Embryology Lec6 Dr.Ban Tongue and Thyroid gland Development · Tongue and Thyroid gland Development . The tongue appears in embryos of approximately 4 weeks . Formation: the anterior

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Embryology

Lec6 Dr.Ban

Tongue and Thyroid gland Development

The tongue appears in embryos of approximately 4 weeks .

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.

Anterior 2/3rd of the tongue :

• sensory innervation of the mucosa is via the lingual branch of the trigeminal

nerve

Innervation of the anterior 2/3rd of the tongue:

• 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 .

• It is indicated by two elevations that develop caudal to the foramen cecum

Posterior 1/3rd of the tongue:

• 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.

• Sensory innervation of the mucosa is mostly via the glossopharyngeal nerve

(and some vagus)

Innervation of the posterior 1/3rd of the tongue:

• 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 pharyngeal 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 in 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 flask-

shape 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.