Skeletal Tissue

Overview of the Skeletal System

The skeletal system is composed of cartilages, bones, and ligaments, each playing a vital role in the structure and function of the human body. It provides support, protection, movement, mineral storage, hematopoiesis, and communication.

• Support: Framework for the body, maintaining shape and posture.

• Protection: Shields vital organs (e.g., skull protects brain, ribs protect thoracic organs).

• Movement: Bones act as levers for muscle action.

• Mineral Storage: Stores calcium (Ca2+) and phosphorus (P).

• Hematopoiesis: Blood cell formation occurs in bone marrow.

• Communication: Facilitates transmission of signals via mineral ions.

Cartilage

Characteristics of Cartilage

Cartilage is a tough, flexible connective tissue that resists compression and tension, especially fibrocartilage. It is avascular and lacks nerve fibers, composed mainly of ground substance, collagen, elastic fibers, and water.

• Ground Substance: Contains glycosaminoglycans (GAGs) such as chondroitin sulfate and hyaluronic acid.

• Cells: Chondroblasts (produce matrix), chondrocytes (maintain matrix), reside in lacunae.

Types of Cartilage

• Hyaline Cartilage: Most abundant, blue-white, glassy appearance. Provides support and shock absorption. Found in articular surfaces, costal cartilages, trachea, larynx, epiphyseal plates, nasal septum.

• Elastic Cartilage: Contains many elastic fibers, allows stretch and recoil. Found in external ear and epiglottis.

• Fibrocartilage: Alternating rows of collagen fibers and chondrocytes. Provides shock absorption and tensile strength. Found in intervertebral discs, menisci, pubic symphysis.

Cartilage Growth

• Interstitial Growth: Cartilage grows from within.

• Appositional Growth: New layers added to the outer surface.

• Most cartilage growth ends after adolescence.

Bone Tissue

Classification and Structure

There are approximately 206 bones in the human skeleton, classified by shape and location. The skeleton is divided into axial (skull, vertebral column, thorax) and appendicular (limbs and girdles) components.

• Long Bones: Cylindrical, found in limbs (except patella, carpals, tarsals).

• Short Bones: Cube-shaped, found in wrist and ankle.

• Sesamoid Bones: Sesame-seed shaped, found in tendons.

• Flat Bones: Sternum, scapulae, ribs, skull.

• Irregular Bones: Complex shapes, vertebrae, pelvis, some skull bones.

Bone Structure

• Compact Bone: External layer, dense and strong.

• Spongy Bone: Internal layer, porous, contains trabeculae.

• Diaphysis: Shaft of long bone, contains medullary cavity.

• Epiphysis: Ends of long bone, contains spongy bone.

• Metaphysis: Region between diaphysis and epiphysis, contains epiphyseal plate/line.

• Periosteum: Covers bone exterior, provides insertion for tendons/ligaments, contains nerve fibers, blood vessels, and osteogenic cells.

• Endosteum: Lines bone interior, contains osteogenic cells.

Bone Cells

• Osteoprogenitor Cells: Stem cells in periosteum/endosteum, differentiate into osteoblasts.

• Osteoblasts: Bone-forming cells, synthesize osteoid, initiate mineralization.

• Osteocytes: Mature bone cells in lacunae, maintain matrix, act as strain sensors.

• Osteoclasts: Bone-resorbing cells, digest matrix with enzymes and acids.

Bone Marrow

• Red Marrow: Hematopoietic, found in medullary cavity (youth), spongy bone (adults).

• Yellow Marrow: Adipose-rich, found in medullary cavity (adults), can revert to red marrow if needed.

Histology of Bone

Compact Bone

Compact bone is organized into osteons (Haversian systems), which are weight-bearing cylinders aligned with the bone's axis.

• Osteon Anatomy: Central canal (blood vessels, nerves), concentric lamellae, osteocytes in lacunae, canaliculi, interstitial and circumferential lamellae, Volkmann's canals.

Spongy Bone

Spongy bone lacks osteons and is composed of trabeculae, oriented to resist stress, with bone marrow between trabeculae.

Chemical Composition of Bone

Organic Components

• 35% of bone, provides flexibility and tensile strength.

• Includes cells and osteoid (ground substance, collagen fibers).

Inorganic Components

• 65% of bone, provides hardness and compression resistance.

