Abnormalities of Teeth qxd

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Hypodontia and Oligodontia

Glossary

:  Anodontia: failure of teeth to develop (same as agenesis of teeth)

Hypodontia:  having less than 6 congenitally missing teeth. 

(partial anodontia)

Oligodontia:  having 6 or more congenitally missing teeth.

Hyperdontia: extra teeth, same as supernumerary teeth, may be single

or multiple as in CCD.

(*To see how the web site OMIM works, log on and enter hypodontia.)

Congenitally missing teeth (hypodontia and oligodontia) are not rare. Generations

of dental students have learned about ectodermal dysplasia, the best know of the

“missing teeth” syndromes. With the discovery of the Pax 9 gene, some light has

been shed on the molecular genetics of congenitally missing teeth. Pax 9 maps to

chromosome #14, it encodes a transcription factor that functions in the develop-

ment of derivatives of the pharyngeal pouches. Mice with Pax 9 mutations have

“craniofacial and limb anomalies and teeth fail to form beyond the bud stage”. A

family with a framshift mutation in Pax 9 had normal deciduous teeth but lacked

permanent molars in both the maxilla and mandible. It is transmitted as a dominant

trait. (Nature Genetics 24;18-19  2000 Jan , also European Journal of Oral Sciences

106; 38-43 1998)

The best known of the missing teeth syndromes is X-linked hypohidrotic ectoder-

mal dysplasia. Ectodermal derivatives such as hair, sweat glands, nails and teeth are

involved. Head hair, eye lashes and brows are sparse, nails are dystrophic and there

is marked oligodontia, rarely total anodontia. Diminished sweat glands leads to the

inability to regulate body temperature, a major disability in warm months in a hot

climate. According to your text ( Neville’s Oral & Maxillofacial Pathology 2nd ed),

there are 150 variants of this syndrome. Dominant, recessive and X-linked inheri-

tance have been reported. 

Slide #7 shows the mildest form of hypodontia, a single congenitally missing tooth,

in this case a bicuspid. Notice the deciduous molar has been retained. Slide #8 illus-

trates the other extreme, a 10 year old child missing virtually all permanent teeth

except 1st molars. Slides #9 and #10 are of a child with ectodermal dysplasia. There

is profound oligodontia and teeth that are present are cone shaped. Note the sparse

scalp hair, brows and lashes. (Remember X-linked disease are milder in females

because they enjoy partial protection thanks to lyonization.)

We will exit the subject of hypodontia/oligodontia by looking at Slide #11, an

example of a cone-shaped lateral incisor, a peg lateral, a form of microdontia. This

may be inherited as a dominant trait. If both parents have “peg laterals”, the

homozygous child will have total anodontia of succedaneous teeth. (Am. J. or Med.

Genetics 26:431-436  1987)