Bone Tissue and the Skeletal System

Overview of the Skeletal System

The skeletal system is a dynamic organ system that provides structural support, protection, and facilitates movement. It is unique in its ability to grow and change throughout life.

• Main components: Bones and cartilage

• Functions: Support, protection, movement, mineral storage, blood cell formation

Cartilage

Structure and Cell Types

Cartilage is a semi-rigid connective tissue that provides flexibility and support in various parts of the body. It is avascular, meaning it lacks blood vessels, and is composed of specialized cells and extracellular matrix.

• Chondroblasts: Cells that produce cartilage matrix

• Chondrocytes: Mature cartilage cells located in small spaces called lacunae

• Matrix: Composed of collagen fibers and proteoglycans

Functions of Cartilage

• Supports soft tissues

• Forms joints and reduces friction

• Serves as a model for bone growth during development

Types of Cartilage

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

Bone Tissue

Structure and Composition

Bones are organs composed primarily of bone tissue, but also contain other tissues such as cartilage, blood vessels, and nerves. Bone tissue is strengthened by calcification, a process where minerals are deposited in the matrix.

• Bone cells: Osteoprogenitor cells, osteoblasts, osteocytes, osteoclasts

• Matrix: Collagen fibers and mineral salts (mainly calcium and phosphate)

• Nutrient foramen: Openings for blood vessels and nerves

Functions of Bone

• Support: Framework for the body

• Protection: Shields vital organs

• Movement: Acts as levers for muscles

• Hemopoiesis: Formation of blood cells in red bone marrow

• Mineral storage: Reservoir for calcium and phosphate

• Triglyceride storage: Yellow bone marrow stores fat

Classification of Bones

Bones are classified by shape and structure, which relate to their functions.

Gross Anatomy of Bone

• Diaphysis: Shaft, provides leverage and weight bearing

• Epiphyses: Ends, provide surface area for joints and muscle attachment

• Metaphysis: Region between diaphysis and epiphysis, contains epiphyseal plate (growth plate)

• Articular cartilage: Hyaline cartilage covering joint surfaces, reduces friction and absorbs shock

• Medullary cavity: Central cavity containing yellow (fat) and red (blood cell formation) bone marrow

Bone Coverings

• Periosteum: Dense irregular connective tissue covering outer bone surface; contains fibrous and cellular layers, osteogenic cells, and is attached via perforating fibers

• Endosteum: Thin membrane lining internal bone surfaces; contains bone cells

Bone Cells

Bone Tissue Types

Compact Bone (Dense Bone)

Compact bone forms the outer surfaces of long and flat bones and provides strength and protection.

• Osteon (Haversian system): Structural and functional unit of compact bone

• Central canal: Contains blood vessels, lymphatics, and nerves

• Lamellae: Concentric rings of bone matrix

• Lacunae: Small spaces housing osteocytes

• Canaliculi: Tiny channels connecting lacunae for cell communication

• Perforating canals: Transverse channels for blood vessels and nerves

Spongy Bone (Trabecular Bone)

Spongy bone is found inside bones and at the ends of long bones. It is composed of a network of trabeculae and contains bone marrow.

• Trabeculae: Lattice-like structures providing strength with minimal weight

• Contains red bone marrow: Site of hemopoiesis

Bone Growth and Development

Interstitial vs. Appositional Growth

• Interstitial growth: Growth in length

• Appositional growth: Growth in thickness

Ossification (Bone Formation)

Ossification is the process of bone tissue formation. There are two main patterns:

• Intramembranous ossification: Bone develops directly from mesenchymal tissue

• Endochondral ossification: Bone develops from a cartilage model

Intramembranous Ossification

• Ossification center forms in mesenchyme

• Osteoblasts secrete osteoid (bone matrix)

• Osteoid undergoes calcification, trapping osteoblasts as osteocytes

• Formation of trabeculae and compact bone on the outside

• Periosteum develops

Endochondral Ossification

• Cartilage model develops; chondroblasts secrete cartilage matrix

• Perichondrium forms around cartilage

• Cartilage model grows via interstitial and appositional growth

• Primary ossification center forms in diaphysis; osteoblasts secrete osteoid

• Perichondrium becomes periosteum

• Secondary ossification centers form in epiphyses

• Articular cartilage remains on joint surfaces; epiphyseal plate (growth plate) remains between diaphysis and epiphysis

• Epiphyseal plate is replaced by epiphyseal line after growth ends

Bone Remodeling and Aging

Bone Remodeling

Bone remodeling is the continuous process of bone formation and resorption, maintaining mineral homeostasis and adapting to stress.

• Regulated by mechanical stress and hormones

• During growth/puberty: bone formation > resorption

• In adults: formation = resorption

• In aging: resorption > formation

Aging and Bone Health

• Reduced collagen production leads to brittle bones

• Loss of calcium and minerals results in thinner, weaker bones

• Osteoporosis: Decrease in bone density, increasing fracture risk

Example: Osteoporosis is common in postmenopausal women due to decreased estrogen, which normally helps maintain bone density.

Additional info: Bone remodeling is essential for healing fractures and adapting to new physical demands.