Bones and Axial Skeleton
Learning Objectives for Bones and Axial Skeleton
L.O. 5.0: Summarize characteristics of bones (features, cells, histology, matrices) and describe growth.
L.O. 5.1: Name the bones of the axial skeleton.
L.O. 5.2: Describe the location of axial skeleton bones.
L.O. 5.3: Identify bone features to describe interactions with neighboring bones and tissues.
L.O. 5.4: Analyze bone histology and explain how microscopic structures result in function.
L.O. 5.5: Describe the skeletal system as an integrated organ system contributing to homeostasis.
L.O. 5.6: Discuss causes and pathophysiology of osteoporosis and preventative steps.
Major Tissue Types of Bones
Osseous Tissue (Bone Tissue):
The primary structural and supportive connective tissue in the human body.
Consists of Compact and Spongy bone.
Marrow: Soft fatty tissue found within bone cavities.
Red Marrow: The site of blood cell production (hematopoiesis).
Yellow Marrow: Serving as a site for fat storage.
Gross Macroscopic Features of Bone
Periosteum:
Composed of dense irregular collagenous connective tissue.
Possesses a rich blood supply that supports nutrition and repair.
Contains neuronal axons, contributing to sensitivity and pain sensation.
Anchored to the bone by perforating (Sharpey’s) fibers.
Compact (Cortical) Bone:
A dense, strong outer layer.
Provides resistance to compression and twisting forces.
Maintains structural integrity and helps prevent fractures.
Microscopic Structure of Spongy (Trabecular) Bone
Trabeculae:
A loosely-gathered meshwork of extensively-branched bone tissue.
Composed of dense collagenous tissue designed to resist stress.
Covered by the endosteum (a thin connective tissue layer).
Forms a protective framework for supporting bone marrow.
Contains concentric lamellae, lacunae with osteocytes, and canaliculi for cell communication.
Nutrient Diffusion: Osteocytes in trabeculae receive nutrients and oxygen via diffusion from nearby marrow blood vessels; therefore, internal canals are not required.
Absence of Structures: Trabeculae lack osteons, central canals, and perforating canals.
Long Bone Anatomy
Epiphyses (Rounded Ends):
Expanded ends of long bones.
Contains abundant spongy bone and marrow.
Covered with hyaline (articular) cartilage to reduce friction at joints.
Metaphysis:
The narrow region situated between the diaphysis and epiphysis.
The major site of bone growth (growth plate region).
Diaphysis (Shaft):
The long central axis of the bone.
Composed of a thick compact bone layer with minimal spongy bone.
Contains the nutrient artery and vein.
Nutrient Foramen: The opening in the bone for blood vessel entry and exit.
Anatomical History and Development Transitions
Epiphyseal Plate (Growth Plate):
A region of hyaline cartilage between the epiphysis and diaphysis.
Responsible for the lengthwise growth of bones during childhood and adolescence.
Epiphyseal Line:
The remnant of the epiphyseal plate observed in adults.
Forms after cartilage ossifies and growth stops.
Developmental Change: The transition from plate (cartilage) to line (bone) typically occurs in early adulthood at both proximal and distal ends.
Extracellular Matrix (ECM) of Osseous Tissue
Inorganic Matrix ():
Provides strength and resistance to compression.
Primarily composed of calcium and phosphate forming hydroxyapatite crystals: .
Bones store approximately of the body's total calcium.
Organic Matrix ():
Provides flexibility and tensile strength.
Osteoid: Represents of the organic matrix secreted by osteoblasts.
Collagen: Collagen strands combine into tropocollagen to create cross-linked fibers.
Ground Substance: Contains proteoglycans, glycosaminoglycans, and glycoproteins that attract and retain water to resist compression.
Helps bind hydroxyapatite crystals to bone cells and the matrix.
Specialized Bone Cell Types
Osteogenic Cells (Osteoprogenitor Cells):
Mitotically active stem cells located in the periosteum and endosteum.
Stem cells that can differentiate into osteoblasts or bone-lining cells.
Osteoblasts:
Bone-forming cells that secrete unmineralized matrix (osteoid).
Osteoid is composed mainly of collagen ( of bone protein) and calcium-binding proteins.
Once surrounded by matrix, they mature into osteocytes.
Osteocytes:
Mature bone cells residing in lacunae that no longer divide.
Function to maintain the bone matrix.
