Skeletal System

Classification of Bones

Skeletal Divisions

• The human skeleton comprises a total of 206 bones.

• Bones are categorized into two main divisions:

  • Axial Skeleton:

    • Consists of 80 bones.

    • Located along the central axis or midline of the body.

    • Illustrated in lighter gray.

    • Components include:

      • Skull: Composed of 29 bones.

      • Vertebral Column: Consists of 26 bones.

      • Thoracic Cage: Composed of 24 ribs and 1 sternum.

  • Appendicular Skeleton:

    • Consists of 126 bones.

    • Located in the limbs and the girdles that attach the limbs to the axial skeleton.

    • Illustrated in bluish-green.

    • Examples of girdles:

      • Pectoral Girdle: Attaches the upper limb.

      • Pelvic Girdle: Attaches the lower limb.

Classification by Bone Shape

• Bones are also classified based on their shape:

  • Long Bones:

    • Found exclusively in the appendicular skeleton.

    • Characterized by a narrow, long central shaft called a diaphysis and two expanded ends called epiphyses.

    • Even small bones with this shape are classified as long bones (e.g., metacarpals, phalanges).

    • Examples in the upper extremity:

      • Humerus: Brachial region.

      • Radius: Lateral antebrachial region (forearm).

      • Ulna: Medial antebrachial region.

      • Metacarpals: Palmar region.

      • Phalanges: Digits.

  • Short Bones:

    • Box-shaped.

    • Examples:

      • Carpals: Carpal region (wrist).

      • Tarsals: Tarsal region (ankle).

  • Flat Bones:

    • Have a broad, flat surface, often slightly curved.

    • Examples:

      • Sternum

      • Scapula

      • Clavicle

      • Bones of the cranium: Frontal bone, Parietal bone, Occipital bone.

      • Ribs

      • Nasal bones (face)

      • Lacrimal bones (medial portion of the orbit)

      • Vomer (inferior nasal region, forms inferior part of nasal septum separating nasal cavities)

      • Coxal bone (hip bone; left and right).

  • Sesamoid Bones:

    • Small, flat bones embedded within a tendon.

    • Most familiar example: Patella (kneecap), which is embedded within the patellar tendon.

  • Irregular Bones:

    • Have complex shapes that do not fit into the long, short, or flat categories.

    • Examples:

      • Vertebrae

      • Temporal bone (cranium)

        • Contains the auditory ossicles: Incus, Malleus, Stapes (smallest bone in the body).

      • Sphenoid bone (floor of the cranium)

      • Ethmoid bone (anterior to sphenoid, forms roof of nasal cavity)

      • Bones of the face:

        • Zygomatic bone (superior buccal region, cheekbone)

        • Maxillae (upper jaw bones)

        • Mandible (lower jaw bone)

        • Palatine bones (posterior portion of the roof of the mouth)

        • Inferior nasal conchae (lateral inferior regions of the nasal cavity)

      • Hyoid bone: (superior cervical region, inferior to mandible).

        • Unusual because it does not form any bony attachments; held in place by ligaments and muscles.

Osseous Tissue and Cells

Histology of Osseous Tissue

• Compact bone exhibits a repeating, ring-shaped structure called an osteon.

• Within an osteon:

  • Concentric Lamellae: Ring-shaped plates of osseous tissue.

  • Osteocytes: Mature bone cells found within spaces called lacunae.

  • Osteocytes are arranged in lamellae, surrounding a central canal.

• Central Canal: Contains blood vessels and nerves.

Bone Matrix Components

• Osteoid: The organic matrix of bone.

  • Primarily composed of collagen fibers, which provide tensile strength to resist pulling forces.

  • Secreted by osteoblast cells.

• Hydroxyapatite: A calcium phosphate hydroxide salt.

  • Mineralizes osteoid.

  • Gives bone hardness to resist compression.

Major Cell Types in Osseous Tissue

• There are four major types of cells:

  • Osteogenic Cells:

    • Stem cells found in bone membranes (periosteum and endosteum).

    • Act as precursors that differentiate into osteoblasts.

  • Osteoblasts:

    • Bone-building cells.

    • Form the bone matrix by secreting osteoid (collagen fibers).

    • As they secrete osteoid and become surrounded by the matrix, they mature into osteocytes.

  • Osteocytes:

    • Mature bone cells.

    • Formed from osteoblasts that have been trapped within the mineralized matrix.

    • Arranged within lacunae in the lamellae.

  • Osteoclasts:

    • Bone-resorbing cells (break down bone).

    • Release acid and enzymes to break down bone matrix, releasing stored calcium.

