Content Reviewers:Yifan Xiao, MD
The tongue is a muscular structure as well as a sensory organ that starts developing alongside the external face around week 4 of intrauterine life.
A fully developed tongue consists of two parts, the anterior two-thirds; and posterior one-third, which is called the root of the tongue; they are separated from each other by a shallow v-shaped groove, known as the terminal sulcus.
The two parts develop separately, which results in them having different nerve supplies.
Around week 4 of embryonic development, as a result of the folding of the embryo along the rostrocaudal axis and the lateral axis, the embryo takes on a more recognizably “human” form—but to be honest, it still looks more like a shrimp than a baby.
At the head end of this little shrimp-like creature, the neural tube expands greatly forming the primitive forebrain, which produces a bulge known as the frontal prominence.
Lateral to the neural tube is the paraxial mesoderm, which partially segments rostrally to form somitomeres and fully segments caudally to form somites, the first in the series being the occipital somites.
At this point, a small pit called the stomodeum forms between the frontal prominence and the developing cardiac bulge, and it will eventually become the oral cavity.
At the same time, six little bulges or thickenings of the mesoderm, sprout from the primitive pharynx to become the branchial, or pharyngeal, arches.
These arches are paired, symmetrical bumps that form on each side on the lateral aspect of the embryo, in a craniocaudal fashion, going from head to tail.
At the same time, neural crest cells from the midbrain and the first two rhombomeres migrate bilaterally to the region and infiltrate the mesoderm bumps where they support the development of embryonic connective tissue needed for craniofacial development, called ectomesenchyme.
The first pharyngeal arch splits up into two processes — the upper maxillary process and the lower mandibular process.
The pharyngeal arches on either side then proceed to grow towards the midline and merge with their counterparts beneath the stomodeum.
Now, the tongue begins to develop around week 4 of intrauterine life.
The anterior two-thirds develops from the first pharyngeal arch, and the posterior one-third develops from the second, third, and fourth pharyngeal arches.
The anterior two-third starts developing when the mesoderm of the first pharyngeal arch proliferates, giving rise to a midline swelling, called the tuberculum impar.
During the following week, the mesoderm of the same arch gives rise to two more bulges, the right and left lateral lingual swellings, just lateral and above the tuberculum impar.
The lateral lingual swellings enlarge, overlap the tuberculum impar, and merge with each other along the midline, giving rise to the mucosa over the anterior two-thirds of the tongue.
Since the mucosa overlying this area of the tongue has its origin from the first pharyngeal arch; it receives its sensory innervation from the lingual branch of the mandibular division of the V cranial nerve (trigeminal nerve).
Now, the area where the two lateral lingual swellings merge, develops into a fibrous septum, which appears as the midline groove over the surface of the tongue, known as the median sulcus.
Similarly, the posterior one-third of the tongue also begins to develop around week 4 as two mesodermal swellings.
The first swelling, known as the copula, develops in the midline of the second and third pharyngeal arches during week 4.