Act as stress or strain sensors responding to mechanical stimuli (increased force or weightlessness).
Communicate with osteoblasts and osteoclasts to regulate remodeling.
Osteoclasts:
Multinucleated cells derived from the macrophage/hematopoietic lineage.
Responsible for bone resorption (breakdown).
Resorption Bays: Shallow depressions on the bone surface where osteoclasts settle.
Ruffled Border: Increases surface area for enzyme release to break down bone.
Lineage and Differentiation Pathways
Osteoblast Lineage: Mesenchymal stem cell $\rightarrow$ Osteoprogenitor cell (regulated by SOX9) $\rightarrow$ Preosteoblast (regulated by RUNX2) $\rightarrow$ Osteoblast (regulated by RUNX2 and OSX) $\rightarrow$ Osteocyte.
Osteoclast Lineage: Hematopoietic stem cell $\rightarrow$ Macrophage (regulated by M-CSF) $\rightarrow$ Osteoclast (regulated by RANKL).
Bone Resorption and Mineral Release
Sealing Zone: Osteoclasts use integrins to attach to the bone surface, isolating the resorption bay.
Chemical Breakdown: Dissolves hydroxyapatite (), releasing calcium and phosphate into the blood.
Absorption: Dissolved minerals and degraded proteins are absorbed into the osteoclast and transported to nearby blood vessels.
Post-Resorption: After completion, osteoclasts undergo apoptosis (die off), and osteoblasts fill the area with new matrix.
Hormonal Regulation of Calcium Plasma Levels
High Plasma Calcium Levels:
Triggers release of Calcitonin from the thyroid gland.
Leads to calcium salts being deposited in the bone.
Low Plasma Calcium Levels:
Triggers release of Parathyroid Hormone (PTH) from the parathyroid glands.
Stimulates osteoclast activity to release calcium from bones.
Increases calcium resorption by the kidneys and absorption by the intestines.
Microscopic Structure of Compact Bone
Osteons: Small, tightly-packed structural units.
Central Canal: Located in the center of each osteon for nerves and blood vessels.
Perforating Canals: Connect osteons to each other.
Canaliculi: Tiny canals connecting adjacent lacunae for osteocyte communication and secretion transport.
Bone Development and Ossification
Ossification (Osteogenesis): The process of bone tissue formation.
Skeleton formation begins at month 2 of fetal development.
Postnatal growth continues until early adulthood.
Remodeling and repair are lifelong processes.
Endochondral Ossification:
Bone forms by replacing hyaline cartilage (primary method for most of the skeleton).
Intramembranous Ossification:
Bone develops from a fibrous membrane (forms flat bones like the skull vault).
Stages of Endochondral Ossification (Long Bone Growth)
Fetal Cartilage Model Forms: Chondroblasts differentiate into osteoblasts; structure is surrounded by perichondrium/periosteum.
Periosteal Bone Collar Forms: Osteoblasts deposit osteoid around the diaphysis; internal cartilage begins to calcify.
Primary Ossification Center (Diaphysis): Blood vessels invade the shaft; chondrocytes die due to lack of nutrients as matrix calcifies; primary center develops.
Secondary Ossification Centers (Epiphyses): Ossification begins in the epiphyses; the medullary cavity forms in the diaphysis.
Bone Replaces Calcified Cartilage: Osteoblasts replace cartilage with early spongy bone; articular cartilage and epiphyseal plates remain.
Epiphyseal Plates Close: Medullary cavity enlarges; growth plates are replaced by bone, forming epiphyseal lines.
Longitudinal Growth Zones in the Epiphyseal Plate
Zone of Reserve Cartilage: Resting hyaline cartilage with small, inactive cells acting as a reserve pool.
Zone of Proliferation: Rapidly dividing chondrocytes form stacks, pushing the epiphysis away from the diaphysis to lengthen the bone.
Zone of Hypertrophy and Maturation: Chondrocytes enlarge and fill with glycogen.
Zone of Calcification: Matrix calcifies; chondrocytes die as nutrients cannot diffuse.
Zone of Ossification: Blood vessels and osteoprogenitor cells invade; osteoclasts remove dead cartilage and osteoblasts lay down bone matrix.
Stages of Intramembranous Ossification (Flat Bone Growth)
Primary Ossification Centers Form: Mesenchymal stem cells differentiate into osteoblasts in vascularized embryonic mesenchyme.
