Development of tongue

A brief description of the development of the human tongue and the associated developmental anomalies.

1. Development of Tongue Presented by: Dr. Swapnaneel Pradhan

2. Contents: Introduction Functions of tongue Development of Tongue Development of muscles of Tongue Developmental Anomalies of Tongue References

3. iNTRODUCTION

4. The tongue is a muscular organ situated in the floor of the mouth. It has an oral part which lies in the mouth, and a pharyngeal part that lies in the pharynx. Oral and pharyngeal parts are separated by a V-shaped sulcus called sulcus terminalis. The tongue is anchored to the floor of the mouth by the lingual frenum.

5. Functions

6. Speech Speech is the product of air leaving the lungs via the mouth. The tongue is responsible for different sounds depending on how it's positioned in the mouth. Since the tongue is so flexible, it can switch positions quickly to make a rapid succession of sounds.

7. Taste Taste buds are responsible for the ability to taste food. There are five different taste sensations: sweet, sour, salty, bitter and savory, according to the National Institute on Deafness and Other Communication Disorders. Taste buds for each of the five sensations are scattered along the tongue's surface, rather than localized in specific sections. All tastes are perceived everywhere.

8. Mastication Mastication is the process of combining ground up food particles with naturally occurring saliva. Mastication happens at the same time as grinding. Grinding is when teeth convert large food particles into small ones. The tongue first positions food between the teeth for chewing before mixing it with saliva.

9. Deglutition After masticating food particles, the tongue helps round them into masses known as bolus. Once the bolus is formed, the tongue pushes them toward the esophagus found in the back of the mouth before finding its way to the stomach for digestion.

10. Development

11. Basic Overview The tongue is divided into two parts: oral part (anterior two third) and pharyngeal part (posterior one-third). The oral part of tongue develops from three swellings associated with first pharyngeal arch. These swellings are two lateral lingual swellings and one median swelling —the tuberculum impar. The pharyngeal part of tongue develops from a median swelling called hypobranchial eminence or copula of His associated with second, third, and fourth pharyngeal arches. Muscles of tongue develop from occipital myotomes .

12. Stages of Development At 4 weeks. Late in the fifth week. Early in the sixth week. Middle of the seventh week. Adult

13. Development of Mucosa

14. Initial Development The tongue develops in the floor of developing mouth from first, second, third, and fourth pharyngeal arches. At the end of the fourth week of intrauterine life (IUL), a small median triangular swelling called tuberculum impar develops in the floor of primitive pharynx, just cranial to foramen cecum. Soon after the appearance of tuberculum impar the two lateral oval swellings called lingual swellings develop one on each side of tuberculum impar. The two lateral lingual swellings are placed slightly distal to the tuberculum impar, hence they are also called distal tongue buds.

15. Initial Development Caudal to tubercular impar a second large median swelling called hypobranchial eminence (copula of His) develops in the floor of primitive pharynx in relation to second, third, and fourth pharyngeal arches. The hypobranchial eminence soon subdivides into large cranial part and small caudal part.

16. Development of Various Parts of Tongue

17. Anterior 2/3rd The two lateral lingual swellings overgrow the tuberculum impar and merge with each other to form anterior two-third of the tongue . The line of fusion of two lingual swellings in the median plane forms median sulcus on the dorsal surface of the tongue. The tuberculum impar does not form any recognizable part of the adult tongue. Since mucous membrane covering the anterior two-third of the tongue develops from the first pharyngeal arch, it is innervated by mandibular branch of trigeminal nerve—the nerve of first arch.

18. Posterior 1/3rd The posterior one-third of the tongue including circumvallate papillae develops from cranial part of hypobranchial eminence . The line of fusion of the anterior two-third and posterior one-third of the tongue is indicated by a V-shaped groove—the sulcus terminalis . Since the mucous membrane of the posterior one-third of tongue (including vallate papillae) develops from the third pharyngeal arch, it is supplied by glossopharyngeal nerve —the nerve of third arch.

19. Posterior most part The posterior most part of the tongue and epiglottis develop from the caudal part of the hypobranchial eminence. Since the mucous membrane of the posterior most part of tongue and epiglottis develop from fourth pharyngeal arch, it is supplied by the superior laryngeal nerve —the nerve of the fourth arch.

20. During this sequence of events, the third arch mesoderm grows over the mesoderm of the second arch and fuses with the mesoderm of the first arch. Thus, second arch gets buried underneath the third arch and thereby gets excluded from the tongue development.

21. Muscles of the tongue

22. Muscles of Tongue The muscles of tongue develop from myoblasts that migrate into developing tongue from the occipital myotomes . The hypoglossal nerve, the nerve of occipital myotomes, accompanies the myoblasts during their migration to the pharyngeal arches and innervates the muscles of tongue as they develop. The migration of the occipital myotomes to the developing tongue explains the course of the hypoglossal nerve. Some muscles of tongue probably develop in situ .

23. Points to Remember Mucous membrane of the tongue is derived from the endoderm of the primitive foregut. Taste buds are formed in relation to the terminal branches of nerves carrying taste sensations. Muscles of the tongue develop from occipital myotomes. Fibro-areolar tissue that binds the tongue muscles develops from mesenchyme of the pharyngeal arches.

24. Relation of Anterior Part of Tongue with the Floor of the Mouth In the region of floor of the mouth, mandibular process forms three structures: (a) lower lip and lower parts of cheeks, (b) lower jaw, and (c) tongue. At first these structures are not discernible from each other. Soon the tongue forms a recognizable swelling in the middle. A sulcus called linguogingival sulcus develops on either side of this swelling and separates the developing tongue from the floor of the mouth.

25. Developmental anomalies of Tongue

26. Aglossia Microglossia Macroglossia Ankyloglossia Cleft tongue Fissured tongue Median Rhomboid Glossitis Benign Migratory Glossitis Hairy tongue Lingual varices Lingual thyroid nodule

27. Thank you

28. References Vishram Singh. Development of Tongue and Thyroid. Textbook of Clinical Embryology. New Delhi: Elsevier; 2012. p. 122 – 125. Inderbir Singh. Alimentary System – I: Mouth, Pharynx and Related Structures. In: V Subhadra Devi, editor. Human Embryology. Eleventh Edition. New Delhi: Jaypee Brothers Medical Publishers (P) Ltd; 2018. p. 168 – 170. Sadler TW, Head and Neck. System-Based Embryology. In: Crystal Taylor, Andrea Vosburgh, editors. Langman’s Medical Embryology. Fourteenth Edition. Philadelphia: Wolters Kluwer; 2019. p. 298 – 299. Schoenwolf , Bleyl , Brauer , Francis-west. Development of the Pharyngeal Apparatus and Face. Larsen’s Human Embryology. Fifth Edition. Philadelphia: Elsevier: 2015. p. 458 – 451.