1. Initiation of tooth development
   1. begins at 37 days of development with formation of primary epithelial bands in upper and lower jaws
      1. each band gives rise to 2 sub-divisions
         1. dental lamina
            1. appears as thickening of oral epithelial adjacent to condensation of ectomesenchyme
            2. 20 areas of enlargement that will form tooth buds
            3. not all will develop at the same time
            4. even at this early stage, crown morphology is already determined
         2. vestibular lamina
2. Bud stage
   1. rounded, localized growth of epithelium surrounded by proliferating mesenchymal cells packed closely beneath and around epithelial buds
   2. enamel organ consists of peripherally located low columnar cells and centrally located polygonal cells
3. Cap stage
   1. condensation of ectomesenchyme immediately subjacent to tooth bud
   2. histodifferentiation begins
   3. enamel organ forms from epithelial outgrowth
      1. enamel organ is avascular and nerves do not enter it
   4. dental papilla forms as ball of condensed ectomesenchymal cells
      1. will form dentin and pulp
      2. peripheral cells adjacent to inner dental epithelium will enlarge and later differentiate into odontoblasts
   5. dental follicle or sac is condensed ectomesenchymal tissue surrounding enamel organ and dental papilla
      1. gives rise to cementum and periodontal ligament
   6. lateral lamina is an extension from dental lamina that is connected to enamel organ
   7. organization of inner and outer dental epithelium
   8. enamel knot is densely packed accumulation of non-dividing cells projecting from inner enamel epithelium into dental papilla
      1. thought to be center for cusp development
         1. cells of inner dental epithelium start proliferating at this area denoted by the enamel knot
      2. temporary structure that is thought to act as reservoir of dividing cells for growing enamel organ
4. Bell stage
   1. continued growth bringing enamel organ to resemble a bell with deepening of the epithelium over the dental papilla
   2. histodifferentiation continues defining ameloblasts and odontoblasts
   3. morphodifferentiation begins (tooth crown assumed final shape)
   4. inner dental epithelium is made up of short columnar cells that border the dental papilla
      1. these cells eventually become ameloblasts that will form the enamel of the tooth crown by differentiating into tall columnar cells
      2. these cells also exert an organizing influence on the underlying mesenchymal cells in the dental papilla
         1. later differentiate into odontoblasts
   5. outer dental epithelium made up of cuboidal cells that cover the enamel organ
      1. organize network of capillaries that will bring nutrition to the ameloblasts
   6. stellate reticulum made up of star-shaped cells with processes that present between the outer and inner dental epithelium
      1. secrete glycosaminoglycans that attract water and swell the cells pushing them apart
      2. still maintain contact with one another thus causing them to become star-shaped
         1. allows them to have a cushion-like consistency that may support and protect the delicate enamel organ
   7. stratum intermedium is cell layer between inner dental epithelium and stellate reticulum that has high alkaline phosphatase activity
      1. assist inner dental epithelium in formation of enamel
   8. peripheral cells of mesenchymal dental papilla differentiate into odontoblasts under organizing influence of inner dental epithelium
      1. assume cuboidal shape, then tall columnar form and acquire specific potential to produce dentin
   9. basement membrane called membrana preformativa separates enamel organ and dental papilla
   10. cervical loop is an area where inner and outer dental epithelium meet at rim of enamel organ
       1. cells continue to divide here until tooth crown attains full size and which after crown formation will give rise to epithelium for root formation
       2. also called the zone of reflection
   11. enamel cord is pattern of enamel knot that extends between innter and outer dental epithelium
   12. dental lamina disintegrates and loses contact with oral epithelium
       1. if these cells persist, cysts can develop in them and prevent eruption, cause tumors, or supernumary teeth
   13. hard tissue formation
       1. for dentinogenesis and amelogenesis to take place normally, the differentiating odontoblasts and ameloblasts will receive signals from each other
          1. reciprocal induction
       2. stages of apposition
          1. elongation of inner dental epithelium
             1. show reverse polarization
             2. nuclei adjacent to stratum intermedium instead of the basement membrane
             3. causes differentiation of odontoblasts
          2. differentiation of odontoblasts
             1. induced by influence from inner dental epithelium to differentiate from ectomesenchymal cells of dental papilla
          3. formation of dentin - largely organic
             1. 1st layer appears at cusp tip and progresses cervically
             2. appearance of large-diameter type III collagen fibrils called Korff's fibers, followed by type I collagen fibers or mantle dentin
             3. 2 step process
             4. formation of collagen matrix
             5. deposition of calcium and phosphate crystals in the matrix
          4. odontoblasts develop stubby processes at side close to inner dental epithelium that extend into forming extracellular matrix
          5. odontoblasts move pulpward, and odontoblast processes (Tomes' fibers) will elongate and become active in dentin matrix formation
             1. initially called predentin before mineralization
          6. differentiation of ameloblasts
          7. formation of enamel proteins that further help with terminal differentiation of odontoblasts
          8. formation of enamel - largely inorganic
             1. 2 step process
             2. production of partially mineralized matrix
             3. influx of additional mineral
             4. Tomes' processes develop at apical end of ameloblasts and as they migrate outward, enamel is left behind
       3. boundary between odontoblasts and inner dental epithelium defines future dentino-enamel junction
5. vascular supply
   1. clusters of blood vessels in dental follicle and papilla
      1. roots form where clustering of vessels appears
6. nerve supply
   1. nerves initially seen in dental follicle during bud to cap stage
   2. after start of dentinogenesis, nerves are also found in the dental papilla
7. formation of permanent dentition
   1. succedaneous teeth
      1. tooth germs form within the dental lamina lingual to the deciduous tooth germ
   2. non-succedaneous teeth
      1. tooth germ forms from dental lamina that extends posteriorly beneath the oral epithelium after jaws have grown