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SBL2 on Tooth Formation
Medicine (A100)
Queen Mary University of London
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Table of Contents
- Introduction:..........................................................................................................................................
- Formation of normal Enamel:...............................................................................................................
- Pre Secretory Stage:......................................................................................................................
- Secretory Stage:.............................................................................................................................
- Maturation Stage:..........................................................................................................................
- Amelogenesis Imperfecta (AI):...............................................................................................................
- The Genetic Code and Mutations:.......................................................................................................
- Conclusion:..........................................................................................................................................
- Bibliography:........................................................................................................................................
Introduction:..........................................................................................................................................
The young boy in this scenario has been diagnosed with a particular condition called X- Linked Amelogenesis Imperfecta, which is a rare inherited disease that seems to affect the enamel layer of deciduous and also permanent set of teeth (Gadhia et al., 2012). In this scientific report I will go through the formation of normal enamel and compare it to this form of malformed enamel, the four main types of AI, reasons that cause this disorder to occur, and also try to explain the various genes and mutations involved behind why the mother and father were not affected.
Formation of normal Enamel:...............................................................................................................
Dental enamel is one of the hardest and most highly mineralised tissue that can be found in the human body and serves as a layer of protection that lies just above the layer of dentine on the outermost of the crowns of all teeth as seen below in (Figure 1). Its main role is to form a barrier and protect the whole tooth from any sort of physical or chemical damage that may otherwise be injurious to the underlying pulp tissues where the blood vessels and nerves are located (Lacruz RS, 2017).
Figure 1 - The anatomy of a tooth, consisting of the various mineralised tissues called the enamel, dentine and cementum which all surround and protect the inner connective tissues called the pulp (Shargorodsky J, 2019).
In terms of the appearance of enamel, it varies from a pale yellow to grey to white tone as it shows in (Figure 2). Near the corners and edges of the teeth where there isn't any sort of dentin underlying, the colour may sometimes be slightly blue or translucent off-white tone (Nanci and Cate, 2008).
Figure 2 - The colour of regular healthy enamel (Kellogg and Latz, 2016)
The formation of enamel, also known as amelogenesis is a very intricate and complex process that is governed by various different factors all working together. It forms a major part of the tooth development process, known as Odontogenesis which begins from the 6th week of the fetal development and takes place at the stomatodeum, which is where the oral cavity starts to develop and the teeth start to erupt (Berkovitz et al., 2009). Although this
section is about enamel formation, in order to understand it better it would be beneficial to go through an overview of the process of odontogenesis.
Odontogenesis is divided into 3 distinct stages; bud, cap and the bell stage. During the first stage, cells called the dental epithelium begin to bud from a thick layer of cells called the dental lamina that forms the inside of both jaws. These cells then eventually develop and evolve to form the tooth germ, which is a mixture of all the soft tissues required for healthy dentition to grow (Colgate, 2018) Following that, is the cap stage in which cells begin to shape and mould to form the outer layer of the tooth, forming a cap like structure that sits on the rest of the tooth bud. This cap is known as the enamel organ as it will soon form different cells called ameloblasts involved in enamel formation. The tooth bud, known as the dental papilla would make up the two interior layers of the tooth; the dentin and the pulp both underlying the enamel. There is also another group of cells called the dental follicle that surrounds the enamel organ and the dental papilla. These sac of cells contains the blood vessels and the nerves (Simmer and Hu, 2001). The final bell stage is very important as both amelogenesis and dentinogenesis (the formation of dentin) take place alongside each other here - a very important concept known as reciprocal induction in which both processes are needed for each other to continue correctly (Nanci, 2014). This stage can then be subdivided into an early stage where there isn't any formation of dentine and the late stage where the first layer of dentine has fully been formed. This is important because the layer of dentine is very important for amelogenesis. In the bell stage, the enamel organ formed grows into a bell shape and it's cells begin to differentiate and change functions for amelogenesis to occur (Lodish, 2016). The figure below shows what the bell stage of odontogenesis looks like:
Figure 3 - The bell stage of odontogenesis (Nanci, 2014)
Amelogenesis itself is also made up of 3 different stages; the presecretory stage, the secretory stage and finally the maturation stage. Amelogenesis begins in the 11th week of fetal development in the bell stage of tooth development (Hu, Chun, Hazzazzi, & Simmer, 2007).
