Bones and Bone Structure: Study Guide for Anatomy & Physiology

Bones and Bone Structure

Functions of the Skeletal System

The skeletal system is essential for structural support, movement, and protection in the human body. It consists of bones, cartilages, ligaments, and other connective tissues that stabilize and interconnect the bones.

• Structural Support: Provides the framework for the body and supports soft tissues.

• Storage of Minerals and Lipids: Stores calcium, phosphorus, and lipids in bone marrow.

• Blood Cell Production: Hematopoiesis occurs in red bone marrow.

• Protection: Shields vital organs (e.g., brain, heart, lungs).

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

Classification of Bones

Bones are classified by their shape and structure, each serving distinct functional roles in the body.

• Sutural Bones: Small, flat, irregular bones found between skull bones.

• Irregular Bones: Complex shapes (e.g., vertebrae, pelvic bones).

• Short Bones: Boxy, found in wrists (carpals) and ankles (tarsals).

• Flat Bones: Thin, parallel surfaces (e.g., skull roof, sternum, ribs, scapulae).

• Long Bones: Long and slender (e.g., humerus, femur, fingers).

• Sesamoid Bones: Small, round, flat bones within tendons (e.g., patella).

Bone Markings

Bone markings are surface features that serve as sites for muscle attachment, articulation, and passage of nerves and blood vessels.

• Projections: For attachment and articulation (e.g., process, ramus).

• Openings and Depressions: For passage of vessels and nerves (e.g., foramen, canal, fossa).

Structure of Long and Flat Bones

Long bones have a tubular shaft (diaphysis), articulating ends (epiphysis), and a connecting zone (metaphysis). Flat bones consist of a core of spongy bone (diploë) between two layers of compact bone.

• Diaphysis: Compact bone surrounding a medullary cavity.

• Epiphysis: Mostly spongy bone.

• Metaphysis: Connects diaphysis and epiphysis.

Bone Tissue

Bone tissue (osseous tissue) is a dense connective tissue composed of specialized cells and a solid extracellular matrix.

• Matrix: Two-thirds calcium phosphate (hydroxyapatite crystals), one-third collagen fibers.

• Cells: Osteogenic cells, osteoblasts, osteocytes, osteoclasts.

• Periosteum: Covers outer surfaces of bones, except at joints.

Types of Bone Cells

Cell Type	Function
Osteogenic (osteoprogenitor) cells	Stem cells for bone repair and growth
Osteoblasts	Produce new bone matrix (osteogenesis)
Osteocytes	Maintain bone matrix, repair damaged bone
Osteoclasts	Absorb and remove bone matrix (osteolysis)

Compact Bone and Spongy Bone

Compact bone and spongy bone have distinct structures and functions, contributing to bone strength and flexibility.

• Compact Bone: Contains osteons (functional units), central canals, lamellae, and lacunae.

• Spongy Bone: Matrix arranged as trabeculae, supports red bone marrow, lighter and can withstand stress from multiple directions.

Distribution of Forces

The femur demonstrates how trabeculae transfer weight and how compact bone resists compression and tension.

Surface Coverings: Periosteum and Endosteum

• Periosteum: Membrane covering outside of bones, involved in growth and repair.

• Endosteum: Incomplete cellular layer lining inner surfaces, active during growth and remodeling.

Bone Formation and Growth

Bone formation (ossification) occurs via two mechanisms: endochondral ossification and intramembranous ossification.

Endochondral Ossification

• Bone replaces cartilage.

• Primary ossification center: Chondrocytes enlarge, matrix calcifies, blood vessels invade.

• Secondary ossification centers: Form in epiphyses.

• Epiphyseal plate: Site of growth in length.

Bone Growth

• Interstitial Growth: Growth in length at epiphyseal plate.

• Appositional Growth: Growth in width by adding lamellae.

Intramembranous Ossification

• Bone forms from mesenchymal cells.

• Produces dermal bones: Flat bones of skull, mandible, clavicles.

• Steps: Mesenchymal cells differentiate, osteoid is secreted and calcified, spicules form, blood vessels invade, remodeling produces compact bone.

Blood Supply to Bones

• Nutrient artery and vein: Supply diaphysis.

• Metaphyseal vessels: Supply epiphyseal cartilages.

• Periosteal vessels: Supply superficial osteons.

Bone Remodeling and Homeostasis

Bone remodeling is a continuous process involving osteocytes, osteoblasts, and osteoclasts. It maintains bone strength and mineral balance.

• Balanced activity: Maintains bone mass.

• Imbalance: Osteoclasts weaken bone; osteoblasts strengthen bone.

Exercise, Nutrition, and Hormones

Bone health is influenced by physical activity, nutrition, and hormones.

• Exercise: Stimulates bone strength and maintenance.

• Minerals: Calcium, phosphorus, magnesium, fluoride, iron, manganese.

• Vitamins: D, C, A, K.

• Hormones: Growth hormone, thyroxine, sex hormones, parathyroid hormone, calcitonin.

Abnormal Bone Development

• Pituitary growth failure: Short bones.

• Gigantism: Tall stature.

• Acromegaly: Thick bones after epiphyseal closure.

• Marfan syndrome: Tall, slender limbs.

Calcium Homeostasis

Bones store 99% of the body's calcium, which is vital for physiological processes. Calcium levels are regulated by hormones.

Hormonal Regulation

• Parathyroid Hormone (PTH): Increases blood calcium by stimulating osteoclasts, increasing intestinal absorption, and decreasing kidney excretion.

• Calcitonin: Decreases blood calcium by inhibiting osteoclasts, increasing kidney excretion, and decreasing intestinal absorption.

Disorders

• Osteomalacia: Weak, flexible bones due to poor mineralization.

• Rickets: Osteomalacia from vitamin D deficiency.

Fractures and Repair

Fractures are cracks or breaks in bones. They are classified by their characteristics and location.

Type	Description
Open (compound)	Projects through skin
Closed (simple)	Internal
Transverse	Break across long axis
Displaced	Abnormal bone arrangement
Compression	Crushed bone
Spiral	Twisting stress
Epiphyseal	At growth plate
Comminuted	Multiple fragments
Greenstick	Partial break
Colles	Distal radius
Pott's	Distal tibia/fibula

Fracture Repair Steps

• Fracture Hematoma: Blood clot forms, bone cells die.

• Callus Formation: Internal and external calluses stabilize fracture.

• Spongy Bone Formation: Osteoblasts replace cartilage with spongy bone.

• Compact Bone Formation: Remodeling restores bone structure.

Effects of Aging on Bones

Aging leads to decreased bone mass and increased risk of fractures.

• Osteopenia: Reduction of bone mass, begins between ages 30-40.

• Osteoporosis: Severe loss of bone mass, common in postmenopausal women.

• Consequences: Fragile limbs, reduced height, tooth loss.