NEET MDS Lessons
Dental Anatomy
Bell stage
The bell stage is known for the histodifferentiation and morphodifferentiation that takes place. The dental organ is bell-shaped during this stage, and the majority of its cells are called stellate reticulum because of their star-shaped appearance. Cells on the periphery of the enamel organ separate into three important layers. Cuboidal cells on the periphery of the dental organ are known as outer enamel epithelium.The cells of the enamel organ adjacent to the dental papilla are known as inner enamel epithelium. The cells between the inner enamel epithelium and the stellate reticulum form a layer known as the stratum intermedium. The rim of the dental organ where the outer and inner enamel epithelium join is called the cervical loop
Other events occur during the bell stage. The dental lamina disintegrates, leaving the developing teeth completely separated from the epithelium of the oral cavity; the two will not join again until the final eruption of the tooth into the mouth
The crown of the tooth, which is influenced by the shape of the internal enamel epithelium, also takes shape during this stage. Throughout the mouth, all teeth undergo this same process; it is still uncertain why teeth form various crown shapes—for instance, incisors versus canines. There are two dominant hypotheses. The "field model" proposes there are components for each type of tooth shape found in the ectomesenchyme during tooth development. The components for particular types of teeth, such as incisors, are localized in one area and dissipate rapidly in different parts of the mouth. Thus, for example, the "incisor field" has factors that develop teeth into incisor shape, and this field is concentrated in the central incisor area, but decreases rapidly in the canine area. The other dominant hypothesis, the "clone model", proposes that the epithelium programs a group of ectomesenchymal cells to generate teeth of particular shapes. This group of cells, called a clone, coaxes the dental lamina into tooth development, causing a tooth bud to form. Growth of the dental lamina continues in an area called the "progress zone". Once the progress zone travels a certain distance from the first tooth bud, a second tooth bud will start to develop. These two models are not necessarily mutually exclusive, nor does widely accepted dental science consider them to be so: it is postulated that both models influence tooth development at different times.Other structures that may appear in a developing tooth in this stage are enamel knots, enamel cords, and enamel niche.
The very first histological evidence of tooth development appear during the second month of intrauterine life. Calcification of deciduous incisors begins at 3-4 months in utero.
TEMPOROMANDIBULAR JOINT
There are three kind of joints:
· Fibrous
Two bones connected with fibrous tissue
Examples
suture (little or no movement)
gomphosis (tooth - PDL - bone)
syndesmosis (fibula & tibia, radius and ulna; interosseous ligament)
· Cartilagenous
Two subtypes:
2a) primary: bone<--->cartilage (costochondral joint)
2b) secondary: bone<-->cartilage<-->FT<-->cartilage<--> bone (pubic symphysis)
· Synovial
Two bones; each articular surface covered with hyaline cartilage in most cases
The bones are united with a capsule (joint cavity)
In the capsule there is presence of synovial fluid
The capsule is lined by a synovial membrane
In many synovial joints there maybe an articular disk
Synovial joints are characterized by the presence of ligaments
Synovial joints are classified according to the number of axes of bone movement: uniaxial, biaxial, multiaxial
the shapes of articulating surfaces: planar, ginglymoid (=hinged), pivot, condyloid
The movement of the joints is controlled by muscles
The temporomandibular joint is a synovial, sliding-ginglymoid joint (humans)
Embryology of the TMJ
Primary TMJ: Meckel's cartilage --> malleus & incal cartilage. It lasts for 4 months.
Secondary TMJ: Starts developing around the third month of gestation
Two blastemas (temporal and condylar); condylar grows toward the temporal (temporal appears and ossifies first)
Formation of two cavities: inferior and upper
Appearance of disk
Bones: glenoid fossa (temporal bone) and condyle (mandible)
Alveolar bone (process)
1. The bone in the jaws that contains the teeth alveoli (sockets).
2. Three types of bone :
a. Cribriform plate (alveolar bone proper)
(1) Directly lines and forms the tooth socket. It is compact bone that contains many holes, allowing for the passage of blood vessels. It has no periosteum.
(2) Serves as the attachment site for PDL (Sharpey’s) fibers.
(3) The tooth socket is constantly being remodeled in response to occlusal forces. The bone laid down on the cribriform plate, which also provides attachment for PDL fibers, is known as bundle bone.
(4) It is radiographically known as the lamina dura.
b. Cortical (compact) bone
(1) Lines the buccal and lingual surfaces of the mandible and maxilla.
