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The premaxilla (or praemaxilla) is one of a pair of small cranial bones at the very tip of the upper jaw of many animals, usually, but not always, bearing teeth. In humans, they are fused with the maxilla. The "premaxilla" of theria has been usually termed as the incisive bone. Other terms used for this structure include premaxillary bone or os premaxillare, intermaxillary bone or os intermaxillare, and Goethe's bone.
It is formed from the fusion of a pair of small at the very tip of the of many animals, usually bearing teeth, but not always. They are connected to the maxilla and the nasals. While Johann Wolfgang von Goethe was not the first one to discover the incisive bone in humans, he was the first to prove its presence across mammals. Hence, the incisive bone is also known as Goethe's bone.
The first ossification centers in the area of the future premaxilla appear during the seventh week above the germ of the second incisor on the outer surface of the nasal capsule. After eleven weeks an accessory ossification center develops into the Alar plate of the premaxilla. Then a premaxillary process grow upwards to fuse with the frontal process of the maxilla; and later expands posteriorly to fuse with the alveolar process of the maxilla. The boundary between the premaxilla and the maxilla remains discernible after birth and a suture is often observable up to five years of age.
It is also common in non-mammals, such as chickens, that premaxilla is derived from medial nasal prominence. However, experiments using mice have shown a different result. The bone that has been called the "premaxilla" (incisive bone) in mice consists of two parts: most of the bone covering the face originates from the maxillary prominence, and only a part of the palate originates from the medial nasal prominence. This may be due to the replacement of most of the incisive bone with septomaxilla in the theria, as following section. In any case, the development and evolution of this region is complex and needs to be considered carefully.
In bilateral cleft lip and palate, the growth pattern of the premaxilla differs significantly from the normal case; Uterus growth is excessive and directed more horizontally, resulting in a protrusive premaxilla at birth.
Reptiles and most non-mammalian have a large, paired, intramembranous bone behind the premaxilla called the septomaxilla. Because this bone is vestigial in Acristatherium (a Cretaceous ) this species is believed to be the oldest known mammal. Intriguingly the septomaxilla is still present in .
However, embryonic and fossil studies in 2021 suggest that the incisive bone, which has been called "premaxilla" in theria, has been largely replaced by septomaxilla; and that only a palatal part of the incisive bone remains a vestige of premaxilla. If this hypothesis is accurate, the bones that have been called "premaxilla" in theria are not entirely homologous to the original premaxilla of other vertebrates. This homology is, however, contended.
The differences in the size and composition in the premaxilla of various families of is used for classification.
The premaxillae of Squamata are fused; this feature can be used to distinguish fossil squamates from relatives.
In the 1790s, Johann Wolfgang von Goethe began studying zoology, and formed the impression that all animals are similar, being bodies composed of vertebrae and their permutations. The human skull is one example of a metamorphosed vertebra, and within it, the intermaxillary bone rests as evidence linking the species to other animals.
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