areas perhaps indicating the role it plays in enamel crystallite elongation. (Part of Amelogenesis)
Figure 4 - The life cycle of an Ameloblast cell (Berkovitz, Moxham and Holland, 2002)
2) Secretory Stage: In the secretory stage, the pre-ameloblasts that had formed in the last stage then differentiate further into mature secretory ameloblasts. These mature ameloblasts then also secrete proteins required for the enamel to be formed. One end of the ameloblasts then forms a cell extension known as the Tomes' process. This process can be seen in the figure below (Figure 5) and also in stage 4a of (Figure 4). The tomes process has a very important role in giving the prism like shape of the enamel as well as the secretion of more enamel proteins(some of those have been listed above already). The proteins are made at the ribosome in the rough endoplasmic reticulum and then packaged/modified at the Golgi apparatus. These products are then transported towards the Tomes' process in vesicles. This initial enamel layer that is forming is then quickly mineralised and then as the enamel matrix continues to lay down the ameloblasts start moving away from the matrix, causing the tomes process to elongate in length (Wöltgens, Lyaruu, Bronckers, Bervoets, & Van Duin, 1995). Over time as more and more enamel matrix is laid down the tomes' process begins to disappear leaving spaces in between. This is depicted in stages 5a of (Figure 4 above). These spaces are then filled by organic materials called the enamel sheath.
Figure 5 - An electron micrograph showing the Tomes’ processes(Berkovitz et al., 2002)
3) Maturation Stage: In the final stage; the maturation stage, the enamel tries to become fully mineralised. The ameloblasts go through many changes with regards to their morphology and histology. The cells begin to reduce and shorten itself in size and length as seen in stages 5b to 6 of (Figure 4), and subsequently causes around 50% of them to completely die out via apoptosis. The morphology of the borders of the cells interchanges from smooth to ruffled, tight junctions begin to appear linking the present ameloblasts together making them completely impermeable. This ruffled border is then involved in the secretion of more proteins such as Kallikrein 4, that causes the organic enamel matrix proteins to degrade creating room for mineralisation(Deposition of ions such as Ca2+ also released by the ruffled border) The ions involved include calcium (Ca2+) and phosphate(PO43-) (Berkovitz et al., 2009).
During the whole process of amelogenesis if there is a particular component that is unable to function normally it will cause the enamel to be malformed or be very weak. This can lead to a variety of different problems for a person, such as being more susceptible to decay and perhaps more harsher conditions in the future. Amelogenesis Imperfecta is a similar disorder which my nephew has been diagnosed with in this scenario.
Amelogenesis Imperfecta (AI):...............................................................................................................
Amelogenesis Imperfecta is an umbrella term that is related with an array of different inherited defects to the enamel causing various structural problems along with aesthetic. The various types of AI differ according to the hardness, surface smoothness, and thickness but most importantly their mode of inheritance (Hu et al., 2007). These conditions are also sometimes known as X-Linked or Autosomal, or dominant and recessive, for which the meanings will be explained in detail below. The terminology AI is only used for enamel malformations and is not concerned with any other sort of health issues (NORD, 2006). This may be the realistic reason to why the boy in the scenario has no other health issues apart from his teeth.
There are 4 main types of AI:
melogenesis Imperfecta Hypomaturation ( (Soames and Southam, 2005)
Figure 7 - Amelogenesis Imperfecta Hypomaturation/Hypoplasia ((Soames and Southam, 2005)
Because the whole process of amelogenesis is very dependent on genetic information and instructions, the genetic code makes up a very important part of the whole process. The genetic code in general and relating to AI will be discussed in more now.
The Genetic Code and Mutations:.......................................................................................................
Genetic code is basically the set of instructions in which proteins are secreted from your genetic material found in your nucleus. The primary subunit of proteins are called amino acids which are coded by four bases found in your DNA. These bases are as follows; Adenine, Cytosine, Guanine, and Thymine. A group of these containing 3 bases is called a codon which would be able to code for 1 specific amino acid. However due to there only being 20
different amino acids found in biological proteins, but there being 64 different codon possibilities, the genetic code is degenerate. This means that more than one codon can code for the same amino acid. For example AGT may code for Glycine but ACT also can code for Glycine. The genetic code is also said to be universal, this means that in all organisms the same DNA code would give rise to the same amino acid.