Osteoid Secretion and Osteocyte Formation: Osteoblasts secrete osteoid which hardens via calcium deposition; trapped cells become osteocytes.
Formation of Early Spongy Bone: Osteoblasts form trabeculae (spicules); mesenchyme differentiates into periosteum.
Development of Spongy Bone and Marrow: Trabecular network enlarges; vascular tissue develops into bone marrow.
Formation of Outer Compact Bone: Osteoblasts in the periosteum deposit matrix to form compact bone on external surfaces, creating a "sandwich" of spongy bone (diplo\u00eb) between compact bone.
Appositional Growth
Increase in bone diameter/thickness by adding tissue at the outer surface.
Osteoblasts in the periosteum secrete matrix to form circumferential lamellae.
Osteoclasts on the inner surface break down bone to prevent it from becoming too heavy.
The Axial Skeleton
Definition: Bones located within the long axis of the body.
Components:
Skull
Auditory Ossicles (internal)
Hyoid Bone
Vertebral Column (Vertebrae, Sacrum, Coccyx)
Ribs
Sternum
The Appendicular Skeleton
Definition: Bones of the body's limbs.
Upper Limbs: Pectoral girdle (Clavicles, Scapulae), Forearm (Ulnae, Radii), Hand (Carpals, Metacarpals, Phalanges).
Lower Limbs: Pelvic girdle (Hip bones), Thigh (Femora, Patellae), Leg (Tibiae, Fibulae), Feet (Tarsals, Metatarsals, Phalanges).
Shape Classifications of Bone
Long Bones: Longer than they are wide.
Short Bones: Equally long and wide (cube-shaped).
Flat Bones: Thin, broad, and commonly curved.
Irregular Bones: Do not fit into other categories (e.g., vertebrae).
Sesamoid Bones: Small, flat, and oval-shaped; found within tendons.
Bone Surface Markings
Depressions (Clefts/Grooves):
Facet: Shallow surface where bones meet for a joint.
Fossa: Deeper indented surface; accepts a rounded surface of another bone.
Fovea: Shallow pit; site for ligament attachment.
Groove: Long, shallow depression for nerves or vessels.
Openings (Holes/Canals):
Canal/Meatus: Passageway through a bone.
Fissure: Slit between bones.
Foramen: Hole for nerves or blood vessels.
Projections (Extensions):
Condyle: Round end fitting into a fossa.
Crest: Ridge for muscle attachment.
Epicondyle: Small projection proximal to a condyle.
Head: Rounded end for a joint.
Process: Any bony projection (e.g., spinous process).
Tubercle: Small rounded projection.
Tuberosity: Larger, more prominent tubercle.
Spine: Sharp, slender projection.
Protuberance: Outgrowth due to muscle pull.
Trochanter: Very large projection (femur only).
Line: Ridge for muscle attachment.
Bones of the Skull
Cranial Bones (8):
Protect the brain and house special sense organs.
Frontal (Singular), Occipital (Singular), Ethmoid (Singular), Sphenoid (Singular).
Temporal (Paired), Parietal (Paired).
Facial Bones (14):
Scaffold for face, anchors for teeth and muscles.
Mandible (Singular), Vomer (Singular).
Maxillary (Paired), Zygomatic (Paired), Nasal (Paired), Lacrimal (Paired), Palatine (Paired), Inferior Nasal Conchae (Paired).
Skull Features and Suffixes/Cavities
Frontal Bone: Supraorbital margins, Supraorbital foramen, Glabella.
Sutures (Parietal Bone): Sagittal (between parietals), Coronal (fronto-parietal), Squamous (parieto-temporal), Lambdoid (parieto-occipital).
Occipital Bone: Foramen magnum (spinal cord pass), Occipital condyles ( articulation), Nuchal lines, External occipital protuberance.
Temporal Bone Regions:
Squamous (Zygomatic process, Mandibular fossa).
Tympanic (External acoustic meatus, Styloid process).
Mastoid (Mastoid process).
Petrous (Middle cranial fossa, Internal acoustic meatus, Jugular foramen, Carotid canal).
Sphenoid Bone: Sella turcica (pituitary gland), Optic canals, Foramen rotundum, ovale, and spinosum, Superior orbital fissure.
Ethmoid Bone: Cribriform plate (smell neurons), Crista galli, Nasal septum, Lateral masses (ethmoid sinuses).
