Bones and Skeletal Tissues: Structure, Function, and Development

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Bones and Skeletal Tissues

Big Picture: Skeletal Cartilage & Bone

The human skeleton initially forms as cartilage, which is later replaced by bone during development. Cartilage persists in regions where flexibility is required throughout life.

Cartilage is avascular (lacks blood vessels) and lacks nerves.

Cartilage Basics

Chondrocytes are the cells found in cartilage, residing in small spaces called lacunae.
The cartilage matrix is rich in water, providing resilience and the ability to withstand compression.
Perichondrium is a dense connective tissue membrane that surrounds cartilage, supplying nutrients and aiding in growth and repair.

Types of Cartilage

There are three main types of cartilage, each with distinct properties and locations:

Hyaline cartilage: Most abundant; provides support with flexibility and resilience. Examples: Articular cartilage (joints), costal cartilage (ribs), nose, trachea.
Elastic cartilage: Contains more elastic fibers, making it flexible. Examples: External ear, epiglottis.
Fibrocartilage: Highly compressible and strong due to thick collagen fibers. Examples: Intervertebral discs, menisci of the knee, pubic symphysis.

Cartilage Growth

Appositional growth: Cartilage grows by adding new layers to the surface, from the perichondrium.
Interstitial growth: Cartilage expands from within, as chondrocytes divide and secrete new matrix.
Calcified cartilage is hardened but is not the same as bone.

The Skeleton: Organization

The adult human skeleton consists of 206 named bones.
It is divided into two major regions:
- Axial skeleton: Forms the long axis of the body (skull, vertebral column, rib cage).
- Appendicular skeleton: Includes the limbs and girdles (shoulder and pelvic girdles).

Bone Classification

Bones are classified by their shapes:

Long bones: Longer than they are wide (e.g., femur, humerus).
Short bones: Cube-shaped (e.g., carpals, tarsals).
Flat bones: Thin, flattened, and often curved (e.g., sternum, skull bones).
Irregular bones: Complicated shapes (e.g., vertebrae, hip bones).

Functions of Bones

Support: Framework for the body and soft organs.
Protection: Shields vital organs (e.g., skull protects brain).
Movement: Acts as levers for muscles.
Mineral storage: Reservoir for minerals, especially calcium and phosphate.
Blood cell formation (hematopoiesis): Occurs in red marrow cavities.

Bone Structure: Gross Anatomy

Compact bone: Dense outer layer; provides strength.
Spongy bone: Internal lattice of trabeculae; spaces may contain red or yellow marrow.

Structure of a Long Bone

Diaphysis: Shaft of the bone; contains the medullary cavity (marrow cavity).
Epiphyses: Expanded ends of the bone; covered with articular cartilage.
Epiphyseal plate: Growth plate in children; becomes the epiphyseal line in adults.

Bone Membranes

Periosteum: Outer membrane; contains blood vessels, nerves, and osteogenic cells.
Endosteum: Lines internal bone surfaces; contains osteogenic cells.

Bone Cells

Osteoblasts: Bone-forming cells; secrete bone matrix.
Osteocytes: Mature bone cells; maintain bone tissue.
Osteoclasts: Bone-resorbing cells; break down bone matrix.

Bone Matrix

Organic component (osteoid): Includes collagen fibers; provides flexibility and tensile strength.
Inorganic component: Mainly mineral salts, especially hydroxyapatite (calcium phosphate crystals), responsible for bone hardness.

Formula for hydroxyapatite:

Compact vs. Spongy Bone

Osteon (Haversian system): Structural unit of compact bone; consists of concentric lamellae around a central canal.
Spongy bone: Organized along lines of stress; contains trabeculae.

Bone Formation (Ossification)

Bone formation is called osteogenesis or ossification. There are two main types:

Endochondral ossification: Bone replaces hyaline cartilage; forms most bones of the skeleton.
Intramembranous ossification: Bone develops from fibrous membranes; forms flat bones (e.g., skull, clavicles).

Bone Growth & Remodeling

Longitudinal growth: Bones grow in length at the epiphyseal plate.
Appositional growth: Bones increase in thickness by adding bone to the surface.
Bone remodeling: Continuous process of bone deposit (by osteoblasts) and bone resorption (by osteoclasts).

Bone Repair

Bone repair after a fracture occurs in four main steps:

Hematoma formation: Blood clot forms at the fracture site.
Fibrocartilaginous callus formation: Soft callus of collagen and cartilage forms.
Bony callus formation: New bone trabeculae appear in the callus.
Bone remodeling: Compact bone replaces spongy bone; bone returns to original shape.

Bone Disorders

Osteomalacia (called rickets in children): Poor bone mineralization, often due to vitamin D deficiency.
Osteoporosis: Loss of bone mass and density, leading to fragile bones.
Paget’s disease: Abnormal bone remodeling, resulting in enlarged and misshapen bones.