Development of the nerve supply to the human tongue

Tongue Growth during Prenatal Development in Korean Fetuses and Embryos

Background:

Prenatal tongue development may affect oral-craniofacial structures, but this muscular organ has rarely been investigated.

Methods:

In order to document the physiology of prenatal tongue growth, we histologically examined the facial and cranial base structures of 56 embryos and 106 fetuses.

Results:

In Streeter’s stages 13–14 (fertilization age [FA], 28 to 32 days), the tongue protruded into the stomodeal cavity from the retrohyoid space to the cartilaginous mesenchyme of the primitive cranial base, and in Streeter’s stage 15 (FA, 33 to 36 days), the tongue rapidly swelled and compressed the cranial base to initiate spheno-occipital synchondrosis and continued to swell laterally to occupy most of the stomodeal cavity in Streeter’s stage 16–17 (FA, 37 to 43 days). In Streeter’s stage 18–20 (FA, 44 to 51 days), the tongue was vertically positioned and filled the posterior nasopharyngeal space. As the growth of the mandible and maxilla advanced, the tongue was pulled down and protruded anteriorly to form the linguomandibular complex. Angulation between the anterior cranial base (ACB) and the posterior cranial base (PCB) was formed by the emerging tongue at FA 4 weeks and became constant at approximately 124°–126° from FA 6 weeks until birth, which was consistent with angulations measured on adult cephalograms.

Conclusions:

The early clockwise growth of the ACB to the maxillary plane became harmonious with the counter-clockwise growth of the PCB to the tongue axis during the early prenatal period. These observations suggest that human embryonic tongue growth affects ACB and PCB angulation, stimulates maxillary growth, and induces mandibular movement to achieve the essential functions of oral and maxillofacial structures.

Angiogenesis and developmental expression of vascular endothelial growth factor in rat lingual papillae

We used an embryological approach to investigate development and microvasculature of lingual papillae, and expression of vascular endothelial growth factor (VEGF) in the rat tongue. Temporal changes in the rat tongue at each developmental stage from embryonic day 13 (E13) to postnatal day 7 (P7) were observed by intravascular injection of India ink and immunohistochemistry using a VEGF antibody. At E13, the primordium of circumvallate papilla was observed among various lingual papillae. VEGF was widely expressed at E16 on the proliferated epithelium and the connective tissue core of circumvallate papilla. Invasion by capillary sprouts forming the lingual papillae was observed at E17. The primordium of fungiform papillae was observed at E14. VEGF was strongly expressed around the basal cells of proliferated epithelial tissues of fungiform papillae at E17. At E18, blind-ended capillary sprouts invaded into connective tissue cores from subepithelial sinusoidal capillaries by sprout angiogenesis. At P1, the invading capillary sprouts formed loops by vascular remodeling. The primordium of foliate papillae was observed at E16. VEGF was slightly expressed, but uniformly at E17 on the epithelium, muscle cells, and fibroblasts of foliate papillae. At E18, vascular density was increased by angiogenesis. The primordium of filiform papillae was observed at E17. It was the last to develop among the lingual papillae. VEGF was expressed in the cytoplasm of grown epithelial cells of filiform papillae at E19, and in blind-ended capillary sprouts formed by angiogenesis in the connective tissue cores at E20. The capillary sprouts formed loops by vascular remodeling at P1. Consequently, VEGF was expressed on the papillary epithelium and connective tissue cores of papillae during development of the papillary epithelium, and invasion by capillary sprouts into each papillae was observed thereafter. These results suggest a close relationship between expression of VEGF and angiogenesis of lingual papillae in the rat.

Development of the sensory nerves to the dorsum of the tongue in staged human embryos

To determine in selected, staged human embryos the time period and manner of formation of the sensory nerves to the mucosa on the dorsum of the tongue, cranial nerves V(3) and IX and their lingual branches were traced microscopically. These two cranial nerves along with cranial nerve VII are present in the dorsal part of their respective pharyngeal arch 1, 2, and 3 at Stage 13. All three nerves grow ventrally at Stage 16, and the lingual branches of V(3) and IX become closer to the developing tongue region. The lingual fibers begin to enter the tongue substance at Stage 17 when they appear as large short branches that terminate 100-400 microm from the dorsal mucosal basement membrane; of the fibers that are in the tissue deep to the surface mucosa, those in the posterior tongue segment terminate the closest and those in the anterior segment the farthest. Chorda tympani nerve fibers are joined with V(3) at Stage 20. At this stage nerve fibers reach the basement membrane in the posterior tongue segment and by Stage 23 they terminate at the basement membrane throughout the tongue, being sparsest in the anterior segment. Primordial papillae appear as small mucosal projections throughout the tongue at Stage 20 and are innervated by Stage 23. The study found that innervation to the dorsum of the tongue develops during the second month of prenatal life. During the 4-week period, Stages 13-23, an innervation pattern was observed based on the relative time fibers terminate at the basement membrane. The pattern begins caudally in the root, then moves rostrally lateral to the middle tongue segment, proceeding then to the tip of the tongue. The last area to become innervated is the medial part of the middle segment or central portion of the tongue.

Course of the peripheral gustatory nerves

The multiple variations of the course of the gustatory nerves still considered possible are discussed. Recent investigations lead to the conclusion that there is only one path for the gustatory fibers for each gustatory area: 1) from the anterior part of the tongue via the tympanic cord and facial nerve to the medulla oblongata; 2) for the posterior part of the tongue in the IX cranial nerve; and 3) from the soft palate via the greater superficial petrosal nerve to the facial nerve. The trigeminal nerve carries no gustatory fibers to the brain.

Spatial distribution of rat fungiform papillae

The objective of this study was to determine the spatial distribution of fungiform papillae on the rat's tongue. Since each fungiform papilla in the rat has a single taste bud, the spatial distribution of fungiform papillae is equivalent to the location of taste buds on the anterior tongue. A mean total number of 187 fungiform papillae per tongue were found which were about equally divided between the two lateral halves of the tongue. Over 50% of the total number of fungiform papillae were located on the tongue tip for an average density of 3.4 papillae/mm2,while the dorsal surface of the tongue had an average density of 1.3 papillae/mm2 of tongue surface. Papillae were absent on the dorsal midline, but a paracentral line of papillae running from anterior to posterior was a consistent finding. Though not identical, the distribution of papillae was essentially the same on different tongues. The functional significance of the papilla distribution is not understood, but electrophysiological experiments show evidence of neural interaction of papillae which are clustered together. The distribution of papillae and the distribution of nerve fibers which innervate them must be evaluated together in order to appreciate the significance of the distribution of fungiform papillae and their associated taste buds.

[Evaluating function and disorders of taste]

In a first anatomical section the peripheral gustatory pathways, their central connections, nuclei and cortical projections are discussed. It is evident, that the gustatory fibres from the posterior part of the tongue run in the IX nerve and those from the soft palate reach the medulla oblongata via the petrosal and facial nerve. For the anterior part of the tongue there obviously exists only one gustatory pathway via the chorda tympani-facial nerve. About the further central pathways of taste fibres is much less known. In a second part the methods of taste testing with different taste solutions and the electrogustometry are described. Their practical use and the pitfalls of testing are considered. The disorders of the taste sense compose a third part. Genetic and endocrine abnormalities as well as the side effects of drugs and radiotherapy and the destruction of taste nerves may lead to gustatory deviations. The possibility of central gustatory disorders, especially the combined loss of taste and smell as a result of trauma are mentioned. A chapter dealing with the therapy of taste disorders and a short outlook on the genetic aspects of this oral sense complete this review.