The pulp periphery: At the outer border of the pulp, odontoblasts and the so-called Hoehl’s cells form continuous layers. These postmitotic cells have the capacity to undergo terminal differentiation. Odontoblasts are implicated in the synthesis of collagen and noncollagenous extracellular matrix components. Some ECM proteins are phosphorylated (SIBLINGs), whereas others are non-phosphorylated. ECM components are implicated in predentin and dentin formation, followed by dentin mineralization. Due to a fixation artifact, the formation of a cell-free layer results in from fixation and dehydration. A cell-free area underlines odontoblasts and Hoehl’s cells, which do not appear on sections after adequate fixative perfusion. Fenestrated capillary loops infiltrate the layer formed by odontoblasts and Hoehl’s cells but do not cross the terminal junctions located between the distal odontoblast cell bodies nor penetrate within the predentin. In contrast, axons infiltrate the odontoblastic layer and penetrate into the predentin. A few axons penetrate into dentin tubules and occupy the inner dentin but are found only in the inner 150 micrometers (Fig. 11.1).

Pulp cells are present in the dental pulp. Fibroblasts (pulpoblasts) and fibrocytes are the prominent cells, with variable cell density. They are elongated, with thin spinous processes. A few macrophages, plasmocytes, mast cells, and leukocytes have been also identified. Pulpoblasts contain dense cytoskeletal proteins, including microfilaments, intermediary filaments, and microtubules. Immunostaining of alpha acetyl tubulin revealed the presence of microtubules (Fig. 11.2a, b) associated in cilium and basal corpuscle (Fig. 11.2c).

Capillaries connect arterioles and venules. Capillaries display continuous thick or thin endothelial lining. Fenestrated endothelial cells contribute to the control and balance between the intravascular compartment and the interstitial tissue. Lymphatic capillaries have been recognized at the pulp periphery. Beneath the odontoblast compartment, the so-called Hoehl subodontoblastic compartment constitutes a second layer of cells, which may differentiate and become a second generation of odontoblasts. The renewal of odontoblasts requires newly differentiated cells, and because odontoblasts are postmitotic cells, there is a need for mesenchymal cells taking origin in the stromal pulp.

Nerve fibrils penetrate into the pulp within the apical region. They are surrounded by a myelin sheath that surrounds the axon. The role of neuropeptides, including substance P, calcitonin gene-related peptide, neurokinin A, neuropeptide Y, and vasoactive intestinal polypeptide has been discovered. Neurotransmitters or neuromodulators presumably play a variety of functions, participating in paracrine, endocrine, and neurocrine forms of communication (Caviedes-Bucheli et al. 2008).

Pulp inflammatory cells include polymorphonuclear neutrophilic leukocytes and mast cells implicated in the defense mechanisms. Interleukin-8 expression is elevated in the irreversibly inflamed dental pulp, but lacking in the normal caries-free pulp (Huang et al. 1999).

CD45+ represented 0.94 % ± 0.65 % of cells obtained from the enzymatic digestion of the whole dental pulp cells. CD16 + CD14+ granulocyte/neutrophils (50.0 % ± 9.08 % represent the major subpopulation in CD 45+ cells, followed by CD3 T lymphocytes (32.58 ± 11 %), CD14+ monocytes (8.93 % ± 5.8 %), and HLA-DR highLin1+ dendritic cells (4.51 % ± 1.12 %). Minor subpopulations included CD3-CD56+ natural killer cells (2.63 % ± 1.15 %) and CD19 B lymphocytes (1.65 % ±0.89 %). In addition cells presenting a phenotype compatible with Foxp3/CD25 are seen in regulatory T lymphocytes (Gaudin et al. 2015).

