Bones

SKELETAL INTRODUCTION

  • Adult skeleton contains 206 bones.

  • Axial skeleton:

    • Composed of the skull, thorax, and vertebral column.

    • Function: protects and supports soft tissues and organs.

  • Appendicular skeleton:

    • Composed of limbs and girdles.

    • Function: leverages for movement.

  • Additional functions include:

    • Mineral and fat storage.

    • Hematopoiesis (blood cell formation).

FUNCTIONS OF SKELETAL SYSTEM

  • Support:

    • The skeletal system acts as a structural backbone for the body.

    • Individual bones or groups of bones serve as a framework for the attachment of soft tissues and organs.

  • Storage of Minerals:

    • Bones store calcium salts, which maintain normal concentrations of calcium and phosphate ions in body fluids.

    • Calcium is the most prevalent mineral in the human body, with approximately 1-2 kg (2.2-4.4 lb) residing in the body, over 98% of which is found in bones.

  • Blood Cell Production:

    • Red blood cells, white blood cells, and platelets are formed in red marrow, located within the internal cavities of several bones.

  • Protection:

    • Skeletal elements encase delicate tissues and organs, such as:

    • Ribs protect the heart and lungs.

    • Skull encloses the brain.

    • Vertebrae shield the spinal cord.

    • Pelvis safeguards delicate digestive and reproductive organs.

  • Leverage:

    • Many skeletal bones act as levers modifying the magnitude and direction of forces generated by muscles, enabling movements from delicate fingertip actions to extensive body positioning changes.

CLASSIFICATION OF BONES BY SHAPE

  • Flat Bones:

    • Thin, parallel surfaces.

    • Function: muscle attachment and organ protection.

  • Wormian/Sutural Bones:

    • Irregular bones located between cranial bones.

    • Varies among individuals.

  • Long Bones:

    • Long and slender, facilitating movement.

  • Irregular Bones:

    • Complex shapes.

    • Comprised of most bones.

  • Sesamoid Bones:

    • Small, flat bones forming within tendons.

    • Function: protect tendons.

  • Short Bones:

    • Small and boxy, serve as stabilizers.

LONG BONE ANATOMY

  • Epiphysis:

    • Composed of spongy (trabecular) bone.

    • Covered by a compact bone layer.

    • Articular cartilage covers joint ends.

    • Red bone marrow present for hematopoiesis.

  • Diaphysis:

    • Central shaft with walls of compact bone.

    • Contains a medullary cavity, lined by endosteum, with yellow bone marrow for adipose storage.

  • Metaphysis:

    • Region where the epiphyseal plate connects.

  • Periosteum:

    • Covers the bone and contains blood vessels and nerves.

    • Site for tendon and ligament attachment.

VASCULARITY FOR GROWTH AND MAINTENANCE

  • Nutrient artery and vein:

    • Generally one per bone, accessing via nutrient foramen in the diaphysis.

    • Supplies osteons in walls and medullary cavity.

  • Metaphyseal and Epiphyseal arteries and veins:

    • Supplies the respective regions of the long bone.

FLAT BONE ANATOMY

  • Cross-section shows spongy bone (diploë) on either side flanked by layers of compact bone.

OSSEOUS CELLS

  • Osteogenic Cells:

    • Mesenchymal stem cells that develop into osteoblasts.

    • Participate in bone repair.

  • Osteoblasts:

    • Immature cells responsible for osteogenesis (bone formation).

    • Secrete osteoid (collagen matrix) and hydroxyapatite (calcium salts).

    • Trapped osteoblasts become osteocytes as the matrix calcifies.

  • Osteocytes:

    • Most common, mature cells found in lacunae (spaces).

    • Separated by lamellae and connected by canaliculi (channels).

    • Maintain bone matrix by regulating mineral concentration through enzyme release.

  • Osteoclasts:

    • Involved in osteolysis (resorption) to liberate stored minerals.

    • Multinucleated cells derived not from osteogenic cells, but from white blood cells (WBCs).

COMPACT BONE

  • Dense and strong; functional unit is the osteon:

    • Composed of concentric rings of calcified matrix called lamellae.

    • Central canal houses blood vessels and nerves.

    • Perforating canals connect with the medullary cavity and periosteum.

    • Osteocytes reside in lacunae (spaces).