(2) Is typical compact bone with a periosteum and contains Haversian systems.
(3) Is generally thinner in the maxilla and thicker in the mandible, especially around the buccal area of the mandibular premolar and molar.
c. Trabecular (cancellous, spongy) bone
(1) Is typical cancellous bone containing Haversian systems.
(2) Is absent in the maxillary anterior teeth region.
3. Alveolar crest (septa)
a. The height of the alveolar crest is usually 1.5 to 2 mm below the CEJ junction.
b. The width is determined by the shape of adjacent teeth.
(1) Narrow crests—found between teeth with relatively flat surfaces.
(2) Widened crests—found between teeth with convex surfaces or teeth spaced apart.
Mandibular Second Deciduous Molar.
-This tooth resembles the lower first permanent molar that is d
istal to it in the dental arch.
-There are two roots and five cusps. The three buccal cusps are all about the same size. This is in contrast to the lower first molar where the 'distal' cusp is smaller that the mesiobuccal and distobuccal cusps.
-The distal of the three buccal cusps may be shifted of onto the distal marginal ridge.
NOTE
-Upper molars have three roots, lowers have two roots.
-Upper and lower second deciduous molars resemble first permanent molars in the same quadrant.
-Upper first deciduous molars vaguely resemble upper premolars. -Lower first deciduous molars are odd and unique unto themselves.
-First deciduous molars (upper and lower) have a prominent bulge of enamel on the buccal at the mesial. These help in determining right and left.
Tooth development is the complex process by which teeth form from embryonic cells, grow, and erupt into the mouth.. For human teeth to have a healthy oral environment, enamel, dentin, cementum, and the periodontium must all develop during appropriate stages of fetal development. Primary teeth start to form between the sixth and eighth weeks in utero, and permanent teeth begin to form in the twentieth week in utero.
Overview
The tooth bud (sometimes called the tooth germ) is an aggregation of cells that eventually forms a tooth.These cells are derived from the ectoderm of the first branchial arch and the ectomesenchyme of the neural crest.The tooth bud is organized into three parts: the enamel organ, the dental papilla and the dental follicle.
The enamel organ is composed of the outer enamel epithelium, inner enamel epithelium, stellate reticulum and stratum intermedium.These cells give rise to ameloblasts, which produce enamel and the reduced enamel epithelium. The location where the outer enamel epithelium and inner enamel epithelium join is called the cervical loop. The growth of cervical loop cells into the deeper tissues forms Hertwig's Epithelial Root Sheath, which determines the root shape of the tooth.
The dental papilla contains cells that develop into odontoblasts, which are dentin-forming cells. Additionally, the junction between the dental papilla and inner enamel epithelium determines the crown shape of a tooth. Mesenchymal cells within the dental papilla are responsible for formation of tooth pulp.
The dental follicle gives rise to three important entities: cementoblasts, osteoblasts, and fibroblasts. Cementoblasts form the cementum of a tooth. Osteoblasts give rise to the alveolar bone around the roots of teeth. Fibroblasts develop the periodontal ligaments which connect teeth to the alveolar bone through cementum.
MAXILLARY LATERAL INCISORS
it is shorter, narrower, and thinner.
Facial: The maxillary lateral incisor resembles the central incisor, but is narrower mesio-distally. The mesial outline resembles the adjacent central incisor; the distal outline--and particularly the distal incisal angle is more rounded than the mesial incisal angle (which resembles that of the adjacent central incisor. The distal incisal angle resembling the mesial of the adjacent canine.
Lingual: On the lingual surface, the marginal ridges are usually prominent and terminate into a prominent cingulum. There is often a deep pit where the marginal ridges converge gingivally. A developmental groove often extends across the distal of the cingulum onto the root continuing for part or all of its length.
Proximal: In proximal view, the maxillary lateral incisor resembles the central except that the root appears longer--about 1 1/2 times longer than the crown. A line through the long axis of the tooth bisects the crown.
Incisal: In incisal view, this tooth can resemble either the central or the canine to varying degrees. The tooth is narrower mesiodistally than the upper central incisor; however, it is nearly as thick labiolingually.
Contact Points: The mesial contact is at the junction of the incisal third and the middle third. The distal contact is is located at the center of the middle third of the distal surface.
Root Surface:-The root is conical (cone-shaped) but somewhat flattened mesiodistally.