• Hydroxyapatite (calcium phosphate) is the main mineral:

• Other minerals: CaCO3, Mg, Na, K, F, SO42-, CO32-, OH-

Diseases

• Rickets: Defective mineralization in children.

• Osteogenesis Imperfecta: "Brittle bone disease" due to abnormal collagen.

Formation of Bony Skeleton

Ossification Processes

• Intramembranous Ossification: Bone develops from fibrous membrane (flat bones, clavicle).

• Endochondral Ossification: Bone replaces hyaline cartilage (most bones).

Endochondral Ossification Steps

1. Perichondrium forms periosteum; osteoprogenitor cells become osteoblasts.

2. Primary ossification center forms in shaft; bony collar develops.

3. Chondrocytes hypertrophy, matrix calcifies, chondrocytes die.

4. Periosteal bud invades, spongy bone forms, medullary cavity created.

5. Secondary ossification centers form in epiphyses around birth.

6. Hyaline cartilage persists as epiphyseal plate and articular cartilage.

Postnatal Bone Growth

Growth in Length of Long Bones

Long bones grow in length during childhood and adolescence via interstitial growth at the epiphyseal plate.

• Zones of Epiphyseal Plate:

  • Zone 1: Reserve cartilage

  • Zone 2: Cell proliferation

  • Zone 3: Cell hypertrophy

  • Zone 4: Calcification

  • Zone 5: Bone deposition

• Epiphyseal plate closure marks end of growth; plate becomes epiphyseal line.

• Bones can widen by appositional growth throughout life.

Bone Remodeling

Remodeling Process

Bone remodeling is a continuous process involving bone formation and resorption, maintaining skeletal integrity and adapting to stress.

• 5-10% of skeleton is remodeled annually.

• Spongy bone replaced every 3-4 years; compact bone every 10 years.

• Bone Formation: Osteoblasts produce osteoid, mineralized after 12-15 days.

• Bone Resorption: Osteoclasts digest matrix with HCl and enzymes.

Control of Remodeling

• Mechanical stress and gravity stimulate osteoblasts (Wolff's Law).

• Calcium homeostasis involves coupled bone formation and resorption.

• Hormones (PTH, calcitriol, calcitonin), cytokines, and growth factors regulate remodeling.

Factors Required for Normal Bone Growth and Maintenance

• Dietary minerals: calcium, phosphorus, magnesium, fluoride, iron, manganese.

• Vitamins: D3 (calcitriol synthesis), C (collagen synthesis), A (osteoblast activity), K, B12.

• Hormones: Growth hormone, thyroxine, sex hormones (stimulate matrix synthesis and epiphyseal plate closure).

Calcium Homeostasis

Roles of Calcium

• Nerve impulse transmission

• Muscle contraction

• Blood coagulation

• Secretion by glands

• Cell division

Calcium Distribution and Blood Levels

• 99% of calcium is stored in bone.

• Normal blood levels: 9-11 mg/100 ml.

• Ionic calcium (Ca2+) in ECF affects membrane potential and excitability.

Calcium Imbalances

Phosphorus

• 85-90% stored in bones.

• Normal blood levels: 3.5-4.0 mg/dl.

• Forms: HPO42-, H2PO4-

Hormonal Regulation of Calcium

• Calcitriol (Vitamin D): Synthesized in skin, liver, kidneys; increases Ca and P absorption in GI tract, resorption from skeleton, reabsorption in kidneys.

• Parathyroid Hormone (PTH): Released when blood Ca decreases; stimulates osteoclasts, increases intestinal absorption (via calcitriol), increases renal reabsorption, lowers serum P by increasing excretion.

• Calcitonin: Secreted by thyroid C cells when blood Ca increases; inhibits osteoclasts, stimulates osteoblasts, increases renal excretion of Ca.

Bone Fractures

Classification of Fractures

Fracture Repair

1. Hematoma formation and granulation tissue

2. Fibrocartilaginous callus formation

3. Spongy bone formation

4. Remodeling to cortical bone

Treatment and Complications

• Closed Reduction: Realignment without surgery.

• Open Reduction: Surgical realignment.

• Complications: Delayed union, mal-union, non-union, infection.

Example: A femoral fracture may be classified as displaced, open, and complete, requiring open reduction and careful monitoring for infection.

Additional info: Academic context was added to clarify bone cell functions, hormonal regulation, and fracture repair steps.