Development of the Tongue
The fusion of the first pharyngeal arch creates a protuberance in the midline known as the tuberculum impar, located on the lower edge of the first pharyngeal arch. The tuberculum impar is labeled as ‘b’ in the figure below.
Later, two swellings develop on either side of the tuberculum impar. These swellings are known as the lateral lingual swellings or prominences. The tuberculum impar and the two lateral swellings extend to form the anterior 2/3 of the tongue. A medial lingual sulcus is formed.
The sensory sensation of touch of the anterior 2/3 of the tongue is carried by the mandibular branch of the trigeminal nerve (CN V) while the taste sensation is carried by the chorda tympani branch of the facial nerve (CN VII). This part of the tongue has both endodermic as well as ectodermic origin.
The posterior 1/3 of the tongue is formed mainly by the third pharyngeal arch with little contribution of the second and fourth pharyngeal arches. The second pharyngeal pouch is however overgrown by the third pharyngeal arch. The fusion of these arches form the copula, which is the posterior 1/3 of the tongue. The touch and taste sensation of this part of the tongue is carried by a branch of the glossopharyngeal nerve (CN IX).
The base of the tongue is formed by the fourth pharyngeal arch and is innervated by a branch of the vagus nerve (CN X) for the touch and taste sensation.
The foramen caecum labeled as ‘c’ in the diagram is basically a remnant of the unpaired endodermic bud, which was the primordium of the thyroid gland. The thyroglossal duct regresses and is not present in an adult. In some cases, where it does not regress, a patent tract is present called the thyroglossal tract which often leads to the formation of a cyst or a fistula.
The entire musculature of the tongue is supplied by the hypoglossal nerve (CN XII).
Major Developmental Anomalies of the Tongue
The major developmental anomalies of the tongue include:
- Aglossia is the complete absence of development of the tongue.
- Microglossia is a small or rudimentary tongue. It is a rare congenital condition in which the sufferers have difficulty in eating and swallowing. Relative microglossia is when the tongue is smaller in structure compared to the jaw bones.
- Macroglossia is an enlarged tongue, which can be either congenital or acquired. It is often associated with Down’s syndrome and Hunter’s syndrome. Aquired macroglossia may be seen in lingual tumours, myxedema, amyloidosis and angioedema.
- Ankyloglossia is a condition characterized by fixation of the tongue to the floor of the mouth. Partial ankyloglossia is also known as tongue tie. Patients have difficulty in speech and swallowing. It may be resolved by surgical intervention.
- Cleft tongue results due to lack of fusion of the lateral lingual swellings.
- Lingual thyroid nodule occurs when the remnants of the thyroid primordium fail to migrate to its normal position, resulting in formation of thyroid nodule in the substance of the tongue. It results in dysphagia, dysphonia and breathing difficulties.