Skeletal System: Bone and Cartilage

Skeletal Cartilages

  • Cartilage is a major component of skeletal tissue, primarily composed of water.

  • It lacks significant nerve input and blood vessels.

  • Three types of cartilage exist in the skeletal system:

Hyaline Cartilage

  • Provides support, flexibility, and resilience; it is the most abundant type.

  • Visually identifiable with a frosted glass appearance.

  • Features chondrocytes in lacunae.

  • Locations:

    • Articular cartilages: Surfaces of mobile joints (e.g., jaw, elbow).

    • Costal cartilages: Rib attachments to the sternum.

    • Respiratory cartilages: Larynx, vocal cords, trachea, and other respiratory passages.

    • Nasal cartilages: Parts of the nose extending from the skull.

Elastic Cartilage

  • Contains elastic fibers, allowing it to withstand repeated bending.

  • Rare in the skeleton.

  • Located in the epiglottis (covers the trachea during swallowing) and the external ear.

Fibrocartilage

  • Highly compressible with high tensile strength.

  • Contains dense collagen fibers.

  • Located in intervertebral discs (shock absorption between vertebrae) and menisci of the knee (shock absorption in the knee joint).

  • All skeletal cartilages are surrounded by a perichondrium (dense irregular connective tissue) that provides nutrients. Nutrients diffuse from blood vessels to bones through this.

Two Types of Cartilage Growth

Appositional Growth

  • Cartilage is surrounded by perichondrium, which contains fibroblasts.

  • Fibroblasts secrete new cartilage tissue onto the existing cartilage base.

  • The perichondrial layer secretes a shell of cartilage.

  • New cartilage comes from the outside.

Interstitial Growth

  • Chondrocytes within the cartilage secrete tissue that expands the cartilage from within.

  • Involves the addition of cartilage tissue from the inside.

Bone (Osseous Tissue)

  • Hard connective tissue.

  • Functions:

    • Supports the body, providing an upright structure.

    • Protects internal organs (e.g., skull protects the brain).

    • Anchors muscles for movement; muscles contract and pull on bones.

    • Stores minerals, especially calcium, which is critical for nervous system and muscular function.

    • Stores fats in yellow bone marrow.

    • Creates new blood cells in red bone marrow.

Bone Classification

Shapes

  • Long, short, flat, irregular, and sesamoid.

Regions

  • Axial skeleton: Long axis of the body (skull, vertebral column, rib cage).

  • Appendicular skeleton: Limbs and girdles (hips, shoulder blades, collarbone) that attach them.

Bone Shapes

Long Bones

  • Cylindrical and long relative to their width.

  • Function as levers for muscle contraction.

  • Examples: Limb bones (except wrist and ankle bones), phalanges.

Short Bones

  • Cube-like overall shape.

  • Provide stability and support.

  • Examples: Carpals (wrist) and tarsals (foot).

Flat Bones

  • Thin, curved plates.

  • Protect internal organs and serve for muscle attachment.

  • Examples: Sternum, skull, ribs, scapula.

Irregular Bones

  • Come in various shapes.

  • Examples: Hip bones, vertebrae, facial bones.

Sesamoid Bones

  • Shaped like sesame seeds and form within tendons.

  • Protect tendons from friction at joints.

  • Examples: Patella (knee), and random formations in the palm or back of the knee.

Bone Tissue Types

  • Compact Bone (Lamellar): Smooth and solid, typically the external layer.

  • Spongy Bone (Trabecular): Internal layer with a network of spines (trabeculae) filled with marrow.

  • Long bones have dense spongy bone at each end, while flat bones have spongy bone (diploe) sandwiched between two layers of compact bone.

Long Bone Structure

  • Epiphysis: Each end of the bone, filled with red marrow.

  • Diaphysis: The shaft of the bone, hollow and filled with yellow marrow (medullary cavity).

  • Articular Cartilages: Hyaline cartilage layer for smooth joint movement.

  • Epiphyseal Line: Forms from the ossified epiphyseal plate (hyaline cartilage).

Connective Tissue Layers

  • Periosteum: Outer layer, rich in nerves and blood vessels.

    • Fibrous layer: Dense irregular connective tissue.

    • Osteogenic layer: Contains bone cells.

  • Endosteum: Internal surface covering the medullary cavity and trabeculae, a major site of growth and repair.

Nutrient Foramen

  • A hole in the center of the diaphysis for blood and nerve supply.

  • Reaches the marrow and spongy bone first.

  • Nerves concentrate at metabolically reactive sites to promote growth.

Bone Marrow

  • Red Marrow: Blood-forming marrow, present in medullary cavity and spongy bone in babies; restricted to flat and irregular bones and heads of femur and humerus in adults.

