Bone development and Joints

• Ossification (osteogenesis)


  Process of bone tissue formation
• Ossification


  1. Formation of bony skeleton

  2. Begins in 2nd month of development

  3. Postnatal bone growth until early adulthood

  4. Bone remodeling and repair lifelong
• Types of ossification

  • Endochondral ossification
  • Intramembranous ossification
• Endochondral ossification

  • Skeletal system starts off as bones made up of hyaline cartilage (cartilage bones)
  • At 9 weeks bone tissue starts to replace hyaline cartilage tissue
  • Continues through adolescence
  • Dominant form of ossification, most bones form from this process
• Intramembranous ossification

  • Bones develops from fibrous membrane
  • Develop from mesenchymal cells (undifferentiated connective tissue)
  • Most flat bones develop from this process, including skull bones, clavicles and manubrium of sternum
• Endochondral ossification

  1. Formation of endochondral bone

  2. Formation of perichondrium

  3. Formation of the bone collar

  4. Development of bone cavities

  5. Bone elongation
• Intramembranous ossification

  1. Formation of fibrous membrane

  2. Appearance of ossification centers

  3. Osteoid secretion and calcification

  4. Woven bone and periosteum formation

  5. Lamellar replacement of woven bone and red marrow formation
• Postnatal growth

  • Interstitial growth
  • Appositional growth
• Interstitial growth


  • AKA longitudinal growth
  • Growth in length of long bones
  • Requires the presence of epiphyseal plates
  • Only occurs from childhood to adolescents
• Appositional growth

  • Increase in bone thickness
  • Occurs throughout life
  • Osteoblasts beneath periosteum secrete bone matrix on external bone
• Interstitial growth

  1. Resting (quiescent) zone

  2. Proliferation (growth) zone

  3. Hypertrophic zone

  4. Calcification zone

  5. Ossification (osteogenic zone)
• Epiphyseal plate closure

  • Bone lengthening ceases
  • Bone of the epiphysis and diaphysis fuses
  • Females ~ 18 yo
  • Males ~ 21 yo
• Appositional growth

  • Occurs through bone remodeling
  • Occurs throughout life
  • Helps with reinforcement when a bone is stressed
• Osteoblasts

  Add osteoid beneath periosteum on external surface of the bone
• Osteoclasts

  Remove bone from the endosteum side
• Hormonal regulation of bone growth

  • Growth hormone
  • Thyroid hormone
  • Testosterone (males) and estrogen (females) at puberty
• Hyposecretion and Hypersecretion

  Of any of these hormones can result in abnormal skeletal growth
• Bone homeostasis

  • We recycle about 5-7% of bone mass each week
  • Older bone becomes more brittle
• Bone remodeling

  1. Bone deposit and bone resorption happening simultaneously

  2. Occurs at the surfaces of both periosteum and endosteum
• Osteoclast function

  • Dig grooves in bone as they break down the matrix
  • Secrete lysosomal enzymes that digest matrix and protons (H+) to convert calcium salts into soluble forms
  • Phagocytize demineralized matrix and dead osteocytes
  • Die through apoptosis once resorption is complete
• Osteoid seam


  Unmineralized band of bone
• Calcification front


  Abrupt transition zone between osteoid seam and older mineralized bone
• Mechanical signals


  Involved in the calcification process, the more stress the faster calcification occurs
• Parathyroid hormone


  Stimulates osteoclasts when blood calcium levels are low
• Calcitonin


  Inhibits osteoclasts, allowing osteoblasts to outpace
• Importance of calcium


  • Functions in nerve impulse transmission, muscle contraction, blood coagulation, secretion by glands and nerve cells, and cell division
  • 1200–1400 grams of calcium in body, 99% as bone minerals, amount in blood tightly regulated
• Wolff's Law


  Bones grow or remodel in response to the demands placed on them
• Bone repair - Fractures

  1. After fracture: blood vessels hemorrhage, hematoma forms

  2. Callus formation: blood vessels regrow, fibroblasts release collagen, osteoblasts lay down new osteoid

  3. Bony callus formation: new trabeculae and spongy bone created

  4. Bone remodeling: bone retakes normal form due to Wolff's law
• Homeostatic imbalances

  • Osteomalacia
  • Rickets
• Osteomalacia

  Bones poorly mineralized, calcium salts not adequate, soft, weak bones, pain upon bearing weight
• Rickets

  Bowed legs and other bone deformities, bone's ends enlarged and abnormally long
• Steps of Repair

  1. After fracture: Blood vessels in bone and periosteum hemorrhage (break), Causes a hematoma to form

  2. Callus formation: Blood vessels regrow into the hematoma, Fibroblasts release collagen fibers to cover the break and reconnect the bones, Osteoblasts start laying down new osteoid

  3. Bony callus formation: New trabeculae starts to form and spongy bone is created

  4. Bone remodeling: Bone retakes its normal form due to Wolff's law, as long as the subject returns to normal activity bone will look like it did before
• Rickets (osteomalacia of children)


  Bowed legs and other bone deformities, Bone's ends enlarged and abnormally long
• Causes of osteomalacia and rickets

  • Vitamin D deficiency
  • Insufficient dietary calcium
  • Kidney disorders
  • Liver disorders
  • Genetic factors
• Osteoporosis


  Group of diseases, Bone resorption outpaces bone deposit, Usually results in brittle bones
• Prevention of osteoporosis

  • Have plenty of calcium in your diet, Especially in early adulthood
  • Reduce carbonated beverage and alcohol consumption, Leaches bone minerals
  • Due plenty of weight-bearing exercises, Wolff's law
• Paget's Disease


  Excessive of haphazard bone deposits, Bone made fast and poorly (called pagetic bone), Usually results a higher spongy to compact bone ratio, Rarely occurs before age 40, Cause unknown, but believed to be viral, Treatment includes calcitonin supplements
• Articulation


  Site where two or more bones meet
• Functions of joints

  • Give the skeleton mobility
  • Hold skeleton together
• Types of joint classifications

  • Functional
  • Structural