Unit 1.4 Bones

• Bone tissue


  Connective tissue
• Ground substance in bone

  Hardened due to mineralization (calcium phosphate)
• Bone classifications by shape


  • Flat bones
  • Long bones
• Epiphyses


  Ends of long bones
• Epiphyseal line


  Remnant of epiphyseal plate, basis of long bone growth
• Diaphysis


  Center portion of long bones
• Nutrient foramen


  Where nerve fibers and blood vessels enter marrow cavity
• Periosteum


  Fibrous outer layer that provides attachment points for tendons and ligaments, osteogenic inner layer important for bone growth and healing
• Perforating (Sharpey's) fibers


  Anchor periosteum to bone
• Flat bones create a sandwich of compact bone with spongy bone center
• Diploë


  Spongy bone center of flat bones
• Hematopoietic tissue


  Red marrow, found around spongy bone trabeculae, mostly in diploë of flat bones in adults
• Bone markings tell a story, many are found and studied in lab
• Bone cell lineages


  • Bone forming/maintaining cells (osteogenesis)
  • Bone breaking down cells (osteolysis)
• How osteoblasts form bone


  1. Bone deposition

  2. Ossification front

  3. Osteoblasts

  4. New osteoid (not mineralized yet)

  5. Older, mineralized bone
• Mineralization of bone


  1. Ossification front

  2. Older, mineralized bone

  3. Hydroxyapatite formation when appropriate Ca2+ and Pi levels are reached
• How osteoclasts remove bone


  1. Bone resorption

  2. Older, mineralized bone

  3. Osteoclasts

  4. H+ and lysosomal enzymes
• Osteon


  Structural unit of compact bone
• Osteocytes


  Bone cells in lacunae, connected by canaliculi for communication and nutrient/waste transfer
• Spongy bone


  • Irregular arrangement of lamellae in trabeculae
• Trabeculae develop along lines of stress, not randomly
• Intramembranous ossification


  1. Bone develops from fibrous membrane, flat bones of skull, most of clavicles, some of mandible

  2. Mesenchymal cells become osteoblasts and secrete osteoid

  3. Osteoid becomes mineralized

  4. Periosteum forms on outside

  5. Osteoblasts of periosteum deposit compact bone
• Endochondral ossification


  1. Bone replaces hyaline cartilage, all other bones

  2. Mesenchyme develops into cartilage model

  3. Primary ossification center forms, perichondrium becomes periosteum

  4. Chondrocytes die, blood vessels and osteoclasts invade

  5. Secondary ossification center(s) form(s)

  6. Cartilage remains only at epiphyseal plate and articular surfaces
• Postnatal bone growth


  1. Growth in length and width

  2. Epiphyseal plate = growth zone, marrow cavity increases, epiphysis increases
• Appositional growth of bone


  Osteoblasts beneath periosteum secrete matrix on external bone surface, osteoclasts on endosteal surface remove bone, buildup exceeds breakdown for stronger, thicker bone
• Bone remodeling occurs throughout growth to maintain proper proportions, collaboration between osteoblasts and osteoclasts
• Wolff's law


  Bone grows or remodels in response to demand (mechanical stress)
• Less bone is needed internally (medullary cavity) during bone remodeling
• Examples of Wolff's law


  • Differences in bone structure between people and chimps
• Opposite of Wolff's law is also true, bone loss in microgravity
• Calcium homeostasis


  Tightly regulated due to other functions, phosphate homeostasis not as much
• Active Vitamin D


  Required for mineral deposition and resorption in bone
• Calcium homeostasis hormones


  Parathyroid hormone, calcitonin
• Mechanism of calcium homeostasis hormones


  Stimulus for release, how they work
• Healing of bone fractures


  Reduction, hematoma formation, fibrocartilaginous callus, bony callus, remodeling
• Osteomalacia and rickets


  Poor mineralization during bone formation, "soft" bones that can "bow", from calcium or vitamin D deficiency
• Osteoporosis


  Weakening of previously constructed bone, compression fractures, kyphosis, affects women more due to sex hormone changes
• Gigantism


  Excessive growth due to hypersecretion of growth hormone
• Regulation of growth


  Hypothalamus secretes GHRH and GHIH, anterior pituitary secretes growth hormone, liver/bone/tissues produce IGFs to stimulate cartilage formation and bone growth
• Acromegaly


  Overgrowth of bones in hands, feet, and face after epiphyseal plate ossification