- Bone tissue review
- Histology of teeth
- Histology of periodontal tissues
- Periodontal Ligament
- Alveolar bone
- Summary of hard tissues
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
Oral histology lab
- by Brian R. MacPherson, Ph.D. and James G. Tieman, Ed.D., at the University of Kentucky (we linked to this in the previous chapter as well)
Microanatomy Web Atlas
- by Gwen V. Childs, Ph.D., FAAA at the
University of Arkansas for medical sciences. Jump to the digestive system/tooth unit.
- by Gwen V. Childs, Ph.D., FAAA at the
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 .
|Components of bone tissue|
|66%||Mineral ECM: Calcium Hydroxyapatite|
|33%||Protein ECM: Collagen|
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 .
|Components of enamel|
|0.0%||Cells (there are no cells)|
|96%||Mineral ECM: Calcium Hydroxyapatite|
|4%||Protein ECM: Amelogenins and Enamelin (not collagen)|
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.
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.
|Components of dentin|
|–||Cells (odontoblasts cell bodies in pulp, odontoblastic processes in dentin)|
|70%||Mineral ECM: Calcium Hydroxyapatite|
|30%||Protein ECM: Collagen, mostly|
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).
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.
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.
|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 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.
|Components of cementum|
Cellular cementum (apical portion)
Acellular cementum (coronal portion)
|50%||Mineral: Calcium Hydroxyapatite|
|50%||Protein: Collagen and glycoprotein mixture|
Fibers in cementum
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.
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).
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).
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 ←.
|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|
Chapter review questions
Periodontal Ligament, a group of specialized connective tissue fibers that attach a tooth to alveolar bone.
A hard connective tissue which is mostly ECM, including collagen fibers and a calcium-phosphate crystal.
A differentiated connective tissue cell that secretes bone ECM, including collagen, calcium and phosphate.
Lakes, or spaces within cartilage of bone connective tissue where cells reside.
When one cell begins to look different from another. This process involves limiting cell fate by altering gene transcription to become more specialized.
The terminally differentiated cell type found in mature bone tissue (both compact and spongy), capable of repairing small amounts of damage.
Rod-like mineralized connective tissue structures found in spongy-bone.
Extra-Cellular Matrix: Materials in the body, but outside of cells. A network of macromolecules, such as fibers, enzymes, and glycoproteins, that provide structural and biochemical support to surrounding cells.
Containing blood vessels
A type of connective tissue composed primarily of adipocytes, which store high-energy triglyceride and other lipid molecules.
Without blood vessels
The basic unit of tooth enamel, 4 μm wide, composed of a tightly packed and organized mass of hydroxyapatite crystals, hexagonal in shape.
Cells present only during the embryonic period that deposit tooth enamel.
Ca5(PO4)3(OH), the inorganic ECM material composed of calcium, phosphate and hydroxide ions that makes up the bulk of bone tissue.
The main structural protein in the ECM of connective tissues
Proteins found in enamel matrix which regulate the initiation and growth of hydroxyapatite crystals during the mineralization of enamel.
Along with amelogenins, a second protein found in enamel, enamelins are necessary for the adhesion of ameloblasts to the surface of the enamel, and they bind to hydroxyapatite to promotes crystallite elongation.
Incremental growth lines seen in enamel.
When cut enamel is viewed under reflected light, curves in the pattern of enamel rods can be seen; these curves strengthen the enamel and prevent cracks from propagating through the tooth.
A process in tooth development in which the teeth enter the mouth and become visible.
Reorganization or renovation of existing tissues, either physiological or pathological. The process can either change the characteristics of a tissue such as in blood vessel remodeling, or result in the dynamic equilibrium of a tissue such as in bone remodeling.
Osteoblasts and osteoclasts working together to remove old bone tissue and replace it with new bone tissue, necessary for the maintenance of healthy bones.
A cell of neural crest origin that is part of the outer surface of the dental pulp, and whose biological function is the formation of dentin.
Arm-like extension of an odontoblast found within dentinal tubules.
