Bones and Bone Structure: Study Notes for Anatomy & Physiology

Bones and Bone Structure

Overview

This chapter provides an in-depth study of the skeletal system, focusing on the structure, classification, and function of bones, as well as the processes of bone formation, growth, and repair. Understanding these concepts is essential for comprehending how the human body maintains support, movement, and mineral homeostasis.

Functions of the Skeletal System

Major Functions

• Structural Support: The skeleton provides a rigid framework that supports the body and maintains its shape.

• Storage of Minerals and Lipids: Bones store essential minerals, primarily calcium and phosphorus, and lipids in yellow bone marrow.

• Blood Cell Production: Red bone marrow produces red blood cells, white blood cells, and platelets in a process called hematopoiesis.

• Protection: Bones protect vital organs and soft tissues (e.g., the skull protects the brain, the rib cage protects the heart and lungs).

• Leverage for Movement: Bones act as levers that muscles pull on to produce movement.

Classification of Bones

Bone Classification by Shape

Bones are classified based on their shapes, which relate to their functions and locations in the body.

• Sutural Bones (Wormian Bones):

  • Small, flat, irregularly shaped bones found between the flat bones of the skull.

  • The number of sutural bones varies among individuals.

• Irregular Bones:

  • Complex shapes that do not fit other categories.

  • Examples: spinal vertebrae, pelvic bones.

• Short Bones:

  • Boxy in appearance, nearly equal in length and width.

  • Examples: carpal bones (wrist), tarsal bones (ankle).

• Flat Bones:

  • Thin with parallel surfaces.

  • Examples: bones of the skull roof, sternum, ribs, scapulae.

• Long Bones:

  • Long and slender, found in limbs.

  • Examples: humerus, femur, bones of arms, legs, palms, soles, fingers, and toes.

• Sesamoid Bones:

  • Small, round, and flat bones found within tendons near joints (e.g., knees, hands, feet).

  • Example: patellae (kneecaps).

  • Location and number can vary between individuals.

Functional Significance: The shape of a bone is closely related to its function. For example, flat bones provide protection, while long bones facilitate movement.

Bone Markings (Surface Features)

• Projections: Sites for attachment of muscles, tendons, and ligaments; also form joints with other bones.

• Openings and Depressions: Allow the passage of blood vessels and nerves.

Example: The condyle is a smooth, rounded projection at the end of a bone, often involved in articulation with another bone.

Structure of a Long Bone

• Diaphysis: The tubular shaft of the bone, composed of compact bone surrounding a central medullary (marrow) cavity.

• Epiphysis: The expanded ends of the bone, consisting mainly of spongy (trabecular) bone.

• Metaphysis: The narrow region connecting the diaphysis and epiphysis.

Structure of a Flat Bone

• Composed of a core of spongy bone (called diploë in the cranium) sandwiched between two layers of compact bone (cortex).

Bone Tissue (Osseous Tissue)

Composition and Characteristics

• Bone tissue is a dense connective tissue made of specialized cells embedded in a solid extracellular matrix.

• The matrix consists of protein fibers (mainly collagen) and deposits of calcium salts.

• Osteocytes (bone cells) reside in small spaces called lacunae within the matrix.

• Lacunae and blood vessels are interconnected by canaliculi, which allow for nutrient and waste exchange.

• The periosteum is a membrane covering the outer surface of bones (except at joints), consisting of an outer fibrous layer and an inner cellular layer.

Bone Matrix

• Two-thirds of the matrix is made of calcium phosphate ($Ca_3(PO_4)_2$), primarily as hydroxyapatite crystals ($Ca_{10}(PO_4)_6(OH)_2$).

• Other minerals include calcium carbonate ($CaCO_3$), sodium, magnesium, and fluoride.

• Hydroxyapatite crystals are hard and brittle, providing resistance to compression.

• One-third of the matrix is made of collagen fibers, which are strong and flexible, providing an organic framework for mineral deposition.

Bone Cells

• Osteogenic (Osteoprogenitor) Cells: Stem cells that divide to produce osteoblasts; located in the periosteum and endosteum; important for fracture repair.

• Osteoblasts: Immature bone cells responsible for osteogenesis (production of new bone matrix); secrete organic matrix (osteoid) and promote mineralization; become osteocytes when surrounded by matrix.

• Osteocytes: Mature bone cells in lacunae; maintain protein and mineral content of matrix; help repair damaged bone; communicate via canaliculi.

• Osteoclasts: Large, multinucleate cells that resorb bone matrix in a process called osteolysis; secrete acids and enzymes to dissolve bone and release stored minerals; not related to other bone cell lineages.

Compact Bone and Spongy Bone

Compact Bone Structure

• Osteon (Haversian System): The basic functional unit of compact bone, consisting of concentric lamellae around a central canal containing blood vessels.

• Perforating (Volkmann's) Canals: Perpendicular channels that also contain blood vessels.

• Lamellae: Layers of bone matrix; concentric lamellae form osteons, interstitial lamellae fill spaces between osteons, and circumferential lamellae are found at the outer and inner surfaces of bone.

• Osteocytes are located in lacunae between lamellae.

Spongy Bone Structure

• Does not contain osteons; instead, the matrix is arranged in a network of trabeculae.

• No blood vessels within the matrix; nutrients reach osteocytes by diffusion from blood vessels in the spaces between trabeculae.

• Spongy bone is lighter than compact bone and can withstand stress from multiple directions.

• Supports and protects red bone marrow (site of blood cell production); some spongy bone contains yellow bone marrow (fat storage).

Coordinated Functions

• In long bones, spongy bone in the epiphyses transfers weight to the compact bone of the shaft, allowing the bone to bear body weight and resist bending.

