Introduction to Human Bones and Cartilage

Introduction to Bones

• Bones are formed by replacing cartilage
• Cartilage is a flexible framework that guides bone development
• Most cartilage is replaced with bone by birth
• Some cartilage remains throughout life and continues to be replaced during childhood

Human Skeleton

• Initially composed of cartilage and fibrous membranes
• These are soon replaced by bone

Skeletal Cartilage

• Sculpted to fit its location
• Primarily composed of water
• Springy and returns to its original shape after compression
• Contains no nerves or blood vessels
• Dense perichondrium surrounds the cartilage
  • Provides additional reinforcement
  • Contains blood vessels that nourish the cartilage

Growth of Cartilage

• Cartilage grows in two ways:
  • Appositional growth: New matrix is laid down on the surface of existing cartilage by cartilage-forming cells in the perichondrium
  • Interstitial growth: Chondrocytes within lacunae divide and secrete new matrix, expanding the cartilage from within

Bone

• Bone is mineralized connective tissue
• Contains four types of cells:
  • Osteoblasts: Bone-forming cells
  • Bone lining cells: Maintain mineral concentration of matrix
  • Osteocytes: Mature bone cells
  • Osteoclasts: Break down bone tissue
• Bone is a highly dynamic organ that continuously remodels
  • Resorption by osteoclasts
  • Formation by osteoblasts

Functions of Bone

• Support for the body and soft organs
• Protection for the brain (skull), spinal cord (vertebrae), and vital organs
• Anchorage for movement as levers for muscle action
• Storage of minerals and growth factors
  • Reservoir for calcium, phosphorus, and growth factors
• Blood cell formation in the red marrow cavities of certain bones
• Triglyceride (fat) storage in bone cavities, used as an energy source
• Hormone production of osteocalcin, which helps regulate insulin secretion, glucose levels, and metabolism

Classification of Bones

• 206 bones in the human skeleton, divided into two groups:
  • Axial skeleton: Skull, vertebral column, and rib cage
  • Appendicular skeleton: Bones of upper and lower limbs, and girdles attaching limbs to the axial skeleton

Classification of Bones by Shape

• Long bones: Longer than they are wide, make up limb bones
• Short bones: Cube-shaped bones found in the wrist and ankle, sesamoid bones (e.g., patella)
• Flat bones: Thin, flat, and slightly curved; include the sternum, scapulae, ribs, and most skull bones
• Irregular bones: Complex shapes, including vertebrae and hip bones

Bone Structure

• Bone is an organ containing different types of tissues
  • Bone (osseous) tissue predominates
  • Other tissues include nervous tissue, cartilage, fibrous connective tissue, muscle cells, and epithelial cells in its blood vessels

Three Levels of Bone Structure

• Gross: Overall structure
• Microscopic: Cellular structure
• Chemical: Composition of bone

Gross Anatomy of Bone

• Compact bone: Dense outer layer that appears smooth and solid
• Spongy bone (trabecular bone): Internal, honeycomb-like structure made up of trabeculae (small, needle-like or flat pieces of bone)
  • Open spaces between trabeculae are filled with red or yellow bone marrow

Flat Bones

• Thin plates of spongy bone covered by compact bone
• Compact bone sandwiched between two connective tissue membranes:
  • Periosteum: Outer layer covering compact bone
  • Endosteum: Inner layer covering compact bone
• Bone marrow scattered throughout spongy bone, no defined marrow cavity
• Hyaline cartilage covers bone surfaces that are part of a movable joint

Long Bones: Structure

• Diaphysis (shaft): Tubular shaft that forms the long axis of the bone, consists of compact bone surrounding a central medullary cavity filled with yellow marrow in adults
• Epiphyses (bone ends): Consist of compact bone externally and spongy bone internally
• Articular cartilage: Covers the articular surfaces
• Epiphyseal line: Remnant of the childhood epiphyseal plate where bone growth occurs

Two Types of Membranes

• Periosteum: White, double-layered membrane that covers external surfaces except joints

  • Contains blood vessels, nerves, and lymphatic vessels that nourish compact bone
  • Anchoring points for tendons and ligaments
  • Contains osteogenic cells

• Endosteum: Delicate connective tissue membrane covering the internal bone surface

  • Covers trabeculae of spongy bone
  • Contains osteogenic cells

Blood Vessels and Nerves

• Bone is well vascularized
• Nutrient artery: Supplies the inner spongy bone and bone marrow
• Epiphyseal arteries and veins: Serve the epiphysis
• Nerves accompany blood vessels

