Human Anatomy & Physiology I: The Skeletal System

Functions of the Skeletal System

  • Support: The skeletal system acts as a structural framework for the whole body.
  • Protection: It provides a physical shield for the most critical internal organs.
  • Movement: When muscles contract, they pulling on the bones to produce movement.
  • Storage (Mineral and Fat): The system serves as a reservoir for minerals, specifically storing and releasing calcium and phosphate as needed.
  • Hematopoiesis: This is the process of blood cell formation, which occurs within the red bone marrow.
  • Acid-base Homeostasis: The system helps maintain the balance of acids and bases in the body.

Classification of Bones by Shape

  • Long Bones: These are defined as bones where the length is greater than the width. Examples include the femur and the humerus.
  • Short Bones: In these bones, the length is nearly equal to the width. Examples include the carpal bones (wrist) and tarsal bones (ankle).
  • Flat Bones: These feature thin surfaces and may be slightly curved. Examples include the bones of the cranium.
  • Irregular Bones: These have elaborate and sometimes complex shapes. Examples include the vertebrae and the sphenoid bone.
  • Contrast of Short, Flat, and Irregular Bones vs. Long Bones:
    • The external surface is composed of compact bone covered by periosteum.
    • The interior is composed of spongy bone.
    • These bones do not have a medullary cavity; only long bones possess this feature.

Macroscopic Anatomy of a Typical Long Bone

  • Diaphysis: This is the shaft of the bone. It is an elongated, usually cylindrical structure that provides leverage and support for weight. It consists of compact bone with thin spicules of spongy bone extending inward.
  • Epiphysis: The end of the bone, classified as either Proximal or Distal. It consists of a thin outer layer of compact bone and an inner region of spongy bone.
  • Metaphysis: The region located between the diaphysis and the epiphysis.
  • Epiphyseal Line and Plate:
    • Epiphyseal Plate: A thin layer of hyaline cartilage located within the metaphysis of growing bones that provides for lengthwise growth.
    • Epiphyseal Line: The remnant of the epiphyseal plate found in adult bones once growth has ceased.
  • Articular Cartilage: A layer of hyaline cartilage covering the epiphysis at joint surfaces. It reduces friction and absorbs shock in movable joints.
  • Periosteum: A tough outer covering of the external bone surface consisting of dense irregular connective tissue. It protects the bone, anchors blood vessels and nerves, and is attached to the bone via collagen fibers known as perforating fibers.
  • Endosteum: A thin layer of connective tissue covering all internal surfaces within the medullary cavity, containing osteoprogenitor cells and osteoblasts.
  • Medullary (Marrow) Cavity: A hollow, cylindrical space within the diaphysis. It contains red bone marrow in children and yellow bone marrow in adults.
  • Bone Marrow Types:
    • Red Bone Marrow: This is hematopoietic (blood-cell forming) tissue composed of reticular connective tissue and adipocytes. In children, it is found in the spongy bone and medullary cavity. In adults, it is restricted to parts of the axial skeleton (skull, vertebrae, ribs, sternum, hip bones) and the proximal epiphyses of the humerus and femur.
    • Yellow Bone Marrow: A fatty substance that results from the degeneration of red marrow as children mature. It can convert back to red bone marrow during severe anemia to facilitate erythrocyte production.

Components of Osseous Tissue and Bone Cells

  • Inorganic Matrix: This component interacts with calcium hydroxide to form salt crystals called calcium phosphate. Other substances included in these crystals are calcium carbonate, sodium, magnesium, sulfate, and fluoride. These crystals deposit around collagen fibers to harden the matrix and provide rigidity.
  • Organic Matrix: Often called the osteoid, this is produced by osteoblasts and contains collagen protein and a semisolid ground substance (proteoglycans and glycoproteins). It provides tensile strength, resisting stretching, and contributes to flexibility.
  • Bone Cell Types:
    • Osteocytes: Mature bone cells derived from osteoblasts that live inside lacunae. They detect stress on the bone and trigger the formation of new bone.
    • Osteoblasts: Immature cells (often referred to as "babies") that develop from osteoprogenitor stem cells. They synthesize and secrete the osteoid. Eventually, they become trapped in the matrix and differentiate into osteocytes.
    • Osteoclasts: Large, multinucleated, phagocytic cells derived from fused bone marrow cells. They feature a ruffled border to increase surface area for bone exposure and are located in pits on the bone surface. They are responsible for bone resorption (breakdown).

