Transcription
The tongue is a muscular structure as well as a sensory organ that starts developing alongside the external face around week four 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 four of embryonic development, as a result of the folding of the embryo along the rostrocotyl 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 rasterly to form somidomeres and fully segments caudally to form somites, the first in the series being the occipital somites.
At this point, a small pit called the stomadeim 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 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 stomadeim.
Now, the tongue begins to develop around week four 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-thirds 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 fifth cranial nerve, the 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 four as two mesodermal swellings. The first swelling, known as the copula, develops in the midline of the second and third pharyngeal arches during week four. The second swelling, known as the hypobranchial eminence, grows from the midline of the third and fourth pharyngeal arches around week five. 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 the anterior portion, forming the complete tongue. The mucosa overlying this area of the tongue receives its sensory innervation from the ninth cranial nerve, the glossopharyngeal nerve.
The region between the anterior two-thirds 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 the 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 tenth cranial nerve, the vagus nerve, via the 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's skeletal muscles. The motor innervation of tongue muscles comes from the 12th cranial nerve, the hypoglossal nerve, except for 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 7 (facial nerve) and 9 (glossopharyngeal nerve) innervate the taste buds. The contribution of cranial nerve 7 is facilitated by its chorda tympani branch, which joins with the lingual branch of the mandibular division of the 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 9.
All right, as a quick recap, the tongue starts developing by week four 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 the 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.