Skeleton

• Skeletal System

  Composed of: Bones, cartilage, ligaments and other connective tissues that connect and stabilize the bones
• Skeletal System


  • Used for: Support, protection, movement, hemopoiesis, storage (mineral and energy)
• Ligament


  Dense regular connective tissue that anchors bone to bone
• Tendon


  Anchors muscle to bone
• Parts of skeletal system

  • Ligament
  • Cartilage
• Types of cartilage

  • Hyaline cartilage
  • Fibrocartilage
  • Articular cartilage
  • Costal cartilage
• Skeletal cartilage

  • Dense extracellular matrix that contains a lot of water
  • Become more gel like
  • Water makes it highly resilient
  • Able to absorb and distribute force
  • Extracellular matrix lacks blood vessels or nerves
• Perichondrium


  Dense connective tissue that encapsulates cartilage
• Perichondrium

  • Highly vascularized for nutrient delivery to cartilage
  • Helps cartilage resist outward expansion
• Lacuna


  Open space in extracellular matrix to house chondrocytes
• Chondrocytes


  Cartilage forming cells
• Cartilage growth pathways

  • Appositional growth
  • Interstitial growth
• Appositional growth

  Due to cartilage forming cells in the perichondrium
• Appositional growth

  1. Stem cells in the perichondrium divide into chondroblasts

  2. Chondroblasts create new matrix towards the already existing cartilage

  3. Adds to the surface
• Interstitial growth

  • Chondrocytes within matrix divide
  • The two new cells start adding matrix and begin separating
• There are 206 named bones
• Subgroups of bones

  • Axial skeleton
  • Appendicular skeleton
• Axial skeleton


  • Long axis of the body
  • Skull, vertebral column and ribcage
  • Used more for structure, support and protection
• Appendicular skeleton


  • Bones of upper and lower limbs
  • Girdles attaching limbs to the axial skeleton
  • Pelvis and pectoral
  • Used more for movement
• Classification of bones by shape

  • Long
  • Short
  • Flat
  • Irregular
• Functions of bone

  • Support
  • Protection
  • Movement
  • Storage
  • Hematopoiesis
  • Hormone production
• Bone


  Organ containing different types of tissue: bone (osseous) tissue, nervous tissue, cartilage, fibrous connective tissue, muscle and epithelial cells
• Levels of bone structure

  • Gross anatomy
  • Microscopic
  • Chemical
• Compact bone


  Dense outer layer, solid, makes up 80% of bone mass
• Spongy bone


  Deep to compact bone, honeycomb appearance, made up of pillars called trabeculae, open spaces filled with marrow, blood vessels and nerves
• Short, irregular and flat bones

  • Composed of a plate of spongy bone (diploe) covered by compact bone
  • Contains two membrane layers of connective tissue: periosteum and endosteum
  • Marrow scattered throughout the spongy bone
  • Does not have a defined marrow cavity like long bones
  • Hyaline cartilage can be found covering articulating surfaces
• Long bones

  • Separated into two regions: diaphysis (shaft) and epiphysis (ends)
  • Diaphysis has compact bone surrounding a medullary cavity
  • Epiphysis has compact bone with internal spongy bone, contains articular cartilage and epiphyseal line
• Periosteum

  • Double layered membrane that covers external surfaces except joints
  • Outer fibrous layer of dense irregular connective tissue
  • Inner osteogenic layer of stem cells (osteogenic cells)
  • Contains many nerve fibers and blood vessels
  • Acts as an anchoring point for tendons and ligaments
• Endosteum

  • Delicate connective tissue membrane covering internal bone surface
  • Covers trabeculae of spongy bone
  • Lines central canals that pass through compact bone
  • Contains osteogenic cells that can differentiate into other bone cells
• Red marrow


  Hematopoietic tissue found in trabecular cavities of spongy bone and medullary cavity of newborns
• Yellow marrow


  Found in medullary cavity of adults, stores triglycerides (fat)
• Bone cell types

  • Bone-lining cells
  • Osteogenic cells
  • Osteoblasts
  • Osteocytes
  • Osteoclasts
• Bone-lining cells

  Inactive osteoblasts that can become activated if needed
• Osteogenic cells

  • Mitotically active stem cells of bone membranes (periosteum and endosteum)
  • One cell becomes another osteogenic cell while the other becomes an osteoblast
• Osteoblasts

  • Bone-forming cells that secrete unmineralized bone matrix (osteoid)
  • Collagen and proteins for calcium binding
  • Actively mitotic
  • Becomes an osteocyte once mature
• Osteocytes

  • Mature bone cells in lacunae
  • Monitor and maintain matrix
  • Senses stress or strain
  • Communicates mechanical stimuli to osteoblasts and osteoclasts
• Osteoclasts


  • Break down calcium matrix
  • Derived from hematopoietic stem cells that create macrophages
  • Plasma membrane is ruffled to increase surface area
  • Increase interaction of bone and digestive enzymes that break down bone
• Compact bone


  • Made up of repeating structural units called osteons
  • Elongated cylinders parallel to long axis of bone
  • Composed of rings of matrix (lamellae) surrounding a central canal (Haversian canal)
  • Central canal acts as a channel for blood vessels and nerves
  • Lamellae have collagen running in different directions to increase resistance to stress
  • Lamellae contain lacunae and canaliculi to allow nutrient passage
  • Central canals connected through perforating canals (Volkmann's canals)
• Canaliculi formation


  1. Osteocytes make dendritic structures with their plasma membrane

  2. Adjacent cells bind tightly together with gap junctions

  3. As matrix is added it separates the cells stretching the dendritic projections

  4. The matrix hardens and the cells are trapped

  5. Cells can then pass nutrients and waste to one another
• Spongy bone


  • Appears like poorly organized pillars of trabeculae
  • Trabeculae grow roughly in direction of most stress
  • Contain lacunae with chondrocytes and canaliculi
  • Lack a central canal, instead capillaries in the endosteum supply nutrients