Introduction

Tongue is a muscular organ situated at the floor of the mouth. Though predominantly muscular, it consists of mucous membrane and fibroareolar stroma.  It is associated with taste, speech, deglutition or swallow, maintaining arch form and tooth position.

Patten initially described the tongue as a sac of mucous membrane that becomes filled with a mass of growing muscle.

Highlights 

A pharyngeal arch arises as a mesodermal thickening in the lateral wall of the foregut and grows ventrally to become continuous with the corresponding arch of the opposite side.

Tongue develops from the 1st, 3rd, 4th pharyngeal arches in the floor of the developing mouth. It takes place during the 4th to 8th week of intrauterine life. 

• Embryologically, fibroareolar stroma tongue is derived from 1st, 3rd, 4th pharyngeal arches. 

• Skeletal muscles develop from Occipital somites – paraxial mesoderm. 

• The mucosa of the tongue is formed from the endoderm of foregut.

Fibroareolar stroma

First arch:

The medial most parts of the mandibular arches proliferate to form two lingual swellings. The lingual swellings are partially separated from each other by another swelling that appears in the midline. This median swelling is called the tuberculum impar. 

Immediately behind the tuberculum impar, the epithelium proliferates to form a downgrowth (thyroglossal duct) from which the thyroid gland develops. The site of this downgrowth is subsequently marked by a depression called the foramen cecum.

Second to fourth arch:

A midline swelling is seen in relation to the medial ends of the second, third and fourth arches. This swelling is called the hypobranchial eminence or copula of His. 

The eminence soon shows a subdivision into a cranial part related to the second and third arches (called the copula) and a caudal part related to the fourth arch which forms the epiglottis.

Anterior 2/3rd:

It is formed by the fusion of 2 inguinal lateral swellings with the tuberculum impar. Few authors say that tuberculum impar does not participate in the formation of the anterior 2/3rd of the tongue.

Hence, the anterior 2/3rd of the tongue is formed from the 1st pharyngeal arch mesoderm.

Posterior 1/3rd:

It is formed from the cranial part of the hypobranchial eminence (copula). In this situation, the second arch mesoderm gets buried below the surface. The third arch mesoderm grows over it to fuse with the mesoderm of the first arch. 

The posterior one-third of the tongue is thus formed by the third arch mesoderm.

Posterior most:

The posterior most part of the tongue is derived from the fourth arch mesoderm.

Sulcus terminalis:

The line of junction of anterior two thirds and posterior one-third of tongue is indicated by an inverted V-shaped sulcus terminalis. The midpoint of this V shaped sulcus terminalis is foramen caecum.

Musculature

There are totally 4 pairs of extrinsic and 4 pairs of intrinsic muscles. 

Extrinsic muscles:

They change the position of the tongue. They originate from various structures and get inserted into the tongue. They primarily involve the tongue movements.

Intrinsic muscles:

They change the shape of the tongue. They are contained within the tongue.

• Superior longitudinal muscle – dorsum concave, shortens the tongue.

• Inferior longitudinal muscle – dorsum convex, shortens the tongue.

• Transverse muscle – make it narrow and elongated.

• Verticalis muscle – make it broad and flattened.

Embryology: 

The above picture depicts the paraxial view of a 5 weeks old fetus where there is migration of occipital somites by forming a hypoglossal cord to the area of future tongue.

The musculature of the tongue is derived from the occipital myotomes. The nerve supply of all muscles of tongue except palatoglossus is by Hypoglossal nerve.

Other structures

The epithelium of the tongue is at first made up of a single layer of cells. Later it becomes stratified and papillae become evident. 

Taste buds are formed in relation to  the terminal branches of the innervating nerve fibers. 

The circumvallate papillae of the tongue develop from the cranial part of hypobranchial eminence and migrate to the anterior aspect of sulcus terminalis. They are supplied by the glossopharyngeal nerve.

Summary