Pulp

1. Four zones—listed from dentin inward

a. Odontoblastic layer

(1) Contains the cell bodies of odontoblasts.

Note: their processes remain in dentinal tubules.

(2) Capillaries, nerve fibers, and dendritic cells may also be present.

b. Cell-free or cell-poor zone (zone of Weil)

(1) Contains capillaries and unmyelinated nerve fibers.

c. Cell-rich zone

(1) Consists mainly of fibroblasts. Macrophages, lymphocytes, and dendritic cells may also be present.

d. The pulp (pulp proper, central zone)

(1) The central mass of the pulp.

(2) Consists of loose connective tissue, larger vessels, and nerves. Also contains fibroblasts and pulpal cells.

2. Pulpal innervation

a. When pulpal nerves are stimulated, they can only transmit one signal pain.

b. There are no proprioceptors in the pulp.

c. Types of nerves:

(1) A-delta fibers

(a) Myelinated sensory nerve fibers.

(b) Stimulation results in the sensation of fast, sharp pain.

(c) Found in the coronal (odontoblastic) area of the pulp.

(2) C-fibers

(a) Unmyelinated sensory nerve fibers.

(b) Transmits information of noxious stimuli centrally.

(c) Stimulation results in pain that is slower, duller, and more diffuse in nature.

(d) Found in the central region of the pulp.

(3) Sympathetic fibers

(a) Found deeper within the pulp.

(b) Sympathetic stimulation results in vasoconstriction of vessels.

Cementum & Cementogenesis

Cementum formation is called cementogenesis and occurs late in the development of teeth. Cementoblasts are the cells responsible for cementogenesis. Two types of cementum form: cellular and acellular.

Acellular cementum forms first. The cementoblasts differentiate from follicular cells, which can only reach the surface of the tooth's root once Hertwig's Epithelial Root Sheath (HERS) has begun to deteriorate. The cementoblasts secrete fine collagen fibrils along the root surface at right angles before migrating away from the tooth. As the cementoblasts move, more collagen is deposited to lengthen and thicken the bundles of fibers. Noncollagenous proteins, such as bone sialoprotein and osteocalcin, are also secreted. Acellular cementum contains a secreted matrix of proteins and fibers. As mineralization takes place, the cementoblasts move away from the cementum, and the fibers left along the surface eventually join the forming periodontal ligmaments.

Cellular cementum develops after most of the tooth formation is complete and after the tooth occludes (in contact) with a tooth in the opposite arch. This type of cementum forms around the fiber bundles of the periodontal ligaments. The cementoblasts forming cellular cementum become trapped in the cementum they produce.

The origin of the formative cementoblasts is believed to be different for cellular cementum and acellular cementum. One of the major current hypotheses is that cells producing cellular cementum migrate from the adjacent area of bone, while cells producing acellular cementum arise from the dental follicle. Nonetheless, it is known that cellular cementum is usually not found in teeth with one root. In premolars and molars, cellular cementum is found only in the part of the root closest to the apex and in interradicular areas between multiple roots.

Amelogenesis and Enamel

Enamel is highly mineralized: 85% hydroxyapatite crystals
Enamel formation is a two-step process
The first step produces partially mineralized enamel: 30% (secretory)
The second step: Influx of minerals, removal of water and organic matrix (maturative)
Again, dentin is the prerequisite of enamel formation (reciprocal induction)
Stratum intermedium: high alkaline phosphatase activity
Differentiation of ameloblasts: Increase in glycogen contents

Formation of the enamel matrix
Enamel proteins, enzymes, metalloproteinases, phosphatases, etc.
Enamel proteins: amelogenins (90%), enamelin, tuftelin, and amelin
Amelogenins: bulk of organic matrix
Tuftelin: secreted at the early stages of amelogenesis (area of the DE junction)
Enamelin: binds to mineral
Amelin

Mineralization of enamel
 No matrix vesicles
Immediate formation of crystallites
Intermingling of enamel crystallites with dentin
"Soft" enamel is formed

Histologic changes

Differentiation of inner enamel epithelium cells. They become ameloblasts
Tomes' processes: saw-toothed appearance
Collapse of dental organ
Formation of the reduced enamel epithelium

Hard tissue formation (Amelogenesis )

Enamel formation is called amelogenesis and occurs in the crown stage of tooth development. "Reciprocal induction" governs the relationship between the formation of dentin and enamel; dentin formation must always occur before enamel formation. Generally, enamel formation occurs in two stages: the secretory and maturation stages. Proteins and an organic matrix form a partially mineralized enamel in the secretory stage; the maturation stage completes enamel mineralization.

In the secretory stage, ameloblasts release enamel proteins that contribute to the enamel matrix, which is then partially mineralized by the enzyme alkaline phosphatase. The appearance of this mineralized tissue, which occurs usually around the third or fourth month of pregnancy, marks the first appearance of enamel in the body. Ameloblasts deposit enamel at the location of what become cusps of teeth alongside dentin. Enamel formation then continues outward, away from the center of the tooth.

In the maturation stage, the ameloblasts transport some of the substances used in enamel formation out of the enamel. Thus, the function of ameloblasts changes from enamel production, as occurs in the secretory stage, to transportation of substances. Most of the materials transported by ameloblasts in this stage are proteins used to complete mineralization. The important proteins involved are amelogenins, ameloblastins, enamelins, and tuftelins. By the end of this stage, the enamel has completed its mineralization.