Major Cavities:
Cranial Cavity: Largest; surrounds brain.
Orbital Cavities: 7 bones (Frontal, Zygomatic, Maxilla, Sphenoid, Ethmoid, Lacrimal, Palatine).
Oral Cavity: Teeth, tongue, hard palate roof.
Nasal Cavity: Passages for air and smell.
The Hyoid Bone
The only bone in the body that does not articulate with any other bone.
Maintained by ligaments (e.g., stylohyoid ligament).
Attachment site for muscles of swallowing and phonation.
The Vertebral Column
Total Vertebrae: segments.
Cervical (): 7 vertebrae; is the Atlas, is the Axis (with dens/odontoid process).
Thoracic (): 12 vertebrae; articulate with ribs.
Lumbar (): 5 vertebrae; largest bodies.
Sacrum: 5 fused segments.
Coccyx: 4 fused segments.
Vertebral Structure:
Body: Primary weight-bearing part.
Vertebral Foramen: Passage for spinal cord.
Vertebral Arch: Formed by pedicles and laminae.
Articular Processes: Contain facets for articulation.
Transverse and Spinous Processes: Muscle attachment sites.
Intervertebral and Herniated Discs
Intervertebral Discs: Cushions that absorb shock and resist grinding; facilitate motion.
Herniated Discs: Occurs when the outer ring weakens and the inner core bulges out. Causes include age, trauma, or awkward motion. Treatments: rest, physical therapy, medication.
Comparison of Vertebrae Classes
Cervical: Small oval body ( lacks body, has dens), triangular foramen, transverse processes contain transverse foramina, most spinous processes are fork-shaped.
Thoracic: Heart-shaped body with costal facets, circular foramen, long transverse processes with articular facets for ribs, long spinous processes pointing inferiorly (look like a giraffe).
Lumbar: Largest kidney-shaped body, flattened triangular foramen, short transverse processes with no facets, thick posteriorly-pointing spinous processes (look like a moose).
Skeletal Changes Throughout Life
Birth: Column is C-shaped (primary thoracic and sacral curves).
Development: Cervical curve appears (lifting head), then lumbar curve (walking).
Adulthood: Column establishes an S-shape.
The Thoracic Cage
Sternum:
Manubrium: Articulates with pectoral girdles and Rib 1.
Body: Articulates with Ribs 2-7.
Xiphoid Process: Abdominal muscle attachment.
Ribs: pairs.
True Ribs (): Direct attachment to sternum via costal cartilage.
False Ribs (): Share costal cartilage.
Floating Ribs (): No attachment to sternum.
Bone Mechanics and Wolff’s Law
Wolff’s Law: Bones adapt and remodel based on the compressive force or load placed on them.
Stress Responses: Consistent stress increases density and strength; absence of loading (e.g., spaceflight/weightlessness) results in bone resorption (increased osteoclast and decreased osteoblast activity).
Fracture Classification and Treatment
Classifications:
Position: Nondisplaced (normal position) vs. Displaced (out of alignment).
Completeness: Complete (all the way through) vs. Incomplete.
Skin Penetration: Open/Compound (skin penetrated) vs. Closed/Simple.
Specific Types: Transverse, Linear, Oblique, Spiral, Greenstick, Comminuted, Avulsion.
Questions & Discussion:
Q6: Describe the type of Fracture shown in the image (Broken skin, Tibia/Fibula).
Response: This is an Open (Compound) Fracture because the skin is penetrated.
Treatment:
Reduction: Realignment (Closed manual manipulation or Open surgical wires/pins).
Immobilization: Cast or traction.
Repair Stages:
Hematoma formation.
Fibrocartilaginous callus formation.
Bony callus formation.
Bone remodeling.
Bone Disorder: Osteoporosis
Pathophysiology: Osteoclast activity outpaces osteoblast activity; often associated with estrogen loss.
Risk Factors: Aged postmenopausal women ( of women 60-70; by age 80), insufficient exercise, diet poor in calcium/protein, smoking, genetics, hyperthyroidism, diabetes, alcohol, and certain medications.
Symptoms: Stooped posture, increased fracture risk.
Treatment: Calcium and Vitamin D supplements, hormone replacement therapy (slows but does not reverse loss), weight-bearing exercise, and reducing alcohol/carbonated beverages.