Skeletal System Study Notes

Skeletal Changes Throughout Life

• Osteoporosis (bone-thinning disease) discussed as a major lifelong skeletal change
  • Affects about $50\%$ of women over age $65$ and $20\%$ of men over age $70$
  • Leads to fragile bones and fractures; vertebral collapse can result in kyphosis (dowager's hump)
  • Estrogen plays a role in health and normal density of the female skeleton
• General aging notes appear (Page 1 shows ages 40, 60, 70 as markers of change)

Curvatures of the Spine and Development

• Primary curvatures: present at birth; convex posteriorly
• Secondary curvatures: develop later in childhood; convex anteriorly
• Abnormal spinal curvatures (scoliosis and lordosis) are often congenital

Fetal to Newborn Skeletal Formation

• In the fetus, long bones are formed from hyaline cartilage; flat bones begin as fibrous membranes
• Both flat and long bone models are converted to bone during development
• Fontanels (soft spots) remain until around age $2$ years

Ossification Centers in a 12-Week-Old Fetus

• Ossification centers listed for multiple bones, including:
  • Parietal bone, Occipital bone, Mandible, Clavicle, Scapula, Frontal bone
  • Ribs, Vertebra, Hip bone, Femur, Radius, Ulna, Humerus, Tibia
• This indicates the early pattern of bone formation via ossification centers

Joint Movement and Synovial Joints: Basic Movements

• Extension: joints straight or extended
• Rotation: head at the neck; other joints can rotate
• Flexion: bending to decrease the angle at a joint
• Abduction: moving away from the midline; examples at hip and shoulder
• Adduction: moving toward the midline; examples at hip and shoulder

Knee Joint: Anatomy Overview

• Key components include:
  • Femur with hyaline cartilage covering articular surfaces
  • Posterior cruciate ligament (PCL)
  • Anterior cruciate ligament (ACL)
  • Menisci (menisci serve as shock absorbers)
  • Tibia and Fibula bones
  • Patellar tendon connecting to the patella
• This configuration supports weight-bearing, stability, and movement

Synovial Joints: Structural Features

• Structural components:
  • Joint capsule (fibrous capsule around joint)
  • Articular (hyaline) cartilage on the ends of articulating bones
  • Joint cavity filled with synovial fluid
  • Ligaments reinforce the joint
• Accessory structures associated:
  • Bursae: flattened fibrous sacs lined with synovial membranes; not part of the joint itself
  • Tendon sheath: elongated bursae surrounding a tendon
• Synovial joints are typically diarthroses (freely movable)

Features and Examples of Synovial Joints

• Articular cartilage covers bone ends; reduces friction
• Fibrous articular capsule encloses joint surfaces
• A joint cavity contains synovial fluid
• Ligaments reinforce the joint
• Examples of common synovial joints include plane, hinge, pivot, condyloid, saddle, and ball-and-socket
• Mobility classifications:
  • Nonaxial (plane)
  • Uniaxial (hinge, pivot)
  • Biaxial (condyloid, saddle)
  • Multiaxial (ball-and-socket)
• Specific joint examples (as per figures):
  • Hinge joints: elbow, knee
  • Plane joints: intercarpal/intertarsal
  • Condyloid joints: radiocarpal
  • Saddle joints: thumb (carpometacarpal #1)
  • Ball-and-socket joints: shoulder, hip

Other Structures Related to Synovial Joints

• Acromion of the scapula, ligaments, bursae, and tendon sheaths contribute to joint stability and movement
• Tendon sheaths and bursae reduce friction between moving structures
• Synovial membrane lines the joint capsule; synovial fluid lubricates the joint

Cartilaginous Joints

• Joints connected by cartilage (cartilaginous joints) are either immovable or slightly movable
• Types:
  • Synchondroses: hyaline cartilage (e.g., epiphyseal plate in growing bones)
  • Symphyses: fibrocartilage (e.g., intervertebral discs, pubic symphysis)
• Examples from figures include:
  • Epiphyseal plate (hyaline cartilage) and intervertebral discs (fibrocartilage)
  • First rib-sternum synchondrosis (immovable in some depictions)
• Intervertebral joints and pubic symphysis are fibrocartilaginous joints providing some mobility

Fibrous Joints

• Immovable joints (sutures, syndesmoses, gomphoses) with collagenic fibers
• Examples:
  • Sutures in the skull (fibrous joints)
  • Syndesmoses (longer fibers) allow slight movement
  • Gomphosis (tooth socket) immovable

