Anatomy, and Types of Endodontic Pathology : Normal Endodontium
The endodontium consists of mineralized (dentin) and nonmineralized (pulp) portions, and encompasses the functional-anatomic relationships of the dentin-pulp system.
The dentin consists of:
• odontoblasts and their processes
• dentinal tubules
• peritubular dentin
• intertubular dentin
• mantle dentin.
The odontoblasts are densely aligned along the inner surface of the dentin. Their processes extend through the entire dentin layer to the dentinoenamel junction, and are up to 5mm long depending on the dentin thickness.
The odontoblasts are interconnected through gap junctions as well as tight junctions. These cells synthesize the primary dentin, consisting of type I collagen, glycoproteins, and glycosaminoglycans.
This organic precursor later becomes mineralized at some distance from the odontoblast layer. The majority of the dentin is referred to circumpulpal dentin and is located between mantle dental and the pulp chamber. Mineralization begins only when the predentin has achieved a certain degree of histologic maturity.
The odontoblastic processes course within the dentinal tubules. Between the cytoplasmic membrane of the processes and the canal wall there is often a periodontoblastic space that contains tissue fluid and collagen fibrils,
as well as dentin matrix.
The very densely mineralized peritubular dentin covers the dentin canal wall. It is not found in predentin. Its thickness depends on patient age, but it may also be laid down as a defense against external influences. Dentinal
tubules with small lumens and thick peritubular walls appear as a translucent zone when viewed in ground sections under a light microscope;
this zone is called sclerosed dentin.
The dentinal tubules are separated from each other by less densely mineralized intertubular dentin.
The peripheral layer of dentin exhibits highly branching odontoblastic processes referred to as mantle dentin. In contrast to the circumpulpal dentin, this layer is less densely mineralized. Dental enamel is attached to the
mantle dentin. As demonstrated by R.M. Frank and coworkers, nerve fibers also extend to the dentinoenamel junction. Thus the vitality of dentin tissue is not the result of hydrodynamic transfer of irritation, as was long believed,
but rather for the most part on direct nerve conduction, i.e., the nerve endings are directly stimulated.
The pulp consists of loose connective tissue. In addition to odontoblasts it contains fibroblasts, replacement cells, and defense cells. The fibroblasts represent the largest cell population, and appear as inactive and active
cells, the latter producing the intercellular substance and collagen precursors. Replacement cells are undifferentiated mesenchymal cells that cannot be differentiated from fibroblasts by shape. These cells may replace odontoblasts as well as defense cells, and assume their functions.