Ans 1 The following are the stages of tooth development : 1. Bud stage 2. Cap stage 3. Bell stage 4. Advanced bell stage 5. Formation of Hertwig s epithelial root sheath BUD STAGE 1. Around the eighth week of embryonic development, the mesenchymal neural crest induces the development of tooth buds at ten locations in the upper and lower dental lamina 2. During the bud stage the dental lamina grows into the mesenchyme in the shape of a bud 3. The basement membrane separates the epithelium of dental lamina from ectomesenchyme. 4. The epithelial cells do not show any change in shape and function. 5. The supporting ectomesenchymal cells are densely packed under the lining epithelium and around the epithelial bud. 6. The enamel organ of budstage have two types of cells. 7. Polygonal cells centrally situated 8. Low columnar cells peripherally situated. 9. The ectomesenchyme of enamel portion of enamel organ is divided due to increased mitotic activity 10. The centrally situated cells rapidly divide and condensed to form dental papilla 11. The ectomesenchyme that surrounds the tooth bud and dental papilla from the dental sac. CAP STAGE 1. Immediately adjacent to epithelial ingrowth the cellular density increases. After continous division and differentiation, the size and shape of enamel organ changes from knob-like to cap-like 2. Invagination of inner surface results from unequal division leading to Cap Stage. 3. It has following layers:-
a) Outer Enamel Epithelium b) Inner Enamel Epiyhelium. c) Stellate Reticulum. 4 Outer and Inner Enamel Epithelium Cells of outer enamel epithelium- cuboidal and along convexity of cap. Cells of outer enamel epithalium- Tall,Columnar and cover concavity of cap. Basement membrane between inner enamel epithelium and dental papilla and also between outer enamel epithelium and dental sac. Hemidesmosomes anchor cells to the basal lamina. 5 Stellate Reticulum Polygonal cells in center of enamel organ between outer and inner enamel epithelium Accumulation of intercellular fluid and osmotic force exerted due to presence of glycosoaminoglycans. Polygonal cells change into star shaped cells since water is drawn into enamel organ. The proteinaceous fluid containing albumin gives a cushion- like consistency to stellate reticulum and protects delicate enamel- forming cells. 6 Dental Papilla The proliferating epithelium of enamel organ influence neural crest cells to proliferate. The ectomesenchymal cells are partly covered by invaginated portion of inner enamel epithelium It is responsible for the formation of dentin and pulp 7 Dental Sac In surrounding areas, cell division takes place and results in condensation and fibrous development in this zone.
BELL STAGE 1 Continous division of ectomesenchymal cells of inner portion of enamel organ 2 Deepening of the epithelial invagination and growth at margins leads to formation of bell shape 3 The enamel organ shows histodifferentiation and morphodifferentiation 4 The shape of crown is determined and controlled by genes, their signalling moleculaes and growth factors 5 It has four layer of epithelial cells a. Inner Enamel Epitheliumo Tall columnar cells which differentiate into specialized cells called Ameloblasts. o These have high glycogen content and induces underlying cells of dental papilla into odontoblasts. b. Stratum Intermediumo Layer of squamous cells present between Inner Emanel Epithelium and Stellate Reticulum. o High degree of metabolic activity. c. Stellate Reticulumo Star shaped cells have long processes which anastomose with the processes of adjacent cells. o It collapses just prior to beginning of enamel formation. d. Outer Enamel Epitheliumo Just before the enamel formation begins, the outer enamel epithelium becomes folded and the capillary network develops in between the folds o This provides rich nutritional blood supply which is required for the intense metabolic activity of the avascular enamel organ o Under the inductive influences of the epithelium, the dental papilla develops the odontoblasts after which the laying down of dentin starts before the inner enamel epithelium lays down the first layer of enamel matrix
ADVANCED BELL STAGE 1. Hard tissues, including enamel and dentin, develop during this stage of tooth development. In prior stages, all of the inner enamel epithelium cells were dividing to increase the overall size of the tooth bud, but rapid dividing stops during this stage at the location where the cusps of the teeth form. 2. The first mineralized hard tissues(enamel and dentin) form at this location. At the same time, the inner enamel epithelial cells change in shape from cuboidal to columnar. 3. As differentiation of the inner enamel epithelium proceeds, cells called preameloblasts form from the inner enamel organ epithelium, adjacent to the dental papilla. These cells induce neural crest cells in the dental papilla to differentiate into preodontoblasts. 4. The preodontoblasts become odontoblasts as they begin to secrete predentin (which will become dentin). The predentin blocks nutrients from moving from the pulp to the preameloblasts. This causes the preameloblasts to become ameloblasts and begin their secretion of enamel. The odontoblasts and ameloblasts move away from each other as the dentin and enamel layers increase in thickness. 5. As this begins to occur, the developing tooth enters the crown stage. 6. Once enamel depostion is completed and the crown is fully formed, the enamel organ collapses and the cells form a sheath called the reduced enamel epithelium that covers the tooth until eruption. 7. Two more features of tooth development are also seen: a. Future dentinoenamel junction: i. forms from the boundary present between the inner enamel epithelium and odontoblasts ii. The first layer of dentin is formed along the future dentinoenamel junction and formation proceeds pulpally and apically. iii. After the formation of first layer of dentin, enamel is laid down over the dentin by the ameloblast and enamel formation proceeds occlusally 8. Hertwig s epithelial root sheath: a. Develops from the cervical portion of the enamel organ 9. Two more features of tooth development are also seen: a. Future dentinoenamel junction:
i. forms from the boundary present between the inner enamel epithelium and odontoblasts ii. The first layer of dentin is formed along the future dentinoenamel junction and formation proceeds pulpally and apically. iii. After the formation of first layer of dentin, enamel is laid down over the dentin by the ameloblast and enamel formation proceeds occlusally 10. Hertwig s epithelial root sheath: a. Develops from the cervical portion of the enamel organ b. Apical extension of cervical loop c. Contains only inner and outer enamel epithelium d. Framework of root formation e. The apical most portion of the root sheath turns inward toward the radicular pulp cavity (that portion of the pulp cavity inside the root) and is called the epithelial diaphragm ROOT FORMATION 1. The root formation is initiated by the disintegration of HERS. The cells in dental papilla are differentiated into odontoblasts and begin to make root dentin. 2. Ectomesenchymal cells in dental follicle migrate onto dentin through disintegrating HERS and become differentiated into cementoblasts and fibroblasts. 3. Cementoblasts synthesize cementum and fibroblasts synthesize collagen fibers for periodontal ligament. The dental follicle cells left outside the HERS are also differentiated into osteoblasts and fibroblasts to form alveolar bone and collagen fibers, respectively. 4. The fibers synthesized by fibroblasts are embedded in cementum and alveolar bone at both ends. As the fibers mature, they are organized forming big bundles and become periodontal ligaments. 5. As the epithelial root sheath breaks down, cells from the dental sac migrate to the surface of the root dentin and differentiate into cementoblasts. These cells lay down cememtum on the surface of the root.