To make proteins, the DNA is firstly transcribed and then it is translated. During the transcription process the DNA double helix unwinds and unzips, and nucleotides present in the nucleoplasm bind to their complementary base on one particular stand of the DNA molecule. This continues for the whole gene, until MRNA is formed. The mRNA leaves the nucleus and undegoes translation to make the peptide. During this process tRNA molecules bring specific amino acids and bind it to their complementary codon. This occurs for the whole of the mRNA molecule until the amino acids join to form the polypeptide.
But sometimes this same DNA can undergo spontaneous random changes known as mutations. These change the structure of DNA and there are various types.
Conclusion:
To conclude, through my research I have covered the numerous amount of mutations that may have occured to the AMELX gene resulting in the secretion of the faulty amelogenin protein. The scenario states that the boy had been diagnosed by the doctor with the X- linked form confirming to me that there has been a mutation on the AMLX gene. This therefore meant that the ameloblast cells made very low quantity of this particular protein, thus quite low amount of enamel was formed in order to prevent attack to his teeth. This may be the reason as to why his teeth are discoloured.
Moreover, the reason that neither the mother or the father was affected was because the AMLX gene that had mutated is found on the X chromosome (Choices, 2016). This means that my sister must have been a carrier of the gene and the father must have been normal. The disorder is therefore recessive, and even though both parents are not affected the child can still be. This is well explained in the image found underneath depicting the way in which unaffected parents can create a child with the condition.
Figure 8 - Image showing X-Linked Recessive Inheritance
Bibliography:
Berkovitz, B. K. B., Holland, G. R. & Moxham, B. J. 2009. Oral Anatomy, Histology and Embryology , Mosby Elsevier
Berkovitz, B., Moxham, B. and Holland, G. (2002). Oral Anatomy, Histology and Embryology. 3rd ed. Edinburgh: Mosby.
CHOICES, N. 2016. Genetics - Genetic inheritance - NHS Choices.
Colgate, Dr Natalie Bradley., Kwon, Hyuk-Jae & Jiang, Rulang. (2018) Development of Teeth. Available at: colgate/en-us/oral-health/basics/mouth-and-teeth- anatomy/odontogenesis-5-stages-of-tooth-development [Accessed December 28, 2019]
Gadhia K, McDonald S, Arkutu N, Malik K (2012) 'Amelogenesis Imperfecta: an introduction' BDJ212:377-379. Available at: nature/articles/sj.bdj.2012.314 [Accessed December 27, 2019]
HU, J. C., CHUN, Y. H., AL HAZZAZZI, T. & SIMMER, J. P. (2007). Enamel formation and amelogenesis imperfecta. Cells Tissues Organs , 186, 78-85.
Kellogg and Latz Dentistry, (2016). Differences in Tooth Enamel. [image] Available at: http :// kelloggandlatz. com / baby - bottle - tooth- decay / [Accessed December 27. 2019].
Lacruz RS, Habelitz S, Wright JT, Paine ML. DENTAL ENAMEL FORMATION AND IMPLICATIONS FOR ORAL HEALTH AND DISEASE. Physiol Rev. 2017;97(3):939–993. Available at: ncbi.nlm.nih/pmc/articles/PMC6151498/ [Accessed December 27, 2019]
Lodish, H. (2015). Molecular cell biology. 7th ed. New York, United States: W.H & Co Ltd.
Nanci, A. and Cate T. (2008). Ten Cate's oral histology. 7th ed. St. Louis, Mo.: Mosby Elsevier.
NANCI, A. (2014). Ten Cate's Oral Histology: Development, Structure, and Function, Elsevier Health Sciences.
NORD, (2006). Amelogenesis Imperfecta. [online] NORD (National Organization for Rare Disorders). Available at: rarediseases/rare-diseases/amelogenesis-imperfecta/ [Accessed 28 Dec. 2019].
SBL2 on Tooth Formation
Module: Medicine (A100)
University: Queen Mary University of London
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