Chapter 6: Bones and Bone Structure
Chapter 6: Bones and Bone Structure
Learning Outcomes (1 of 2)
6.1 Describe the major functions of the skeletal system.
6.2 Classify bones according to shape and structure, giving examples of each type, and explain the functional significance of each of the major types of bone markings.
6.3 Identify the cell types in bone, and list their major functions.
6.4 Compare the structures and functions of compact bone and spongy bone.
6.5 Compare the mechanisms of endochondral ossification and intramembranous ossification.
Learning Outcomes (2 of 2)
6.6 Describe the remodeling and homeostatic mechanisms of the skeletal system.
6.7 Discuss the effects of exercise, nutrition, and hormones on bone development and the skeletal system.
6.8 Explain the role of calcium as it relates to the skeletal system.
6.9 Describe the types of fractures, and explain how fractures heal.
6.10 Summarize the effects of the aging process on the skeletal system.
6.1 Functions of Skeletal System (1 of 2)
Skeletal System Includes:
Bones of the skeleton
Cartilages, ligaments, and other connective tissues
6.1 Functions of Skeletal System (2 of 2)
Primary Functions of the Skeletal System:
Support: Provides a framework for the body, supporting soft tissues and anchoring organs.
Storage of Minerals and Lipids: Acts as a reservoir for minerals, particularly calcium and phosphorus; storage of lipids in adipocytes in yellow marrow.
Blood Cell Production: Hematopoiesis occurs in the red marrow of certain bones, producing red and white blood cells.
Protection: Shields vital organs (e.g., the skull protects the brain, and the rib cage protects the heart).
Leverage: Serves as a system of levers that muscles use to produce movement.
6.2 Classification of Bones (1 of 8)
Classification Criteria:
By Shape
By Structure
6.2 Classification of Bones (2 of 8)
Bone Shapes:
Sutural: Small, flat, oddly shaped bones found between flat bones of the skull.
Irregular: Have complex shapes; e.g., spinal vertebrae, pelvic bones.
Short: Box-like bones; e.g., carpal (wrist) and tarsal (ankle) bones.
Flat: Thin, parallel surfaces; e.g., parietal bones of the skull, sternum, ribs, scapulae.
Long: Relatively long and slender; e.g., humerus, femur.
Sesamoid: Small, round, typically flat bones developed within tendons near joints; e.g., patella.
6.2 Classification of Bones (3 of 8)
Sutural Bones (Wormian Bones):
Characteristics: Small, flat, irregularly shaped, found between the flat bones of the skull.
Size range: From a grain of sand to a quarter.
Appearance: Borders resemble jigsaw puzzle pieces.
Irregular Bones:
Characteristics: Complex shapes with short, flat, notched, or ridged surfaces.
Examples: Vertebrae in the spinal column, bones in the pelvis, various skull bones.
6.2 Classification of Bones (4 of 8)
Short Bones:
Appearance: Box-like; examples include the carpal bones and tarsal bones.
Flat Bones:
Appearance: Thin, with parallel surfaces; examples include the skull roof, sternum, ribs, and scapulae.
Function: Provide protection and extensive surface area for muscle attachment.
6.2 Classification of Bones (5 of 8)
Long Bones:
Description: Long and slender, found in arms, legs, palms, soles, fingers, and toes.
Example: Femur (largest and heaviest bone).
Sesamoid Bones:
Description: Typically small, round, and flat; develop within tendons near joints.
Variance: Number varies by individual; universally includes patella (kneecap).
6.2 Classification of Bones (6 of 8)
Bone Markings (Surface Features):
Projections: Sites for muscle, tendon, and ligament attachment.
Openings and Depressions: Allow passage of blood vessels and nerves.
6.2 Classification of Bones (7 of 8)
Structure of a Long Bone:
Diaphysis (shaft): Composed of compact bone, with a central medullary (marrow) cavity.
Epiphysis (ends): Mostly spongy bone (trabecular bone).
Metaphysis: Junction where diaphysis meets epiphysis.