SHORT ESSAYS Ans 1. Hunter Schrerger Bands 1. It is an optical phenomenon caused by changes of rod direction (the wavy course). 2. They are seen clearly by longitudinal ground section viewed by reflected light at cervical 2/3. 3. The phenomenon appears as dark and light alternating bands, starting from the D.E.J. and ends shortly from the outer surface of enamel where the enamel rods run straight to the surface. 4. The dark bands (Diazones) absorb the light where the light bands (Parazones) reflect the light. 5. If the light passes from the opposite side, the light and dark bands will be reversed. 6. The dark bands correspond to the cross sectional enamel rods (diazones) and the light bands represent the longitudnally sectioned interrod enamel (parazones)
Ans 2. Types of dentin are: 1. Primary dentin a. Mantle dentin b. Circumpulpal dentin 2. Secondary dentin 3. Tertiary dentin 4. Peritubular dentin 5. Intertubular dentin PRIMARY DENTIN Represents all of the dentin formed prior to root completion. 1. Mantle dentin is the first formed dentin in the crown underlying the dentinoenamel junction. It is the outer or most peripheral part of the primary dentin & is about 20um thick. The fibrils found in this zone are perpendicular to the dentinoenamel junction. 2. Circumpulpal dentin forms the remaining primary dentin or bulk of the tooth. Represents all of the dentin formed prior to root completion. The fibrils are much smaller in diameter & are more closely packed together. Slightly more mineral content than mantle dentin. SECONDARY DENTIN 1. A narrow band of dentin bordering the pulp and representing the dentin formed after root completion. 2. Contains fewer tubules than primary dentin. 3. There is usually a bend in the tubules where primary and secondary dentin interface.
REPARATIVE DENTIN 1. Also known as tertiary or response dentin. 2. If by extensive abrasion, erosion, caries, or operative procedures the odontoblast processes are exposed or cut, the odontoblasts die or, if they live, deposit reparative dentin. 3. Origin of the new odontoblast is from undifferentiated perivascular cell. 4. Has fewer & more twisted tubules than normal dentin. 5. Sometimes, a combination of osteodentin & atubular dentin is seen. 6. Odontoblasts lay down 1.4 um/day. PERITUBULAR DENTIN 1. The dentin that immediately surrounds the dentinal tubules. 2. It is more highly mineralized than intertubular dentin. 3. It is twice as thick in outer dentin (approx. 0.75um) than in inner dentin (0.4um). 4. By its growth, it constricts the dentinal tubules to a diameter of 1um near the dentinoenamel junction. 5. Organic matrix is lost along with mineral after decalcification. 6. The calcified tubule wall has an inner organic lining termed the lamina limitans, high in glucosaminoglycans (GAG). INTERTUBULAR DENTIN 1. Forms the main body of dentin. 2. It is located between the dentinal tubules or, more specifically, between the zones of peritubular dentin. 3. Its organic matrix is retained after decalcification. 4. About one-half of its volume is organic matrix, specifically collagen fibers. The fibrils range from 0.5 to 0.2um in diameter and exhibit crossbanding at 64um intervals, which is typical for collagen. ANS 3. Palmer and FDI system of notation: Palmer notation system
1. In 1861 Adolph Zsigmondy of Vienna introduced the symbolic system for permanent dentition. He then modified it for the primary dentition in 1874. The symbolic system is now commonly referred to as the Palmer notation system or Zsigmondy system. 2. The arches are divided into quadrants with the entire dentition being notated as follows Primary teeth- EDCBA ABCDE EDCBA ABCDE Permanent teeth- 87654321 12345678 87654321 12345678 FDI system (Federation Dentaire Internationale) The FDI system is a two digit system that has been adopted by WHO and also IADR. In this system the first digit indicates the quadrant and the second digit indicates the tooth within the quadrant. 1 to 4 and 5 to 8 as the first digit indicates permanent and primary dentition respectively. 1 to 8 and 1 to 5 as the second digit indicates permanent and primary teeth respectively. Primary teeth- 55 54 53 52 51 61 62 63 64 65 85 84 83 82 81 71 72 73 74 75 Permanent teeth- 18 17 16 15 14 13 12 11 21 22 23 24 25 26 27 28 48 47 46 45 44 43 42 41 31 32 33 34 35 36 37 38