Mandibular First Deciduous Molar

-This tooth doesn't resemble any other tooth. It is unique unto itself.

-There are two roots.

-There is a strong bulbous enamel bulge buccally at the mesial.

- the mesiolingual cusps on this tooth is the highest and largest of the cusps.

FORMATION OF THE ROOT AND ITS ROLE IN ERUPTION

- As dentin and enamel is deposited the shape of the future crown appears.

- The cells just superficial to the horizontal diaphragm start to proliferate and grow pushing the horizontal diaphragm down into the mesenchym.

- This forms a tube.

- This tube is the epithelial root sheath of Hertwig's.

- The mesenchym cells lying inside the tube nearest to the epithelial root sheath are induced to differentiate into odontoblasts, which then start to deposit dentin.

- After the first dentin of the root has been laid down the inner epithelial cells of the sheath start to deposit an enameloid substance called intermediate cementum.

- The root sheath cells then separate from the intermediate cementum and breaks up in a network of epithelial strands.

- The mesenchym on the outside comes into contact with the intermediate cementum and differentiate into

cementoblasts, which will deposit the cementum.

- This cementum traps the collagenic fibres, of the periodontal ligament, which are also formed.

- Epithelium of the root sheath persists as epithelial rests of Malassez. Because the epithelium of the root sheath forms from enamel epithelium it can develop into ameloblasts which will deposit enamel pearls.

- There is little space for the root to develop.

- To create space the crown is pushed out.

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.

HISTOLOGIC CHANGES OF THE PULP

Regressive changes

Pulp decreases in size by the deposition of dentin.
This can be caused by age, attrition, abrasion, operative procedures, etc.
Cellular organelles decrease in number.

Fibrous changes

They are more obvious in injury rather than aging. Occasionally, scarring may also be apparent.

Pulpal stones or denticles

They can be: a)free, b)attached and/or c)embedded. Also they are devided in two groups: true or false. The true stones (denticles) contain dentinal tubules. The false predominate over the the true and are characterized by concentric layers of calcified material.

Diffuse calcifications

Calcified deposits along the collagen fiber bundles or blood vessels may be observed. They are more often in the root canal portion than the coronal area.

Histology of the Cementum

Cementum is a hard connective tissue that derives from ectomesenchyme.

Embryologically, there are two types of cementum:
Primary cementum: It is acellular and develops slowly as the tooth erupts. It covers the coronal 2/3 of the root and consists of intrinsic and extrinsic fibers (PDL).
Secondary cementum: It is formed after the tooth is in occlusion and consists of extrinsic and intrinsic (they derive from cementoblasts) fibers. It covers mainly the root surface.

Functions of Cementum

It protects the dentin (occludes the dentinal tubules)
It provides attachment of the periodontal fibers
It reverses tooth resorption

Cementum is composed of 90% collagen I and III and ground substance.
50% of cementum is mineralized with hydroxyapatite. Thin at the CE junction, thicker apically.

Dentin

1. Composition

a. Inorganic (70%)—calcium hydroxyapatite crystals.

b. Organic (30%)—water and type I collagen.

2. Types of dentin

a. Primary dentin

(1) Dentin formed during tooth development, before completion of root formation.

It constitutes the majority of dentin found in a tooth.

(2) It consists of a normal organization of dentinal tubules.

(3) Circumpulpal dentin

(a) The layer of primary dentin that surrounds the pulp chamber.  It is formed after the mantle dentin.

(b) Its collagen fibers are parallel to the DEJ.

b. Secondary dentin

(1) Dentin formed after root formation is complete.

(2) Is deposited unevenly around the pulp chamber, forming along the layer of dentin closest to the pulp.

It therefore contributes to the decrease in the size of the pulp chamber as one ages.

(3) It consists of a normal, or slightly less regular, organization of dentinal tubules. However,

as compared to primary dentin, it is deposited at a slower rate.

