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THYROID GLAND

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Title: THYROID GLAND, TONGUE & SALIVARY GLAND Author: User Last modified by: Dr Khan Created Date: 3/3/2007 7:36:56 AM Document presentation format – PowerPoint PPT presentation

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Title: THYROID GLAND


1
THYROID GLAND TONGUE
  • 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

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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

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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

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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

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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

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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

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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

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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

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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

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

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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

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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

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Nerve Supply of the Tongue
  • The 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

35
Nerve Supply of the Tongue
  • The 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
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