Anat Cell Biol.  2019 Dec;52(4):426-435. 10.5115/acb.19.111.

Topographical variations of the incisive canal and nasopalatine duct in human fetuses

• Kim JH
• Shibata S
• Abe H
• Murakami G
• Rodríguez-Vázquez JF

• KMID: 2466696
• DOI: http://doi.org/10.5115/acb.19.111

Abstract

The incisive canal for nerves and vessels is generally thought to run along a suture between the incisive bone (IN) and maxilla. In contrast, there was a report saying the canal passes through the IN or primary palate in human fetuses. Examination of sagittal and frontal sections from 69 fetuses (31 of gestational age [GA] 9-15 weeks and 38 of GA 26-34 weeks) showed that the canal often penetrated the IN at the nasal half of its course and that, in other fetuses, the canal penetrated the IN along its entire course, irrespective of involvement of the nasopalatine duct. Canals developing in and corresponding to parts of the suture resulted in partial enlargement of the thin and tight sutures, which contained loose tissue, vessels, nerves and even a duct. Small processes of the IN were identified as upper irregular parts continuous with inferior main masses of bone in frontal sections but as bone fragments in sagittal sections. In some sections, a thin layer of the maxilla along the canal covered the medial or inferior aspect of the IN. Therefore, the incisive canal with or without duct exhibited a spectrum of variations in topographical relation to the IN-maxillary border. Because the primitive oronasal communication passes through the suture, the nasopalatine duct may have originated from the secondary developed elongation of the nasal epithelium at midterm. A large incisive fossa along the midline on the oral surface of the palate might make a macroscopic finding of variants difficult even in adults.

Keyword

Incisive canal; Nasopalatine duct; Nasopalatine nerve; Maxilla; Human fetus

MeSH Terms

Adult
Fetus*
Gestational Age
Humans*
Maxilla
Nasal Mucosa
Palate
Sutures

Figure

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  Fig. 1 An incisive canal passing along the suture between the incisive bone and the maxilla in a midterm fetus. Hematoxylin and eosin staining. Frontal sections of a specimen of crown rump length 84 mm. Panels A–E were prepared at the same magnification. Panel A shows the most posterior site in the figure. Panel F schematically demonstrates the topographical relationship between the incisive bone (blue) and the maxilla (orange), with the dotted lines showing a paired inferior elongation of the nasal epithelium. The arrows in panels A and B indicate the nasal epithelial elongation. A duct was absent. Stars indicate the incisive-maxillary suture distant from the canal. Panels B and C show dilation of the suture corresponding to the canal. Scale bar=1 mm. A, B, or C in circles indicate a tooth bud of the medial incisor, lateral incisor, or canine, respectively. AC, alar cartilages of the anterior nose; MX, maxilla; NS, nasal septum; IN, incisive bone.

• 

  Fig. 2 Presence of an intra-incisive canal in the nasal half, with the oral half appearing to pass through the incisive-maxillary suture in a midterm fetus. Hematoxylin and eosin staining. Frontal sections of a specimen of crown rump length 77 mm. Panels A–F were prepared at the same magnification. Panel A shows the most posterior site in the figure. Panel G schematically demonstrates the topographical relationship between the incisive bone (blue) and the maxilla (orange), with the dotted lines showing a paired inferior elongation of the nasal epithelium. Panels A and B show that paired loose spaces or canals penetrate the incisive bone (IN), followed by inferior elongation of the nasal epithelium (arrows). However, the lower half of the incisive canal is sandwiched by the incisive bone and maxilla (MX). Stars indicate the incisive-maxillary suture distant from the canal. The epithelial elongation continues to a thin duct (inset at the rightupper in panel D) and connects with an epithelial pearl in the incisive fossa. Scale bar=1 mm. A, B, or C in circles indicate a tooth bud of the medial incisor, lateral incisor, or canine, respectively. AC, alar cartilages of the anterior nose; NS, nasal septum; pearl, palatal epithelial pearl.

