Embryological Development of the Tongue

Medical question written for a National Board Dental Exam test site.

Which of the following statements about embryologic development of the tongue are true?

A.  The tongue develops from the first branchial arch.
B.  Occipital somites migrate into the floor of the mouth to form the musculature of the tongue.
C.  The tongue’s somatic, sensory, and special sensory afferent nerves, and musculature arise together from the first and second branchial arches.
D.  The second branchial arch retains importance during development.
E.  The tongue begins to develop at 9 weeks.

Answer & Explanation:

The answer is B.

The tongue begins to develop at about four weeks. The pharyngeal arches meet in the midline beneath the primitive mouth. Local proliferation of the mesenchyme then gives rise to a number of swellings in the floor of the mouth. A midline swelling, the hypobranchial eminence, develops from the third arch mesenchyme. The first arch develops lingual swellings, along with a central tuberculum impar. The hypobranchial eminence overgrows the second arch, forms the root of the tongue, and fuses with the lingual swellings and the tuberculum impar. It also gives rise to the mucosal covering the root and posterior third of the tongue.

The tongue separates from the floor of the mouth by a downgrowth of ectoderm around its periphery that subsequently degenerates to form the lingual sulcus and gives the tongue mobility. The muscles of the tongue have a different origin: they arise from the occipital somites that migrate forward into the tongue area, carrying the hypoglossal nerve XII.

Each branchial arch has the same basic architecture. The inner aspect is covered by endoderm (ectoderm in the case of the first arch because it forms in front of the buccopharyngeal membrane, which delineates the junction of stomatodeum ectoderm with the foregut endoderm). The outer surface is covered by ectoderm. The central core consists of mesenchyme derived from the lateral plate mesoderm, which is surrounded by mesenchyme derived from the neural crest. The neural crest mesenchyme (also termed echtomesenchyme) condenses to form a bar of cartilage, the arch cartilage.

Each arch also contains an artery and a nerve. The nerve consists of a motor and a sensory component. The sensory nerve has two branches. One branch supplies the epithelium covering the anterior half of that arch. Another branch passes forward to supply the epithelium covering the posterior half of the preceding arch. The nerve of the first branchial arch contains the fifth cranial nerve. The second arch contains the seventh nerve. The third arch contains the ninth cranial nerve. Thus, the first arch musculature gives rise to the muscles of mastication that are innervated by the trigeminal nerve V, and the second arch gives rise to the muscles of facial expression that are innervated by the facial nerve VII.

(Choice A) is incorrect. The tongue develops from the first three branchial arches although the second arch ceases to be of importance. The third arch mesenchyme rapidly overgrows the second arch.

(Choice C) is incorrect. Because the mucosa of the anterior two thirds of the tongue is derived from the first arch, it is supplied by the arch’s trigeminal nerve V. Similarly, the mucosa of the posterior third of the tongue is derived from the third arch supplied by this arch’s hypoglossal nerve IX. These two nerves supply general sensory nerves only; special sensory taste nerves are supplied by facial nerve VII via the chora tympani.

The second branchial arch (Choice D) does not remain important during the remainder of the tongue’s development after the tongue’s mucosal covering has been formed from the first and third branchial arches.

The tongue begins to develop (Choice E) at four weeks. By the ninth week, the tongue is elevated and has begun growing forward.

References:

Putz, R. & Pabst, R. (1997). Sobatta, Atlas of Human Anatomy Volume 1, Head, Neck, and Upper Limb (12th Edition). Munchen, Germany. Williams and Wilkins.

Liebgott, B. (2001). The Anatomical Basis of Dentistry (2nd Edition). US. Mosby & Co.

Ten Cate, A.R. (1998). Oral Histology: Development, Structure, and Function (5th Edition). US. Mosby-Year Book Company.

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