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.


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 PDL. 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)

Bone tissue is deposited in layers by osteoblasts. These cells become trapped inside lacunae between layers of bone tissue, and differentiate into osteocytes. Bone tissue can either be compact and made of osteons, or spongy bone made of trabeculae. Bone tissue is almost entirely ECM, summarized in Table 4.1. Bone tissue is highly vascular—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 adipose connective tissue.

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 acellular and avascular. Enamel matrix is deposited in columns called enamel rods by cells called ameloblasts. Enamel is the strongest substance in the human body, due to its high mineral content, listed in Table 4.2. Like bone tissue, the ECM is mostly calcium hydroxyapatite, but instead of collagen fibers, enamel contains proteins including amelogenins and enamelins.

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, ameloblasts work on a circadian rhythm, depositing ECM at a regular pace for 4 days, then changing speed. The changes in speed lead to changes in enamel density, which can be seen as horizontal Lines of Retzius (or Striae of Retzius). Another pattern is visible because ameloblasts do not create an enamel rod in a perfectly straight line, but change direction slightly over days. This leads to the pattern known as Hunter-Schreger bands. The border between enamel and underlying dentin is a distinct line named the dentin-enamel junction (DEJ). Ameloblasts are lost during tooth eruption, and are therefore not found in the adult tissue. As a result, enamel does not undergo remodeling the way the remodeling unit 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 avascular, but it should not be called acellular. Dentin matrix is deposited by cells called odontoblasts. 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 odontoblastic process that extends nearly the entire thickness of the ECM that cell secreted. The odontoblastic process secretes more dentin (in a soft form that hardens later), as well as a fluid called dentinal fluid into the space known as a dentinal tubule. Dentin is not as strong as enamel, because of its lower mineral content, listed in Table 4.3. Dentin makes up the greatest bulk of teeth.

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 Lines of Retzius, dentin is deposited at different rates over days, which creates bands called the Imbrication lines of von Ebner. These lines run perpendicular to the dentinal tubules. Exceptionally-pronounced imbrication lines are named the Contour lines of Owen, 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 Tomes' granular layer. 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 mantle dentin, 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 areolar connective tissue. The most common cell types found within the pulp core are fibroblasts and mesenchymal stem cells. The pulp houses the blood, nerve and lymphatic supplies for each tooth. The pulp has 4 layers that appear distinct when using an H&E 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 ECM of cementum differs significantly enough from enamel that the two can be differentiated by their color (cementum is yellowish in hue, enamel has no color). Both cementum and dentin are yellowish, making those two difficult to differentiate by color. The ECM of cementum is deposited by cementoblasts, 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 remodeling, as it does not involve the activity of resorptive cells  in addition to cementoblasts.  Cementum incorporates more fluoride than the other hard tissues.

Cementum, like dentin and enamel, is an avascular tissue, but it can contain cells. There are two types of cementum based on histological (visual) studies from the 1830s: cellular cementum and acellular cementum. 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 cellular cementum, cementoblasts become trapped within the ECM and differentiate into cementocytes. In the mature tissue, cementocytes are found within lacunae, similar to bone tissue. However, canaliculi only extend in the direction of the (vascular) PDL. In the opposite direction is avascular dentin, which cannot provide cementocytes with nutrients. Cellular cementum is found in the apical half of the root.

Acellular cementum does not contain cementocytes, 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 (CDJ), is less distinct than the DEJ. 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 glycoproteins 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 periodontitis and orthodontia is complex and requires discussion of morphogens and the differentiation of resorptive cells. 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 periodontium is best left until we discuss the lineage 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

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

Cellular cementum contains intrinsic fibers and possibly extrinsic fibers. Intrinsic collagen 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 scaffold). 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 (PDL) is a region of dense regular connective tissue that connects the cementum to alveolar bone. Sharpey's fibers extend from one side of the PDL into cementum, and on the other side of the PDL into alveolar bone. Like the CDJ, 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 epithelial rests of Malassez. The PDL has a much higher degree of vascularity 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

Alveolar bone contains compact bone tissue, with a semi-unique feature of many tiny holes through which Sharpey's fibers from the PDL 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 cribriform plate, 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 periosteum is a layer of dense regular connective tissue that surrounds bone tissue. In addition to collagen fibers, it contains osteoblasts and osteoclasts involved in bone remodeling. Similar cells exist for cementum and dentin, but not enamel. Cementoclasts and odontoclasts 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 matrix metalloproteinase family. Unlike osteoclasts, they are not active continuously in a remodeling unit, they have no baseline activity and are typically only activated during tooth exfoliation.

Odontoclasts and cementoclasts likely differentiate from the same bone marrow stem cells that become osteoclasts. 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 hormone and growth factor signals. Regulation may also be very short-range: binding of the stem cell to proteins or glycoproteins within the ECM of each target tissue, providing the stem cell with positional information. 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 resorption does not always match bone resorption. For instance, in osteoporosis, it is bone resorption, not tooth root resorption, that leads to the loss of teeth. During tooth exfoliation, 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 cementoclasts are distinct cells found in the PDL and that they play a role in the maintenance of cementum throughout life. Other publications suggest cementoclasts are the same cell type as odontoclasts and osteoclasts. 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 exfoliation and orthodontic movement.

Summary of hard tissues


Table 4.6: Summary of hard tissues. * = denotes significant variation in percentages found in the literature depending on whether cementum is measured by weight or volume, or whether the CDJ is included or not.
Hard tissue Components
Enamel 96% Mineral: Calcium Hydroxyapatite
1% Protein: Amelogenins and Enamelins (not collagen)
3% water
Dentin 70% Mineral: Calcium Hydroxyapatite
20% Protein: Collagen, mostly
10% water
Cementum 50%* Mineral: Calcium Hydroxyapatite
50%* Protein: Collagen and glycoprotein mixture


Bone 60% Mineral: Calcium Hydroxyapatite
25% Protein: Collagen
15% water
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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Histology and Embryology for Dental Hygiene Copyright © 2020 by Laird C Sheldahl is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

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