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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 A tooth ( : teeth) is a hard, calcified structure found in the jaws (or mouths) of many vertebrates and used to break down food. Some animals, particularly carnivores and omnivores, also use teeth to help with capturing or wounding prey, te ...
. In humans, they are fused with the
maxilla The maxilla (plural: ''maxillae'' ) in vertebrates is the upper fixed (not fixed in Neopterygii) bone of the jaw formed from the fusion of two maxillary bones. In humans, the upper jaw includes the hard palate in the front of the mouth. The ...
. The "premaxilla" of therian mammal 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.


Human anatomy

In
human anatomy The human body is the structure of a human being. It is composed of many different types of cells that together create tissues and subsequently organ systems. They ensure homeostasis and the viability of the human body. It comprises a head ...
, the premaxilla is referred to as the incisive bone (') and is the part of the maxilla which bears the incisor teeth, and encompasses the anterior nasal spine and alar region. In the
nasal cavity The nasal cavity is a large, air-filled space above and behind the nose in the middle of the face. The nasal septum divides the cavity into two cavities, also known as fossae. Each cavity is the continuation of one of the two nostrils. The nasal ...
, 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. The
incisive foramen In the human mouth, the incisive foramen (also known as: "''anterior palatine foramen''", or "''nasopalatine foramen''") is the opening of the incisive canals on the hard palate immediately behind the incisor teeth. It gives passage to blood ves ...
is bound anteriorly and laterally by the premaxilla and posteriorly by the palatine process of the maxilla. It is formed from the fusion of a pair of small cranial bones at the very tip of the jaws of many animals, usually bearing
teeth A tooth ( : teeth) is a hard, calcified structure found in the jaws (or mouths) of many vertebrates and used to break down food. Some animals, particularly carnivores and omnivores, also use teeth to help with capturing or wounding prey, te ...
, but not always. They are connected to the maxilla and the nasals. While
Johann Wolfgang von Goethe Johann Wolfgang von Goethe (28 August 1749 – 22 March 1832) was a German poet, playwright, novelist, scientist, statesman, theatre director, and critic. His works include plays, poetry, literature, and aesthetic criticism, as well as tr ...
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.


Incisive bone and premaxilla

Incisive bone is a term used for mammals, and it has been generally thought to be
homologous Homology may refer to: Sciences Biology *Homology (biology), any characteristic of biological organisms that is derived from a common ancestor *Sequence homology, biological homology between DNA, RNA, or protein sequences * Homologous chrom ...
to premaxilla in non-mammalian animals. However, there are counterarguments. According to them, the incisive bone is a novel character first acquired in therian mammals as a composition of premaxilla derived from medial nasal prominence and septomaxilla derived from maxillary prominence. In the incisive bones, only the palatine process corresponds to the premaxilla, while the other parts are the septomaxilla. Based on this, the incisive bone is not completely
homologous Homology may refer to: Sciences Biology *Homology (biology), any characteristic of biological organisms that is derived from a common ancestor *Sequence homology, biological homology between DNA, RNA, or protein sequences * Homologous chrom ...
to the non-mammalian premaxilla. This was hypothesized by Ernst Gaupp in 1905 and demonstrated by developmental biological- and paleontological experiments in 2021. This issue is still under debate.


Embryology

In the embryo, the nasal region develops from neural crest cells which start their migration down to the face during the fourth week of gestation. A pair of symmetrical nasal placodes (thickenings in the epithelium) are each divided into medial and lateral processes by the nasal pits. The medial processes become the
septum In biology, a septum (Latin for ''something that encloses''; plural septa) is a wall, dividing a cavity or structure into smaller ones. A cavity or structure divided in this way may be referred to as septate. Examples Human anatomy * Interat ...
,
philtrum The philtrum ( la, philtrum from Ancient Greek ''phíltron,'' lit. "love charm"), or medial cleft, is a vertical indentation in the middle area of the upper lip, common to therian mammals, extending in humans from the nasal septum to the tuber ...
, and premaxilla. 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 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 the
alveolar process The alveolar process () or alveolar bone is the thickened ridge of bone that contains the tooth sockets on the jaw bones (in humans, the maxilla and the mandible). The structures are covered by gums as part of the oral cavity. The synonymous te ...
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 The palate () is the roof of the mouth in humans and other mammals. It separates the oral cavity from the nasal cavity. A similar structure is found in crocodilians, but in most other tetrapods, the oral and nasal cavities are not truly s ...
originates from the medial nasal prominence. This may be due to the replacement of most of the incisive bone with septomaxilla in the therian mammal, 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 A cleft lip contains an opening in the upper lip that may extend into the nose. The opening may be on one side, both sides, or in the middle. A cleft palate occurs when the palate (the roof of the mouth) contains an opening into the nose. The t ...
, 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.


