Download Embryology Lec6 Dr.Ban Tongue and Thyroid gland Development

Embryology
Lec6
Dr.Ban
Tongue and Thyroid gland Development
The tongue appears in embryos of approximately 4 weeks .
Anterior 2/3rd of the tongue :
Formation: the anterior 2/3rd of the tongue is derived from median(tuberculum
impar) and lateral tongue buds that arise from the floor of the 1st pharyngeal arch
and then grow rostrally.The tongue buds are then invaded by occipital myoblasts
that form the intrinsic muscles of the tongue.
Innervation of the anterior 2/3rd of the tongue:
•
sensory innervation of the mucosa is via the lingual branch of the trigeminal
nerve
•
taste bud innervation is via the chorda tympani branch of the facial nerve,
except for the taste buds in the circumvallate papilla that present in the most
posterior part of the anterior 2/3rdof the tongue these are innervated by the
glossopharyngeal nerve.
•
motor innervation of the intrinsic tongue muscles is via the hypoglossal
nerve .
Posterior 1/3rd of the 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.
• As the tongue develops the copula is gradually overgrown by the
hypopharyngeal eminence and disappears.
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• The line of fusion of the anterior and posterior parts of the tongue is roughly
indicated by a V-shaped groove called terminal sulcus.
• The foramen cecum (which is related to the development of the thyroid
gland) is found in the midline of terminal sulcus.
Innervation of the posterior 1/3rd of the tongue:
•
Sensory innervation of the mucosa is mostly via the glossopharyngeal nerve
(and some vagus)
•
Taste innervation is mostly via the glossopharyngeal nerve (and some vagus)
•
Motor innervation of the intrinsic skeletal muscles is via the hypoglossal
nerve .
Ventral portion of the pharynge al arches seen from above showing deve lopment of the
tongue. A. 5 weeks. B. 5 months,note the foramen cecum, site of origin of the thyroid
primordium.
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Abnormalties
Macroglossia
Macroglossia is the medical term for an unusually large tongue. Sever enlargement
of the tongue can cause cosmetic and functional difficulties in speaking, eating,
swallowing and sleeping. Macroglossia is uncommon, and usually occurs in
children. There are many causescan be associated with a number of genetic
abnormalities including: trisomy 21 (Down syndrome), acromegaly. Treatment is
dependent upon the exact cause.
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Microglossia
This is a condition where the size of the tongue is abnormally small. Cases of
complete absence of the tongue have been reported. Fortunately, it is a rare
condition. Obviously, a tiny tongue will pose many difficulties related to speech
and swallowing. There is no treatment for this condition, and the affected person
will have to train their tongue to the best of their abilities.
Ankyloglossia(Tongue-Tie)
Ankyloglossia (tongue-tie) is the general clinical term for the short lingual frenulum
(less than 2 cm), that limits the range of movement of the tongue, This is
associated with speech development disorders and feeding disorders.In the most
common form of ankyloglossia, the frenulum extends to the tip of the tongue.
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Thyroid gland
The thyroid gland appears as an epithelial proliferation in the floor of the pharynx
between the tuberculum impar and the copula at a point later indicated by the
foramen cecum .Subsequently, the thyroid descends in front of the pharyngeal gut
asa bilobeddiverticulum . During this migration, the thyroid remains connected to
the tongue by a narrow canal, the thyroglossal duct. This duct later disappears.
With further development, the thyroid gland descends in front of the hyoid bone
and the laryngeal cartilages. It reaches its final position in front of the trachea in
the seventh week. By then, it has acquired a small median isthmus and two lateral
lobes. The thyroid begins to function at approximately the end of the third month,
at which time the first follicles containing colloid become visible. Follicular cells
produce the colloid that serves as a source of thyroxine and triiodothyronine.
Parafollicular, or C, cells derived from the ultimobranchialbody ,serve as a source
of calcitonin.
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Thyroglossalduct and thyroid abnormalities
A thyroglossal cyst may lie at any poínt along the migratory pathway of the thyroid
gland but is always near or in the midline of the neck. As indicated by its ñame, it
is a cystic remnant of the thyroglossal duct. Although approximately 50% of these
cysts are close to or just inferior to the body of the hyoid bone , they may also be
found at the base of the tongue or close to the thyroid cartilage.Sometimes, a
thyroglossal cyst is connected to the outside by a fistulous canal, a
thyroglossalfístula. Such a fistula usually arises secondarily after rupture of a cyst
but may be present at birth.
Aberrant thyroid tissue may be found anywhere along the path of descent of the
thyroid gland. It is commonly found in the base of the tongue, just behind the
foramen cecum, and is subject to the same diseases as the thyroid gland itself.
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A. The thyroid primordium arises as an epithelial diverticulum in the midline of the pharynx
immediately caudal to the tuberculum impar. B. Position of the thyroid gland in the adult. Broken
line, the path of migration.
Thyroid gland:
•
The thyroid gland is the first endocrine gland to develop in embryo.
•
It begins to form about 3–4 weeks after fertilization
•
By seventh week it migrates to the base of the neck, passing anterior to the hyoid bone.
•
During migration, the thyroid remains connected to the tongue by a narrow canal, the
thyroglossal duct
•
The proximal opening of the thyroglossal duct persists as a small pit in the tongue, the
foramen cecum found i n the midline at the terminal sulcus of the tongue.
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Thymus
The thymus is a soft, roughly triangular specialized primary lymphoid organ of the
immune system.Located in the upper chest region,It has two distinct but identical
lobes that are each surrounded by a tough, fibrous capsule. Within each lobe is a
superficial region of tissue called the cortex and a histologically distinct deep
region called the medulla.Within the thymus, T cells or T lymphocytes mature.
The two main components of the thymus, the lymphoid thymocytes and the thymic
epithelial cells, have distinct developmental origins.
The thymic epithelium is the first to develop, and appears in the form of two flaskshape endodermal diverticula, which arise, one on either side, from the third
branchial pouch (pharyngeal pouch).
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The thymocytes and epithelium meet and join with connective tissue. The
pharyngeal opening of each diverticulum is soon obliterated, but the neck of the
flask persists for some time as a cellular cord. By further proliferation of the cells
lining the flask, buds of cells are formed, which become surrounded and isolated
by the invading mesoderm. Additional portions of thymus tissue are sometimes
developed from the fourth branchial pouches.
Unlike most organs that grow until the age of maturity, the thymus enlarges
throughout childhood but slowly shrinks from the onset of puberty and throughout
adulthood. As the thymus shrinks, its tissues are replaced by adipose tissue.
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