Chapter_7-_Skeletal_System__2024_

Introduction to Bones

  • Bones are essential organs of the skeletal system made up of diverse tissues:

    • Bone tissue

    • Cartilage

    • Dense connective tissue

    • Blood

    • Nervous tissue

  • Bones serve multiple crucial functions:

    • Provide structural support and protection to softer tissues.

    • Serve as attachment points for muscles.

    • House blood-producing cells.

    • Store inorganic salts.

7.1: Bone Shape and Structure

  • Variability in Bones:

    • Bones differ in size and shape.

    • Commonalities include structure, development, and function.

Bone Classification by Shape

  • Long Bones:

    • Long and narrow with expanded ends.

  • Short Bones:

    • Cubelike; length equals width, includes sesamoid bones embedded in tendons.

  • Flat Bones:

    • Plate-like with broad surfaces.

  • Irregular Bones:

    • Varied shapes, often interconnected with other bones.

Parts of a Long Bone

  • Epiphysis: Expanded end of the bone.

  • Diaphysis: Shaft of the bone.

  • Metaphysis: Widened area between diaphysis and epiphysis.

  • Articular Cartilage: Covers the epiphysis.

  • Periosteum: Dense connective tissue that encloses the bone.

  • Compact (Cortical) Bone: Forms the wall of the diaphysis.

  • Spongy (Cancellous) Bone: Located in epiphyses.

  • Medullary Cavity: Hollow chamber in diaphysis containing marrow.

  • Endosteum: Lines internal spaces and cavities.

Microscopic Structure of Bone

  • Mature bone cells are called osteocytes.

  • Osteocytes reside in small cavities known as lacunae.

  • Nutrient and waste exchange occurs through canaliculi (tiny passageways).

  • The extracellular matrix consists mainly of:

    • Collagen fibers: Provide resilience.

    • Inorganic salts: Contribute to hardness.

Compact vs. Spongy Bone

  • Compact Bone:

    • Composed of cylindrical units called osteons.

    • Strong, solid, and weight-bearing.

    • Resists compression.

  • Spongy Bone:

    • Composed of trabeculae (branching plates).

    • Exhibits some flexibility.

    • Nutrients diffuse through canaliculi.

7.2: Bone Development and Growth

  • Bone development starts prenatally, continuing into adulthood.

  • Bones form by replacing existing connective tissue:

    • Intramembranous Bones: Formed from primitive connective tissue.

    • Endochondral Bones: Formed from hyaline cartilage models.

Intramembranous Ossification

  • Occurs in broad, flat bones like skull plates, clavicles, and sternum.

  • Process:

    • Mesenchymal cells differentiate into osteoblasts.

    • Osteoblasts deposit bone matrix, becoming osteocytes.

Endochondral Ossification

  • Most of the skeleton begins as hyaline cartilage.

  • Process involves:

    • Growth and degeneration of cartilage models.

    • Replacement by bone matrix from osteoblasts, initially forming spongy bone and later compact bone.

Growth at the Epiphyseal Plate

  • Diaphysis is separated from epiphysis by the epiphyseal plate responsible for lengthwise growth.

  • Layers of cartilage in the epiphyseal plate:

    1. Zone of Resting Cartilage: Anchors the epiphyseal plate.

    2. Zone of Proliferating Cartilage: Young cells undergoing mitosis.

    3. Zone of Hypertrophic Cartilage: Older cells left behind, thickening the plate.

    4. Zone of Calcified Cartilage: Contains dead cartilage cells and calcified matrix.

7.3: Bone Function

  • Major functions include:

    • Giving shape to the body.

    • Supporting body structures.

    • Protecting vital organs.

    • Aiding movement in conjunction with muscles.

    • Producing blood cell formations and storing inorganic salts.

Blood Cell Formation

  • Hematopoiesis occurs in red bone marrow, producing:

    • Red blood cells,

    • White blood cells,

    • Platelets.

Inorganic Salt Storage

  • Approximately 70% of bone matrix is composed of inorganic salts.

  • Important salts include:

    • Hydroxyapatite (calcium phosphate),

    • Magnesium, sodium, potassium, and carbonate ions.

  • Osteoporosis is a condition resulting from decreased mineralization.

Hormonal Control of Blood Calcium

  • The balance of calcium is regulated by hormones:

    • Calcitonin released when blood calcium is high, promoting deposition into bones.

    • Parathyroid hormone (PTH) released when blood calcium is low, promoting bone breakdown.

Factors Affecting Bone Development

  • Key factors include:

    • Nutrition (e.g., Vitamin D, A, C)

    • Hormonal balance

    • Physical Exercise.

Clinical Application - Fractures

  • Types of Fractures:

    • Simple (closed)

    • Compound (open)

    • Greenstick, fissured, comminuted, transverse, oblique, spiral fractures.

Steps in Fracture Repair

  1. Hematoma forms (blood clot).

  2. Cartilaginous callus develops.

  3. Bony callus formation.

  4. Remodeling restores bone to original shape.

Life-Span Changes in Bones

  • Decrease in height and calcium levels with age.

  • Osteoclasts outnumber osteoblasts leading to weaker bones, especially in postmenopausal women.

  • Increased risk of fractures, notably hip and vertebral compressions.