4 Histology of tooth and periodontal tissues


Histology and animation of bone
Figure 4.1: Histology and illustration of compact bone tissue, highlighting cells and layers of ECM.

Overview

This chapter briefly covers the histology of the hard tissues enamel, dentin, cementum and bone, as well as the soft tissues of pulp and the . The definitions and clinical considerations of these tissues are covered in chapters on the development of these tissues (Chapters 8 through 11)

For more practice using histology images, we share these useful links

Histology of bone tissue (short review)

is deposited in layers by . These cells become trapped inside between layers of bone tissue, and into . Bone tissue can either be compact and made of osteons, or spongy bone made of . Bone tissue is almost entirely , summarized in Table 4.1. Bone tissue is highly —compact bone contains central canals and perforating canals, whereas in spongy bone the space between bony trabeculae is filled with red bone marrow or highly vascular .

Table 4.1: Components of bone tissue.
Components of bone tissue
<1% Osteocytes
66% Mineral ECM: Calcium Hydroxyapatite
33% Protein ECM: Collagen

Histology of teeth

animation of enamel rods
Figure 4.2: The ECM of enamel is laid down in rods next to other rods, each rod is secreted by one cell. In contrast, bone tissue is deposited in a circular layer upon circular layer, each cell working only in one layer.

Histology of enamel

Enamel shares some mineral characteristics with bone tissue, but it is and . Enamel matrix is deposited in columns called by cells called . Enamel is the strongest substance in the human body, on account of its high mineral content, listed in Table 4.2. Like bone tissue, the is mostly , but instead of fibers, enamel contains proteins including and .

Table 4.2: Components of enamel.
Components of enamel
0.0% Cells (there are no cells)
96% Mineral ECM: Calcium Hydroxyapatite
4% Protein ECM: Amelogenins and Enamelin (not collagen)

lines in enamel
Figure 4.3: Cross-section of tooth enamel, highlighting the Striae of Retzius and Hunter-Schreger bands. Image credit: “Tooth of Paranthropus robustus SKX 21841 from Swartkrans" by Didier Descouens, is licensed CC BY 3.0 / boxes and animation added

During embryonic development, work on a circadian rhythm, depositing at a regular pace for 4 days, then changing speed. The changes in speed leads to changes in enamel density, which can be seen as horizontal (or Striae of Retzius). Another pattern is visible because ameloblasts do not create an in a perfectly straight line, but change direction slightly over days. This leads to the pattern known as . The border between enamel and underlying dentin is a distinct line named the Dentin-Enamel Junction (). Ameloblasts are lost during , and are therefore not found in the adult tissue. As a result, enamel does not undergo the way the maintains bone tissue throughout life.


electron micrograph of dentinal tubules
Figure 4.4: Surface view of dentin. Image credit: "Dentinal tubule occlusion of dentine discs after treatment" by Peiyan Yuan, is licensed CC BY SA 3.0 / labels, arrows and animation added

Histology of dentin

Dentin, like enamel, is , but it should not be called . Dentin matrix is deposited by cells called . These cells—or more accurately, their cell bodies– are found within the pulp cavity, immediately adjacent to the dentin. Each odontoblast has an arm-like protrusion called an that extends nearly the entire thickness of the that cell secreted. The odontoblastic process secretes more dentin (in a soft form that hardens later), as well as a fluid called into the space known as a . Dentin is not as strong as enamel, because of its lower mineral content, listed in Table 4.3.

Table 4.3: Components of dentin.
Components of dentin
Cells (odontoblasts cell bodies in pulp, odontoblastic processes in dentin)
70% Mineral ECM: Calcium Hydroxyapatite
30% Protein ECM: Collagen, mostly

Histology of dentin pulp
Figure 4.5: Histology of dentin, highlighting the cell bodies of odontoblasts and the imbrication lines of Von Ebner, Image credit: "histological section of tooth" by Doc. RNDr. Josef Reischig, CSc. is licensed under CC BY-SA 3.0 / labels and animations added

Similar to the , dentin is deposited at different rates over days, which creates bands called the . These lines run perpendicular to the . Exceptionally-pronounced imbrication lines are named the , and occur with changes in nutrition (such as during childbirth).

illustration of dentin
Figure 4.6: Illustration of Tomes’ granular layer in root dentin (small dark spots)

Root dentin has a layer of speckles near the border with the cementum named . It has no known function. It is a good landmark when looking at dentin under the microscope, its presence indicates you are viewing root dentin, not , and a thin layer of cementum should be close by.


animation of pulp regions
Figure 4.7: Illustration of the histology of the pulp chamber.

