Bones & Bone Structure

Functions of the Skeletal System

The skeletal system is composed of bones, cartilages, ligaments, and other connective tissues. It serves several essential functions in the human body:

• Support: Provides a structural framework for the body.

• Storage of minerals & lipids: Stores calcium, phosphate, and fat in bone marrow.

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

• Calcium regulation: Maintains blood calcium levels.

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

• Leverage/movement: Bones act as levers for muscles to produce movement.

Classification of Bones by Shape

Bones are classified according to their shape, which relates to their function and location:

• Long Bones: Found in arms, legs, palms, soles, fingers.

• Flat Bones: Roof of skull, sternum, ribs, scapulae.

• Short Bones: Carpals (wrist), tarsals (ankle).

• Irregular Bones: Vertebrae, pelvis, some skull bones.

• Sesamoid Bones: Patella, small bones near joints.

• Sutural Bones: Small, irregular bones found between skull sutures; vary in number, shape, and position.

Bone Structure

Structure of a Long Bone

Long bones have a distinct anatomy that supports their function:

• Diaphysis (shaft): The central, elongated portion; contains the medullary (marrow) cavity.

• Epiphysis (ends): Proximal and distal ends, mostly spongy bone.

• Metaphysis: Region between diaphysis and epiphysis; contains the growth plate (cartilage plate).

• Articular Cartilage: Hyaline cartilage covering joint surfaces; reduces friction.

Periosteum and Endosteum

The periosteum and endosteum are connective tissue membranes that cover bone surfaces:

• Periosteum: Surrounds and isolates bone; has a fibrous layer (mixes with tendons/ligaments via Sharpey’s fibers) and a cellular layer (growth/repair).

• Endosteum: Incomplete layer of osteoprogenitor cells; lines marrow cavity and central canal.

Microscopic Structure: Osteons and Lamellae

Compact bone is organized into osteons (Haversian systems), which are cylindrical structures:

• Osteon: Basic unit of compact bone; consists of concentric lamellae surrounding a central canal.

• Lamellae: Layers of bone matrix; types include concentric, interstitial, and circumferential lamellae.

• Lacunae: Small spaces housing osteocytes.

• Canaliculi: Tiny channels connecting lacunae for nutrient/waste exchange.

Types of Bone Tissue

Cortical (Compact) Bone vs. Trabecular (Spongy) Bone

Bones contain two main types of tissue:

• Compact Bone: Dense, forms the external surface; few pores/spaces.

• Spongy (Cancellous) Bone: Contains many pores/spaces; organized into trabeculae (bars/plates).

Organization of Compact Bone

Compact bone is highly organized for strength and nutrient delivery:

• Osteons: Cylindrical units with concentric lamellae.

• Central Canal: Contains blood vessels and nerves.

• Volkmann’s (Perforating) Canals: Connect central canals to each other and to the periosteum.

Organization of Spongy Bone

Spongy bone is less organized, with a network of trabeculae:

• Trabeculae: Bands and arches of bone matrix.

• Red Bone Marrow: Fills spaces between trabeculae; site of hematopoiesis.

• Blood Vessels: Supply nutrients to osteocytes.

Bone Marrow

Red vs. Yellow Bone Marrow

Bone marrow is found within the cavities of bones and serves different functions:

• Red Bone Marrow: Forms red blood cells; found in proximal epiphysis of humerus/femur, sternum, pelvis, ribs, skull, scapulae, vertebrae.

• Yellow Bone Marrow: Stores fat (adipose tissue); energy reserve; found in other bones.

Bone Cells

Types of Bone Cells

Bone tissue contains several specialized cell types:

• Osteoprogenitor Cells: Stem cells that differentiate into osteoblasts; found in endosteum, central canal, periosteum.

• Osteoblasts: Bone-forming cells; synthesize organic matrix (osteoid).

• Osteocytes: Mature bone cells; maintain bone matrix and calcium homeostasis.

• Osteoclasts: Bone-resorbing cells; remove calcium from bone; multinucleated.

Bone Development & Growth

Ossification Processes

Ossification is the process of converting other tissue to bone. There are two main types:

• Intramembranous Ossification: Forms bones from embryonic tissue (mesenchyme); occurs in skull, mandible, clavicle.

• Endochondral Ossification: Forms bones from hyaline cartilage; most bones develop this way.

Intramembranous Ossification Steps

• Mesenchymal cells cluster and differentiate into osteoblasts.

• Osteoblasts lay down osteoid.

• Osteoblasts become osteocytes when surrounded by matrix.

• Spicules interconnect, trapping blood vessels.

• Spongy bone forms, later remodeled to compact bone.

Endochondral Ossification Steps

• Bone growth from hyaline cartilage model (~7 weeks development).

• Long bones grow through cartilage until puberty.

• At puberty, cartilage is replaced by bone; epiphyses fuse to diaphysis.

• Epiphyseal plate becomes epiphyseal line (ossified).

Appositional Bone Growth

Appositional growth increases bone diameter in adults:

• Bone formation occurs at the surface, creating new osteons.

• Ridges enlarge and fuse, trapping blood vessels.

• Bone continues to increase in diameter.

Blood Supply to Bone

Blood Vessels in Bone

Bones are highly vascularized to support growth and maintenance:

• Nutrient arteries: Supply blood throughout diaphysis.

• Periosteal artery: Supplies blood to superficial osteons.

• Metaphyseal artery: Supplies blood to epiphyseal plate (metaphysis).

Bone Remodeling & Calcium Storage

Bone Remodeling

Bones remodel continuously, with up to 20% replaced per year:

• Exercise stimulates osteoblast activity and increases bone density.

• Inactivity weakens bone.

• Hormones influencing bone growth: growth hormone, calcitriol, sex hormones, thyroxine, calcitonin, parathyroid hormone (PTH).

Calcium Regulation

Calcium homeostasis is maintained by bone cells and hormones:

• 99% of body’s calcium is stored in the skeletal system.

• Calcitonin: Decreases blood calcium levels.

• PTH: Increases blood calcium levels.

• Calcitriol: Works with PTH to absorb calcium from the digestive system; made in kidneys from Vitamin D3.

Fracture Repair

Steps in Fracture Repair

Bone repair depends on adequate blood flow and survival of cells in periosteum/endosteum:

1. Hematoma forms to control bleeding.

2. Cells divide to form callus (external: cartilage/bone; internal: spongy bone).

3. Cartilage replaced by spongy bone.

4. Extra material is remodeled over months to years.

Fracture Categories

• Open (Compound) Fracture: Bone projects through skin.

• Closed (Simple) Fracture: Bone remains internal.

• Displaced Fracture: Bone fragments out of alignment.

• Epiphyseal Fracture: Involves epiphyseal plate; can affect growth.

Common Fracture Patterns

• Greenstick Fracture: Incomplete break, common in children.

• Spiral Fracture: Caused by twisting forces.

• Pott’s Fracture: Ankle fracture.

• Colles’ Fracture: Distal radius fracture.

• Compression Fracture: Bone is crushed.

Effects of Aging on Bone

Osteopenia and Osteoporosis

Bone density naturally decreases with age:

• Osteopenia: Natural decrease in bone density; more common in women (menopause = decreased estrogen).

• Osteoporosis: Severe decrease in bone density; affects daily function; risk higher in females, especially postmenopausal women.

• Onset can be delayed and progression slowed with calcium, vitamin D, and exercise.

• Bones most affected: epiphyses of long bones, vertebrae, mandible, maxilla.