Joint Classifications: Structural and Functional

• Structural classifications:
  • Fibrous joints: bones united by collagenic fibers; examples: sutures, syndesmoses, gomphoses; immobile (synarthrosis) or slightly mobile
  • Cartilaginous joints: bones united by cartilage; examples: synchondroses, symphyses; immobile or slightly movable (amphiarthrosis depending on type)
  • Synovial joints: joints with a synovial cavity; freely movable (diarthrosis)
• Functional classifications:
  • Synarthroses: immovable joints
  • Amphiarthroses: slightly movable joints
  • Diarthroses: freely movable joints
• A compact Summary (Table 5.3 concepts):
  • Structural class and mobility are linked: fibrous (sutures, syndesmosis, gomphosis) — mostly immobile or slightly mobile; cartilaginous (synchondroses, symphysis) — immobile or slightly movable; synovial — freely movable

Skeletal System Numbers and Basic Architecture

• The adult human skeleton has 206 bones
• Skull bones: 22 (bones of cranium and face; mandible is the only freely movable joint with the skull)
• Hyoid bone: unique as it does not articulate with other bones
• Middle ear bones: 6 (three per ear)
• Vertebral column: 26 bones in typical adult: 7 cervical, 12 thoracic, 5 lumbar; sacrum (fused, 5) and coccyx (fused, 4–5)
• Thoracic cage (rib cage): True ribs (7 pairs), False ribs (3 pairs), Floating ribs (2 pairs)
• Appendicular skeleton details: pelvic girdle (coxal bones: ilium, ischium, pubis) and pectoral girdle (clavicle and scapula)
• Pectoral girdle attaches upper limbs to axial skeleton via sternoclavicular joint and acromioclavicular joint

The Axial Skeleton

• Components: skull (cranium and facial bones), vertebral column, bony thorax (ribs and sternum)
• The thoracic cage protects major organs (heart and lungs) and supports upper limbs
• The vertebral column provides the longitudinal axis of the body and houses the spinal cord

The Vertebral Column: Regions and Features

• Regions and counts:
  • Cervical vertebrae: $7$
  • Thoracic vertebrae: $12$
  • Lumbar vertebrae: $5$
  • Sacrum: fusion of $5$ vertebrae
  • Coccyx: fusion of $3-5$ vertebrae
• Intervertebral discs separate vertebrae and allow movement; posterior bodies form vertebral arch with vertebral foramen
• Normal curvatures include primary (thoracic and sacral) and secondary (cervical and lumbar) curves
• Scoliosis, kyphosis, and lordosis are common abnormal curvatures discussed in the slides

The Fetal and Infant Skull

• The fetal skull is large relative to total body length; fontanels allow brain growth and skull flexion during birth
• Fontanels: anterior, posterior, sphenoidal, mastoid (posterolateral) fontanels
• Fontanel conversion to bone occurs within around $24$ months after birth
• Ossification centers appear in the fetal skull; sutures remain between bones

The Hyoid Bone and Paranasal Sinuses

• Hyoid bone: does not articulate with other bones; serves as a moveable base for the tongue; aids in swallowing and speech
• Paranasal sinuses: frontal, ethmoid, sphenoid, maxillary
  • Functions: lighten the skull, provide resonance and amplification to voice

The Pelvis and Lower Limbs

• The Pelvic Girdle:
  • Formed by two coxal bones (each composed of ilium, ischium, and pubis), joined at the pubic symphysis and acetabulum
  • Supports upper body weight, protects reproductive organs, urinary bladder, and portions of the large intestine
• Female pelvis differences (to accommodate birth):
  • Inlet larger and more circular; pelvic brim shapes; false pelvis wider; pubic arch angle greater (>$90^ ext{o}$ in many cases)
  • The ilia flare more laterally; sacrum shorter and less curved; ischial spines shorter and farther apart; outlet larger
  • Bones lighter and thinner; true birth canal wider
• The Lower Limb: bones include femur, patella, tibia, fibula; tarsals, metatarsals, and phalanges
  • The femur: heaviest, strongest bone; features include head and neck, greater and lesser trochanters, intertrochanteric crest, linea aspera, medial and lateral condyles, epicondyles, patellar surface
  • The tibia (shinbone) is the larger, medial bone; the fibula is slender and does not bear weight
  • The knee joint integrates the femur, tibia, fibula, patella, and associated ligaments and menisci
  • The ankle and foot contain seven tarsals (including calcaneus and talus), five metatarsals, and fourteen phalanges; arches of the foot include two longitudinal arches and one transverse arch