Bone Surface Coverings

• Periosteum: Membrane covering the outside of bones; consists of an outer fibrous layer and an inner cellular layer; continuous with joint capsules and tendons; functions in isolation, vascular supply, and bone growth/repair.

• Endosteum: Incomplete cellular layer lining internal bone surfaces (medullary cavity, trabeculae, central canals); contains osteogenic cells, osteoblasts, and osteoclasts; active during growth, repair, and remodeling.

Bone Formation and Growth

Ossification and Calcification

• Ossification (Osteogenesis): The process of bone formation.

• Calcification: Deposition of calcium salts, necessary for ossification; can occur in other tissues as well.

• Two main forms: Endochondral ossification and Intramembranous ossification.

Endochondral Ossification

1. Chondrocytes in the center of the cartilage model enlarge and die as the matrix calcifies.

2. Blood vessels grow around the cartilage; perichondrial cells become osteoblasts and form a bone collar.

3. Blood vessels penetrate the cartilage; fibroblasts become osteoblasts and produce spongy bone at the primary ossification center.

4. Remodeling creates a medullary cavity; bone thickens; cartilage at epiphyses is replaced by bone.

5. Secondary ossification centers form in the epiphyses.

6. Epiphyses fill with spongy bone; epiphyseal plate (cartilage) remains between diaphysis and epiphysis for growth.

7. At puberty, ossification overtakes cartilage production, leading to epiphyseal closure and formation of the epiphyseal line.

Bone Growth

• Interstitial Growth: Growth in length at the epiphyseal plate; new cartilage forms on the epiphyseal side and is replaced by bone on the diaphyseal side.

• Appositional Growth: Growth in width; osteoblasts add new bone to the outer surface, while osteoclasts enlarge the medullary cavity.

Intramembranous Ossification (Dermal Ossification)

1. Mesenchymal cells differentiate into osteoblasts, which secrete osteoid that calcifies to form bone matrix.

2. Osteoblasts become osteocytes; bone grows in struts (spicules).

3. Blood vessels grow between spicules, accelerating growth; spicules interconnect, trapping blood vessels.

4. Osteoblasts near blood vessels continue depositing bone, forming a plate of spongy bone.

5. Remodeling produces compact bone on the outer surfaces; periosteum forms from connective tissue.

Example: Flat bones of the skull, mandible, and clavicles are formed by intramembranous ossification.

Blood and Nerve Supply to Bones

• Bones receive blood from nutrient arteries/veins (diaphysis), metaphyseal vessels (epiphyseal cartilage), and periosteal vessels (superficial osteons).

• Lymphatic vessels and sensory nerves are present in the periosteum, endosteum, and bone tissue.

Bone Remodeling

Remodeling and Homeostasis

• Bone remodeling is the continuous recycling and renewal of bone matrix, involving osteocytes, osteoblasts, and osteoclasts.

• Remodeling allows bones to adapt to stress, repair damage, and maintain mineral homeostasis.

• Balance between osteoblast and osteoclast activity is essential; imbalance can lead to weakened or excessively strong bones.

Exercise, Nutrition, and Hormones

Effects on Bone Development

• Physical activity and weight-bearing exercise stimulate bone strength and maintenance; inactivity leads to bone loss.

• Essential nutrients: calcium, phosphorus, magnesium, fluoride, iron, manganese.

• Vitamins: D (for calcium absorption), C (for collagen synthesis), A, K, and B12 (for osteoblast activity and protein synthesis).

• Hormones: Growth hormone, thyroxine, sex hormones (estrogen, testosterone), parathyroid hormone (PTH), and calcitonin regulate bone growth and calcium homeostasis.

Disorders:

• Pituitary growth failure: Inadequate growth hormone; short stature.

• Gigantism: Excess growth hormone before puberty; tall stature.

• Acromegaly: Excess growth hormone after epiphyseal closure; thickened bones.

• Marfan syndrome: Excessive cartilage formation; tall, slender limbs.

Calcium Homeostasis

Role of Calcium in the Skeletal System

• Bones store 99% of the body's calcium, which is vital for physiological processes such as nerve impulse transmission and muscle contraction.

• Calcium ion concentrations in body fluids must be tightly regulated.

Hormonal Regulation

• Parathyroid Hormone (PTH): Increases blood calcium by stimulating osteoclast activity, increasing intestinal absorption (via calcitriol), and decreasing renal excretion.

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

Disorders: Osteomalacia (soft bones due to poor mineralization), Rickets (osteomalacia in children, often due to vitamin D deficiency).

Fractures and Bone Repair

Types of Fractures

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

• Closed (Simple) Fracture: Bone does not penetrate the skin.

• Other types: transverse, displaced, compression, spiral, epiphyseal, comminuted, greenstick, Colles, Pott’s fracture.

Steps in Fracture Repair

1. Fracture Hematoma Formation: Blood clot forms at the site; bone cells die.

2. Callus Formation: Cells from endosteum and periosteum form internal (spongy bone) and external (cartilage and bone) calluses to stabilize the fracture.

3. Spongy Bone Formation: Osteoblasts replace cartilage of the external callus with spongy bone.

4. Compact Bone Formation: Remodeling restores bone structure; repaired bone may be slightly thicker and stronger.

Effects of Aging on Bones

Osteopenia and Osteoporosis

• Osteopenia: Inadequate ossification (bone mass reduction) beginning between ages 30 and 40; more pronounced in women.

• Osteoporosis: Severe bone mass loss compromising function; bones become brittle and fragile; more common in postmenopausal women due to decreased sex hormones.

• Commonly affects epiphyses, vertebrae, and jaw bones, leading to fragile limbs, reduced height, and tooth loss.

Additional info: Osteoporosis can also be a secondary effect of certain cancers that stimulate osteoclast activity.