Microscopic Structure of Compact and Spongy Bone

  • Compact Bone:
    • Dense outer layer of the bone.
    • Organized into functional units called Osteons (Haversian systems).
    • Contains central canals for blood vessels and nerves.
  • Spongy Bone:
    • Inner, lightweight portion of the bone.
    • Composed of a lattice of "little bone beams" called trabeculae.
    • Contains red bone marrow.
    • Lacks osteons and central canals.
  • Detailed Osteon Components:
    • Osteon (Haversian System): The basic unit of compact bone, resembling a tree trunk.
    • Lamellae: Concentric rings of bone connective tissue matrix surrounding the central canal.
    • Central (Haversian) Canal: A hole in the center of the osteon containing blood vessels and nerves.
    • Lacunae: Tiny spaces found between the lamellae where ostecytes reside.
    • Canaliculi: Tiny channels that connect lacunae, allowing osteocytes to communicate and share nutrients.

Comparison of Bone Formation (Ossification)

  • Intramembranous Ossification:
    • Bone forms directly from mesenchyme (no cartilage precursor).
    • Produces flat bones of the skull, some facial bones, the mandible, and the center of the clavicle.
    • Steps: Formation of ossification center (Mesenchyme → Osteoprogenitor → Osteoblast); Calcification of osteoid (Osteoblasts become trapped and turn into osteocytes); Formation of woven bone (immature) and periosteum; Replacement of woven bone with lamellar (mature) bone.
  • Endochondral Ossification:
    • Bone replaces a hyaline cartilage model.
    • Produces most bones in the body.
    • Steps: Hyaline cartilage model forms; Cartilage calcifies and a bone collar forms (chondrocytes die); Primary ossification center forms in the diaphysis (shaft); Secondary ossification centers form in the epiphyses (ends); Bone replaces cartilage everywhere except the articular cartilage and the epiphyseal growth plates.

Bone Growth and Hormonal Regulation

  • Interstitial Growth (Length): Occurs at the epiphyseal plate in the metaphysis. Chondrocytes divide and enlarge, pushing the ends away from the shaft. Cartilage is replaced by bone. This continues until late teens or early 20s when the plate ossifies into a line.
  • Appositional Growth (Width): Occurs at the periosteum. Osteoblasts add bone to the outside while osteoclasts remove bone from the inside of the medullary cavity, thickening the bone while maintaining the marrow space.
  • Hormonal Influences:
    • Growth Hormone (GH): Stimulates growth of bone and cartilage; promotes length.
    • Insulin-like growth factors (IGFs): Produced by the liver via GH stimulation to trigger epiphyseal plate growth.
    • Thyroid Hormone: Regulates bone metabolism and growth plate activity.
    • Estrogen and Testosterone: Trigger puberty growth spurts but eventually cause the epiphyseal plates to close.
    • Glucocorticoids: High levels slow growth and increase bone loss.
    • Serotonin: High levels can reduce osteoblast formation and lower bone density.

Bone Remodeling and Homeostasis

  • Remodeling: Continuous process of old bone removal (resorption) and new bone addition (deposition). It repairs damage and maintains calcium levels.
  • Bone Deposition: Osteoblasts add minerals and osteoid.
  • Bone Resorption: Osteoclasts break down bone to release calcium and phosphate into the blood.
  • Fracture Repair Steps: 1. Hematoma → 2. Soft callus → 3. Hard callus → 4. Remodeling.
  • Calcium Homeostasis:
    • Low Blood Calcium: Parathyroid Hormone (PTH) is released → increases osteoclast activity → increases resorption → raises blood calcium.
    • High Blood Calcium: Calcitonin is released → decreases osteoclast activity → calcium is stored in bone → blood calcium falls.
  • Diet and Age:
    • Vitamin D is required for calcium absorption.
    • Vitamin C is required for collagen production.
    • As adults age, bone resorption begins to exceed deposition, leading to loss of mass.

