THE DECIDUOUS DENTITION

I. The Deciduous Dentition

-It is also known as the primary, baby, milk or lacteal dentition.

diphyodont, that is, with two sets of teeth. The term deciduous means literally 'to fall off.'

  There are twenty deciduous teeth that are classified into three classes. There are ten maxillary teeth and ten mandibular teeth. The dentition consists of incisors, canines and molars.

Clinical importance of cementum

1) Deposition of cementum continues throughout life.
The effects of the continuous deposition of cementum are the maintenance of total length of the tooth (good) and constriction of the apical foramen (bad).
2) With age, the smooth surface of cementum becomes more irregular due to calcification of some ligament fiber bundles. This is referred to as spikes.

Behavior of cementum in pathologic conditions

1. Errors in development. These are usually genetic.

a. Variability of the individual teeth. In general, the teeth most distal in any class are the most variable.

b. Partial or total anodontia. missing teeth in children,

c. Supernumerary teeth.

d. Microdontia

e. Macrodontia

F. Microdontia

2. Errors in skeletal alignment. Malpositioned jaws disrupt normal tooth relationships.

3. Soft tissue problems.

-Ocasionally, the proper eruption of a tooth is prevented by fibrous connective tissue over the crown of the tooth.

-In the mixed dentition, the deciduous second molars have a special importance for the integrity of the permanent dentition. Consider this: The first permanent molars at age six years erupt distal to the second deciduous molars.

-Permanent posterior teeth exhibit physiological mesial drift, the tendency to drift mesially when space is available. If the deciduous second molars are lost prematurely, the first permanent molars drift anteriorly and block out the second premolars.

An incisor diastema may be present. The plural for diastema is diastemata.

-Important: The deciduous anteriors--incisors and canines are narrower than their permanent successors mesiodistally.

-Important: The deciduous molars are wider that their permanent successors mesiodistally.

-This size difference has clinical significance. The difference is called the leeway space.

The leeway space in the lower arch is approximately 3.4 mm.

-The leeway space in the upper arch is approximately 1.8 mm. In normal development, the leeway space is taken up by the mesial migration of the first permanent molars.

TOOTH MORPHOLOGY

Descriptive anatomy

• Median sagittal plane: the imaginary plane in the center that divides right from left.
• Median line: an imaginary line on that plane that bisects the dental arch at the center.
• Mesial: toward the center (median) line of the dental arch.
• Distal: away from the center (median) line of the dental arch.
• Occlusal plane: A plane formed by the cusps of the teeth. It is often curved, as in a cylinder. We will speak often of the occlusal surface of a tooth.
• Proximal: the surface of a tooth that is toward another tooth in the arch.
• Mesial surface: toward the midline.
• Distal surface: away from the midline.
• Facial: toward the cheeks or lips.
• Labial: facial surface of anterior teeth (toward the lips).
• Buccal: facial surfaceof anterior teeth (toward the cheeks).
• Lingual: toward the tongue.
• Occlusal: the biting surface; that surface that articulates with an antagonist tooth in an opposing arch.
• Incisal: cutting edge of anterior teeth.
• Apical: toward the apex, the tip of the root.

Structure

There are 3 pairs

 The functional unit is the adenomere.

The adenomere consists of secreting units and an intercalated duct, which opens, in a striated duct.

An secreting unit can be:

- mucous secreting

- serous secreting

THE SECRETING UNIT

THE CELLS

Serous cells

(seromucus cells=secrete also polysaccharides), They have all the features of a cell specialized for the synthesis, storage, and secretion of protein
 Pyramidal, Nuclei are rounded and more centrally placed,  In the basal 1/3 there is an accumulation of Granular EPR,  In the apex there are proteinaceous secretory granules,  Cells stain well with H & E (red),  Between cells are intercellular secretory capillaries

Rough endoplasmic reticulum (ribosomal sites-->cisternae)
Prominent Golgi-->carbohydrate moieties are added
Secretory granules-->exocytosis
The secretory process is continuous but cyclic
There are complex foldings of cytoplasmic membrane
The junctional complex consists of: 1) tight junctions (zonula occludens)-->fusion of outer cell layer, 2) intermediate junction (zonula adherens)-->intercellular communication, 3)desmosomes-->firm adhesion