6.2 Classification of Bones (8 of 8)
Structure of Flat Bones:
Example: Parietal bones of the skull.
Composition: Spongy bone (diploë) sandwiched between two layers of compact bone.
6.3 Bone Tissue (1 of 9)
Bone Tissue:
Described as dense, supportive connective tissue.
Contains specialized cells and features a solid extracellular matrix loaded with collagen fibers.
6.3 Bone Tissue (2 of 9)
Characteristics of Bone:
Dense matrix from calcium salt deposits.
Osteocytes: Bone cells located within lacunae organized around blood vessels.
Canaliculi: Narrow passageways enabling nutrient, waste, and gas exchange.
Periosteum: Covers outer surfaces of bones (except at joints), consisting of outer fibrous and inner cellular layers.
6.3 Bone Tissue (3 of 9)
Bone Matrix Composition:
Major mineral: Calcium phosphate, making up about two-thirds of bone mass.
Interacts with calcium hydroxide to form hydroxyapatite crystals.
Contains other calcium salts (e.g., calcium carbonate) and various ions (e.g., magnesium).
A non-calcified bone matrix appears normal but is overly flexible.
6.3 Bone Tissue (4 of 9)
Bone Matrix Proteins:
One-third of bone mass consists of collagen fibers which contribute to matrix strength.
6.3 Bone Tissue (5 of 9)
Bone Cells Composition:
Represent only 2% of total bone mass.
Four Types:
Osteogenic cells: Stem cells dividing to form osteoblasts.
Osteoblasts: Immature cells producing new bone matrix during osteogenesis (ossification).
Osteocytes: Mature bone cells that maintain the bone matrix.
Osteoclasts: Multinucleate cells that absorb and remove bone matrix, aiding in remodeling.
6.3 Bone Tissue (6 of 9)
Osteogenic Cells:
Also called osteoprogenitor cells; mesenchymal stem cells.
Located in the inner cellular layer of the periosteum and endosteum; help in fracture repair.
6.3 Bone Tissue (7 of 9)
Osteoblasts:
Function: Produce new bone matrix during the process of osteogenesis.
Produce osteoid: Unmineralized bone matrix formed before mineralization occurs.
When surrounded by matrix, osteoblasts mature into osteocytes.
6.3 Bone Tissue (8 of 9)
Osteocytes:
Mature bone cells that reside in lacunae, do not divide.
Possess cytoplasmic extensions through canaliculi; main functions include:
Maintaining protein and mineral content of matrix.
Assisting in repairing damaged bone.
6.3 Bone Tissue (9 of 9)
Osteoclasts:
Large multinucleate cells that resorb and dissolve bone matrix.
Secrete acids and protein-digesting enzymes, critical for bone remodeling and mineral release (osteolysis essential for homeostasis).
Derive from the same stem cells that produce monocytes and macrophages.
6.4 Compact Bone and Spongy Bone (1 of 6)
Osteon: Functional unit of compact bone.
Central Canal: Contains blood vessels.
Perforating Canals: Perpendicular channels carrying blood vessels into deeper bone and marrow.
Lamellae: Layers of bone matrix; classified into:
Concentric Lamellae: Surround the central canal.
Interstitial Lamellae: Fill spaces between osteons.
Circumferential Lamellae: Found on outer and inner bone surfaces.
6.4 Compact Bone and Spongy Bone (2 of 6)
Characteristics of Spongy Bone:
Lacks osteons; matrix forms an open network of trabeculae.
Contains red bone marrow filling trabecular spaces: produces blood cells and contains blood vessels supplying nutrients to osteocytes through diffusion.
Yellow Bone Marrow: Located in other areas of spongy bone for fat storage.
6.4 Compact Bone and Spongy Bone (3 of 6)
Weight-Bearing Bones:
Trabeculae in the femur’s epiphysis transfer forces from the pelvis to the compact bone of the femoral shaft, causing compression on the medial side and tension on the lateral side.
6.4 Compact Bone and Spongy Bone (4 of 6)
Periosteum:
Membrane covering the outside of bones except within joint cavities.
