THYROID GLAND & TONGUE

1. THYROID GLAND & TONGUE By: Dr. Mujahid Khan

2. Development of Thyroid • The thyroid gland is the first endocrine gland to develop in embryo • It begins to form about 24 days after fertilization • It develops from a median endodermal thickening in the floor of a primordial pharynx • Thickening soon forms a small outpouching called thyroid primordium

4. Development of Thyroid • As the embryo and tongue grow, the developing thyroid gland descends in the neck, passing ventral to the developing hyoid bone and laryngeal cartilages • For a short time the thyroid gland is connected to the tongue by a narrow tube, the thyroglossal duct

6. Development of Thyroid • At first the thyroid primordium is hollow but it soon becomes solid and divides into right and left lobes • The two lobes are connected by the isthmus of the thyroid gland • Isthmus lies anterior to the developing second and third tracheal rings • By seventh week it assumes the definitive shape and has reached its final site in the neck

8. Development of Thyroid • The thyroglossal duct has normally degenerated by seventh week • The proximal opening of the thyroglossal duct persists as a small pit in the tongue, the foramen cecum • A pyramidal lobe extends upward from the isthmus in about 50% of people

10. Development of Thyroid • The pyramidal lobe may be attached to the hyoid bone by fibrous tissue or smooth muscle, the levator of thyroid gland • The pyramidal lobe and the associated smooth muscle represent a persistent part of the distal end of the thyroglossal duct

11. Histogenesis of Thyroid • The thyroid primordium consists of a solid mass of endodermal cells • The cellular aggregation later breaks up into a network of epithelial cords • By the tenth week the cords have divided into small cellular groups • A lumen soon forms in each cell cluster and the cells become arranged in a single layer around the lumen • During the eleventh week colloid begins to appear in these structures, called thyroid follicles • Iodine concentration and synthesis of thyroid hormones can be demonstrated

12. Thyroglossal Duct Cysts & Sinuses • Cyst may form anywhere along the course followed by the thyroglossal duct during descent of the primordial thyroid gland from the tongue • Normally the thyroglossal duct atrophies and disappear • A remnant of it may persist and form a cyst in the tongue or in the anterior part of the neck

14. Thyroglossal Duct Cysts & Sinuses • It usually lies just inferior to the hyoid bone • Most thyroglossal duct cysts are observed by the age of 5 years • The swelling produced is usually develops as a painless, progressively enlarging, moveable mass • The cyst may contain some thyroid tissue

15. Thyroglossal Duct Cysts & Sinuses • Following infection of a cyst, a perforation of the skin occurs forming a thyroglossal duct sinus • It usually opens in the median plane of the neck, anterior to the laryngeal cartilages

16. Development of Tongue • A median triangular elevation appears in the floor of the primordium pharynx near the end of 4th week, just rostral to the foramen cecum • This swelling or median tongue bud is the first indication of tongue development • Soon two oval distal tongue buds develop on each side of the median tongue bud

18. Development of Tongue • The three lingual buds result from the proliferation of mesenchyme in ventromedial parts of the first pair of pharyngeal arches • The distal tongue buds rapidly increase in size, merge with each other, and overgrow the median tongue bud • The merged distal tongue buds form the anterior two-thirds (oral part) of the tongue

20. Development of Tongue • Fusion of the distal tongue buds is indicated by a middle groove, the median sulcus of the tongue and internally by the fibrous lingual septum • Median tongue bud forms no recognizable part of the adult tongue

21. Formation of Posterior third of Tongue • It is indicated by two elevations that develop caudal to the foramen cecum • Copula: Forms by fusion of the ventromedial part of the second pair of pharyngeal arches • The hypopharyngeal eminence: develops caudal to the copula from mesenchyme in the ventromedial parts of the third and fourth pairs of arches

23. Formation of Posterior third of Tongue • As the tongue develops the copula is gradually overgrown by the hypopharyngeal eminence and disappear • As a result, the pharyngeal part of the tongue develops from the rostral part of the hypopharyngeal eminence • The line of fusion of the anterior and posterior parts of the tongue is roughly indicated by a V-shaped groove called terminal sulcus

25. Formation of Posterior third of Tongue • Pharyngeal mesenchyme forms the connective tissue and vasculature of the tongue • Most of the tongue muscles are derived from myoblasts that migrate from the occipital myotomes • The hypoglossal nerve (CN Ⅻ) accompanies the myoblast during their migration and innervates the tongue muscles as they develop • The entire tongue is within the mouth at birth, its posterior third descends into the oropharynx by 4 years of age

26. Papillae and Taste Buds • Lingual papillae appear towards the end of the eighth week • The vallate and foliate papillae appear first, close to the terminal branches of the glossopharyngeal nerve (CN Ⅸ) • The fungiform papillae appear later near termination of chorda tympani branch of the facial nerve

28. Papillae and Taste Buds • The most common lingual papillae, known as filiform papillae because of their threadlike shape, develop during early fetal period (10-11 weeks) • They contain afferent nerve endings sensitive to touch • Taste buds develop during 11-13 weeks • Most taste buds form on the dorsal surface of the tongue

30. Papillae and Taste Buds • Fetal responses in the face can be induced by bitter tasting substances at 26-28 weeks, indicating that the reflex pathways between taste buds and facial muscles are established by this age

31. Nerve Supply of the Tongue • The development of tongue explains its nerve supply • The sensory supply to the mucosa of almost the entire anterior two-thirds of the tongue is from the lingual branch of the mandibular division of the trigeminal nerve • This nerve is the nerve of first pharyngeal arch and this arch forms the median and distal tongue buds

32. Nerve Supply of the Tongue • Facial nerve is the nerve of second pharyngeal arch • Its chorda tympani branch supplies the taste buds in the anterior two-thirds of the tongue except the vallate papillae • The facial nerve does not supply any of the tongue mucosa, except for taste buds in the oral part of the tongue

34. Nerve Supply of the Tongue • The vallate papillae in the oral part of the tongue are innervated by glossopharyngeal nerve (CN Ⅸ) of the third pharyngeal arch • This is due to the reason that mucosa of posterior two third of the tongue is pulled slightly anteriorly as the tongue develops • The posterior third of the tongue is innervated mainly by the glossopharyngeal nerve, which is a nerve of third pharyngeal arch

35. Nerve Supply of the Tongue • The superior laryngeal branch of the vagus nerve (CN Ⅹ) of the fourth arch supplies small area of the tongue anterior to the epiglottis • All muscles of the tongue are supplied by the hypoglossal nerve (CN Ⅻ), except for palatoglossus, which is supplied from pharyngeal plexus by fibers arising from the vagus nerve