THYROID GLAND TONGUE Dr Mujahid Khan Development of

THYROID GLAND & TONGUE Dr. Mujahid Khan

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

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

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

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

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

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

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

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

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

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

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

Development of Tongue Ø 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 Ø Median tongue bud forms no recognizable part of the adult tongue

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

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

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

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

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

Papillae and Taste Buds Ø 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

Nerve Supply of the Tongue Ø The development of tongue explains its nerve supply Ø 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 Ø This nerve is the nerve of first pharyngeal arch and this arch forms the median and distal tongue buds

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

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

Nerve Supply of the Tongue Ø The superior laryngeal branch of the vagus nerve (CN Ⅹ) of the fourth arch supplies small area of the tongue anterior to the epiglottis Ø 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