Pulp cells are present for a limited period of time. Odontoblasts (Fig. 11.3a, b) are postmitotic cells that survive the initial period. Later, the so-called Hoehl’s cell layer becomes apoptotic and displays a turn over more rapidly than the odontoblasts (Fig. 11.3c, d). Within the dental pulp, some pulpoblasts underwent apoptosis; their nuclei display condensed chromatin (Fig. 11.3e). Cytoplasmic inclusions contribute to the restricted aging of pulpoblasts. After the complete degradation of pulp cells, apoptotic bodies are present in the pulp extracellular matrix. They are destroyed by phagocyte or by macrophages (Fig. 11.3f). Apoptosis contributes actively to the pulp defense mechanisms.

Carious pulps can be classified as being at the transitional stage, or displaying partial pulpitis, or total pulpitis and/or total necrosis (Di Nicolo et al. 2000).

If caries progresses slowly, there is time to form reactionary dentin or tertiary dentin (Bjorndal 2008). Pulp can be affected or infected. Bacteria are rarely seen in the unexposed pulp, whereas they are often sees in infected and necrotic pulps (Massler and Pawlak 1977). Microorganisms can reach the pulp via the dentin tubules. Often apoptotic pulpal cells at some distance from cavity preparations may display fragmentation of nuclei (Fig. 11.6a, b).

Such rapidly progressing carious lesions lead to the formation of an atubular dentin or to the complete absence of tertiary dentin. This rapidly progressing lesion leads to pulp necrosis and furthermore to periapical lesion formation. The formation of reactionary or reparative dentin is representative of the pulp response to the carious lesion. Dentin sclerosis, dead tracts, or reparative dentin are correlated with sex, age, and the type of surface location of the lesion (Stanley et al. 1983). Dentin sclerosis results from aging. The formation of translucent zone is observed in response to caries of the slow type and other mild irritations. The production of reparative dentin is directly related to the carious decay. Slowly progressing caries may become arrested caries lesions, with occlusion of the tubules by mineral deposits contributing to the formation of a “transparent zone” subjacent to the mineralized carious dentin. In this zone needle and rhombohedral crystals have been identified, together with hydroxyapatite and whitlockite crystals. They occlude the lumen of the tubules (Fig. 11.8a, b).

A gradual degradation of the dental pulp is seen. The density of the odontoblast layer slowly decreases (Fig. 11.7a), and finally, between the mineralized dentin and the pulp, an empty border is gradually inhabited.

Odontoblasts are postmitotic cells. After development of carious lesions, under pro-inflammatory stimuli, dental pulp cells can differentiate and produce reactionary or reparative dentin. The pro-inflammatory cytokine tumor necrosis factor α (TNFα) may be a mediator involved in dental pulp cell differentiation toward an odontoblastic phenotype. TNF-α-stimulated pulp cells display increased expression of DPP, DSP, DMP-1, and osteocalcin. The TNF-α differentiation of dental pulp cells toward an odontoblastic phenotype occurs via p38 and is negatively regulated by MMP-1 expression (Paula-Silva et al. 2009). Although there are no specific odontoblastic markers, osteocalcin, osteonectin, alkaline phosphatase, bone sialoprotein, and DSPP have been used as indicators of odontoblastic differentiation. Studies have shown that MMP-2 and MMP-9 inhibition is necessary for dentin matrices to mineralize alters dentin remodeling (Fanchon et al. 2004).

Bacterial invasion occurs in dentin that is stained bright red by 0.5 % basic fuchsine – propylene glycol solution, whereas caries-affected dentin that is bacteria-free stains pink. Collagen fibrils in infected dentin have lost their cross-banded appearance in transmission electron micrographs, indicating they are irreversibly denatured (Kuboki et al. 1977).

Reactionary dentin and reparative dentin are both strongly immunopositive for osteopontin (OPN), a phosphorylated protein of the SIBLING family, also implicated in intracellular cell signaling and inflammatory process (Aguiar and Arana-Chavez 2007). DMP-1 and collagen were associated and seem to be essential for reactionary and reparative dentin formation (Aguiar and Arana-Chavez 2010) (Figs. 11.6, 11.7, 11.8, 11.9 and 11.10).