• 

  Fig. 3 A midterm fetus, in which almost the entire course of the canal takes an intra-incisive course. Hematoxylin and eosin staining. Frontal sections of a specimen of crown rump length 70 mm. Panels A–E were prepared at the same magnification. Panel A shows the most posterior site in the figure. Panel F schematically demonstrates the topographical relationship between the incisive bone (blue) and the maxilla (orange), with the dotted lines showing a paired inferior elongation of the nasal epithelium. Panel C displays that the epithelial elongation (arrows) is sandwiched by parts of the incisive bone (IN). Panel E exhibits candidate nasopalatine nerves (nerves) as well as a tight tissue in the midline suture. Stars indicate the incisive-maxillary suture distant from the canal, although the lateral part is unclear in panels A, C, and D. Scale bar=1 mm. B or C in circles indicate a tooth bud of the lateral incisor, or canine, respectively. AC, alar cartilages or the anterior nose; MX, maxilla; NS, nasal septum; pearl, palatal epithelial pearl; VM, vomer; VNO, vomeronasal organ.

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  Fig. 4 Incisive canal and the incisive-maxillary suture in sagittal sections of two midterm fetuses with ducts. Hematoxylin and eosin staining. Panels A–D display a specimen of crown rump length (CRL) 82 mm, and panels E–G exhibit a specimen of CRL 100 mm. All panels were prepared at the same magnification. Panels A and E show the most lateral sites in each of the specimens. Panels D and G exhibit the presence of an epithelial pearl in the incisive fossa of both specimens. The smaller specimen showed a simple morphology, in which the suture (stars) was partly dilated to contain a duct (C). The larger specimen showed a complicated morphology, in which the suture (stars) appears to merge with an intra-incisive canal (triangles) in panel F. In panel G, however, the intra-incisive canal (anterior; triangles) and the suture (posterior; stars) are separated by a process of the incisive bone (IN). This morphology looks like a branching of the suture. Scale bar=1 mm. B in circles indicates a tooth bud of the lateral incisor. MX, maxilla; pearl, palatal epithelial pearl; VM, vomer.

• 

  Fig. 5 Frontal sections of three late-stage fetuses with and without ducts. Hematoxylin and eosin staining. Panels A–C displays a specimen of crown rump length (CRL) 230 mm with a duct; panels D–F exhibits a specimen of CRL 250 mm without a duct, and panels G–I show a specimen of CRL 270 mm with a duct. All panels were prepared at the same magnification. Panels A, D and G exhibit the most posterior site in each specimen. In all three specimens, an epithelial elongation of the nose extended into loose tissue of the incisive canal. Stars indicate the incisive-maxillary suture. Panels B and C display an incisive canal passing along the suture between the incisive bone (IN) and the maxilla (MX). Panels E and F show an intra-incisive canal on the left-hand side, rather than a canal between the incisive bone and maxilla on the right-hand side. Panel H and I show bilateral intra-incisive canals. Nerves are seen in the canal as well as in the incisive bone (panel F). Scale bar=1 mm. B or C in circles indicate a tooth bud of the lateral incisor, or canine, respectively. pearl, palatal epithelial pearl; VM, vomer.

• 

  Fig. 6 Sagittal sections of a late-stage fetus of crown rump length 255 mm with a canal along the suture. Hematoxylin and eosin staining. Panel A displays the most lateral site in the specimens. This specimen exhibits a simple morphology, in which an incisive canal containing nerves and loose tissue (C, D) corresponds to parts of a suture (stars) between the incisive bone (IN) and maxilla (MX). A duct is absent. Stars indicate the suture, which is narrow and tight in panels A and B. All panels were prepared at the same magnification. Scale bar=1 mm. A or B in circles indicate a tooth bud of the medical incisor, or lateral incisor, respectively.

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  Fig. 7 Sagittal sections of a late-stage fetus of crown rump length (CRL) 290 mm with an intra-incisive canal and duct. Hematoxylin and eosin staining. Panel A displays the most lateral site in the specimens. The course of the intra-incisive canal with a duct is intra-incisive laterally (A) and along a suture between the incisive bone (IN) and maxilla (MX) medially (B, C). The suture (stars) turns posteriorly in upper sites (B), but the posterior part appears to be a branch of suture. On the lateral side of the canal and duct between the IN and MX, the suture is narrow and closed (stars in panel A). An inset in panel B shows a higher magnification view of the duct. Panels A–C were prepared at the same magnification. Scale bar=1 mm. B in circles indicates a tooth bud of the lateral incisor. AC, alar cartilages of the anterior nose; PA, palatine bone; VM, vomer.

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