Bones and Bone Structure (Lecture)
Section 1: Introduction to the Structure and Growth of Bones
Learning Outcomes
6.1 Describe the two main divisions of the skeleton and list the major functions of the skeletal system.
6.2 Classify bones according to their shapes, identify major types of bone markings, and explain the functional significance of bone markings.
6.3 Identify the parts of a typical long bone and describe its internal structures.
6.4 Identify types of cells in bone and list their major functions.
6.5 Compare structures and functions of compact bone and spongy bone.
6.6 Describe the process of appositional bone growth.
6.7 Describe the process of endochondral ossification.
6.8 Describe the process of intramembranous ossification.
6.9 Discuss various abnormalities of bone formation and growth.
Module 6.1: The Skeletal System Divisions
Total Number of Bones: ~206 bones in the adult human body.
Axial Skeleton (80 bones):
Composed of bones of the skull, thorax, and vertebral column.
Forms the longitudinal axis of the body.
Appendicular Skeleton (126 bones):
Bones of the limbs and girdles attaching them to the axial skeleton.
Associated Structures: Includes cartilages, ligaments, and other connective tissues.
Functions of the Skeletal System
Support: Provides a framework for the body.
Mineral and Lipid Storage: Reservoir for minerals (e.g., calcium, phosphate).
Blood Cell Production: Hematopoiesis occurs in the bone marrow.
Protection: Shields vital organs (e.g., cranial bones protect the brain).
Leverage: Facilitates movement by serving as levers operated by muscles.
Module 6.2: Bone Classification by Shape
Six Categories of Bone Type
Flat Bones:
Thin, roughly parallel surfaces.
Examples: Cranial bones, sternum, ribs, scapulae.
Functions: Protect underlying tissues; provide surface area for muscle attachment.
Sutural Bones (Wormian Bones):
Irregular bones formed between cranial bones.
Vary in number, size, and shape.
Long Bones:
Longer and slender.
Examples: Bones of the limbs (e.g., humerus, femur).
Irregular Bones:
Complex shapes with short, flat, notched, or ridged surfaces.
Examples: Vertebrae, bones of the pelvis, facial bones.
Sesamoid Bones:
Small, flat, shaped like sesame seeds; develop within tendons.
Examples: Patella (kneecap).
Short Bones:
Small and boxy.
Examples: Carpals in wrists, tarsals in ankles.
Bone Markings and Functional Significance
Bone Markings: Surface features of bones indicating function.
Elevations/Projections:
Muscle, tendon, and ligament attachment; involved in joints.
Depressions/Grooves/Tunnels:
Path for blood vessels and nerves.
Specific Bone Markings
Elevations/Projections:
Process: Any projection or bump.
Tubercle: Small, rounded projection.
Tuberosity: Small, rough projection broad area.
Trochlea: Smooth, grooved articular process like a pulley.
Condyle: Smooth, rounded articular process.
Trochanter: Large, rough projection.
Facet: Small, flat articular surface.
Crest: Prominent ridge.
Line: Low ridge, delicate.
Spine: Pointed or narrow process.
Ramus: Extension making an angle with the rest.
Depressions/Grooves/Tunnels:
Canal (Meatus): Large passageway.
Sinus: Chamber within bone.
Foramen: Small, rounded passageway for vessels/nerves.
Fissure: Elongated cleft or gap.
Sulcus: Deep, narrow groove.
Fossa: Shallow depression.
Module 6.3: Internal Structure of Long Bones
Features of Long Bones:
Epiphysis: Expanded area at each end, primarily spongy bone covered by compact bone.
Metaphysis: Connects epiphysis to shaft.
Diaphysis (Shaft): Contains the medullary cavity filled with red and yellow marrow.
Articular Cartilage: Covers articulating surfaces of epiphyses.
Blood Supply and Innervation of Bone
Rich blood supply required for growth and maintenance.
Blood Vessels: Nutrient artery and vein supply blood, along with metafyseal arteries/veins.
Lymphatic Vessels: Collect lymph from bone.
Sensory Nerves: Innervate diaphysis, medullary cavity, and epiphyses.
Module 6.4: Osteogenic Cells and Bone Matrix
Osteogenic (Osteoprogenitor) Cells: Mesenchymal stem cells that give rise to osteoblasts.
Osteoblasts:
Produce unmineralized matrix (osteoid) and mineralize it.