    • Activated by parathyroid hormone to help maintain blood calcium homeostasis.

    • Important for bone remodeling and fracture repair by removing damaged tissue.

Bone Membranes

• Periosteum:

  • Superficial membrane covering the outer surfaces of bones.

  • Contains osteogenic cells.

• Endosteum:

  • Inner membrane lining internal bone surfaces, including the medullary cavity, central canals, and perforating canals.

  • Contains osteoclast cells.

Structure and Locations of Compact and Spongy Bone

Compact Bone

• Location: Superficial layer of bone organs.

• Appearance: Dense, smooth, solid.

• Microscopic Structure:

  • Osteon: The repeating structural and functional unit.

    • Consists of a central canal (also known as Haversian Canal) surrounded by concentric lamellae.

    • Central Canal: Contains blood vessels and nerves; runs parallel to the long axis of a long bone for maximum strength.

    • Concentric Lamellae: Circular plates of osseous tissue surrounding the central canal.

  • Interstitial Lamellae: Plates of osseous tissue wedged between osteons.

  • Circumferential Lamellae: Form the outer and inner layers of compact bone.

    • Outer Circ. Lamellae: Most superficial outer layer.

    • Inner Circ. Lamellae: Lining the deep surface of compact bone.

  • Perforating Canals (Volkmann's Canals):

    • Run perpendicular to central canals.

    • Contain blood vessels and nerves.

    • Connect from the surface deep into the bone and into the medullary cavity.

• All canals in compact bone are lined with endosteum.

Spongy Bone (Trabecular Bone)

• Location: Deep within bones, surrounding the medullary cavity, and filling the epiphyses of long bones.

• Appearance: Has many small holes (trabecular cavities) between irregularly arranged struts of bone.

• Structure:

  • Trabeculae: Irregularly arranged branching struts or plates of bone.

  • Trabecular Cavities: Spaces between trabeculae; contain bone marrow, blood vessels, and nerves.

• Trabecular cavities are lined with endosteum, which contains osteoclasts and osteoblasts.

Flat Bone Structure

• Consists of outer layers of compact bone surrounding an inner layer of spongy bone.

• Overall surface is broad, flat, and often slightly curved (e.g., cranium).

Anatomy of Long Bones

Major Components

• Epiphysis (Proximal and Distal):

  • Expanded ends of the long bone.

  • Mostly composed of spongy bone.

  • Articular Cartilage: Covers the articular surface where a joint is formed.

• Diaphysis:

  • Narrow shaft of the long bone.

  • Outer layer composed of compact bone.

  • Medullary Cavity: Space inside the diaphysis, filled primarily with yellow bone marrow in adults.

    • Lined with spongy bone and endosteum.

• Metaphysis:

  • Region at the border between the diaphysis and epiphyses.

  • Contains the epiphyseal line in mature bone.

• Epiphyseal Line:

  • A remnant of the epiphyseal growth plate (or epiphyseal plate).

  • Composed of compact bone at the diaphysis-epiphysis border.

  • The epiphyseal growth plate was a layer of hyaline cartilage where bones grew longer during adolescence through interstitial growth.

• Periosteum:

  • Outer membrane surrounding the bone.

  • Has a rich blood supply (periosteal vessels).

  • Contains osteogenic cells (stem cells) that differentiate into osteoblasts (bone-building cells), which then mature into osteocytes.

• Endosteum:

  • Membrane lining the medullary cavity, trabecular cavities, central canals, and Volkmann's canals.

  • Contains osteoclast cells, which break down bone matrix to release calcium and maintain calcium homeostasis.

Bone Formation (Ossification)

Definition of Ossification

• The process of bone formation, where existing tissue is replaced with osseous tissue.

Types of Ossification

1. Intramembranous Ossification

• Process: Bone develops directly within a fibrous membrane (e.g., within the dermis of the skin).

• Steps:

  • Ossification Center Formation: Osteoblasts differentiate within embryonic connective tissue (mesenchyme) and begin secreting osteoid, forming an ossification center.

  • Bony Spicule Growth: As osteoid surrounds osteoblasts, they mature into osteocytes. Connections of osseous tissue, known as bony spicules, grow and connect to surrounding blood vessels.

  • Trabeculae Formation: Bone spicules grow together to form trabeculae, creating a mesh-like structure of spongy bone with blood vessels now within the trabecular cavities.

  • Compact Bone Formation: Osteoblasts organize around the surface into the newly formed periosteum and ossify the superficial layer, forming compact bone.

• Examples of bones formed this way: Parietal bones, Frontal bone, Clavicle, Patella.

