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In biology, dactyly is the arrangement of digits (fingers and toes) on the hands, feet, or sometimes wings of an animal. It comes from the Greek word δακτυλος, meaning "finger". more...

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Sometimes the ending "-dactylia" is used. The adjectival forms end with "-dactyl" or "-dactylous".

By number

Pentadactyly is the condition of having five digits on each limb. All land vertebrates are descended from an ancestor with a pentadactyl limb, although many groups of species have lost or transformed some or all of their digits.

Tetradactyly is the condition of having four digits on a limb, as in amphibians and many birds

Tridactyly is the condition of having three digits on a limb, as in some birds and ancestors of the horse such as Protohippus and Hipparion.

Bidactyly or didactyly is the condition of having two digits on each limb, as in the Two-toed Sloth, Choloepus didactylus. In humans this name is used for an abnormality in which the middle digits are missing, leaving only the thumb and fifth finger.

Monodactyly is the condition of having a single digit on a limb, as in modern horses.

Syndactyly is a condition where two or more digits are fused together. It occurs normally in some mammals, such as the siamang. It occurs as a rare abnormality in humans.

In birds

Anisodactyly is the most common arrangement of digits in birds, with three toes forward and one back. This is common in songbirds and other perching birds, as well as hunting birds like eagles, hawks, and falcons.

Syndactyly in birds is like anisodactyly, except that the third and fourth toes (the outer and middle forward-pointing toes) are fused together, as in the Belted Kingfisher, Ceryle alcyon.

Zygodactyly (from Greek ζυγον, a yoke) is an arrangement of digits in birds, with two toes facing forward (digits 2 and 3) and two back (digits 1 and 4). This arrangement is most common in arboreal species, particularly those that climb tree trunks or clamber through foliage. Zygodactyly occurs in the woodpeckers and flickers, nuthatches, and parrots.

Heterodactyly is like zygodactyly, except that it is digits 3 and 4 that point forward and digits 1 and 2 that point back. This is only found in trogons.

Other terms

An excess of digits is called hyperdactyly or polydactyly, such as in the extremely rare case that a person has six fingers or toes on a single hand or foot.

A lack of digits not caused by an amputation is called hypodactyly.

Ectrodactyly is the congenital absence of all or part of one or more fingers or toes. This term is used for a range of conditions from aphalangia (in which the some of the phalanges or finger bones are missing), to adactyly (the absence of a digit).


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Ectodermal dysplasias
From Dental Assistant, The, 5/1/04 by Ronald J. Jorgenson


The ectodermal dysplasias are a complex group of disorders that should be recognized by all members of the dental profession. After all, there are dozens of such disorders and the teeth are involved in a significant way in many of them. A brief review of embryology and genetics will help dental assistants recognize the disorders and appreciate how to deal with the dental manifestations.

For years the consensus opinion has been that the ectodermal dysplasias (EDs) are inherited disorders that affect two or more derivatives of the embryonic ectoderm. To say that the EDs are inherited means that they are caused by mutations of various genes and in families are transmitted across the generations in one or another of the patterns of inheritance: autosomal dominant, autosomal recessive, or X-linked. To say that derivatives of the embryonic ectoderm are involved means that the hair, nails, teeth, sweat glands, and other such structures are abnormally formed.

As the title of this article suggests, there are many types of ED. The first comprehensive book on the subject (1) listed more than one hundred types. Since the book's publication several dozen more have been reported in the medical and dental literature. The types of ectodermal dysplasia are distinguished by the specific permutation of ectodermal defects and precise mode of inheritance. The words specific and precise were chosen carefully in the preceding sentence. The specific combination of ectodermal defects is different from one ED to the next. One type may involve specifically the hair, teeth and sweat glands (Figure One), while another type might involve specifically the hair and nails (Figure Two). There is no overlap among the different types of ED. In other words, all people affected by the type of ED shown in Figure One have pretty much the same clinical features, while those affected by the type of ED shown in Figure Two have pretty much the same features, different than those shown in Figure One. And, the mode of inheritance for the types of ED is precise enough that patterns of inheritance can be used to help distinguish among the types and can be used to determine the risk for relatives of an affected person to be affected. Of course, nothing is simple in genetics, and genetic heterogeneity complicates inheritance for some types of ED. Still, most EDs are inherited in one precise way or another.


Common Types of Ectodermal Dysplasia

The earliest type of ED to be recognized in the world's literature was hypohidrotic ectodermal dysplasia (HED). (2) Early reports, and some current ones, call this type of ED anhidrotic, but since affected people do sweat, albeit in limited amounts, hypohidrotic (meaning reduced sweating) is more appropriate than anhidrotic (meaning total absence of sweating). This type of ED is characterized by sparse blonde hair, a significant reduction in the number of teeth, inability to sweat normally in response to heat, and some minor features such as increased skin pigmentation around the eyes and a distinctive facial appearance (Figure One). For many years, HED was thought to be inherited only as an X-linked recessive trait, but recent work has shown that it also may be inherited as an autosomal dominant and autosomal recessive trait. The genetic defect responsible for many cases of this type of ED has been identified, making DNA diagnostic studies possible.

