Chapter outline

bone

Histology of bone tissue (review)

Bone tissue is deposited in layers by osteoblasts. These cells become trapped within 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, composed of:

66% Mineral: Calcium Hydroxyapatite
33% Protein: Collagen

Histology of Teeth

enamel rods

Histology of enamel

Enamel matrix is deposited in columns called enamel rods by cells called ameloblasts. Enamel is is the strongest substance in the human body, on account of its high mineral content:
96% Mineral: Calcium Hydroxyapatite
4% Protein: Amelogenins and enamelins, not collagen

enamel
Tooth of Paranthropus robustus SKX 21841 from Swartkrans by Didier Descouens, is liscensed CC BY 3.0
Ameloblasts have a circadian rhythm, deposting ECM at a regular pace for 4 days, then changing speed. The change in speed leads to a change in enamel density, which can be seen as Lines (or striae) of Retzius . Another pattern can be seen because an ameloblast does not create an enamel rod perfectly parallel, but changes directions slightly over days. This leads to the pattern known as Hunter Schreger bands. The border between enamel and underyling dentin is a distinct line named the Dentino-enamel Junction (DEJ).

dentin
Dentinal tubule occlusion of dentine discs after treatment by Peiyan Yuan, is liscensed CC BY SA 3.0

Histology of dentin

Dentin matrix is deposited by cells named odontoblasts. These cells can be found within the pulp cavity, immediately adjacent to the dentin. Each odontoblast has an arm-like extension running the entire thickness of the dentin it produced called an odontoblastic process. This process can secrete more dentin, as well as a fluid called dentinal fluid into the space known as a dentinal tubule. Dentin can either be peri-tubular dentin found lining each dentinal tubule, or inter-tubular dentin found between the tubules. Dentin is not as strong as enamel, being composed of:
70% Mineral: Calcium Hydroxyapatite
30% Protein: Collagen, mostly

dentin
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 perpindicular to the dentinal tubules. Exceptionally-pronounced imbrication lines are named Contour lines of Owen, and occur with exceptional changes in nutrition (such as childbirth).

dentin
Root dentin has a layer of speckles near the border with the cementum named Tome's granular layer. It has no known function, and no underlying cause... but it has a name!

enamel rods

Histology of pulp

The pulp chamber is composed of an inderterminate type of connective tissue, although it most resembles areolar CT. 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 under the microscope:
Odontoblast layer Odontoblast cell bodies
Cell-free zone Not actually free of cells, they just aren't visible
Cell-rich zone Cells are visible and densely packed
Pulp core Location of capillaries, nerve endings and lymphatic vessels

cementum

Histology of cementum

There are two types of cementum based on their appearance. Cellular cementum contains mature cementocytes within lacunae, similar to bone tissue. Cellular cementum can be 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. Cementum is produced by cells called cementoblasts, which differentiate into cementocytes in cellular cementum. The border between cementum and dentin-- the Dentino-cemental junction-- is less distinct than the DEJ because cementum and dentin have a shared lineage, whereas dentin and enamel do not. Cementum is slightly less strong as dentin, being composed of:
50% Mineral: Calcium Hydroxyapatite
50% Protein: Collagen and proteoglycan mixture

Histology of periodontal tissues

cementum

Histology of the perdiodontal ligament

The Periodontal Ligament (PDL) is a region of mostly dense regular CT that connects the cementum to alveolar bone. Sharpey's fibers extend from the PDL into cementum. Like the dentino-cemental junction, the border between cementum and PDL is blurred due to shared lineage. Clusters of rogue epithelial cells can be found within the PDL which are named the epithelial rests of Malassez.

periodontum

Histology of alveolar bone

Alveolar bone is compact bone tissue, with a semi-unique feature of containing dozens of tiny holes through which Sharpey's fibers from the PDL insert, as well as larger holes for the nerves and blood and lymphatic vessels than exit bone tissue and into the apical foramen of the tooth.

Summary of hard tissues

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 proteoglycan mixture
Bone 66% Mineral: Calcium Hydroxyapatite
33% Protein: Collagen