Development of palatine tonsils, tongue and thyroid gland

Palatine tonsils

Origin: 2nd pharyngeal arch endoderm

Endoderm proliferates into buds ⇒ Canalizes ⇒ Forms crypts ⇒ Invaded by lymphocytes

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Tongue

1. Muscles of tongue – 3 occipital myotomes of paraxial mesoderm

• (1st occipital myotome forms extraoccular muscles of eye)
• The 3 remaining myotomes drag the hypoglossal nerve with them

2. Mucous membrane:

• Anterior 2/3 – 1st pharyngeal arch
• From two lingual swellings and tuberculum impar

• Posterior 1/3 – endoderm of 3rd pharyngeal arch
• Endoderm of 2,3 and 4 arch fuse to form Hypobronchial eminence
• A groove divides the hypobronchial eminence into:

1. Upper part – Posterior 1/3 tongue mucous membrane
2. Lower part – Forms epiglottis

Anterior and Posterior fuse at sulcus terminalis ( v shape)

Congenital anomalies:

• Aglossia – absence of tongue
• Macroglossia, microglossia
• Bifid tongue
• Glossoschisis – cleft tongue
• Tongue tie (ankyloglossia) – frenulum is till tip of tongue, can’t protrude tongue

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Thyroid gland

1. Endodermal thickening around foramen cecum
2. Forms thyroid primodium between tuberculum impar and hypobronchial eminance
3. Thyroid primodium invaginated – bilobed diverticulum
4. Descends infront of hyoid bone and laryngeal cartilage
5. Connected to dorsum of tongue by thyroglossal duct
6. Finally reaches infront of thyroid cartilage and upper trachea
7. Thyroglossal duct obliterated

Congenital anomalies:

• Thyroid agenesis – absence, leads to stunted physical and mental growth
• Lingual thyroid – Fails to descend
• Thyroglossal cyst – thyroglossal duct unobliterated
• Thyroglossal fistula – cyst may rupture
• Ectopic positions – Sublingual, retrohyoid, retrosternal etc.