Ameloblasts are
cells present only during tooth development that deposit
tooth enamel
Tooth enamel is one of the four major tissues that make up the tooth in humans and many other animals, including some species of fish. It makes up the normally visible part of the tooth, covering the crown. The other major tissues are dentin, ...
, which is the hard outermost layer of the
tooth
A tooth ( : teeth) is a hard, calcified structure found in the jaws (or mouths) of many vertebrates and used to break down food. Some animals, particularly carnivores and omnivores, also use teeth to help with capturing or wounding prey, ...
forming the surface of the crown.
Structure
Each ameloblast is a columnar cell approximately 4 micrometers in diameter, 40 micrometers in length and is hexagonal in cross section. The secretory end of the ameloblast ends in a six-sided pyramid-like projection known as the
Tomes' process Tomes's processes (also called Tomes processes) are a histologic landmark identified on an ameloblast, cells involved in the production of tooth enamel. During the synthesis of enamel, the ameloblast moves away from the enamel, forming a project ...
. The angulation of the Tomes' process is significant in the orientation of
enamel rods, the basic unit of tooth enamel. Distal terminal bars are junctional complexes that separate the Tomes' processes from ameloblast proper.
Development
Ameloblasts are derived from oral
epithelium
Epithelium or epithelial tissue is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. It is a thin, continuous, protective layer of compactly packed cells with a little intercellul ...
tissue of
ectoderm
The ectoderm is one of the three primary germ layers formed in early embryonic development. It is the outermost layer, and is superficial to the mesoderm (the middle layer) and endoderm (the innermost layer). It emerges and originates from t ...
al origin. Their differentiation from preameloblasts (whose origin is from inner enamel epithelium) is a result of signaling from the
ectomesenchymal cells of the
dental papilla. Initially the preameloblasts will differentiate into presecretory ameloblasts and then into secretory ameloblasts which lay down the tooth enamel. The differentiation from preameloblasts to ameloblasts occurs during the first stage of
amelogenesis, called the pre-secretory (or inductive) phase.
The ameloblasts will only become fully functional after the first layer of dentin (predentin) has been formed by
odontoblasts
In vertebrates, an odontoblast is a cell of neural crest origin that is part of the outer surface of the dental pulp, and whose biological function is dentinogenesis, which is the formation of dentin, the substance beneath the tooth enamel on ...
. The cells are part of the reduced enamel epithelium after enamel maturation and then subsequently undergo
apoptosis
Apoptosis (from grc, ἀπόπτωσις, apóptōsis, 'falling off') is a form of programmed cell death that occurs in multicellular organisms. Biochemical events lead to characteristic cell changes ( morphology) and death. These changes in ...
before or after tooth eruption.
These stages occur during the third and final stage of
amelogenesis, called the maturation phase.
There are various factors which can affect the differentiation and development of ameloblasts, causing abnormalities to form within the tooth structure. One example is the
BMP (bone morphogenetic protein,) which has an important role in ameloblast differentiation. When Follistatin, a
BMP inhibitor, is over expressed in the epithelium of developing teeth, the ameloblasts do not differentiate and no
enamel forms. Another example includes the conditional deletion of Dicer-1 in the epithelium of developing teeth may cause impaired differentiation of ameloblasts which results in deficient
enamel formation.
Life cycle
The life cycle of ameloblasts
consists of six stages:
# Morphogenic stage
# Organizing stage
# Formative (secretory) stage (Tomes' processes appear)
# Maturative stages
# Protective stage
# Desmolytic stage
The
murine
The Old World rats and mice, part of the subfamily Murinae in the family Muridae, comprise at least 519 species. Members of this subfamily are called murines. In terms of species richness, this subfamily is larger than all mammal families ex ...
ALC (ameloblast like cell)
cell line
An immortalised cell line is a population of cells from a multicellular organism which would normally not proliferate indefinitely but, due to mutation, have evaded normal cellular senescence and instead can keep undergoing division. The cell ...
is of ameloblastic origin.
