Composition and function of structures

Question 1

Describe the composition of enamel

Answer 1

· 96% hydroxyapatite

• 2% organic material
• 2% water
• Mineral ions may be replaced with other ions like Mg, Carbonate and F
• It is organised in rods/ prisms and inter-rod enamel (inter-prismatic substance)

Question 2

Describe the three stages of enamel formation

Answer 2

Pre- secretary:
· In the bell stage, the IEE cells differentiates to form ameloblasts. Simultaneously, IEE send signalling molecules to the dental papilla to differentiate into odontoblasts
· Once dentine is formed, it stimulates ameloblasts to form the enamel matrix; the organic component of enamel

Secretary:
· In this stage, ameloblasts develop a distal extension called the Tome’s process, which has a proximal and distal part
· Ribosomes within ameloblasts secrete enamel proteins, which are packagedas secretory granules from Golgi body, and then secreted by the proximal portion of the Tome’s process
· The first layer of enamel is laid down on dentine and is mineralised immediately; thus it doesn’t contain enamel rods
· Rod and inter-rod enamel are a result of the Tome’s process
* Once the Tome’s process disappears, the ameloblasts resemble how they looked initially. Thus, the last layer of enamel formed has no rods
* The enamel from the Tome’s process is deposited incrementally, resulting in lines called “Stria of Retzius” are visible in the enamel

Maturation:
· 25% ameloblasts undergo apoptosis, and then, the organic matrix and water are mineralised. This is achieved by a process called modulation
· In modulation, the ameloblasts undergo cyclic changes. They become ruffle ended ameloblasts and smooth ended ameloblasts
· Ruffle ended ameloblasts possess enzymes, which are secreted by their ruffle ended borders, designed to degrade in bulk the enamel matrix proteins
· As these proteins are broken into fragments of peptides, they diffuse into the smooth ended ameloblasts through leak junctions
· Calcium ATPase and Ca binding proteins from the ruffle ended ameloblasts help pump Ca ions into the degrading matrix, enabling mineralisation of the enamel matrix

Question 3

Describe the composition of dental pulp.

Answer 3

· The dental pulp is a soft connective tissue that normally is not mineralized
· Pulp cells are constantly renewed with a rapid turnover
· The principal cells of the pulp are the odontoblasts, fibro- blasts, undifferentiated ectomesenchymal cells, macro- phages, and other immunocompetent cells

It has four zones histologically;
1. Pulp core:which is in the centre of the pulp chamber with many cells and an extensive vascular supply. Very similar to the cell-rich zone
2. Cell rich zone: contains fibroblasts and undifferentiatedmesenchymal cells
3. Cell free zone: rich in capillaries and networks
4. Odontoblastic layer: containsodontoblasts which lies next to predentine

Question 4

Describe the composition of dentine

Answer 4

• 70% hydroxyapatite
• 20% organic matrix
• 10% water

Question 5

Describe the process of dentinogenesis

Answer 5

· Before differentiating into ameloblasts, the IEE secretes growth factors and signaling molecules which influence the differentiation of dental papilla cells to pre- odontoblasts. This happens in the bell stage
· Pre-odontoblasts grow and develop numerous organelles to form odontoblasts
· Dentine has two compartments. Mantle dentin, which is the layer closest to the enamel, and circumpulpal dentine
· Once odontoblasts are fully differentiated, they start to secrete collagenous and non-collagenous proteins in the extracellular region towards the IEE
· After the odontoblasts have formed the mantle dentine, they develop the odontoblastic process known as Tome’s fibers which keeps elongating as more organic matrix is deposited while the odontoblasts retreat towards the pulp
· In fully formed dentine, the Tome’s fibres are firmly embedded within dentinal tubules. Mantle dentine does not have dentine tubules
· The organic matrix is subsequentlymineralised
· Once the mantle dentine is secreted and mineralised, circumpulpal dentine isdeveloped

Question 6

Describe the mineralisation process of dentinogenesis

Answer 6

· There is a lag period between when dentine matrix is deposited then mineralised, resulting in the appearance of the pre-dentine layer
· Theodontoblasts secrete membrane bound matrix vesicles which contain enzymes that bind calcium and inorganic phosphate that initiate hydroxyapatite crystals
· These matrix vesicles are only found in mantle dentine

Question 7

Describe primary dentine

Answer 7

· Primary dentin makes up most of the tooth
· It is the thick bulk between enamel and pulp
· Primary dentin is the type of dentin a tooth starts out with
· It can also be called circumpulpal (surrounding the pulp) dentin
· Mantle dentin, a harder but thinner layer separates primary dentin from the tooth crown

Question 8

Describe secondary dentine

Answer 8

· Secondary dentin is made after a tooth has already appeared in themouth
· This type of dentin is made throughout life
· It is found between pulp and primary dentin

Question 9

Describe the composition of cementum

Answer 9

· Slightly softer than dentine
· 45-50% hydroxyapatite crystals
· 50 - 55% water and organic substances
· Organic substances include collagen and proteoglycans

Question 10

Describe the formation of cementum

Answer 10

the Hertwig Epithelial Root Sheath (HERS) must fragment
· Once the root sheath disintegrates, the root dentin comes into contact with the undifferentiated mysenchymal cells of the dental sac/ follicle
· This stimulates the activation of cementoblasts to begin cementogenesis
· In cementogenesis, cementoblasts lay down cementoid which is calcified to form cementum
* Sometimes, the cementoid may become trapped in the rapidly calcifying cementum, and become cementocytes

Question 11

Describe the composition of PDL (6)

Answer 11

· 70% water

· 5 Principal fibres

• Loose connective tissue (extracellular matrix, cells, nerves and blood vessels)
• Blast and clast cells (fibroblast, defence cells and undifferentiated mesenchymal cells)
• Oxytalan fibres (elastic in nature and inserts into cementum)
• Cell rest of Malassez (remaining HERS that didn’t disappear)

Question 12

Describe the composition of the alveolar bone

Answer 12

• 67% hydroxyapatite crystals
• 33% organic material like collagen and non-collagenous material
• Cellular component of bone consists of osteoblasts, osteocytes and osteoclasts
• Cancellous bone and cortical bone