skeletal system

Skeletal System Overview

  • Instructor: Prof. B. Estrella
  • Course: A&P1
  • Source: IT

What is a Bone?

  • Definition:
    • Bones are the organs of the skeletal system.
    • They comprise the skeletal system alongside cartilage, tendons, and ligaments.
    • Note: Tendons are associated with both the muscular and skeletal systems.

Tissues Found in Bones

  • Epithelial Tissue:
    • Present? No.
  • Connective Tissue:
    • Present? Yes!
    • Reason: Forms the structure of bones.
  • Nervous Tissue:
    • Present? Not really.
    • Explanation: Sensory nerves accompany blood vessels entering bones via foramina, innervating and vascularizing the periosteum, endosteum, and marrow.
  • Muscular Tissue:
    • Present? No.
    • Reason: Muscles attach to bones but are not part of bones.

Types of Bones

  • Categories of Bones:
    • Long Bone
    • Short Bone
    • Irregular Bone
    • Flat Bone

Functions of Bones

  • Support:
    • Provides support to the body.
  • Hematopoiesis:
    • Produces blood cells in red and yellow bone marrow.
  • Protection:
    • Shields internal organs and soft tissues.
  • Mineral Storage:
    • Stores calcium and phosphorus.

Bone Growth

  • Initial Count:
    • Infants have approximately 300 bones; adults have 206.
  • Composition:
    • Infant bones are primarily cartilage, which is replaced by hard bone through the deposition of calcium.
  • Growth Area:
    • Bones grow at the growth plate at each end.
  • Growth Cessation:
    • Growth stops around ages 25 to 30 as growth plates seal, leaving epiphyseal lines.

Continued Growth and Remodeling

  • Even after length growth ceases, bones can increase in thickness or diameter.
    • Response to stress from increased muscle activity or weight.
  • Last Bone to Mature:
    • The clavicle.

Everyday Life of Bones

  • Dynamic Nature:
    • Bones may appear hard and dry, resembling non-living objects, but they are dynamic and constantly remodeled.
  • Resorption:
    • The process of breaking down old bone to release calcium and phosphorus into the bloodstream, particularly when calcium levels are low.

Bone Remodeling

  • Definition:
    • A complex process where old bone is replaced with new bone.
  • Necessity:
    • Important for:
    • Fracture repair.
    • Mechanical adaptation.
    • Calcium homeostasis.
  • Age-Related Changes:
    • After age 30, the resorption rate exceeds new bone creation, leading to bone mass loss.
  • Preventing Bone Loss:
    • Regular exercise, avoiding alcohol, a diet high in calcium, and maintaining normal Vitamin D levels.

Stages of Embryonic Development

  • Stages:
    • Zygote
    • 2 Cell Stage
    • 4 Cell Stage
    • 8 Cell Stage
    • Morula (72 hours)
    • Blastocyst (5 days)

Embryonic Layers

  • Germ Layers:
    • Ectoderm:
    • Develops into the central and peripheral nervous systems, sensory organs, epidermis, hair, and nails.
    • Mesoderm:
    • Forms skeleton, muscles, connective tissues, heart, blood vessels, and kidneys.
    • Endoderm:
    • Forms internal organs, parts also derived from ectoderm and mesoderm.

Ossification Process

  • Phases of Ossification:
    • Step 1: Degenerating hypertrophic cartilage.
    • Step 2: Perichondrium and capillary development.
    • Step 3: Formation of primary ossification center in the diaphysis.
    • Step 4: Formation of secondary ossification centers in epiphyses.
    • Step 5: Bone replaces cartilage in all areas except articular cartilage and epiphyseal plates.
    • Step 6: Osteogenesis concludes with ossification of epiphyseal plates into epiphyseal lines.

Types of Bone Cells

  • Osteogenic Cells:
    • Undifferentiated cells that can develop into osteoblasts.
  • Osteoblasts:
    • Form bone tissue, secrete matrix, cannot divide, and mature into osteocytes.
  • Osteocytes:
    • Mature bone cells involved in monitoring bone health and orchestrating remodeling.
  • Osteoclasts:
    • Fused white blood cells involved in bone resorption, critical for breakdown and renewal of bone tissue.

Bone Homeostasis

  • Equilibrium Maintenance:
    • A balance between osteoblasts (bone formation) and osteoclasts (bone resorption) is vital to maintain bone tissue health.

Bone Repair Mechanism

  • Repair Process:
    • Begins about a week post-injury.
    • Soft Callus Formation:
    • A soft callus replaces the blood clot formed during the inflammation phase.
    • The callus stabilizes the bone but lacks the strength needed for full use.
    • Gradually, the soft callus hardens over subsequent weeks.

Bone Marrow: Structure and Function

  • Definition:
    • A soft, jelly-like tissue located in the spongy bone of the marrow cavity of long bones and select others.