Transform into osteocytes.
Osteocytes:
Mature bone cells maintaining the matrix, occupying lacunae communicated via canaliculi.
Osteoclasts:
Responsible for remodeling and releasing minerals (osteolysis).
Bone Matrix Composition
Collagen Fibers: Accounts for ~1/3 of bone weight, providing flexibility.
Calcium Phosphate ():
Accounts for ~2/3 of bone weight.
Forms crystals (hydroxyapatite, ) which strengthen the matrix.
Module 6.5: Bone Structure Types
Compact Bone:
Comprised of osteons (Haversian systems), concentric lamellae around central canals.
Strong along length, connected by canaliculi.
Spongy Bone:
Composed of trabeculae (struts/plates), lacking blood vessels inside matrix.
Contains red bone marrow between trabeculae.
Module 6.6: Bone Growth Processes
Appositional Bone Growth
Increases diameter via periosteum.
Osteogenic cells differentiate into osteoblasts, adding circumferential lamellae
Osteoclasts remodel inner surfaces to enlarge the medullary cavity.
Variants:
Periosteum: Fibrous outer and cellular inner layer, active in growth and repair.
Endosteum: Cellular layer lining medullary cavity, involved in growth and repair/destruction of matrix.
Module 6.7: Ossification Processes
Endochondral Ossification:
Process of Cartilage Replacement: Hyaline cartilage model replaced by bone via ossification.
Steps:
Cartilage model enlarges; chondrocytes enlarge and die.
Blood vessels grow, osteoblasts form around the shaft.
Blood vessels penetrate allowing spongy bone production at primary ossification centers.
Medullary cavity forms and continues growth.
Secondary ossification centers form in epiphyses, leading to spongy bone formation.
Module 6.8: Intramembranous Ossification
Formation via Mesenchymal Cells: Differentiation into osteoblasts without a cartilage model, primarily in dermal layers.
Bones formed include roofing skull bones, jaws, collar bones, and patella.
Steps include clustering osteoblasts, forming and mineralizing osteoid matrix, and developing blood vessels that fuse spicules of bone.
Module 6.9: Clinical Aspects of Bone Growth
Abnormalities:
Pituitary Growth Failure: Leads to shorter bones due to inadequate growth hormone.
Achondroplasia: Affects growth at epiphyseal cartilage, leading to shorter stocky limbs.
Marfan Syndrome: Excessive cartilage leads to tall stature and connective tissue issues.
Fibrodysplasia Ossificans Progressiva (FOP): Abnormal bone formation outside the normal skeletal structures.
Section 2: Physiology of Bones
Learning Outcomes
6.10 Identify minerals stored in bones and organs involved in calcium homeostasis.
6.11 Discuss hormones affecting bone growth and homeostasis mechanisms.
6.12 Identify fracture types and healing processes.
Module 6.10: Bones as Mineral Reservoirs
Minerals in Bone: Primarily calcium and phosphate, key for physiological processes (e.g., muscle contractions).
Calcium plays a crucial role, about 1-2 kg present in the body, predominantly stored in bones.
Maintaining Calcium Levels
Hormonal Control: Involves intestines, bones, and kidneys.
Intestines: Absorb calcium and phosphate.
Bones: Osteoclasts and osteoblasts assist in calcium balance.
Kidneys: Regulate calcium and phosphate levels in urine.
Module 6.11: Hormones Regulating Calcium Ion Metabolism
Hormones Involved:
Parathyroid Hormone (PTH): Increases blood calcium by stimulating osteoclasts, enhancing intestinal absorption, and promoting kidney reabsorption.
Calcitonin: Decreases blood calcium by inhibiting osteoclasts, decreasing intestinal absorption, and inhibiting kidney reabsorption.
Module 6.12: Fracture Healing Process
Fracture Definition: A crack or break in the bone from mechanical stress.
Steps in Healing:
Formation of a fracture hematoma.
Callus formation (internal network of spongy bone and external stabilization).
Replacement of cartilage with spongy bone.
Replacement of spongy bone with compact bone over time.
Types of Fractures
Closed (Simple): Internal; only visible in x-rays.
Open (Compound): Projects through the skin, risk of infection.
Specific Types:
Transverse, spiral, displaced, nondisplaced, compression, greenstick, comminuted, epiphyseal, Pott's, and Colles fractures.