osteocytes.

  • Yellow Marrow:

    • Found in some bones, acts as fat storage material.

Bone (Osseous) Tissue Composition

  • Matrix Minerals:

    • 2/3 of bone matrix is calcium phosphate, Inorganic

    • Matrix Proteins:

    • 1/3 of bone matrix is protein fibers, primarily type I collagen. Is organic

Osteocytes

  • Maintain protein and mineral content of the matrix.

  • Assist in the repair of damaged bone.

  • 'Live' in lacunae and communicate through canaliculi using gap junctions.

  • Do not divide.

Osteoblasts

  • Responsible for osteogenesis (bone formation).

  • Synthesize and release proteins and components of the bone matrix; the initial unmineralized matrix is referred to as osteoid.

  • Secretes RANKL (Receptor Activator of Nuclear Factor Kappa-Β Ligand) and OPG (Osteoprotegerin).

Osteoprogenitor Cells

  • Mesenchymal stem cells that undergo mitosis to form osteoblasts.

  • Important in the healing process of fractures, located in the inner periosteum and endosteum.

Osteoclasts

  • Large cells containing approximately 50 nuclei.

  • Function to break down bone matrix; dissolve matrix by secreting acids and enzymes through a process known as osteolysis.

Osteoclast Formation

  • Activation occurs via RANK/RANKL interaction, enabling the differentiation and fusion into multi-nucleated osteoclasts.

  • Osteoblasts produce OPG to inhibit RANKL, thereby modulating bone resorption; the OPG/RANKL ratio influences the extent of bone degradation.

  • Note: Estrogen increases OPG production, affecting bone density.

How Osteoclasts Catabolize Bone

  • Bone dissolution results in term called resorption.

  • Hydrogen ions from the osteoclast membrane are secreted into the space between the osteoclasts and bone surface, leading to the formation of hydrochloric acid which dissolves minerals.

  • Enzyme, acid phosphatase, digests collagen during the breakdown process.

Bone Development

  • Human Bones Growth Timeline: Continues until approximately age 25.

  • Osteogenesis: Formation of bone.

  • Ossification: Overall process of new bone material laying down, mainly involving calcification of the osteoid matrix.

  • Abnormal calcifications (ectopic calcifications) can occur in various locations outside the skeleton, like lungs and arteries.

Types of Ossification

  • Intramembranous Ossification:

    • Begins around the 8th week of fetal development.

    • Involves mesenchymal tissue and leads to the formation of flat bones of the skull, clavicle, sternum, and parts of the mandible and zygomatic bones.

  • Endochondral Ossification:

    • Involves the replacement of cartilage with bone. Most skeleton bones form through this process including long bones, vertebrae, ribs, scapulae, and pelvic girdle.

Endochondral Ossification Details

  • Begins with a developing cartilage model, leading to the formation of the primary ossification center where bone replaces cartilage.

  • Vascular invasion occurs, creating marrow cavities, allowing for further ossification.

  • Involves expansion into epiphyses with formation of secondary ossification centers.