Evolutionary variation

Forming the oral edge of the upper jaw in most jawed vertebrates, the premaxillary bones comprise only the central part in more primitive forms. They are fused in blowfishes and absent in
cartilaginous fishes Chondrichthyes (; ) is a class that contains the cartilaginous fishes that have skeletons primarily composed of cartilage. They can be contrasted with the Osteichthyes or ''bony fishes'', which have skeletons primarily composed of bone tissue ...
such as sturgeons. Reptiles and most non-mammalian
therapsid Therapsida is a major group of eupelycosaurian synapsids that includes mammals, their ancestors and relatives. Many of the traits today seen as unique to mammals had their origin within early therapsids, including limbs that were oriented mor ...
s have a large, paired, intramembranous bone behind the premaxilla called the septomaxilla. Because this bone is
vestigial Vestigiality is the retention, during the process of evolution, of genetically determined structures or attributes that have lost some or all of the ancestral function in a given species. Assessment of the vestigiality must generally rely on co ...
in ''
Acristatherium ''Acristatherium yanensis'' is an extinct basal eutherian from the Early Cretaceous (early Aptian, about ) Lujiatun Bed of the Yixian Formation. It was described on the basis of a single specimen (holotype) from Beipiao, Liaoning, China, by Ya ...
'' (a Cretaceous
eutheria Eutheria (; from Greek , 'good, right' and , 'beast'; ) is the clade consisting of all therian mammals that are more closely related to placentals than to marsupials. Eutherians are distinguished from noneutherians by various phenotypic t ...
n) this species is believed to be the oldest known
theria Theria (; Greek: , wild beast) is a subclass of mammals amongst the Theriiformes. Theria includes the eutherians (including the placental mammals) and the metatherians (including the marsupials) but excludes the egg-laying monotremes. C ...
n mammal. Intriguingly the septomaxilla is still present in
monotreme Monotremes () are prototherian mammals of the order Monotremata. They are one of the three groups of living mammals, along with placentals ( Eutheria), and marsupials (Metatheria). Monotremes are typified by structural differences in their bra ...
s. However, embryonic and fossil studies in 2021 suggest that the incisive bone, which has been called "premaxilla" in therian mammals, has been largely replaced by septomaxilla; and that only a palatal part of the incisive bone remains a vestige of premaxilla. If this theory is accurate, the bones that have been called "premaxilla" in therian mammals are not entirely homologous to the original premaxilla of other vertebrates. The differences in the size and composition in the premaxilla of various families of bats is used for classification.


History

In 1573, Volcher Coiter was the first to illustrate the incisive suture in humans.
Pierre Marie Auguste Broussonet Pierre Marie Auguste Broussonet (28 February 1761 – 17 January 1807) was a French naturalist who contributed primarily to botany. He was born in Montpellier, where he was educated, and travelled to Morocco, Spain, the Canary Islands, and Souther ...
and Félix Vicq-d'Azyr were the first to describe the incisive bone as a separate bone within the skull in 1779 and 1780, respectively. In the 1790s,
Johann Wolfgang von Goethe Johann Wolfgang von Goethe (28 August 1749 – 22 March 1832) was a German poet, playwright, novelist, scientist, statesman, theatre director, and critic. His works include plays, poetry, literature, and aesthetic criticism, as well as tr ...
began studying
zoology Zoology ()The pronunciation of zoology as is usually regarded as nonstandard, though it is not uncommon. is the branch of biology that studies the animal kingdom, including the structure, embryology, evolution, classification, habits, and ...
, and formed the impression that all animals are similar, being bodies composed of
vertebrae The spinal column, a defining synapomorphy shared by nearly all vertebrates, Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristi ...
and their permutations. The human skull is one example of a
metamorphosed Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, causi ...
vertebra, and within it, the intermaxillary bone rests as evidence linking the species to other animals.


References

{{Tetrapod osteology Vertebrate anatomy