glands to develop, on approximately the 24th day of
gestation
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? It begins its development from a median endodermal
thickening in the floor of the primitive pharynx just
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caudal to the future site of the tuberculum imparDescent of the Thyroid Gland
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? Thickening forms a downgrowth, thyroiddiverticulum, which grows into the underlying
mesoderm, and as the embryo elongates and the tongue
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grows, the diverticulum descends in front of the neck
and pharyngeal gut
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? The diverticulum is connected to the tongue by anarrow canal, the thyroglossal duct, which opens in
the tongue via the foramen cecum, which persists as a
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vestigial pit on the tongue
..Cont
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? Diverticulum grows rapidly and forms 2 lobes? By week 7 of embryonic development, it reaches anterior to
the trachea, having acquired a small median isthmus and 2
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lateral lobes. By then, the thyroglossal duct usually hasdisappeared
? A pyramidal lobe of the thyroid may be observed in as many
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as 50% of patients. This lobe represents a persistence of the
inferior end of the thyroglossal duct that has failed to
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obliterate.? THE THYROID GLAND begins to function at about the end of
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month 3, at which time, the first follicles containing colloid canbe seen
? AT FIRST, THE THYROID PRIMORDIUM is made up of a solid
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mass of entodermal cells
? It later breaks up into a network of epithelial cords or plates by
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invasion of the surrounding mesenchyme
? By week 10, the cords have divided into small cellular groups,
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and a lumen forms in each cellular cluster. The cells thenarrange themselves in a single layer around the lumen
? During week 11, colloid is seen in these follicle structures, and
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even thyroxine can be demonstrated
REMNANTS OF THE THYROGLOSSAL DUCT
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? The normal remains of the thyroglossal duct are thevestigial foramen cecum (of the tongue) and the
functional pyramidal lobe of the thyroid gland
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Congenital malformations
? THYROGLOSSAL DUCT CYSTS AND SINUSES
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? Cysts can form anywhere along the course of the developing
thyroglossal duct during descent of the developing thyroid
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gland from the tongue? Remnants of the duct may persist and give rise to cysts in the
tongue or in the midline of the neck, usually below the hyoid
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bone
ACCESSORY THYROID TISSUE
? Very rarely the thyroid fails to descend from the tongue area
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resulting in a lingual thyroid
? Incomplete descent, which is rare, may result in a cervical
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thyroid that is seen in the neck at or just below the hyoid bone? Accessory thyroid tissue often is fully functional, originates
from remnants of the thyroglossal duct, thus can be found
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anywhere from the level of the tongue to where the thyroid
gland comes to rest in the neck
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Ectopic Thyroid tissue and Agenesis? Found in thorax in relation to trachea and bronchi or even
oesophagus
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? Believed to arise from endodermal cells displaced during
formation of laryngotracheal tube.
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? Agenesis of the Thyroid? Failure of development of thyroid gland may also occur