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PrecursorPremaxilla_header_cell_0_2_0 median nasal prominencePremaxilla_cell_0_2_1

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. Premaxilla_sentence_0

In humans, they are fused with the maxilla and usually termed as the incisive bone. Premaxilla_sentence_1

Other terms used for this structure include premaxillary bone or os premaxillare, and intermaxillary bone or os intermaxillare. Premaxilla_sentence_2

Human anatomy Premaxilla_section_0

In humans, 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. Premaxilla_sentence_3

In the nasal cavity, the premaxillary element projects higher than the maxillary element behind. Premaxilla_sentence_4

The palatal portion of the premaxilla is a bony plate with a generally transverse orientation. Premaxilla_sentence_5

The incisive foramen is bound anteriorly and laterally by the premaxilla and posteriorly by the palatine process of the maxilla. Premaxilla_sentence_6

Embryology Premaxilla_section_1

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. Premaxilla_sentence_7

A pair of symmetrical nasal placodes (thickenings in the epithelium) are each divided into medial and lateral processes by the nasal pits. Premaxilla_sentence_8

The medial processes become the septum, philtrum, and premaxilla. Premaxilla_sentence_9

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. Premaxilla_sentence_10

After eleven weeks an accessory ossification center develops into the alar region of the premaxilla. Premaxilla_sentence_11

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. Premaxilla_sentence_12

The boundary between the premaxilla and the maxilla remains discernible after birth and a suture is often observable up to five years of age. Premaxilla_sentence_13

In bilateral cleft 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. Premaxilla_sentence_14

Evolutionary variation Premaxilla_section_2

Forming the oral edge of the upper jaw in most jawed vertebrates, the premaxillary bones comprise only the central part in more primitive forms. Premaxilla_sentence_15

They are fused in blowfishes and absent in cartilaginous fishes such as sturgeons. Premaxilla_sentence_16

Reptiles and most non-mammalian therapsids have a large, paired, intramembranous bone behind the premaxilla called the septomaxilla. Premaxilla_sentence_17

Because this bone is vestigial in Acristatherium (a Cretaceous eutherian) this species is believed to be the oldest known therian mammal. Premaxilla_sentence_18

Intriguingly the septomaxilla is still present in monotremes. Premaxilla_sentence_19

The differences in the size and composition in the premaxilla of various families of bats is used for classification. Premaxilla_sentence_20

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