SPONGY BONE (CANCELLOUS BONE)

  • Located in areas of minimal stress or where stress is applied from multiple directions.

  • Osteocytes and lacunae exist within a network of trabecular matrix.

  • Provides lightweight structure while red bone marrow is situated between trabeculae for hematopoiesis.

FORMATION OF BONE (OSSIFICATION)

  • Bone formation occurs via replacing a hyaline cartilage “template” or directly from connective tissue.

  • At birth, the majority (but not all) structures have been converted to bone.

BONE FORMATION MECHANISMS

  • Endochondral Ossification:

    • Replaces hyaline cartilage with bone, contributing to lengthening of bones.

    • Responsible for forming long, short, and irregular bones.

    • Increased activity of osteoblasts relative to chondroblasts during puberty leads to narrowing of the epiphyseal plate.

  • Intramembranous Ossification:

    • Mesenchymal stem cells differentiate into osteoblasts within connective tissue.

    • Responsible for forming most flat bones in the skull.

INTRAMEMBRANOUS OSSIFICATION PROCESS

  • Mesenchymal cells cluster, differentiating into osteoblasts and forming ossification centers.

  • Osteoblasts secrete osteoid, capturing themselves to become osteocytes.

  • As osteoid is deposited around capillaries, trabecular matrix forms.

  • Compact bone develops over the spongy bone, and blood vessels condense to form red marrow.

ENDOCHONDRAL OSSIFICATION PROCESS

  • Mesenchymal cells differentiate into chondrocytes, forming a cartilage model of the future skeleton and the perichondrium.

  • Capillaries invade the cartilage, causing the perichondrium to convert into periosteum.

  • The periosteal collar develops, along with the primary ossification center.

  • Cartilage and chondrocytes grow at the end of the bone, leading to secondary ossification centers forming.

  • Cartilage persists at the epiphyseal plate and joint surfaces as articular cartilage, allowing for growth.

LONGITUDINAL GROWTH

  • The epiphyseal plate is paramount for longitudinal bone growth, involving:

    • Chondrocytes dividing and secreting matrix at epiphyseal side, while degenerating at the diaphyseal side.

    • Osteoblasts migrate to convert cartilage into bone, progressively replacing cartilage with bone matrix.

LONGITUDINAL GROWTH CHANGES

  • The progression from an epiphyseal plate to an epiphyseal line marks the conclusion of longitudinal bone growth in early adulthood, as chondrocytes cease division.

APPOSITIONAL GROWTH (Bone Remodeling)

  • Involves increasing the diameter of existing bones, not creating new bones.

  • Osteogenic cells differentiate into osteoblasts under the periosteum, depositing circumferential lamellae.

  • Trapped osteoblasts become osteocytes, while deeper lamellae are recycled into osteons.

  • Osteoclasts remove matrix at the inner surface, enlarging the medullary cavity.

REGULATING BLOOD CALCIUM

  • Bones serve as a reservoir for various ions, predominantly Ca2+.

  • Calcium's Importance:

    • Essential for muscle function and neuron activity.

  • Hormonal Regulation:

    • Affects:

    • Intestinal calcium absorption.

    • Osteoblast and osteoclast activity.

    • Urine composition in kidneys, influencing bone formation/resorption.

BONE FRACTURES

  • Definition: A fracture is a crack or break in a bone and can be classified based on:

    • Whether the fracture leads to skin penetration:

    • Simple (closed): bone breaks cleanly, not penetrative.

    • Compound (open): broken ends protrude through the skin.

    • Orientation of the break:

    • Transverse: break perpendicular to impact axis.

    • Linear: break parallel to long axis of bone.

    • Position of the bone ends:

    • Non-displaced: bone ends stay aligned.

    • Displaced: bone ends shift from their normal alignment.

TYPES OF BONE FRACTURES

  • Closed Fracture: skin remains intact.

  • Open Fracture: skin is broken.

  • Transverse Fracture: fracture is straight across the long axis.

  • Spiral Fracture: caused by twisting motion, segments are pulled apart.

  • Comminuted Fracture: multiple breaks in one bone.

  • Impacted Fracture: one bone fragment is driven into another due to compression.

  • Greenstick Fracture: one side of the bone is broken and the other side bends.

  • Oblique Fracture: occurs at an angle other than 90 degrees to the long axis.