Which of the following is a function of red bone marrow? - hematopoiesis

  • Yellow Marrow: Stored fat, converted from red marrow, can revert to red marrow during anemia.

Bone Markings

  • Regions where bones are not smooth, including raised surfaces, dips, depressions, or holes.

    • Articulations (Joints): Flat surfaces where two bones meet; one is convex and the other is concave.

    • Projections: Raised areas for tendon and ligament attachment.

    • Depressions/Openings: Holes or grooves for blood vessels and nerves.

Bone Cells

  • Osteogenic Cells: Stem cells that differentiate into other bone cell types, mitotically active, located in periosteum and endosteum.

  • Osteoblasts: Create bone tissue by secreting proteins of the bone matrix, which adheres to calcium phosphate and calcium carbonate to become hydroxyapatite.

  • Osteocytes: Mature, trapped cells that sense mechanical stress and communicate to regulate bone remodeling; not mitotically active.

  • Osteoclasts: Break down bone tissue, giant multinucleate cells derived from white blood cells, balance osteoblast activity.

  • Bone Lining Cells: Maintain bone matrix.

Compact Bone Structure

  • Tightly packed osteons (Haversian systems) with lamellae (fibers of calcified matrix) running in different directions for strength.

  • Central Canal (Haversian Canal): Contains blood, lymphatic vessels, and nerves.

  • Perforating Canals (Volkmann's Canals): Connect central canals perpendicularly.

  • Canaliculi: Connect central canals to osteocytes in lacunae. Osteocytes communicate via gap junctions.

Which of the following is found within compact bone, but not within spongy bone? - CENTRAL CANAL ( a central canal is found within each cylindrical osteon comprising compact bone.)

Spongy Bone Structure

  • Trabeculae: Architectural struts that support bones under pressure; arranged along lines of stress.

  • Canaliculi and lamellar rings are present; osteocytes reside within.

Chemical Composition of Bone

  • Organic Components (35%):

    • Cells and osteoid (ground substance and collagen fibers).

    • Collagen fibers with sacrificial bonds that break under stress to dissipate energy and prevent bone fracture.

  • Inorganic Components (65%):

    • Hydroxyapatite: Tightly packed, provides hardness and resists compression.

Bone Development

  • Ossification (Osteogenesis): Begins at 6-7 weeks.

Endochondral Ossification

  • Replacement of cartilage with bone.

  • Mesenchymal cells differentiate into chondrocytes, surrounded by osteoblasts forming a bone collar.

  • Primary ossification center forms within the cartilage.

  • Cartilage dies due to lack of nutrients (no blood vessels), allowing blood vessel invasion and spongy bone formation.

  • Contributes to articular cartilages and epiphyseal line.

  • Forms most long bones and structures below the skull.

Intramembranous Ossification

  • Direct development of bone from connective tissue (no cartilage template).

  • Mesenchymal stem cells become osteoblasts and secrete osteoid matrix.

  • Osteoblasts trap themselves and become osteocytes.

  • Blood vessels invade, osteoblasts build around them, leading to spongy bone formation.

  • Osteoblasts densify on the outside to create compact bone.

  • Forms skull and clavicle bones.

Bone Growth

  • Bones grow longer and thicker.

  • Appositional Growth: Cells on the surface (periosteum) secrete layers of bone tissue, thickening the bone.

  • Interstitial Growth: Cartilage secretes around chondrocytes, creating more cartilage, which is then replaced by bone.

Epiphyseal Plate Zones

  • Reserve (Resting) Zone: Small, inactive chondrocytes adhered to the epiphysis.

  • Proliferative Zone: Cells divide, increasing cartilage length.

  • Hypertrophic Zone: Chondrocytes enlarge (hypertrophy), outgrowing lacunae.

  • Calcification Zone: Chondrocytes die as lacunae degrade, matrix deteriorates, and blood vessels invade.

  • Ossification Zone: Osteoblasts and osteoclasts remodel bone.

Hormonal Control of Bone Growth

  • Growth Hormone (pituitary gland): Promotes chondrocyte proliferation and osteoblast activity.

  • Thyroxine (thyroid gland): Promotes osteoblast and bone matrix synthesis.

  • Estrogen/Testosterone: Promote osteoblast activity, with estrogen triggering epiphyseal plate conversion to epiphyseal line.

  • Parathyroid Hormone (parathyroid gland): Stimulates osteoclasts and kidney calcium retention.

  • Calcitonin (thyroid gland): Inhibits osteoclast activity (limited role in humans).

Bone Remodeling

  • Bones are entirely replaced approximately every 10 years through a process of resorption and deposition.

