Crown stage

Hard tissues, including enamel and dentin, develop during the next stage of tooth development. This stage is called the crown, or maturation, stage by some researchers. Important cellular changes occur at this time. In prior stages, all of the inner enamel epithelium cells were dividing to increase the overall size of the tooth bud, but rapid dividing, called mitosis, stops during the crown stage at the location where the cusps of the teeth form. The first mineralized hard tissues form at this location. At the same time, the inner enamel epithelial cells change in shape from cuboidal to columnar. The nuclei of these cells move closer to the stratum intermedium and away from the dental papilla.

The adjacent layer of cells in the dental papilla suddenly increases in size and differentiates into odontoblasts, which are the cells that form dentin. Researchers believe that the odontoblasts would not form if it were not for the changes occurring in the inner enamel epithelium. As the changes to the inner enamel epithelium and the formation of odontoblasts continue from the tips of the cusps, the odontoblasts secrete a substance, an organic matrix, into their immediate surrounding. The organic matrix contains the material needed for dentin formation. As odontoblasts deposit organic matrix, they migrate toward the center of the dental papilla. Thus, unlike enamel, dentin starts forming in the surface closest to the outside of the tooth and proceeds inward. Cytoplasmic extensions are left behind as the odontoblasts move inward. The unique, tubular microscopic appearance of dentin is a result of the formation of dentin around these extensions.

After dentin formation begins, the cells of the inner enamel epithelium secrete an organic matrix against the dentin. This matrix immediately mineralizes and becomes the tooth's enamel. Outside the dentin are ameloblasts, which are cells that continue the process of enamel formation; therefore, enamel formation moves outwards, adding new material to the outer surface of the developing tooth.

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

Cementum

Composition

a. Inorganic (50%)—calcium hydroxyapatite crystals.

b. Organic (50%)—water, proteins, and type I collagen.

c. Note: Compared to the other dental tissues, the composition of cementum is most similar to bone; however, unlike bone, cementum is avascular (i.e., no Haversian systems or other vessels are present).

Main function of cementum is to attach PDL fibers to the root surface.

Cementum is generally thickest at the root apex and in interradicular areas of multirooted 

Types of cementum

a. Acellular (primary) cementum

(1) A thin layer of cementum that surrounds the root, adjacent to the dentin.

(2) May be covered by a layer of cellular cementum, which most often occurs in the middle and apical root.

(3) It does not contain any cells.

b. Cellular (secondary) cementum

(1) A thicker, less-mineralized layer of cementum that is most prevalent along the apical root and in interradicular (furcal) areas of multirooted teeth.

(2) Contains cementocytes.

(3) Lacunae and canaliculi:

(a) Cementocytes (cementoblasts that become trapped in the extracellular matrix during cementogenesis) are observed in their entrapped spaces, known as lacunae.

(b) The processes of cementocytes extend through narrow channels called canaliculi.

(4) Microscopically, the best way to differentiate between acellular and cellular cementum is the presence of lacunae in cellular cementum.

Histology of the Pulp

PARTICIPATING CELLS

1. Odontoblasts (body and process)
Most distinctive cells of the pulp
Single layer
The cells are columnar in the coronal portion, cuboidal in the middle portion, flat in the apical portion

Individual odontoblasts communicate with each other via junctions. The number of odontoblasts corresponds to the number of dentinal tubules.
The lifespan of an odontoblast equals the one of a vital tooth.
The morphology of the odontoblasts reflects their functional activity.
(There are three stages that reflect the functional activity of a cell: active, transitional and resting)

The odontoblastic process

2. Fibroblasts
Most numerous cells
Produce collagen fibers and ground substance
Ground substance consists of: proteoglycans and glycoproteins
Again, active and resting cells
Fibroblasts have also capability to degrade collagen

3. Undifferentiated mesenchymal cells A pool of cells from which connective tissue cells can derive.
They are reduced with age.

4. Endothelial cells, Schwann cells, pericytes and immunocompetent cells

MATRIX

It is composed of fibers and ground substance
55% of the fibers are Type I collagen. 45% of the fibers are Type III collagen.
The ground substance is gelatinous in the coronal aspect and more fibrous in the apical.

VASCULARITY

Superior and inferior alveolar arteries that derive from the external carotids
Afferent side of the circulation: arterioles
Efferent side of the circulation: venules
Lymphatics

Small, blind, thin-walled vessels in the coronal region of the pulp and exit via one or two larger vessels.
 

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.

ERUPTION

. Root completion (approximately 50% of the root is formed when eruption begins)

Generally mandibular teeth erupt before maxillary teeth,

Primary teeth

I. Emerge into the oral cavity as follows:

           Maxillary                       Mandibular

Central Incisor                          7½ months                     6 months

Lateral incisor                           9 months                       7 months

Canine                                     18 months                      16 months

First Molar                               14 months                     12 months

Second Molar                          24months                       20 months

The sequence of  primary  tooth development is central incisor, lateral incisor, first molar, second molar

3. Hard tissue formation begins between 4 and 6 months in utero

4. Crowns completed between 1½ and 10 months of age

5. Roots are completed between I½ and3 yearsof age 6 to 18 months after eruption

6. By age 3 years all of the primary and permanent teeth (except for the third molars) are in some stage of development

7. Root resorption of primary teeth is triggered by the pressure exerted by the developing permanent tooth; it is followed by primary tooth exfoliation in sequential patterns

8. The primary dentition ends when the first permanent tooth erupts