Histology of pulp

The pulp chamber is composed of an indeterminate type of connective tissue, although it most resembles . The most common cell types found within the pulp core are and . The pulp houses the blood, nerve and lymphatic supplies for each tooth. The pulp has 4 layers that appear distinct when using an stain,  listed in Table 4.4.

Table 4.4: The four layers of the pulp chamber and their predominant features.
Layer Predominant feature
Odontoblast layer Odontoblast cell bodies
Cell-free zone Not actually free of cells, cells aren’t visible
Cell-rich zone Cells are visible and densely packed
Pulp core Location of capillaries, nerve endings and lymphatic vessels

Histology of periodontal tissues

animation of the PDL
Figure 4.8: Illustration of cementum, and surrounding tissues.

Histology of cementum

Cementum forms a thin layer (between 50 to 200 μM, see below) over the surface of root dentin, which is roughly the width of a hair. The of cementum differs significantly enough from enamel that the two can be differentiated by their color (cementum is yellowish in hue), whereas no significant color difference exists between cementum and dentin. The ECM of cementum is deposited by , which can be found on the surface of cementum throughout life. As a result, the thickness of cementum can triple in thickness between the ages of 10 and 70 years of age. This is not the same as bone , as it does not involve the activity of   in addition to cementoblasts.  Cementum incorporates more fluoride than the other hard tissues.

Cementum, like dentin and enamel, is an tissue, but it can contain cells. There are two types of cementum based on histological (visual) studies from the 1830s: and . More recent advances in molecular biology and genetics are yielding newer concepts, but the old classification system is still commonly used in clinical practice.

In , become trapped within the and into . In the mature tissue, cementocytes are found within , similar to . However, only extend in the direction of the (vascular) . In the opposite direction is dentin, which cannot provide cementocytes with nutrients. Cellular cementum is found in the apical half of the root.

does not contain , and can be found in the coronal half of the root as well as underneath cellular cementum in the apical portion (cementum is around 50 μM thick in the coronal region and 200 μM thick in the apical region).

The border between cementum and dentin, the Cementum-Dentin junction (), is less distinct than the . Cementum is slightly less strong than dentin, and is composed of the materials listed in Table 4.5. Some publications list the mineral component of cementum to be 60 to 65% (further reading here). The difference between these numbers reflects whether a layer of mostly un-mineralized at the CDJ is included or excluded. The glycoproteins are specific to cementum, therefore we have chosen to report measurements that include them.

Regardless of how we measure the mineral content of cementum, loss of bone, dentin and cementum in response to and is complex and requires discussion of and the of . Similarly, discussion of whether cementum should be categorized as a part of the tooth (based on its location and appearance) or a part of the is best left until we discuss the of tooth and periodontal tissues.

Table 4.5: Components of cementum
Components of cementum

Cellular cementum (apical portion)

Acellular cementum (coronal portion)

50% Mineral: Calcium Hydroxyapatite
50% Protein: Collagen and glycoprotein mixture

Fibers in cementum

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Fig. 4.9: Schematic of a cementoblast wrapping Principal Fibers (PF) with wing-like processes, then encircling those fibers with Intrinsic Fibers (IF) secreted by finger-like processes. Image credit: "Schematic diagram depicting how cementoblasts produce intrinsic fibers around principal fibers on extrinsic fiber-rich CIFC." by Tsuneyuki Yamamoto et al, Jpn Dent Sci Rev is licensed under CC BY-NC-ND 4.0

contains which travel perpendicular to the surface of the root, and are likely secreted by both and . These fibers connect all the way to alveolar bone, and represent the ends of the of the embedded within cementum (extrinsic fibers may be called , the same as bone tissue).

contains intrinsic fibers and possibly extrinsic fibers. are secreted by cementoblasts, and do not extend beyond cementum. They are oriented parallel to the root surface and are mainly involved in the repair of cementum (a ). More recent publications subdivide cellular cementum into Cellular Intrinsic Fiber Cementum and Cellular Mixed Stratified Cementum, based on the amount of extrinsic collagen fibers present. Cementum that does not contain extrinsic collagen fibers (closer to the cervical region) does not contribute to the attachment to bone tissue.

animation of the PDL
Figure 4.10: Illustration of the PDL, and surrounding tissues.