    • Resorption (Osteoclast Activity): Dissolves bone matrix using acid ions, lysosomes, and phagocytosis of osteocytes.

    • Deposition (Osteoblast Activity): Secrete osteoid, bind calcium salts, and promote calcium phosphate formation.

Influences on Bone Remodeling

Hormonal Control

  • Negative feedback loop involving parathyroid hormone for low blood calcium and calcitonin for high blood calcium.

Mechanical Stress

  • Bones sense force and remodel based on stress.

  • Stronger bones develop in the dominant hand due to increased usage.

  • Trabeculae formation occurs along lines of stress, such as in the head of the femur.

  • Larger projections form for larger muscles.

Bone Fractures

  • Any broken bone; requires physician intervention to ensure proper alignment during healing.

  • Open Reduction: Requires surgery to set the bone.

  • Closed Reduction: Does not require surgery; bone is set in-office.

Fracture Classification

  • Completeness of break, shape of break, and whether it has broken through the skin.

Types of Fractures

  • Closed vs. Open: Whether the bone has broken through the skin.

  • Transverse: Straight across the bone.

  • Spiral: Twisting fracture around the bone's surface.

  • Comminuted: Multiple bone fragments.

  • Impacted: One end of the bone is driven into the other.

  • Greenstick: Incomplete fracture common in children.

  • Oblique: Fracture at an angle.

Bone Fracture Healing

  • Hematoma Formation: Blood clot surrounds the break.

  • Fibrocartilaginous Callus Formation: New connective tissue and cartilage fill the hematoma (internal and external calli).

  • Bony Callus Formation: Fibrocartilage is replaced with spongy bone.

  • Remodeling: Bone is sculpted back into its correct shape.

Bone Diseases

Paget's Disease

  • Occurs in adults over 40, where osteoclasts destroy bone faster than osteoblasts can create it.

  • Causes weak, brittle, and misshapen bones.

  • Symptoms include pain, increased fracture rates, bone deformities, headaches, and hearing loss.

Osteogenesis Imperfecta (Brittle Bone Disease)

  • Genetic disease present from birth due to a mutation in collagen production.

  • Bones are constantly fractured and become misshapen.

  • Treated with stabilizing rods and environmental/activity management.

  • Also affects skin and musculature.

Osteoporosis

  • Age-related disorder where bone mass decreases due to overactive osteoclasts.

  • Associated with women in menopause due to decreased estrogen.

  • Increases the risk of dangerous fractures and complications.

Osteosarcoma

  • Bone cancer, most prevalent in teenagers and young adults during epiphyseal line ossification.

  • Mutated osteoblasts result from high hormone levels promoting bone growth.

Dictionary meanings :

Osteomalacia, a condition characterized by softening of the bones due to inadequate mineralization, is primarily linked to deficiencies in minerals like calcium and phosphate

Osteoporosis is characterized by decreased bone density, making bones more fragile and susceptible to fractures, not increased bone density.

Rickets is a bone disease in children caused by a deficiency in vitamin D, calcium, or phosphate, not vitamin A.

Ligaments connects bones and reinforce joints, allowing required movements while restricting motion in other directions.

Joints provide mobility of the skeleton ( joints have two fundamental functions: they give our skeleton mobility, and they hold it together, sometimes playing g a protective role in the process.)

Functional classification;

Synarthroses; immovable joints ( restricted in the axial skeleton)

Amphiarthroses; slightly movable

Diaryhroses; freely movable joints (limbs)

The skull consists of 8 cranial bones and 14 facial bones, 22 in all.

  • the cranial bones, or cranium, enclose and protect the fragile brain and furnish attachment sister for head and neck k muscle. The facial bones:

  • Forms the framework of the face

  • Contain cavities for the special sense organ of sight, taste, and smell

  • Provide opening for the passage of air and food

  • Secure teeth

  • Anchor the facial muscles of expression, which we use to show our feelings

Most skull bones are flat bones. Except for the mandible, which connected to the rest of the skull by freely movable joint

Adult bones in the skull are firmly united by interlocking joints called sutures;

  • coronal

  • Sagittal

  • Squamous

  • Lambdoid sutures

  • Connect cranial bones

The cranium can be divided into a vault and a base;

  • cranial vault, forms the superior, lateral, and posterior aspect of the skull, as well as the forehead

  • Cranial base forms the skulls’s inferior aspect. Internally, prominent bony ridges divide the base into thread distinct “Steps” or fossae- the any, middle, and posterior cranial fossae

The space enclosed by the cranial vault and cranial base is called the cranial cavity and is occupied by the brain


The vertebral column (spine) consists of 26 irregular bones connected in such a way that a flexible, curved structure