(4) Although the dentinal tubules in secondary dentin can be continuous with those in primary

dentin, there is usually a tubular angle change between the two layers.

c. Tertiary (reparative, reactive) dentin

(1) Dentin that is formed in localized areas in response to trauma or other stimuli such as caries, tooth wear, or dental work.

(2) Its consistency and organization vary. It has no defined dentinal tubule pattern

d. Mantle dentin
 

(1) The outermost layer of dentin
(2) Is the first layer of dentin laid down by odontoblasts adjacent to the DEJ.

(3) Is slightly less mineralized than primary dentin.

(4) Has collagen fibers that are perpendicular to the DEJ.

(5) Dentinal tubules branch abundantly in this area.

e. Sclerotic (transparent) dentin

(1) Describes dentinal tubules that have become occluded with calcified material .

(2) Occurs when the odontoblastic processes retreat, filling the dentinal tubule with calcium phosphate crystals.

(3) Occurs with aging.

f. Dead tracts

(1) When odontoblasts die, they leave behind empty dentinal tubules, or dead tracts.

(2) Occurs with aging or trauma.

(3) Empty tubules are potential paths for bacterial invasion.

3. Structural characteristics and microscopic features:
 

a. Dentinal tubules

(1) Tubules extend from the DEJ to the pulp chamber.

(2) The tubules taper peripherally (i.e., their diameters are wider as they get closer to the pulp). Since the tubules are distanced farther apart at the periphery, the density of tubules is greater closer to the pulp.

(3) Each tubule contains an odontoblastic process or Tomes’ fiber.

Odontoblastic processes are characterized by the presence of a network of microtubules, with

Occasional mitochondria and vesicles present.

Note: the odontoblast’s cell body remains in the pulp chamber.
 

(4) Coronal tubules follow an S-shaped path, which may result from the crowding of  odontoblasts as they migrate toward the pulp during dentin formation.

b. Peritubular dentin (intratubular dentin)

(1) Is deposited on the walls of the dentinal tubule, which affects (i.e., narrows)the diameter of the tubule .

(2) It differs from intertubular dentin by lacking a collagenous fibrous matrix. It is also more mineralized than intertubular dentin.
 

c. Intertubular dentin

(1) The main part of dentin, which fills the space between dentinal tubules

 (2) Is mineralized and contains a collagenous matrix.

d. Interglobular dentin

(1) Areas of hypomineralized or unmineralized dentin caused by the failure of globules or calcospherites to fuse uniformly with mature dentin.

(2) Dentinal tubules are left undisturbed as they pass through interglobular dentin; however,

No peritubular dentin is present.

(3) Interglobular dentin is found in the:

(a) Crown—just beneath the mantle dentin.

(b) Root—beneath the dentinocemental junction, giving the root the appearance of a granular

layer (of Tomes).

e. Incremental lines

(1) Dentin is deposited at a daily rate of approximately 4 microns.

(2) As dentin is laid down, small differences in collagen fiber orientation result in the formation of incremental lines.

(3) Called imbrication lines of von Ebner.

(a) Every 5 days, or about every 20 µm, the changes in collagen fiber orientation appear more

accentuated. This results in a darker staining line, known as the imbrication line of von

Ebner.

(b) These lines are similar to the lines of Retzius seen in enamel.

f. Contour lines of Owen

(1) An optical phenomenon that occurs when the secondary curvatures of adjacent dentinal tubules coincide, resulting in the appearance of lines known as contour lines of Owen.
 

(2) Contour lines of Owen may also refer to lines that appear similar to those just described; however, these lines result from disturbances in mineralization.

g. Granular layer of Tomes

(1) A granular or spotty-appearing band that can be observed on the root surface adjacent to the dentinocemental junction, just beneath the cementum.

The mixed dentition

I. Transition dentition between 6 and 12 years of age with primary tooth exfoliation and permanent tooth eruption

2. Its characteristic features have led this to be called the ugly duckling stage because of

a. Edentulated areas

b. Disproportionately sized teeth

c. Various clinical crown heights

d. Crowding

e. Enlarged and edematous gingiva

f. Different tooth colors