Hematopoietic Tissue in Bone (Red Marrow)

• Found within trabecular cavities of spongy bone and diploë (spongy bone) of flat bones
• In newborns, medullary cavities and all spongy bone contain red marrow
• In adults, red marrow is located in the heads of the femur and humerus, but most active areas of hematopoiesis are flat bone diploë and some irregular bones
• Yellow marrow can convert to red marrow if a person becomes anemic

Bone Markings and Features

• Projections: Outward bulges of bone, often exaggerated due to muscle pull or joint modifications
• Depressions: Bowl or groove-like cut-outs serving as passageways for vessels and nerves, or playing a role in joints
• Openings: Holes or canals in bone serving as passageways for blood vessels and nerves

Microscopic Anatomy of Bone

• Five major cell types:

  • Osteoprogenitor cells (osteogenic cells): Undifferentiated cells with high mitotic activity
  • Osteoblasts: Secrete unmineralized bone matrix (osteoid)
  • Osteocytes: Mature bone cells that maintain bone matrix and act as stress sensors
  • Bone-lining cells: Flat cells that help maintain the matrix
  • Osteoclasts: Multinucleated cells that function in bone resorption

• The presence of these cells makes bone a dynamic living tissue, continuously resorbing and depositing bone (remodeling)

Bone Remodeling

• Highly complex process involving three phases:
  • Initiation of bone resorption by osteoclasts
  • Transition (reversal period) from resorption to new bone formation
  • Bone formation by osteoblasts
• Coordinated actions of osteoclasts, osteoblasts, osteocytes, and bone lining cells

Cells Involved in Bone Remodeling

• Osteogenic cells (osteoprogenitor cells): Found in the deep layers of the periosteum and marrow, differentiate into osteoblasts or bone-lining cells
• Osteoblasts: Actively mitotic, responsible for bone formation, secrete osteoid
• Osteocytes: Mature bone cells in lacunae, maintain bone matrix and act as stress sensors
• Bone-lining cells: Found on bone surfaces, believed to help maintain matrix
• Osteoclasts derived from hematopoietic stem cells, responsible for bone resorption

Microscopic Anatomy of Compact Bone

• Although it appears solid, it contains tiny passageways for nerves and blood vessels
• Compact bone (lamellar bone) consists of:
  • Osteons (Haversian systems): Structural units of compact bone
  • Canals and canaliculi: Passageways for blood vessels and nerves
  • Interstitial and circumferential lamellae: Layers of bone matrix surrounding the Haversian canals

Osteon (Haversian System)

• Elongated cylinder running parallel to the long axis of bone
• Contains several rings of bone matrix called lamellae
  • Collagen fibers run in different directions in adjacent rings, providing strength and resisting twisting
• Bone salts found between collagen fibers

Canals and Canaliculi

• Central (Haversian) canal: Runs through the core of an osteon, contains blood vessels and nerve fibers
• Perforating canals: Connect the blood vessels and nerves of the periosteum, medullary cavity, and central canal
• Lacunae: Small cavities that contain osteocytes
• Canaliculi: Hairlike canals that connect lacunae to each other and to the central canal

Interstitial and Circumferential Lamellae

• Interstitial lamellae: Remnants of osteons cut by bone remodeling
• Circumferential lamellae: Extend around the entire surface of the diaphysis, help long bone resist twisting

Microscopic Anatomy of Spongy Bone

• Appears poorly organized but is actually organized along lines of stress
• Trabeculae confer strength to bone
• No osteons present, but trabeculae contain irregularly arranged lamellae and osteocytes

Chemical Composition of Bone

• Organic components:
  • Osteogenic cells, osteoblasts, osteocytes, bone lining cells, osteoclasts, and osteoid
  • Osteoid makes up one-third of the organic bone matrix and consists of ground substance and collagen fibers
• Inorganic components:
  • Hydroxyapatites (mineral salts): Make up 65% of bone mass, mainly calcium phosphate crystals
  • Contribute to bone hardness and resistance to compression

Bone Remodeling

• Old bone is replaced by new bone in a cycle of three phases:
  • Resorption by osteoclasts
  • Transition from resorption to bone formation
  • Formation by osteoblasts

Bone Cells: Summary

This quiz explores the formation and growth of bones and cartilage in the human skeleton. It covers key concepts including the transition from cartilage to bone, the structure of skeletal cartilage, and the processes of cartilage growth. Test your knowledge on these fundamental aspects of human anatomy.