Red Bone Marrow

  • Function:
    • Responsible for hematopoiesis (blood cell formation).
  • Components:
    • Hematopoietic stem cells (HSCs) differentiate into:
    • Red blood cells
    • White blood cells
    • Platelets

Changes in Bone Marrow with Age

  • Children:
    • Possess mostly red marrow, found throughout most bones.
  • Adults:
    • Red marrow is replaced by yellow marrow with maturity.
    • Appearance of yellow marrow attributed to the abundance of adipose cells.
  • Location of Red Marrow in Adults:
    • Found primarily in:
    • Skull
    • Vertebrae
    • Ribs
    • Sternum
    • Proximal femur and humerus

Yellow Bone Marrow

  • Function:
    • Serves primarily as fat storage.
    • Ceases hematopoiesis, except in emergencies.

Microscopic Bone Structure

  • Compact Bone:
    • Composed of Haversian systems.
    • Features a central canal (Haversian canal) surrounded by concentric rings of matrix.
    • Contains osteocytes within their lacunae.

Osteocyte Nourishment

  • Nutrition Mechanism:
    • Through a network called the lacunar-canalicular system, which interconnects canals (canaliculi) transporting nutrients from blood vessels to osteocytes situated in lacunae.

Haversian System

  • Description:
    • Composed of Haversian canals within compact bone, located internally to the periosteum and endosteum.

Structure of Long Bone

  • Layers (Superficial to Deep):
    • Periosteum
    • Compact Bone
    • Endosteum
    • Spongy Bone
    • Articular Cartilage
    • Metaphysis
    • Diaphysis
    • Medullary Cavity containing yellow marrow

Periosteum

  • Function and Structure:
    • Covers the outer surface of bones (except at joint surfaces) and contains blood vessels, nerves, and lymphatics necessary for bone nourishment.
    • Tendons and ligaments attach to it.

Endosteum

  • Definition:
    • Membrane lining the marrow cavity and Haversian canals.

Spongy Bone Structure

  • Comparison with Compact Bone:
    • Situated deep to compact bone, less hard than compact but still firm.

Joints

  • Definition:
    • Points where two bones connect.
  • Characteristics:
    • Smooth cartilage facilitates movement.
    • Some joints are movable, while others are fixed.

Joints Structure

  • Components of Synovial Joint:
    • Smooth cartilage
    • Synovial membrane (produces synovial fluid)
    • Ligaments

Ligaments vs. Tendons

  • Ligament:
    • Connects bone to bone (the 'L' is in 'b').
  • Tendon:
    • Connects muscle to bone.

Joints Classification

  • Structural Types of Joints:
    • Fibrous Joints:
    • Immovable or slightly movable, connected by fibrous connective tissue (e.g., skull sutures).
    • Cartilaginous Joints:
    • Partially movable, joined by cartilage (e.g., intervertebral discs).
    • Synovial Joints:
    • Freely movable with fluid-filled capsules (e.g., knee joint).

Functional Classification of Joints

  • Synarthrosis:
    • Immovable joints.
  • Amphiarthrosis:
    • Slightly movable joints.
  • Diarthrosis:
    • Freely movable joints.

Types of Synovial Joints

  • Ball-and-Socket:
    • One bone fits into a socket-like depression in another (e.g., hip, shoulder).
  • Hinge:
    • One bone moves back-and-forth like a door (e.g., elbow, knee).
  • Pivot:
    • One bone rotates around another (e.g., neck).
  • Gliding:
    • Bones slide over each other (e.g., wrist).
  • Condyloid:
    • Oval-shaped movement (e.g., wrist).
  • Saddle:
    • Combination of movements (e.g., thumb).

Bone Disorders

  • General Information:
    • Disorders are often due to abnormalities in bone resorption.
    • Men's bones are typically denser than women's due to testosterone effects.
    • Men experience faster healing; however, they are also more likely to sustain fractures due to sports activities.

Osteoporosis

  • Definition:
    • Condition that weakens bones, making them brittle.
  • Risk Factors:
    • Can result in fractures from moderate stress (bending or coughing).
    • Affects all races but is most prevalent in post-menopausal white and Asian women.

Osteopenia

  • Description:
    • A precursor to osteoporosis, involves slower bone formation than resorption.
    • Causes:
    • Low calcium intake
    • Smoking
    • Estrogen deficiency
    • Certain medications

Osteopetrosis

  • Description:
    • A rare inherited disorder leading to bone brittleness and deformities.
    • May cause scoliosis, vision problems, fracture risks, infections, and stunted growth.

Osteolysis

  • Description:
    • Condition where bones become thin and weak, often associated with the use of prosthetics (referred to as "cement disease").

Bone Cancer

  • Description:
    • An abnormal growth of tumors in bone tissue, possible causes include:
    • Genetic conditions
    • Radiation therapy
    • Previous tumors
    • Metastasis from other cancers

Ainhum

  • Description:
    • A rare condition causing autoamputation of a toe due to pressure from toughening tissues.
    • Symptoms begin with slight pain due to nerve pressure.
    • Prevalent among individuals of African descent and those with a history of walking barefoot since childhood.