MANDIBULAR SECOND MOLAR

Facial: When compared to the first molar, the second molar crown is shorter both mesiodistally and from the cervix to the occlusal surface. The two well-developed buccal cusps form the occlusal outline. There is no distal cusp as on the first molar. A buccal developmental groove appears between the buccal cusps and passes midway down the buccal surface toward the cervix.

Lingual: The crown is shorter than that of the first molar. The occlusal outline is formed by the mesiolingual and distolingal cusps.

Proximal: The mesial profile resembles that of the first molar. The distal profile is formed by the distobuccal cusp, distal marginal ridge, and the distolingual cusp. Unlike the first molar, there is no distal fifth cusp.

Occlusal: There are four well developed cusps with developmental grooves that meet at a right angle to form the distinctive "+" pattern characteristic of this tooth.

Contact Points; When moving distally from first to third molar, the proximal surfaces become progressively more rounded. The net effect is to displace the contact area cervically and away from the crest of the marginal ridges.

Roots:-The mandibular second molar has two roots that are smaller than those of the first molar. When compared to first molar roots, those of the second tend to be more parallel and to have a more distal inclination.

Dentin

Composition: 70% inorganic, 20% organic, 10% water by weight and 45%, 33%, and 22% in volume respectively
Hydroxyapatite crystals and collagen type I
Physical characteristics: Harder than bone and softer than enamel
Yellow in color in normal teeth
Radiographic appearance: More radiolucent than enamel

Primary (circumpulpal) dentin: forms most of the tooth
Mantle dentin: first dentin to form; forms the outline of dentin in the adult tooth
Predentin: lines the innermost portion of dentin (faces the pulp)
Secondary dentin: after root formation dentin continues to form, continuous to primary dentin but with structural irregularities
Tertiary dentin: reactive or reparative dentin; may or may not have characteristics of primary dentin; produced in the area of an external stimulus; osteodentin

Dentin is formed by cells called odontoblasts.
These cells derive from the ectomesenchyme and produce the organic matrix that will calcify and become the dentin.
Formation of dentin initiates formation of enamel.
The formation of dentin starts during late bell-stage in the area of the future cusp.

First coronal dentin and then root dentin.

Completion of dentin does not occur until about 18 months after eruption of primary and 2-3 years after eruption of permanent teeth.

The rate of dentin development varies.

The role of the internal (inner) dental (enamel) epithelium
Cuboidal - Columnar (reverse polarization)
Ectomesenchymal cells of the dental papilla become preodontoblasts - odontoblasts
Acellular zone disappears

Histologic features of dentin
Odontoblasts
Dentinal tubules
Extend through the entire thickness of dentin
S-shaped (primary curvatures) path in the crown, less S-shaped in the root, almost straight in the cervical aspect
Secondary curvatures
Tubular microbranches
Presence of fluid
 

Intratubular dentin
Dentin in the tubule that is hypermineralized

The term peritubular dentin should not be used
 

Sclerotic dentin
Dentinal tubules that are occluded with calcified material
Most likely a physiologic response
Reduction of permeability of dentin
 

Intertubular dentin
Dentin between the tubules
 

Interglobular dentin
Areas of unmineralized or hypomineralized dentin
The defect affects mineralization and not the architecture of dentin
 

Incremental lines
Lines of von Ebner: lines associated with 5-day rythmic pattern of dentin deposition
Contour lines of Owen: Originally described by Owen they result from a coincidence of the secondary curvatures between neighboring dentinal tubules.
 

Granular Layer of Tomes
Seen only in ground sections in the root area covered by cementum
Originally, they were thought to be areas of hypomineralization
They are true spaces obtained by sections going through the looped terminal portions dentinal tubules

DE junction :Scalloped area

Enamel tissue with incremental lines of Retzius and dentin tissue with parallel, curved dentinal tubules are in contact at the irregular dentino-enamel junction. The junction often has a scalloped-shaped morphology

DC junction Dentin Cemental Junction

MANDIBULAR SECOND BICUSPID

Facial: From this aspect, the tooth somewhat resembles the first, but the buccal cusp is less pronounced. The tooth is larger than the first.

Lingual: Two significant variations are seen in this view. The most common is the three-cusp form which has two lingual cusps. The mesial of those is the larger of the two. The other form is the two-cusp for with a single lingual cusp. In that variant, the lingual cusp tip is shifted to the mesial.

Proximal: The buccal cusp is shorter than the first. The lingual cusp (or cusps) are much better developed than the first and give the lingual a full, well-developed profile.

Occlusal: The two or three cusp versions become clearly evident. In the three-cusp version, the developmental grooves present a distinctive 'Y' shape and have a central pit. In the two cusp version, a single developmental groove crosses the transverse ridge from mesial to distal

Contact Points; Height of Curvature: From the facial, the mesial contact is more occlusal than the distal contact.The distal marginal ridge is lower than the mesial marginal ridge

Root Surface:-The root of the tooth is single, that is usually larger than that of the first premolar  

the lower second premolar is larger than the first, while the upper first premolar is just slightly larger than the upper second

There may be one or two lingual cusps