Histology of the Periodontal Ligament

The Periodontal Ligament () is a region of that connects the cementum to . extend from one side of the PDL into cementum, and on the other side of the PDL into alveolar bone. Like the , the border between cementum and PDL is blurred due to their shared lineage. Clusters of rogue epithelial cells can be found within the PDL which are named the . The PDL has a much higher degree of than other ligaments. It also has a more diverse and numerous population of cells (compare Fig. 4.14 to the link to dense regular connective tissue above).

illusrtation of the periodontium
Figure 4.11: Illustration of alveolar bone, and nearby tissues. Look for Tomes’ granular layer as a landmark, if needed.

Histology of alveolar bone

contains compact bone tissue, with a semi-unique feature of many tiny holes through which from the insert, as well as larger holes for the nerves, blood and lymphatic vessels that exit bone tissue and enter the apical foramen of a tooth. This makes the surface of the alveolar sockets appear porous, which is almost unique for the surface of bones. The porous surface is called the , which is not to be confused with the other region of compact bone riddled with tiny holes, the cribriform plate of the ethmoid bone (whose holes are for olfactory nerve endings, connecting the nasal cavity to the brain).

Resorptive cells in hard tissues

The is a layer of that surrounds bone tissue. In addition to fibers, it contains and involved in . Similar cells exist for cementum and dentin, but not enamel. and are capable of demineralizing cementum and dentin, respectively. Like osteoclasts, these cells express genes involved in acid secretion as well as digestive enzymes, including members of the family. Unlike osteoclasts, they are not active continuously in a remodeling unit, they have no baseline activity and are typically only activated during tooth .

and likely from the same bone marrow that become . The signals that determine whether the multi-potent stem cell differentiates into an osteoclast, odontoclast or cementoclast are currently not well understood. The body may regulate this process by long or medium-range and signals. Regulation may also be very short-range: binding of the stem cell to proteins or within the of each target tissue, providing the stem cell with . While roughly identical to osteoclasts under the microscope, odontoclasts and cementoclasts have important physiological differences. For instance, the hormone calcitonin inhibits osteoclasts but triggers odontoclast activity during root resorption. The significance of this detail is that root loss does not always match bone loss. For instance, in osteoporosis, it is bone loss, not tooth root loss, that leads to the loss of teeth. During tooth , however, root dentin is resorbed but alveolar bone undergoes both resorption and deposition, to form a pathway through bone above the permanent tooth and fill in alveolar socket space below.

Some publications suggest are distinct cells found in the and that they play a role in the maintenance of cementum throughout life. Other publications suggest cementoclasts are the same cell type as and . Without further study, it is hard to tell whether a cementoclast is a distinct cell type, or an osteoclast found near cementum. The difference may be semantic, like asking whether your teacher is still a teacher when you see them in the grocery store, or are they just another shopper? The mechanism of resorption is the same (acids and proteinases), but whether these cell are regulated the same is an important question during tooth and .

Summary of hard tissues

Hard tissue Components
Enamel 96% Mineral: Calcium Hydroxyapatite
4% Protein: Amelogenins and Enamelins (not collagen)
Dentin 70% Mineral: Calcium Hydroxyapatite
30% Protein: Collagen, mostly
Cementum 50% Mineral: Calcium Hydroxyapatite
50% Protein: Collagen and glycoprotein mixture
Bone 67% Mineral: Calcium Hydroxyapatite
33% Protein: Collagen
scientific figure of tooth histology
Figure 4.14: Summary of the histology of mouse tooth and periodontal tissues using various staining techniques. Legend: AEFC: acellular extrinsic fiber cementum, CIFC = cellular intrinsic fiber cementum, H&E, TB and AB-NFR: different tissue stains. Image credit: "Serial sections of the 44 dpn mouse mandibular first molar were stained with (a–d) H&E, (e–h) TB and (i–l) AB-NFR" by Brian L Foster is licensed under CC BY-NC-ND 4.0

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