The Pectoral Girdle and Upper Limb

• Pectoral girdle components: clavicle (collar bone) and scapula (shoulder blade)
• Clavicle features: sternal end and acromial end; connects to the sternum and scapula
• Scapula features: acromion, coracoid process, glenoid cavity
• Upper limb bones: humerus (with greater/lesser tubercles, anatomical and surgical necks, deltoid tuberosity, medial/lateral epicondyles), radius, ulna, carpals (scapula to hand region), metacarpals, and phalanges

The Bones of the Hand and Forearm

• Forearm bones: radius and ulna with radioulnar joints; proximal radioulnar joint near the elbow; distal radioulnar joint near the wrist
• Carpals: eight bones (scaphoid, lunate, triquetrum, pisiform, trapezium, trapezoid, capitate, hamate) arranged in two rows
• Metacarpals: five bones in the palm; Phalanges: distal, middle, and proximal phalanges in fingers

The Bones of the Lower Limbs

• Femur: proximal and distal structures; major processes and lines for muscle attachment
• Tibia and Fibula: two bones of the lower leg; tibia bears most weight; fibula provides muscle attachment and stability
• Patella: kneecap; articulates with the femur; surface features for ligaments and tendons
• Tarsals: seven bones including calcaneus (heel) and talus; supports weight transfer to the foot
• Metatarsals and Phalanges: form the sole and toes; arches provide shock absorption and propulsion

Arches of the Foot and Lower Limb Stability

• Foot bones form three arches: two longitudinal arches (medial and lateral) and one transverse arch
• Arches distribute body weight and absorb shock during movement

The Thoracic Cage (The Bony Thorax)

• Components: sternum, true ribs ( $7$ pairs ), false ribs ( $3$ pairs ), floating ribs ( $2$ pairs )
• The sternum landmarks: manubrium, body, xiphoid process; sternal angle
• Ribs articulate with thoracic vertebrae and costal cartilages; costal cartilages connect ribs to the sternum
• The bony thorax forms a protective cage for vital organs and supports shoulder girdles and upper limbs

The Skull: Cranium and Facial Bones

• Skull comprises two sets: cranium and facial bones
• Bones join via sutures; mandible is the only movable joint with the skull
• The inferior view shows features such as the foramen ovale, carotid canal, jugular foramen, and others
• The superior view highlights the sella turcica, cribriform plate, optic canals, and various foramina
• The skull includes multiple bones: frontal, parietal, temporal, occipital, sphenoid, ethmoid (and facial bones like maxilla, zygomatic, nasal, lacrimal, palatine, vomer, inferior nasal concha, etc.)
• Paranasal sinuses (frontal, ethmoid, sphenoid, maxillary) lie within cranial bones and contribute to lightening the skull and voice resonance

The Hyoid Bone and Fontanels

• Hyoid bone serves as a movable base for the tongue and aids in swallowing and speech; unique in that it does not articulate with other bones
• Fontanels in the fetal/neonatal skull allow brain growth and skull deformation during birth; fontanels convert to bone within ~$24$ months after birth

The Vertebral Column: Structure and Curvature

• Normal spinal curvatures include primary and secondary curves
  • Primary curvatures: thoracic and sacral (present at birth)
  • Secondary curvatures: cervical and lumbar (develop after birth)
• Curvatures such as scoliosis (lateral), kyphosis (thoracic), and lordosis (lumbar) are noted

Intervertebral Discs and Spinal Neighbors

• Intervertebral discs sit between vertebrae; they contain anulus fibrosus and nucleus pulposus components (as shown in the lumbar view diagrams)
• Vertebrae are named by region (cervical, thoracic, lumbar) and by specific position (e.g., atlas C1, axis C2 in cervical region)

The Skeletal System: Axial vs Appendicular

• Axial skeleton includes skull, vertebral column, and thoracic cage
• Appendicular skeleton includes girdles and limbs: pectoral girdle (clavicle and scapula), upper limbs (humerus, radius, ulna, carpals, metacarpals, phalanges), pelvic girdle (coxal bones), lower limbs (femur, tibia, fibula, talus, calcaneus, tarsals, metatarsals, phalanges)
• The adult skeleton contains 206 bones in total

Bone Cells and Remodeling

• Three main bone cell types:
  • Osteocytes: mature bone cells
  • Osteoblasts: bone-forming cells
  • Osteoclasts: bone-destroying cells
• Remodeling involves osteoblasts and osteoclasts working together to resorb and form bone; calcium release and bone density regulation are related to parathyroid hormone signals