Composed of an outer fibrous layer and an inner cellular layer.
Fibers intermingle with those of tendons; perforating fibers strengthen the bond with bone.
6.4 Compact Bone and Spongy Bone (5 of 6)
Functions of Periosteum:
Isolates bone from surrounding tissues.
Provides a pathway for blood vessels and nerves.
Participate in bone growth and repair processes.
6.4 Compact Bone and Spongy Bone (6 of 6)
Endosteum:
An incomplete cellular layer lining the medullary cavity.
Active in bone growth, repair, and remodeling; covers trabecular surfaces and lines central canals of compact bone.
Composed of a flattened layer of osteogenic cells.
6.5 Bone Formation and Growth (1 of 7)
Bone Development:
Ossification (osteogenesis): The process of bone formation.
Calcification: Deposition of calcium salts that occurs during ossification.
Two primary types of ossification:
Endochondral Ossification: Most bones form this way.
Intramembranous Ossification: Occurs in certain flat bones.
Note: Some human bones continue to grow until age 25.
6.5 Bone Formation and Growth (2 of 7)
Endochondral Ossification:
Mechanism for forming most bones; begins with a primary ossification center in hyaline cartilage, eventually replaced by bone through several key steps:
Step 1: Cartilage enlarges; chondrocytes near the center grow significantly, leading to calcification.
Step 2: Blood vessels grow around the edges (perichondrium cells convert to osteoblasts).
Step 3: Blood vessels penetrate the central cartilage region, and fibroblasts turn into osteoblasts, leading to spongy bone formation.
6.5 Bone Formation and Growth (3 of 7)
Continuation of Endochondral Ossification:
Step 4: Remodeling creates a medullary cavity; bone near epiphyses is replaced by shafts of bone, allowing increase in length and diameter.
Step 5: Peristaltic activities lead to secondary ossification centers in the epiphyses.
Step 6: Eventually, the epiphyses are filled with spongy bone, while the metaphyseal region, the growth plate, persists between diaphysis and epiphysis, managing ongoing growth.
6.5 Bone Formation and Growth (4 of 7)
Completion of Endochondral Ossification:
Step 7: At puberty, cartilaginous growth slows, and osteoblast activity increases, leading to epiphyseal closure. The region of previous cartilage becomes the distinct epiphyseal line.
6.5 Bone Formation and Growth (5 of 7)
Interstitial Growth: Process that increases bone length via:
Development of secondary ossification centers and eventual epiphyseal closure.
Previous sites of epiphyseal cartilage visible as epiphyseal lines on X-rays after closure.
6.5 Bone Formation and Growth (6 of 7)
Appositional Growth: Growth in width of bones; involves:
Thickening and strengthening the long bones by adding circumferential lamellae on outer surfaces, while osteoclasts remodel the inner surface to enlarge the medullary cavity.
6.5 Bone Formation and Growth (7 of 7)
Intramembranous Ossification (dermal ossification):
Produces dermal bones such as the mandible and clavicles through five primary steps:
Mesenchymal cells cluster, differentiate into osteoblasts, and begin secreting organic matrix substances (osteoid).
Osteoblasts become trapped in bony pockets and transform into osteocytes as the bone expands through spicule formation.
Blood vessels grow amidst spicules, increasing growth rate significantly.
Continued deposition by osteoblasts leads to plates of spongy bone containing blood vessels.
The remodeling forms diploë with a covering of compact bone around it.
6.6 Bone Remodeling (1 of 1)
Bone Remodeling: A lifelong process of maintenance involving:
Recycling bone matrix.
Key players: osteocytes, osteoblasts, and osteoclasts.
Importance of balanced activities:
If removal outpaces replacement, bones weaken.
If deposition is predominant, bones strengthen.
6.7 Exercise, Nutrition, and Hormones (1 of 4)
Effects of Exercise:
Promotes mineral recycling, adaptation of bones to stress.
Bones subjected to stress become thicker and stronger, especially with weight-bearing activities.
