NEET MDS Lessons
Dental Anatomy
FORMATION OF THE PERMANENT DENTITION
Twenty deciduous tooth buds are formed initially.
Proliferative activity of the dental lamina during the bell stage that leads to formation of permanent tooth buds (cap stage) lingual of each deciduous tooth germ.
Molars have no predecessors; they are formed by posterior proliferation of the dental lamina.
HARD TISSUE FORMATION
Hard tissue formation starts at the late stages of the bell stage.
Differentiatioin of cells into odontoblasts and ameloblasts.
The cells of the inner dental epithelium will become ameloblasts.
The cells of the dental papilla opposite to the inner dental epithelium will become odontoblasts.
Dentin is formed before enamel.
Dentin initiates the formation of enamel.
ROOT FORMATION
The root of the tooth is composed by dentin and cementum.
Dentinogenesis is initiated by the odontoblasts.
Odontoblasts are formed as epithelial cells continue to proliferate from the cervical loop as a double layer of cells known as Hertwig's root sheath.
TOOTH SHAPE
The shape of the crowns results from the interaction of inner dental epithelium and the dental papilla.
The cells of the inner dental epithelium have a programmed proliferation.
This internal program determines the tooth form.
The fate of the dental lamina
Rests of Serres
The rest of Serres are rests of the dental lamina identified in the gingival soft tissues.
They are round to ovoid aggregates of epithelial cells that have clear cytoplasm (glucogen rich).
They result from early breakup of the dental lamina during bell stage.
Rests of Malassez
The rests of Malassez result from breakup of the Hertwig's root sheath during root formation.
They can be identified in the periodontal ligament and are responsible for the development of radicular cysts.
Tooth development is commonly divided into the following stages: the bud stage, the cap, the bell, and finally maturation. The staging of tooth development is an attempt to categorize changes that take place along a continuum; frequently it is difficult to decide what stage should be assigned to a particular developing tooth. This determination is further complicated by the varying appearance of different histological sections of the same developing tooth, which can appear to be different stages.
Bud stage
The bud stage is characterized by the appearance of a tooth bud without a clear arrangement of cells. The stage technically begins once epithelial cells proliferate into the ectomesenchyme of the jaw. The tooth bud itself is the group of cells at the end of the dental lamina.
Maxillary First Deciduous Molar.
-The notation is B or I.
-It looks a bit like an upper 1st premolar.
-There are three roots.
-It has a strong bulbous enamel bulge that protrudes buccally at the mesial.
-It is the smallest of the deciduous molars in crown height and in the mesiodistal dimension.
Transient structures during tooth development
Enamel knot: Thickening of the internal dental epithelium at the center of the dental organ.
Enamel cord: Epithelial proliferation that seems to divide the dental organ in two.
Review the role of these two structures
Enamel niche: It is an artifact that is produced during section of the tissue. It occurs because the dental organ is a sheet of proliferating cells rather than a single strand. It looks like a concavity that contains ectomesenchyme.
Differences Between the Deciduous and Permanent Teeth
1. Deciduous teeth are fewer in number and smaller in size but the deciduous molars are wider mesiodistally than the premolars. The deciduous anteriors are narrower mesiodistally than their permanent successors. Remember the leeway space that we discussed in the unit on occlusion?
2. Their enamel is thinner and whiter in appearance. Side by side, this is obvious in most young patients.
3. The crowns are rounded. The deciduous teeth are constricted at the neck (cervix).
4. The roots of deciduous anterior teeth are longer and narrower than the roots of their permanent successors.
5. The roots of deciduous molars are longer and more slender than the roots of the permanent molars. Also, they flare greatly.
6. The cervical ridges of enamel seen on deciduous teeth are more prominent than on the permanent teeth. This 'bulge' is very pronounced at the mesiobuccal of deciduous first molars.
G. Deciduous cervical enamel rods incline incisally/occlusally.
MANDIBULAR LATERAL INCISORS
The mandibular incisor is a little wider mesiodistal than the mandibular central incisor, and the crown is slightly longer from the incisal edge to the cervical line.
Facial Surface:-The facial surface is less symmetrical than the facial surface of the mandibular central incisor. The incisal edge slopes upward toward the mesioincisal angle, which is slightly less than 90°. The distoincisal angle is rounded. The mesial border is more nearly straight than the distal border.
Lingual Surface:- The incisal portion of the lingual surface is concave. The cingulum is quite large but blends in smoothly with the rest of the surface.
Root Surface:-The root is single and extremely flattened on its mesial and distal surfaces.
Formation and Eruption of Deciduous Teeth.
-Calcification begins during the fourth month of fetal life. By the end of the sixth month, all of the deciduous teeth have begun calcification.
-By the time the deciduous teeth have fully erupted (two to two and one half years of age), cacification of the crowns of permanent teeth is under way. First permanent molars have begun cacification at the time of birth. -Here are some things to know about eruption patterns:
(1) Teeth tend to erupt in pairs.
(2) Usually, lower deciduous teeth erupt first. Congenitally missing deciduous teeth is infrequent. Usually, the lower deciduous central incisors are thefirst to erupt thus initiating the deciduous dentition. The appearance of the deciduous second molars completes the deciduous dentition by 2 to 2 1/2 years of age.
- Deciduous teeth shed earlier and permanent teeth erupt earlier in girls.
- The orderly pattern of eruption and their orderly replacement by permanent teeth is important.
- order for eruption of the deciduous teeth is as follows:
(1) Central incisor.........Lower 6 ½ months, Upper 7 ½ months
(2) Lateral incisor.........Lower 7 months, Upper 8 months
(3) First deciduous molar...Lower 12-16 months, Upper 12-16 months
(4) Deciduous canine........Lower 16-20 months, Upper 16-20 months
(5) Second deciduous molar..Lower 20-30 months, Upper 20-30 months