Human dentition consists
of 20 deciduous teeth and 32 permanent teeth. Deciduous teeth are
shed during childhood to give way for the permanent teeth. Replacement
of teeth is common among animals. However, it does not happen in the commonly
used laboratory animal, the mouse. Mouse has 16 teeth which serve it for
whole of its life.
Different types of teeth
are classified according to their position and shape. The tooth series
in each jaw and on both sides ("quadrants") are the same even though the
shapes of individual teeth in the jaws vary.
Incisors (two in each
quadrant) and canines (one in each quadrant) are the front teeth.
Premolars (two in
each quadrant) and molars (three in each quadrant) are the posterior
Deciduous dentition has no
premolars and only two molars in each guadrant.
Mouse has only one
incisor in each quadrant, and behind a gap ("diastema"), three molars.
do teeth develop?
The basic development of
all teeth happens in the same way and bears similarities to the development
of many other epithelial appendages like hair, glands, and limbs (see illustrations).
In embryo, oral epithelial cells (or cells of the epithelial dental lamina)
grow into the underlying mesenchymal tissue. Subsequently, the epithelial
compartment, the enamel organ, and the mesenchymal tissues, the
papilla and the dental follicle, grow and form the final shape
of the tooth ("morphogenesis"). Different stages of tooth development are
called the bud, cap or bell stages according to the shape of the
tooth germ. Mesenchymal cells of the dental papilla that are adjacent to
the enamel organ, differentiate to odontoblasts and start to secrete
the inner hard tissue of teeth. Epithelial cells adjacent to differentiating
odontoblasts differentiate to ameloblasts, and secrete enamel,
the outer cover of teeth. Finally the roots develop and the tooth erupts
into the oral cavity. Secondary teeth are formed in the similar fashion,
but the development happens very slowly.
is the development regulated?
Research has shown that
tooth development is regulated by interactions of the epithelial and
mesenchymal tissues. The tissues interact reciprocally, sending signals
to each other. These signals are most often small proteins that are secreted
by one tissue and received by another. The cells interpretation of a received
signals then determines its response, growth, gene expression or even cell
death. It is obvious that there are subtle differences of these signals
during development of different teeth. Similar signals and responses are
used during development of many organs.
in tooth development
Scientists all over the
world study expression of different genes during development. The existing
knowledge of genes expressed during tooth development has been collected
to a WWW-database,
expression in tooth. The functions of genes are studied by using
laboratory animals (mouse and rat) or looking for gene defects that cause
abnormal tooth development in humans.
of tooth development
Numerous genetic and environmental
factors may cause abnormalities in tooth development. These may include
defects of structure
(e.g. abnormal enamel, amelogenesis imperfecta, or abnormal dentin, dentinogenesis
imperfecta or dentin dysplasia)
abnormal position of
reduced size and abnormal
of teeth (e.g. "peg-shaped" incisors, taurodontism, or short root anomaly)
missing of one or more
teeth (hypodontia, oligodontia, tooth agenesis).
Hypodontia refers to
congenital lack of a few teeth. The population frequency is over 5 % (missing
of wisdom teeth not included) (see publications).
Missing of one or more teeth
is perhaps our most common congenital malformation. More than 20 % of us
lack one or more wisdom teeth (third molars). More than five percent of
us lack one or more second premolars or upper second (lateral) incisors.
Lack of a large amount of teeth, though, is much more rare.
Oligodontia refers to
congenital lack of more than six teeth (wisdom teeth not included). The
population frequency is low, especially for cases when absence of teeth
is the only malformation ("isolated" cases). Most often oligodontia appears
as part of some congenital syndrome that affects several organ systems.
Anodontia refers to complete
lack of teeth, which is very rare.
ectodermal dysplasias, i.e.
defects of skin, hair, nails, teeth and ectodermal glands
oral clefting (cleft lip, cleft
palate, or cleft lip and palate)
Rieger syndrome, Char syndrome
Tooth agenesis, also
used as partial or selective tooth agenesis, may refer to all of
Most commonly missing
teeth are the third molars (wisdom teeth), second premolars and permanent
upper second (lateral) incisors.
Most rarely missing
teeth are the upper first (central) incisors. Missing of lower second (lateral)
incisors, all canines, first premolars and first molars or any of the deciduous
teeth is also rare.
Shapes and positions
of the existing teeth may also be abnormal in association with missing
teeth. The features often seen include "peg-shaped" upper second incisors,
taurodontism and malpositions (see publications).
of congenitally missing teeth
factors like virus infections, toxins and radio- or chemotherapy may cause
missing of permanent teeth. However, most of the cases are caused by genetic
factors. The heritability of congenitally missing teeth has been shown
in many studies. The genetic factors may be dominant or recessive and it
is obvious that in many cases multiple genetic (and environmental) factors
are acting together. The importance of genetic factors is shown by appearance
of multiple cases among relatives (familial clustering) and higher concordance
in identical than in non-identical twins.
of congenitally missing teeth has been shown both in hypodontia and
oligodontia. However in both cases the amount and identity of missing
teeth may vary between relatives. In hypodontia, the variability may extend
to no teeth actually missing ("reduced penetrance"). The variability is
probably caused by other genetic and environmental factors, and in some
cases the etiology is analogous to multifactorial traits.
An example of recessive
inheritance is given by recessive incisor hypodontia
(RIH). In this condition described by us, a recessive gene causes congenital
missing of several incisors, including lower permanent incisors and often
decidusous incisors, too) the inheritance is recessive.
for congenitally missing teeth
We already know several
genes which, when defective, cause congenitally missing teeth. The known
gene defects include mostly those that cause a multi-organ syndrome
(genes EDA, EDAR, EDARADD, IKKgamma, p63, IRF6, PITX2, TFAPB2, SHH,
OFD1). Only two genes are known so far where defects cause isolated
tooth agenesis. Dominant loss-of function mutations in MSX1
cause oligodontia. Identication of gene defects that cause isolated hypodontia,
the most common type of congenitally missing teeth, has been much more
Mutations in different genes
may cause different types of oligodontia (i.e. different sets of teeth
are missing). For example, all individuals with a mutation in MSX1 lack
all second premolars and third molars (and a variable number of other permanent
teeth). Typically, mutations in PAX9 cause agenesis of most permanent molars
(and again, a variable number of other permanent teeth). These differences
presumably reflect different functions of these genes during development
It is remarkable that all
genes mentioned above participate in signaling between cells or
of cell activity. Mutations that cause tooth agenesis have not been found
in genes that code structural proteins.