In biology, the term “supernumerary” describes an organ or structure present in a quantity beyond the normal number. These structures result from deviations from the standard body plan that occur during development before birth. The term is derived from Latin, with “super” meaning beyond and “numerus” meaning number. It is a broad descriptor that can apply to a wide range of additional parts, from those visible on the body’s exterior to those located internally.
Common Supernumerary Structures in Humans
A well-known supernumerary structure in humans is an extra tooth, a condition known as hyperdontia. These teeth can appear anywhere in the dental arch but are most frequently found in the upper jaw between the two front teeth; this specific type is called a mesiodens. The prevalence in permanent dentition ranges from 0.1% to 3.8%. The shape of these teeth can vary, with some being conical or peg-shaped, while others may be larger with multiple cusps.
Extra fingers or toes, a condition called polydactyly, are another common example. An extra digit can range from a small, soft tissue nub to a fully formed and functional finger or toe. This condition is most often seen on the little finger side of the hand or, less commonly, on the thumb side. Some extra digits contain bone but no joints, while others are complete with nails and the ability to move.
Supernumerary nipples, also called polythelia, have a prevalence between one in 18 and one in 40 people. These additional nipples appear along the embryonic “milk lines,” which are two vertical lines running from the armpit to the groin on each side of the body. They are often mistaken for moles and can range in completeness from a simple patch of hair to a miniature, milk-bearing breast. While present at birth, they might not be noticed until hormonal changes during puberty or pregnancy cause them to darken or swell.
Developmental and Genetic Origins
The formation of supernumerary structures is rooted in processes that occur during embryonic development. These additional body parts arise when a group of cells, such as a tooth or limb bud, splits or divides abnormally, causing two or more structures to grow where only one would. For instance, hyperactivity of the dental lamina—the band of tissue from which teeth develop—is a widely accepted explanation for the formation of extra teeth.
Genetics also plays a role in the occurrence of these traits. Some supernumerary conditions are inherited and run in families, often following an autosomal dominant pattern, which means only one parent needs to carry the gene for the trait to be passed on. In other cases, multiple supernumerary structures are associated with specific genetic syndromes. For example, multiple extra teeth are sometimes seen in individuals with conditions like cleidocranial dysplasia or Gardner’s syndrome.
Supernumerary Features in the Animal Kingdom
Supernumerary structures are not limited to humans and are observed across the animal kingdom. A well-known example is polydactyly in cats, famously associated with the “Hemingway cats.” This trait, which results in cats having more than the usual number of toes, is caused by a dominant genetic mutation. A cat only needs to inherit one copy of the gene to express the trait.
Another example is found in frogs, which are sometimes discovered with extra limbs. In many of these cases, the cause is not genetic but environmental. Infection by a parasitic flatworm, Ribeiroia ondatrae, during the tadpole stage can lead to the development of additional legs. The parasite burrows into the developing limb buds of the tadpole, causing a physical disruption that can lead to the limb splitting and growing into multiple, often malformed, legs.
Medical Considerations and Management
The medical significance of a supernumerary structure varies depending on its type, location, and development. Many instances, such as an isolated supernumerary nipple, are benign and do not require medical intervention. These are often addressed for cosmetic reasons if the individual chooses, and while surgical removal is an option, it is not medically necessary.
Other structures may cause functional or health-related problems. Supernumerary teeth, for example, can lead to dental issues like crowding, an improper bite, or the impaction of adjacent permanent teeth. In these situations, extraction of the extra tooth is often recommended to prevent further complications, based on a clinical and radiographic examination.
The management of polydactyly is decided on a case-by-case basis. A small, non-functional digit might be removed with a simple procedure, sometimes by tying a ligature around its base to cut off blood flow. More complex, fully formed digits that share bones or tendons with the hand or foot require surgery for functional or cosmetic reasons. The timing of such surgeries is in early childhood.