Treatment for HED is successful. Wigs may be used to mask the sparseness of hair, although many affected males notice a slight increase in the amount of hair as they age. Dentures may be made for children as young as 3-4 years of age to replace the missing teeth. Dental implants may be successful at somewhat later ages. Finally, cooling devices and learned behavior in hot weather may be successful in preventing overheating.

Clouston Ectodermal Dysplasia

Not long after HED was documented in the literature. Dr. Clouston in Canada described a type of ED in which sweat function was normal. (3) This led to the first classification of the EDs: the hidrotic type (Clouston syndrome) and the anhidrotic (sic) type (HED). (4)

People with the Clouston syndrome have characteristic thin, dry hair, and thick nails that are often discolored (Figure Two). Dental abnormalities may be seen in the syndrome, although the predominant features involve the hair and nails. Sweat function is normal. Unlike HED, but like most of the ED's, Clouston syndrome is inherited only in one way: autosomal dominant.

Ectrodactyly-Ectodermal Dysplasia-Clefting (EEC) Syndrome

It seems appropriate to describe a complex ED syndrome at this juncture to illustrate that the ectoderm does not develop in a vacuum. There are subtle, but meaningful interactions of the ectoderm and mesoderm that lead to syndromes in which derivatives of the ectoderm and mesoderm coexist. Still, many of these complex syndromes are treated as types of ED for classification purposes and for peer support--a person with the EEC syndrome, for instance, might seek support from both the National Foundation for Ectodermal Dysplasias and from any one of the cleft palate support groups.

People with the EEC syndrome have the characteristics of HED: sparse blonde hair, absence of most teeth, and inability to sweat normally in response to heat (Figure Three). They also have cleft lip and/or cleft palate and ectrodactyly (lobster claw deformity of hands or feet). While EEC syndrome is thought to be inherited as an autosomal dominant trait, there is some question about how many types of EEC syndrome there are and how it is related to other syndromes that share some or all of its features.


The Importance of Ectodermal Dysplasia in Dental Practice

Abnormalities of the teeth are some of the most consistent features of the EDs. Teeth may be congenitally absent as in HED, may be malformed as in otodental dysplasia, (5) or only the enamel may be affected as in the tricho-dento-osseous syndrome. (6) Many cases, therefore, are first diagnosed in dental offices by dentists, hygienists, or assistants who know what to look for inside the oral cavity and make proper associations with physical features outside the oral cavity.

Whether the diagnosis is made in the dental office or elsewhere, the dentist and dental staff will be involved in discussions about the manifestations and treatment for the EDs. It is critical, then, for dental professionals to know something about the range of EDs, that treatment is possible early in life, that dentures can be made for children, that implants work for congenital malformations as well as for replacement of teeth lost for other reasons, and that dental treatment is often necessary for habilitation or rehabilitation of people affected by this important group of disorders.

The dentist and dental staff must also recognize the genetic basis for these disorders, and be able to provide genetic counseling. After all, the EDs are genetic disorders and the first case seen in the office may be the harbinger of other cases in practices that see more than one member of a family. If the dental staff chooses not to provide counseling, referral to a genetic counselor is imperative.


The EDs represent a large group of disorders, one or more of which will certainly be seen in every dental office. Each member of the team in a dental office must be trained to recognize the dental and nondental manifestations of the EDs for patients to receive early and appropriate diagnosis, treatment and counseling. The EDs are as much "dental disorders" as they are "medical disorders" and must be managed at the entry point to the health professions, which for the EDs is often the dental office.


(1.) Freire-Maia N and Pinheiro M: Ectodermal Dysplasia: A Clinical and Genetic Study. AR Liss, New York, 1984.

(2.) Thurman J: Two cases in which the skin, hair and teeth were very imperfectly developed. Proc R M Chirurg Soc 31: 71-82, 1848.

(3.) Clouston HR: A hereditary ectodermal dystrophy. Can Med Assoc J 21: 18-31, 1929.

(4.) Clouston HR: The major forms of hereditary ectodermal dysplasia. Can Med Assoc J 40: 1-7, 1939.

(5.) Levin LS and Jorgenson RJ: Oto-dental dysplasia--a previously undescribed syndrome. Birth Defects 10(4): 310-312, 1974.

(6.) Lichtenstein JR, Warson R, Jorgenson RJ, Dorst J and McKusick VA: The tricho-dento-osseous (TDO) syndrome. Am J Hum Genet 24: 568-582, 1972.


Hypohidrotic ED (HED)

Limited ability to sweat

Sparse blonde

Reduction in number of teeth

Increased pigmentation near eyes

Distinctive facial appearance

Clouston ED (CED)

Thin dry hair

Thick nails (often discolored)

Occasional dental abnormalities

Ectrodactyl-ED-Clefting (ECC)

Characteristics of HED

Absence of most teeth

Inability to sweat normally

Cleft lip or cleft palate

Lobster claw deformity of hands or feet

Ronald J. Jorgenson, DDS, PhD, FACMG

Chairman, Scientific Advisory Board:

National Foundation for Ectodermal Dysplasias

410 Fast Main, PO Box 114

Mascoutah, Illinois 62258-0114

Phone (618) 566-2020 Fax (618) 566-4718

E-Mail Web Site

COPYRIGHT 2004 American Dental Assistants Association
COPYRIGHT 2004 Gale Group

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