The lymph or ECF found within dentinal tubules, surrounding the odontoblastic process
Tiny canals that run through dentin, from the pulp cavity up to near the DEJ or CDJ
Incremental lines in the peritubular dentin of the tooth that correspond to the daily rate of dentin formation.
Exceptionally-pronounced imbrication lines of von Ebner
A layer of dark granules that lie parallel to the outer surface of root dentin.
The outer layer of dentin closest to enamel, contains few odontoblastic processes.
A loose connective tissue composed of fibroblasts and other cell type, plus ground substance and all three fiber types.
The basic connective tissue cell type capable of secreting ECM components, including fibers and ground substance proteins.
Multipotent cells of a connective tissue that can differentiate into a variety of cell types, including osteoblasts, fibroblasts, chondrocytes, myoblasts, blood cells and adipocytes.
Hematoxylin and Eosin stain, typically turns proteins pink and DNA purple.
Differentiated cell that deposits cementum matrix.
Cells capable of removing a tissue, such as osteoclasts, cementoclasts and odontoclasts.
Cementum containing cells and collagen fibers anchoring the tooth to alveolar bone.
Cementum covering the cervical portion of the tooth root, important for attachment of the periodontal ligament (PDL) to the root surface.
Terminally differentiated cell found within mature cementum lacunae.
Microscopic canals between the lacunae of ossified bone or cementum, containing thin cell processes linking distant cells together.
A protein that has had sugar residues attached to it in the Golgi apparatus and is often secreted.
Inflammation that damages soft tissue and bone that anchors the teeh, including the gingiva plus PDL, cementum or alveolar bone tissue.
The treatment of irregularities in the teeth (especially of alignment and occlusion) and jaws, including the use of braces.
A substance whose non-uniform distribution governs the pattern of tissue development and pattern formation.
Specialized tissues that surround and support the teeth, including gingival tissue, PDL, cementum and alveolar bone.
The developmental history of a differentiated cell traced back to the embryonic cell from which it arises.
Ends of the principal fibers of the periodontal ligament embedded within cementum.
The main principal fiber group is the alveolodental ligament, which consists of five fiber subgroups: alveolar crest, horizontal, oblique, apical, and interradicular on multirooted teeth. Principal fibers other than the alveolodental ligament are the transseptal fibers.
Bundles of strong type I collagen that anchor the periosteum to bones by penetrating the outer layers of compact bone tissue
Fibers found within cementum oriented parallel to the root surface, mainly involved in the repair of cementum.
Extracellular matrix material involved in the repair of injured and missing tissues, allows stem cells to migrate into the injured area. It is usually replaced during regeneration.
A type of connective tissue found in tendons and ligaments that is composed primarily of parallel collagen fibers, secreted by fibroblasts, with very little ground substance.
The part of the jaws that holds the teeth.
Clusters of epithelial cells found within the PDL, remnants of Hertwig's Epithelial Root Sheath (HERS).
Sieve-like region of compact bone of the alveolar sockets or ethmoid bone.
A layer f dense regular connective tissue surrounding bones, also containing osteoblasts, osteo-progenitor cells and some osteoclasts.
A cell derived from bone marrow stem cells capable of demineralizing bone tissue by the secretion of acids and enzymes, releasing calcium into the blood.
Resorptive cell capable of demineralizing cementum by the secretion of acids and enzymes.
Resorptive cell capable of demineralizing dentin by the secretion of acids and enzymes.
Enzymes that degrade all kinds of extracellular matrix proteins, or process a number of bioactive molecules. They play a major role in cell proliferation, migration, differentiation, angiogenesis, and apoptosis.
A general term for peeling or shedding, in humans it refers to the loss of dead epithelial cells or structures (hair, teeth).
Undifferentiated or partially differentiated cells that can differentiate into various cell types, and proliferate to produce more of the same stem cell.
Signaling molecules secreted directly into the blood which travel throughout the body.
A signaling molecule capable of stimulating cell proliferation, wound healing, and cellular differentiation.
In pattern formation, the development of spatial organization in the embryo that results from cells differentiating at specific positions, which requires that the cells have positional values as in a coordinate system