Mucus cells

Pyramidal,  Nuclei are flattened and near the base,  Have big clear secretory granules

Cells do not stain well with H & E (white)

Production, storage, and secretion of proteinaceous material; smaller enzymatic component
-more carbohydrates-->mucins=more prominent Golgi
-less prominent (conspicuous) rough endoplasmic reticulum, mitochondria
-less interdigitations
 

Myoepithelial cells

Star-shaped, Centrally located nucleus, Long cytoplasmic arms - bound to the secretory cells by desmosomes, Have fibrils like smooth muscle, Squeeze the secretory cell

One, two or even three myoepithelial cells in each salivary and piece body, four to eight processes
Desmosomes between myoepithelial cells and secretory cells myofilaments frequently aggregated to form dark bodies along the course of the process. The myoepithelial cells of the intercalated ducts are more spindled-shaped and fewer processes
Ultrastructure very similar to that of smooth muscle cells (myofilaments, desmosomal attachments)
 

Functions of myoepithelial cells
-Support secretory cells
-Contract and widen the diameter of the intercalated ducts
-Contraction may aid in the rupture of acinar cells of epithelial origin

Ductal system

Three classes of ducts:
Intercalated ducts

They have small diameter; lined by small cuboidal cells; nucleus located in the center. They have a well-developed RER, Golgi apparatus, occasionally secretory granules, few microvilli. Myoepithelial cells are also present. Intercalated ducts are prominent in salivary glands having a watery secretion (parotid).
Striated ducts

They have columnar cells, a centrally located nucleus, eosinophilic cytoplasm. Prominenty striations that refer to indentations of the cytoplasmic membrane with many mitochondria present between the folds. Some RER and some Golgi. The cells have short microvilli.
The cells of the striated ducts modify the secretion (hypotonic solution=low sodium and chloride and high potassium). There is also presence of few basal cells.
Terminal excretory ducts

Near the striated ducts they have the same histology as the striated ducts. As the duct reaches the oral mucosa the lining becomes stratified. In the terminal ducts one can find goblet cells, basal cells, clear cells. The terminal ducts alter the electrolyte concentration and add mucoid substance.

Connective tissue
Presence of fibroblasts, inflammatory cells, mast cells, adipose cells
Extracellular matrix (glycoproteins and proteoglycans)
Collagen and oxytalan fibers
 

 Nerve supply
The innervation of salivary glands is very complicated. There is no direct inhibitory innervation. There are parasympathetic and sympathetic impulses, the parasympathetic are more prevalent.
The parasympathetic impulses may occur in isolation, evoke most of the fluid to be excreted, cause exocytosis, induce contraction of myoepithelial cells (sympathetic too) and cause vasodialtion. There are two types of innervation: epilemmal and hypolemmal. There are beta-adrenergic receptors that induce protein secretion and L-adrenergic and cholinergic receptors that induce water and electrolyte secretion.

Hormones can influence the function of the salivary glands. They modify the salivary content but cannot initiate salivary flow.

Age changes

Fibrosis and fatty degenerative changes
Presence of oncocytes (eosinophilic cells containing many mitochondria)

Clinical considerations

Role of drugs, systemic disorders, bacterial or viral infections, therapeutic radiation, obstruction, formation of plaque and calculus.

    - Rich capillary networks surround the adenomeres.

As root and cementum formation begin, bone is created in the adjacent area. Throughout the body, cells that form bone are called osteoblasts. In the case of alveolar bone, these osteoblast cells form from the dental follicle. Similar to the formation of primary cementum, collagen fibers are created on the surface nearest the tooth, and they remain there until attaching to periodontal ligaments.

Like any other bone in the human body, alveolar bone is modified throughout life. Osteoblasts create bone and osteoclasts destroy it, especially if force is placed on a tooth. As is the case when movement of teeth is attempted through orthodontics, an area of bone under compressive force from a tooth moving toward it has a high osteoclast level, resulting in bone resorption. An area of bone receiving tension from periodontal ligaments attached to a tooth moving away from it has a high number of osteoblasts, resulting in bone formation.