Long Bone Formation and Growth

• Longitudinal growth occurs at the epiphyseal plates (growth plates):
  • Cartilage grows on the epiphyseal side; cartilage is eventually replaced by bone on the diaphyseal side
• The growth process involves:
  • Articular cartilage at the ends
  • A thriving remodeling process within the shaft
  • Growth in width via appositional growth (bone formation on the outer surface)
• In embryo/fetus, models start as hyaline cartilage or a cartilage model with a bone collar and early spongy bone formation

Ossification and Skeletal Development

• Bones begin as hyaline cartilage in embryos and are later replaced by bone through endochondral ossification; some flat bones arise from fibrous membranes via intramembranous ossification
• Epiphyseal plates allow for lengthwise growth in childhood; as growth ends, an epiphyseal line remains as a remnant
• Cartilage remains in isolated areas such as bridge of the nose, parts of ribs, and joints

Microscopic Anatomy of Bone

• Bone tissue features:
  • Osteocytes embedded in lacunae within lamellae
  • Central (Haversian) canal containing blood vessels and nerves
  • Canaliculi: tiny channels radiating from the central canal to lacunae, forming a transport system for nutrients
• Osteons (Haversian systems) are the basic units of compact bone; perforating (Volkmann's) canals connect osteons perpendicularly to the central canal
• Periosteum: outer fibrous membrane that covers the bone; inner osteogenic layer contains osteoblasts and is linked to Sharpey fibers that anchor to the bone
• Endosteum lines the medullary cavity and contains cells involved in growth and repair

Bone Markings and Bone Surface Features

• Projections (e.g., head, facet, condyle, ramus, trochanter, tuberosity, crest, spine) serve as sites of muscle and ligament attachment
• Depressions and openings (e.g., foramen, fossa, meatus, sinus, notch) provide passages for nerves and blood vessels
• Examples include features on the skull, vertebrae, and long bones

Summary of Bone Classifications and Functions

• Functions of the skeletal system include movement, support, protection of organs, mineral storage (calcium and phosphorus), blood cell formation (bone marrow), and hormonal signaling via osteoblast/osteoclast activity
• The science of bones is osteology

Learning Objectives (Reinforced)

• Group the bones into axial and appendicular divisions
• Name the cranial and facial bones, vertebral column, thorax, shoulder, upper limb, pelvis, and lower limb
• Discuss the functions of the skeletal system
• Classify bones by shape and location
• Describe histological features of bone tissue
• Recognize stages in bone fracture healing and types of fractures

Quick References and Numbers

• Vertebral counts: $7$ cervical, $12$ thoracic, $5$ lumbar; sacrum $5$ fused; coccyx $3-5$ fused
• Ribs: true $7$ pairs, false $3$ pairs, floating $2$ pairs
• Skull bones: 22 + 1 hyoid + 6 middle ear bones = 29 (excluding other tiny sutural bones) but total skull-related count is commonly cited as 28-29; overall body total is 206 bones
• Fontanels: typically close and convert to bone by around $24$ months
• Arches of the foot: two longitudinal arches and one transverse arch
• Pelvic inlet and pubic arch dimensions play a key role in differentiating male vs female pelvis

Notes on Figures and Slide References

• Many slides reference figures to illustrate: ossification centers, types of joints, anatomy of the knee, skull sutures, vertebral regions, and bones of the limbs
• Figures illustrate specific bones and landmarks (e.g., femoral head/neck, acetabulum, tibial tuberosity, calcaneus, talus, carpals, metacarpals, phalanges, scapula, clavicle, sternum, ribs, vertebrae, pelvis, skull sutures, fontanels, paranasal sinuses)

Quick Terminology Recap

• Osteology: study of bones
• Osteocytes, Osteoblasts, Osteoclasts: bone cell types
• Endochondral Ossification, Intramembranous Ossification: two primary bone formation processes during development
• Epiphyseal Plate/Line: site of longitudinal growth in bones; plate is hyaline cartilage; line is the reminant of growth after growth stops
• Intervertebral Discs: fibrocartilaginous joints between vertebrae
• Fontanel: soft spots on a infant skull that allow brain growth and skull molding during birth
• Hyoid Bone: unique bone that does not articulate with others; base for tongue
• Paranasal Sinuses: air-filled spaces that lighten the skull and modulate voice
• Pelvic Girdle: formed by ilium, ischium, and pubis; supports the upper body and bears weight
• Arch Types in Foot: medial and lateral longitudinal arches, transverse arch
• Joint Classifications: structural vs functional; fibrous, cartilaginous, synovial; synarthrosis, amphiarthrosis, diarthrosis