Development of the tongue
Definitions & Key takeaways
The tongue develops from the first four pharyngeal arches at around week 4 of the intrauterine life. Pharyngeal arches are paired embryological and primitive structures, which give rise to different structures as the embryo develops.
The anterior two-thirds of the tongue develops from the tuberculum impar (a medial swelling from the first pharyngeal arch); and two lateral lingual swellings. Next, the median sulcus forms from the growth of the lateral lingual swellings, which merge along the midline over the tuberculum impar. The posterior one-third develops from the second pharyngeal arch, and the third and fourth pharyngeal arch. The muscles of the tongue develop from the myoblasts (embryologic precursors of muscles) that have originated from the occipital somites (blocks of mesodermal tissue that give rise to different structures, including muscles and bones)
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.
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 pharyngeal arches are separated externally by small clefts on the pharyngeal wall called branchial grooves, and internally by corresponding depressions called pharyngeal pouches.
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).
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.
The second swelling, known as hypobranchial eminence, grows from the midline of the third and fourth pharyngeal arches around week 5.
In the following weeks, the hypobranchial eminence begins to grow upwards, and along the way, it grows over the copula. The hypobranchial eminence then goes on to become the mucosa over the posterior one-third of the tongue, which grows upwards and fuses with anterior portion, forming the complete tongue.
The mucosa overlying this area of the tongue receives its sensory innervation from the IX cranial nerve (glossopharyngeal nerve).
The region between anterior two-third and posterior one-third contains a V-shaped depression called the terminal sulcus.
At the tip of the terminal sulcus, the endoderm descends downwards and develops into the thyroid gland. The descent of the endoderm creates a tiny sac-like structure called foramen cecum.
The posterior-most part of the tongue develops from a third median swelling, arising from the fourth pharyngeal arch. This area of the tongue receives its innervation from the X cranial, or vagus, nerve via internal laryngeal nerve.
Now, as the lingual swellings are developing, the occipital somites that come from the paraxial mesoderm, migrate into the developing tongue.
The occipital somites give rise to myoblasts, which go on to develop into the tongue skeletal muscles. The motor innervation of tongue muscles comes from the XII cranial nerve (hypoglossal nerve); except the palatoglossus muscle.
And, as a final touch, the taste buds start to sprout over the surface of the tongue around week 8 and finish differentiating into the different types around week 11 to week 13.
Cranial nerves VII, or facial nerve, and IX, or glossopharyngeal nerve, innervate the taste buds. The contribution of cranial nerve VII is facilitated by its chorda tympani branch, which joins with the lingual branch of the mandibular division of trigeminal nerve carrying afferent special sensation of the anterior two-thirds of the tongue.
The posterior one-third special sensory component is carried by cranial nerve IX. ##Summary All right, as a quick recap… The tongue starts developing by week 4 of intrauterine life from the first four pharyngeal arches.
The anterior two-thirds of the tongue develops from the first arch mesenchyme as the tuberculum impar, and two lateral lingual swellings.
The lateral lingual swellings grow over tuberculum impar and merge along the midline eventually forming the median sulcus.
The posterior one-third develops from the second pharyngeal arch as the copula, and the third and fourth pharyngeal arch as the hypobranchial eminence.
Finally, the tongue muscles derive from the occipital somites and taste buds finish appearing over the tongue around week 13.
anterior two-thirds of the tongue develops from the first Artemis and kind as the tuberculum impar and two lateral lingual swellings, the lateral lingual swellings grow over tuberculum impar and merge along the midline eventually forming the median sulcus.
The posterior 1/3 develops from the second pharyngeal Arch as the copula and the third and fourth pharyngeal Arch as the hypobranchial eminence.
Finally the tongue muscles derived from the occipital somites and taste buds finish appearing over the tongue
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