Skeletal Cartilage Types

Types and Locations of Skeletal Cartilage

Cartilage is a specialized connective tissue found throughout the skeletal system, providing support, flexibility, and shock absorption. There are three main types of skeletal cartilage, each with distinct properties and locations in the human body.

• Hyaline cartilage: Supports and protects. Found in the trachea and ends of long bones.

• Elastic cartilage: Supports and protects, but is flexible. Located in the outer ear.

• Fibrocartilage: Provides support, protection, and absorbs shock. Found between vertebrae and in pubic bones.

Key Terms: Cartilage, Hyaline, Elastic, Fibrocartilage

Classification of Bones

Bone Classification by Location and Shape

Bones are classified based on their location in the body and their shape, which reflects their function and structure.

• By location:

  • Axial skeleton: Skull, vertebral column, thoracic cage (supports and protects vital organs)

  • Appendicular skeleton: Limbs and girdles (pectoral and pelvic girdles)

• By shape:

  • Long bones: Longer than wide (e.g., femur, humerus)

  • Short bones: Cube-shaped (e.g., carpals, tarsals)

  • Flat bones: Thin, flat, slightly curved (e.g., skull, ribs, sternum)

  • Irregular bones: Complicated shapes (e.g., vertebrae, hip bones)

Example: The femur is a long bone found in the thigh, while the sternum is a flat bone in the chest.

Functions of Bone

Major Functions of Bone Tissue

Bones perform several essential functions in the human body:

• Protection: Protects vital organs (e.g., skull protects the brain)

• Mineral storage: Stores calcium (Ca) and phosphorus (P)

• Blood cell formation: Occurs in red bone marrow (hematopoiesis)

• Fat storage: Yellow bone marrow stores fat

• Movement and support: Provides attachment for muscles and supports body structure

Structure of Typical Bone: Compact and Spongy Tissue

Bone Structure and Associated Features

Bones consist of two main types of tissue: compact bone and spongy bone, each with unique structural features.

• Epiphysis: Spongy tissue, epiphyseal plate (site of bone growth in length)

• Diaphysis: Medullary cavity, yellow marrow (contains cavity for fat storage)

• Membranes: Periosteum (outer covering), endosteum (inner lining), Sharpey's fibers (connect periosteum to bone)

Hematopoiesis

Blood Cell Formation in Bone

Hematopoiesis is the process of forming blood cells, which occurs in red bone marrow. The location of active hematopoiesis changes with age:

• Adults: Sternum, ribs, vertebrae, pelvis, proximal ends of femur and humerus

• Infants: Long bones

Microscopic Anatomy of Bone

Osteon and Canal Systems

The osteon (Haversian system) is the structural unit of compact bone, consisting of concentric lamellae arranged around a central canal.

• Three types of canals:

  • Haversian canal: Central canal in osteon (contains blood vessels and nerves)

  • Volkmann's canal: Perpendicular to Haversian canal (connects osteons)

  • Canaliculi: Tiny canals connecting osteocytes for nutrient exchange

• Bone cells:

  • Osteocytes: Mature bone cells (maintain bone matrix)

  • Osteoblasts: Immature bone-forming cells

  • Osteoclasts: Bone-resorbing cells (break down bone)

Bone Development

Ossification Processes

Bones develop through two main processes: intramembranous and endochondral ossification.

• Intramembranous ossification: Formed from fibrous connective tissue. Produces flat bones (e.g., skull, mandible, clavicle).

• Endochondral ossification: Formed from hyaline cartilage. Produces long bones (e.g., femur, humerus).

Steps of Ossification

Endochondral and Intramembranous Ossification

Bone formation involves several steps, differing between intramembranous and endochondral ossification.

• Intramembranous:

  1. Mesenchymal cells cluster and differentiate into osteoblasts

  2. Development of ossification center

  3. Formation of periosteum and compact bone

• Endochondral:

  1. Mesenchymal cells become chondrocytes → hyaline cartilage

  2. Growth of cartilage model

  3. Development of primary ossification center – blood vessels invade, osteoblasts replace cartilage with bone in the diaphysis

  4. Osteoclasts break down bone – medullary cavity forms

  5. Development of secondary ossification center – epiphyses appear

  6. Formation of articular cartilage and epiphyseal plate

Postnatal Bone Growth

Interstitial and Appositional Growth

After birth, bones grow in length and diameter through two processes:

• Interstitial growth: Occurs within bone (epiphyseal plate); lengthening by chondrocytes dividing and pushing the ends of the bone apart.

• Appositional growth: Occurs on the surface of bone; increases thickness as osteoblasts add new bone tissue to the outer surface.

Hormonal Mechanisms for Calcium Homeostasis

Regulation by Calcitonin and Parathyroid Hormone (PTH)

Calcium levels in the blood are tightly regulated by hormones:

• Calcitonin: Produced by the thyroid gland; lowers blood calcium levels by stimulating osteoblasts and calcium deposition.

• PTH (Parathyroid Hormone): Produced by the parathyroid gland; increases blood calcium levels by stimulating osteoclasts to release calcium from bone and increasing calcium reabsorption in kidneys and vitamin D activation in intestines.

Bone Fractures and Repair

Steps in Bone Healing

Bone repair after fracture involves several stages:

1. Formation of hematoma and granulation tissue: Blood vessels break, hematoma forms, granulation tissue develops.

2. Formation of soft callus: Fibroblasts and chondroblasts produce soft callus, bridging the broken bone ends.

3. Formation of hard callus: Osteoblasts produce spongy bone, which is later remodeled into hard bone.

4. Remodeling of bone: Hard callus is remodeled by osteoclasts and osteoblasts.

Homeostatic Imbalances of Bone

Common Bone Disorders

Several disorders can affect bone health and development:

• Osteoporosis: Bones become weak and brittle due to loss of bone mass (aging, hormone changes, lack of vitamin D/calcium).

• Rickets: Children lacking vitamin D; soft and weak bones.

• Osteomalacia: Similar to rickets, but in adults; bone pain and muscle weakness.

• Achondroplastic dwarfism: Genetic disorder affecting bone growth; mutation affects cartilage formation at growth plates, resulting in short stature (shorter limbs).

Additional info: Vitamin D is essential for calcium absorption and bone mineralization. Deficiency can lead to rickets in children and osteomalacia in adults.