Bone Degeneration: Rapid bone loss can occur; inactivity can lead to up to one-third of bone mass loss in weeks.
6.7 Exercise, Nutrition, and Hormones (2 of 4)
Nutritional and Hormonal Effects on Bone:
Minerals: Essential mineral intake includes calcium, phosphorus, magnesium, fluoride, iron, and manganese.
Calcitriol and Vitamin D: Calcitriol is derived from Vitamin D (cholecalciferol) and is crucial for calcium and phosphate absorption in the intestines.
6.7 Exercise, Nutrition, and Hormones (3 of 4)
Continued Nutritional and Hormonal Effects:
Vitamin C: Necessary for collagen synthesis and stimulating osteoblast differentiation.
Vitamin A: Promotes osteoblast activity.
Vitamins K and B12: Needed for bone protein synthesis.
6.7 Exercise, Nutrition, and Hormones (4 of 4)
Hormones Influencing Bone Growth:
Growth hormone and thyroxine stimulate bone growth.
Sex hormones: Estrogen and testosterone boost osteoblast activity.
Parathyroid hormone and calcitonin regulate calcium ion homeostasis.
6.8 Calcium Homeostasis (1 of 4)
Skeleton and Calcium Storage:
Bones serve as the largest reservoir, storing 99% of the body’s calcium, vital for various physiological functions.
6.8 Calcium Homeostasis (2 of 4)
Hormonal Regulation of Calcium:
Calcium concentration must be finely tuned; involved hormones include:
Parathyroid hormone (PTH): Raises blood calcium levels by enhancing osteoclast activity, increasing intestinal absorption, and decreasing kidney excretion.
6.8 Calcium Homeostasis (3 of 4)
Calcitonin: Secreted by the thyroid gland; functions to lower blood calcium levels by:
Inhibiting osteoclast activity.
Enhancing calcium excretion by kidneys and reducing intestinal absorption.
6.9 Fractures (1 of 4)
Definition of Fractures:
Fractures refer to cracks or breaks in bones caused by physical stress.
Types: Open (compound) and closed (simple).
6.9 Fractures (2 of 4)
Major Types of Fractures:
Transverse: A straight break across the bone.
Displaced: Ends of the broken bone are not aligned.
Compression: Bone is crushed, often seen in vertebrae.
Spiral: Caused by twisting stress.
Epiphyseal: Fracture occurs at the growth plate.
Comminuted: Bone shatters into numerous pieces.
Greenstick: Incomplete fracture often seen in children.
Colles: Fracture of the distal radius caused by falling on an outstretched hand.
Pott’s: Fracture of the ankle involving both bones.
6.9 Fractures (3 of 4)
Fracture Healing Process:
Step 1: Fracture hematoma formation; blood clot establishes a fibrous network and causes cell death in the surrounding area.
Step 2: Callus formation; cells from endosteum and periosteum migrate, creating internal and external calluses that stabilize the break.
Step 3: Spongy bone formation; callus cartilage is gradually replaced with spongy bone.
Step 4: Compact Bone formation; the final repair may lead to a slightly denser bone than normal.
6.10 Effects of Aging on Skeletal System (1 of 3)
Aging and Bone Density:
Bones typically become thinner and weaker with age, a condition called osteopenia beginning typically around ages 30-40:
Women: lose 8% bone mass per decade.
Men: lose 3% per decade.
Most affected areas: epiphyses, vertebrae, and jaws, possibly leading to fragile limbs, reduced heights, and tooth loss.
6.10 Effects of Aging on Skeletal System (2 of 3)
Osteoporosis:
A severe condition characterized by significant bone mass reduction, affecting normal functionalities:
Prevalence in adults over 45:
29% of women
18% of men
Hormonal influences: Osteoporosis accelerates post-menopause due to decreased sex hormones.
6.10 Effects of Aging on Skeletal System (3 of 3)
Cancer and Bone Loss:
Cancerous tissues can release osteoclast-activating factors, resulting in increased osteoclast activity and significant osteoporosis.