| Premaxilla | |
|---|---|
 Skull ofSpinosaurus aegyptiacus, premaxilla in orange | |
 Human premaxilla and its sutures | |
| Details | |
| Precursor | Median nasal prominence |
| Identifiers | |
| TA98 | A02.1.12.031 |
| TA2 | 833 |
| FMA | 77231 |
| Anatomical terminology | |
Thepremaxilla (orpraemaxilla) is one of a pair of smallcranial bones at the very tip of the upperjaw of many animals, usually, but not always, bearingteeth. In humans, they are fused with themaxilla. The "premaxilla" oftherian mammals has been usually termed as theincisive bone. Other terms used for this structure includepremaxillary bone oros premaxillare,intermaxillary bone oros intermaxillare, andGoethe's bone.
| Incisive bone | |
|---|---|
 The bony palate and alveolar arch. (Premaxilla is not labeled, but region is visible.) | |
| Details | |
| Identifiers | |
| Latin | os incisivum |
| TA98 | A02.1.12.031 |
| TA2 | 833 |
| FMA | 77231 |
| Anatomical terms of bone | |
Inhuman anatomy, the premaxilla is referred to as theincisive bone (os incisivum) and is the part of the maxilla which bears theincisor teeth, and encompasses the anterior nasal spine and alar region. In thenasal cavity, the premaxillary element projects higher than the maxillary element behind. The palatal portion of the premaxilla is a bony plate with a generally transverse orientation. Theincisive foramen is bound anteriorly and laterally by the premaxilla and posteriorly by the palatine process of the maxilla.[1]
It is formed from the fusion of a pair of smallcranial bones at the very tip of thejaws of many animals, usually bearingteeth, but not always. They are connected to the maxilla and the nasals. WhileJohann 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 asGoethe's bone.[2]
Incisive bone is a term used for mammals, and it has been generally thought to behomologous to premaxilla in non-mammalian animals. However, there are counterarguments. According to them, the incisive bone is a novel character first acquired intherian mammals as a composition of premaxilla derived frommedial nasal prominence andseptomaxilla derived frommaxillary prominence. In the incisive bones, only the palatine process corresponds to the premaxilla, while the other parts are theseptomaxilla. Based on this, the incisive bone is not completelyhomologous to the non-mammalian premaxilla. This was hypothesized byErnst Gaupp in 1905[3] and demonstrated by developmental biological- and paleontological experiments in 2021.[4] This issue is still under debate.
In the embryo, the nasal region develops fromneural crest cells which start their migration down to the face during the fourth week of gestation. A pair of symmetrical nasalplacodes (thickenings in theepithelium) are each divided into medial and lateral processes by the nasal pits. The medial processes become theseptum,philtrum, and premaxilla.[5]
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 thealar region 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 thealveolar 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.[1]
It is also common in non-mammals, such as chickens, that premaxilla is derived frommedial 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 themaxillary prominence, and only a part of thepalate originates from themedial nasal prominence.[4] This may be due to the replacement of most of the incisive bone withseptomaxilla in thetherian mammal, as following section. In any case, the development and evolution of this region is complex and needs to be considered carefully.
In bilateralcleft lip and palate, the growth pattern of the premaxilla differs significantly from the normal case;in utero growth is excessive and directed more horizontally, resulting in a protrusive premaxilla at birth.[6]
Forming the oral edge of the upper jaw in mostjawed vertebrates, the premaxillary bones comprise only the central part in more primitive forms. They arefused inblowfishes and absent incartilaginous fishes such assturgeons.[7]
Reptiles and most non-mammaliantherapsids have a large, paired, intramembranous bone behind the premaxilla called theseptomaxilla. Because this bone isvestigial inAcristatherium (a Cretaceouseutherian) this species is believed to be the oldest knowntherian mammal. Intriguingly the septomaxilla is still present inmonotremes.[8][9]
However, embryonic and fossil studies in 2021 suggest that the incisive bone, which has been called "premaxilla" intherian mammals, has been largely replaced by septomaxilla; and that only a palatal part of the incisive bone remains a vestige of premaxilla.[4] If this hypothesis is accurate, the bones that have been called "premaxilla" intherian mammals are not entirely homologous to the original premaxilla of other vertebrates. This homology is, however, contended.[10]
The differences in the size and composition in the premaxilla of various families ofbats is used for classification.[11]
The premaxillae ofsquamates are fused; this feature can be used to distinguish fossil squamates from relatives.[12]
In 1573,Volcher Coiter was the first to illustrate the incisive suture in humans.Pierre Marie Auguste Broussonet andFélix Vicq-d'Azyr were the first to describe the incisive bone as a separate bone within the skull in 1779 and 1780, respectively.[2]
In the 1790s,Johann Wolfgang von Goethe began studyingzoology, and formed the impression that allanimals are similar, being bodies composed ofvertebrae and their permutations. The human skull is one example of ametamorphosed vertebra, and within it, the intermaxillary bone rests as evidence linking the species to other animals.[13]
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