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Feb 10, 2016




THYROID GLAND & TONGUE. Dr. Mujahid Khan. 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 - PowerPoint PPT Presentation
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  • THYROID GLAND & TONGUEDr. Mujahid Khan

  • Development of ThyroidThe 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

  • Development of ThyroidAs 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

  • Development of ThyroidAt 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

  • Development of ThyroidThe 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

  • Development of ThyroidThe 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

  • Histogenesis of ThyroidThe 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

  • Thyroglossal Duct Cysts & SinusesCyst 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

  • Thyroglossal Duct Cysts & SinusesIt 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

  • Thyroglossal Duct Cysts & SinusesFollowing 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

  • Development of TongueA 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

  • Development of TongueThe 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

  • Development of TongueFusion 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

  • Formation of Posterior third of TongueIt 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

  • Formation of Posterior third of TongueAs 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

  • Formation of Posterior third of TonguePharyngeal 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 ) 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

  • Papillae and Taste BudsLingual 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 )

    The fungiform papillae appear later near termination of chorda tympani branch of the facial nerve

  • Papillae and Taste BudsThe 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

  • Papillae and Taste BudsFetal 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 TongueThe 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

  • Nerve Supply of the TongueFacial 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

  • Nerve Supply of the TongueThe vallate papillae in the oral part of the tongue are innervated by glossopharyngeal nerve (CN ) 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

  • Nerve Supply of the TongueThe superior laryngeal branch of the vagus nerve (CN ) 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 ), except for palatoglossus, which is supplied from pharyngeal plexus by fibers arising from the vagus nerve