Divisions of the Skeletal System

  • Axial Skeleton (80 bones): Supports and protects the brain, spine, and thoracic organs. Includes the skull, thoracic cage (ribs/sternum), and vertebral column.
  • Appendicular Skeleton (126 bones): Primarily for movement. Includes the limbs and the girdles that attach them to the axial skeleton.
    • Pectoral Girdle: Scapula and Clavicle.
    • Upper Limb: Humerus, Ulna, Radius, Carpals, Metacarpals, Phalanges.
    • Pelvic Girdle: Coxal bones.
    • Lower Limb: Femur, Tibia, Fibula, Tarsals, Metatarsals, Phalanges.

Regional Anatomy and Bone Markings

  • Skull Markings: Mastoid process, Styloid process, Foramen magnum, Occipital condyles, External acoustic meatus, Zygomatic arch, Mental protuberance, Alveolar process.
  • Vertebrae Markings: Body, Spinous process, Transverse process, Vertebral foramen, Intervertebral foramen, Dens (C2), Transverse foramina (Cervical).
  • Appendicular Markings: Acromion, Coracoid process, Glenoid cavity, Greater/Lesser tubercles, Deltoid tuberosity, Trochlea, Capitulum, Olecranon, Styloid process (radius/ulna), Greater/Lesser trochanter, Tibial tuberosity, Medial/Lateral malleoli.
  • Orbital and Nasal Features:
    • Orbit: Encloses and protects eyes.
    • Nasal Cavity: Encloses nasal passages.
    • Paranasal Sinuses: Air-filled chambers that humidify/warm air, lighten the skull, and provide voice resonance.
  • Vertebral Column Classes:
    • Cervical: Small, light, have transverse foramina. C1 (Atlas) supports the head ("Yes" motion). C2 (Axis) has the dens ("No" motion).
    • Thoracic: Have costal facets for ribs.
    • Lumbar: Largest bodies for weight bearing.
    • Sacrum: 5 fused vertebrae.
    • Coccyx: 4 fused vertebrae (tailbone).
  • Intervertebral Discs: Composed of an outer annulus fibrosus (fibrocartilage) and an inner nucleus pulposus (gel-like). They absorb shock and allow spinal bending.

Classification and Dynamics of Joints

  • Functional Classification (Degree of Movement):
    • Synarthroses: Immovable (e.g., skull sutures, teeth joints).
    • Amphiarthroses: Slightly mobile (e.g., symphysis, syndesmoses).
    • Diarthroses: Freely mobile (all synovial joints).
  • Structural Classification (Anatomical Features):
    • Fibrous: Bones held by dense regular connective tissue; no joint cavity. Types: Gomphosis (teeth), Suture (skull), Syndesmosis (interosseous membrane).
    • Cartilaginous: Held by cartilage; no joint cavity. Types: Synchondrosis (hyaline cartilage), Symphysis (fibrocartilage pad).
    • Synovial: Contains a joint cavity and is freely movable (diarthrosis). Features: Articular cartilage, Articular capsule (fibrous layer and synovial membrane), Synovial fluid, and Ligaments.
  • Synovial Joint Types (By Shape and Movement):
    • Uniaxial (1 plane): Plane (gliding), Hinge (convex/concave), Pivot (rotation).
    • Biaxial (2 planes): Condylar (oval convex/concave), Saddle.
    • Multiaxial (3 planes): Ball and socket (spherical head/socket; most mobile, e.g., shoulder/hip).
  • Joint Movements:
    • Flexion/Extension: Decreasing/increasing the angle between bones.
    • Abduction/Adduction: Moving away from/toward the midline.
    • Plantar flexion/Dorsiflexion: Pointing toes down/moving toes toward leg.
    • Pronation/Supination: Palm posterior (down)/Palm anterior (up).
    • Inversion/Eversion: Turning the sole medially/laterally.