2. Endochondral Ossification

• Process: Bone formation from a pre-existing hyaline cartilage model.

• Steps:

  • Cartilage Model Growth: The hyaline cartilage model (surrounded by perichondrium) grows. Deep chondrocytes become starved of nutrients, the surrounding matrix calcifies, and the chondrocytes die, leaving spaces.

  • Primary Ossification Center: Blood vessels grow into these spaces, carrying osteogenic cells that differentiate into osteoblasts. Osteoblasts secrete osteoid, forming the primary ossification center within the diaphysis. The perichondrium differentiates into a periosteum.

  • Medullary Cavity Formation: Osteoclast cells migrate in with the blood and begin breaking down some of the newly formed bone in the center of the primary ossification center, creating the medullary cavity.

  • Secondary Ossification Centers: The process repeats in the developing epiphyses. Chondrocytes calcify, enlarge, die, and leave spaces. Blood vessels invade, bringing osteogenic cells that differentiate into osteoblasts and secrete osteoid, forming secondary ossification centers in both epiphyses.

  • Compact Bone and Growth Plates: The periosteum's osteogenic cells form a superficial layer of compact bone. At birth, the long bone still has hyaline cartilage at the epiphyseal plate (for growth) and as articular cartilage (for joint surfaces).

Bone Growth

Longitudinal (Interstitial) Bone Growth

• Mechanism: Occurs at the epiphyseal growth plates to increase bone length.

• Zones of the Epiphyseal Plate:

  • Zone of Proliferation: Chondrocytes divide (mitosis) to produce more chondrocytes, causing the cartilage plate to grow.

  • Zone of Hypertrophy: Cells enlarge and mature.

  • Zone of Calcification: Matrix calcifies, chondrocytes die, leaving spaces.

  • Zone of Ossification: Osteogenic cells migrate into the spaces, differentiate into osteoblasts, and secrete osteoid, replacing cartilage with osseous tissue. New bone forms at the plate's end closest to the diaphysis (metaphysis).

• Cessation of Growth: During puberty, sex hormones stimulate increased osteoblast activity, causing rapid ossification of the epiphyseal growth plates. The cartilage is completely replaced by bone, ending longitudinal growth.

• Epiphyseal Line: The ossified remnant of the epiphyseal plate.

• Achondroplasia: The most common form of dwarfism, caused by mutations that disrupt cartilage growth, impairing longitudinal bone growth.

Appositional Bone Growth

• Mechanism: Increases the width (thickness) of bones.

• Process:

  • Osteoblast cells produce new bone superficially (on the outer surface).

  • Osteoclast cells break down bone to expand the medullary cavity deep within the bone.

Endocrine Regulation of Bone Growth

• Growth Hormone (GH): Secreted by the anterior pituitary gland.

  • Stimulates both longitudinal and appositional bone growth.

  • Deficiency during childhood: Leads to pituitary dwarfism (proportionate smaller size).

  • Excessive GH during childhood: Leads to gigantism (overall larger size).

  • Excessive GH after epiphyseal plate closure: Leads to acromegaly.

    • Stimulates appositional growth, causing growth of jaws (increased teeth spacing) and increased width/size of hands and feet.

    • Does not increase bone length because epiphyseal plates are closed.

• Sex Hormones (Estrogen and Testosterone):

  • Released during puberty.

  • Stimulate an increased rate of ossification at the epiphyseal growth plates.

  • Lead to the eventual closure of the epiphyseal plates, forming the epiphyseal line.

Bone Remodeling and Osteoporosis

Bone Remodeling Process

• Definition: A continuous, lifelong process where new bone is formed by osteoblasts, and old bone is simultaneously broken down and reabsorbed by osteoclasts.

• Purposes:

  • Maintain proper bone proportions during growth and adolescence.

  • Repair damaged bone (e.g., fractures).

  • Strengthen bone in response to mechanical stresses.

  • Maintain blood calcium homeostasis.

• Wolff's Law: A fundamental principle of bone remodeling stating that bone adapts to strengthen areas experiencing the most mechanical stress.

• Calcium Homeostasis: Parathyroid hormone stimulates osteoclasts, releasing calcium from bone into the blood. This can weaken bone if calcium release outpaces deposition.

Osteoporosis

• Definition: A reduction in bone mass resulting in decreased bone density, making bones light, fragile, and prone to fracturing.

• Cause: Occurs when osteoclast activity (bone breakdown for calcium release) is higher than osteoblast activity (calcium deposition for bone strengthening).

• Effects: Bones become fragile and light. Can lead to an exaggerated curvature of the thoracic region (kyphosis or