1. Morphogenic Stage
In this morphogenic stage the morphology of the cell is short, columnar with large oval nuclei. The golgi apparatus and centrioles are located in the proximal end of the ameloblast and mitochondria are dispersed throughout the cytoplasm.
2. Organizing Stage
In this stage the ameloblast cell become longer and the nucleus migrates towards the proximal end.In contrast to this, golgi apparatus and centrioles migrate towards the distal end.This change is referred to as ''"REVERSAL OF POLARITY"'' .During this stage odontoblast starts laying down dentin.
''Reversal of Nutrition''- as long as the ameloblast is in contact with the dental papilla it receives nutrient material from the blood vessels of the tissue but due to formation of this dentin the original source of nutrition is cut off and ameloblast is supplied by capillaries penetrating the outer enamel epithelium. This change in nutrition source is referred to as reversal of nutrition.
3. Formative Stage
In this stage formation of enamel matrix begins during the formation of enamel matrix the ameloblast retain approximately the same length.
4. Maturative Stage
After the formation of enamel matrix mineralisation of enamel takes place which is known as maturation. During this stage the ameloblasts are slightly reduced in length. The stratum intermedium cells lose their cuboidal shape and assumed to be as spindle shape. During this stage ameloblasts also exhibits microvilli at their distal extremities.
5. Protective Stage
In this stage enamel is completely developed and fully calcified. Now the cell layers forms a stratified epithelial covering of enamel which is known as reduced enamel epithelium. This reduced enamel epithelium protects the mature enamel.
6. Desmolytic Stage
In this stage the reduced enamel epithelium proliferates and induce atrophy. The reduced enamel epithelium releases enzymes which destroy the connective tissue this process is known as desmolysis.
Function
Ameloblasts are
cells which secrete the
enamel proteins
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...
enamelin and
amelogenin which will later mineralize to form enamel, the hardest substance in the human body.
Ameloblasts control ionic and organic compositions of enamel. It is theorized that a circadian clock (24-hour) probably regulates enamel production on a daily cycle by the ameloblasts (similar to osteoblasts in production of bone tissue).
Ameloblasts adjust their secretory and resorptive activities to maintain favorable conditions for biomineralization.
Clinical significance
These cells are sensitive to their environment. One common example is illustrated by the neonatal line, a pronounced incremental
line of Retzius found in the primary teeth and in the larger cusps of the permanent first molars, showing a disruption in enamel production when the person is born. High fevers in childhood are also an example of bodily stressors causing interruptions in enamel production.
Another possible example of this sensitivity (stress response pathway activation) may be the development of
dental fluorosis after childhood exposure (between the ages of 2 to 8 years old) to excess consumption of fluoride, an elemental agent used to increase enamel hardness and as a result, prevent dental caries.
See also
*
Ameloblastin
*
Ameloblastoma
*
Amelogenesis imperfecta
*
Dentin
Dentin () (American English) or dentine ( or ) (British English) ( la, substantia eburnea) is a calcified tissue of the body and, along with enamel, cementum, and pulp, is one of the four major components of teeth. It is usually covered by e ...
*
Enamel
*
Odontoblast
*
Tooth development
Tooth development or odontogenesis is the complex process by which teeth form from embryonic cells, grow, and erupt into the mouth. For human teeth to have a healthy oral environment, all parts of the tooth must develop during appropriate st ...
*
List of human cell types derived from the germ layers
This is a list of cells in humans derived from the three embryonic germ layers – ectoderm, mesoderm, and endoderm.
Cells derived from ectoderm
Surface ectoderm Skin
* Trichocyte
* Keratinocyte
Anterior pituitary
* Gonadotrope
* Corti ...
References
External links
*
{{Authority control
Non-terminally differentiated (blast) cells
Animal cells
Human cells