veterinary medicine

1. Thyroid EmbryologyThyroid Embryology
Stephanie JohnsonStephanie Johnson
PGY 2PGY 2

2. Initial embryologyInitial embryology
Thyroid:Thyroid:
 11stst
endocrine glandendocrine gland
 Day ~24th gestationDay ~24th gestation
 Proliferation ofProliferation of endodermalendodermal cells oncells on
midline pharyngeal floormidline pharyngeal floor
 Between 2 key structures:Between 2 key structures:
– tuberculum impartuberculum impar
– CopulaCopula
……called “called “foramen cecumforamen cecum””

4. Initial embryologyInitial embryology
Thyroid:Thyroid:
 Initially develops caudal to theInitially develops caudal to the
“tuberculum impar”“tuberculum impar” (median tongue bud)(median tongue bud)
 From the 1From the 1stst
pharyngeal archpharyngeal arch
 Midline on floor of developing pharynxMidline on floor of developing pharynx
 Eventually helps form the tongue as the 2Eventually helps form the tongue as the 2
lateral lingual swellings overgrow itlateral lingual swellings overgrow it

5. Initial embryologyInitial embryology
 Foramen cecum begins ant to the copula,Foramen cecum begins ant to the copula,
aka ‘aka ‘hypobranchial eminencehypobranchial eminence ’’
 Median embryologic swelling = mesodermMedian embryologic swelling = mesoderm
that arises from thethat arises from the secondsecond pharyngealpharyngeal
pouch (although the third and fourthpouch (although the third and fourth
pouches are also involved)pouches are also involved)
 SummarySummary - thyroid gland originates from- thyroid gland originates from
b/wb/w 11stst
and 2and 2ndnd
pharyngeal pouchespharyngeal pouches

6. Initial embryologyInitial embryology
““thyroid primordium”thyroid primordium”
 initial thyroid precursorinitial thyroid precursor
 starts as a simple midline thickeningstarts as a simple midline thickening
 forms thyroid diverticulumforms thyroid diverticulum
 initially hollowinitially hollow
 later solidifies and becomes bilobedlater solidifies and becomes bilobed

8. Descent of thyroid glandDescent of thyroid gland
 Initial descent starts ant to pharyngeal gutInitial descent starts ant to pharyngeal gut
 Thyroid still connected to the tongue via theThyroid still connected to the tongue via the
thyroglossal ductthyroglossal duct
 Later - tubular duct solidifies then obliteratesLater - tubular duct solidifies then obliterates
entirely (during gestational weeksentirely (during gestational weeks 7-107-10))
 Foramen cecumForamen cecum = opening of the= opening of the
thyroglossal duct into the tonguethyroglossal duct into the tongue

9. Descent of thyroid glandDescent of thyroid gland
Pyramidal lobe:Pyramidal lobe:
 in up to 50% peoplein up to 50% people
 persistence of the inferior end of thepersistence of the inferior end of the
thyroglossal duct (fx to obliterate)thyroglossal duct (fx to obliterate)
 may be attached to the hyoid bonemay be attached to the hyoid bone
(~thryoglossal duct cyst) OR may be(~thryoglossal duct cyst) OR may be
incorporatedincorporated intointo a thyroglossal ducta thyroglossal duct
cystcyst

10. Descent of thyroid glandDescent of thyroid gland
 Further descent – ant (ventral) to theFurther descent – ant (ventral) to the
hyoid bone and laryngeal cartilagehyoid bone and laryngeal cartilage
 DuringDuring descent – thyroid forms itsdescent – thyroid forms its
mature shape: median isthmusmature shape: median isthmus
connecting 2 lateral lobesconnecting 2 lateral lobes
 Descent COMPLETE at 7Descent COMPLETE at 7thth
gestationalgestational
weekweek

11. Parafollicular cellsParafollicular cells
 AkaAka C cellsC cells
 Secrete calcitonin (Ca regulation)Secrete calcitonin (Ca regulation)
 FromFrom ultimobranchial bodyultimobranchial body (last(last
structure derived from the branchialstructure derived from the branchial
pouches)pouches)
 Ultimobranchial body – fromUltimobranchial body – from 55thth
pharyngeal pouchpharyngeal pouch

12. Parafollicular cellsParafollicular cells
 Migrating cells from the neural crest regionMigrating cells from the neural crest region
infiltrateinfiltrate  ultimobranchial body (UBB)ultimobranchial body (UBB)
 This struct (neural crest + UBB) =This struct (neural crest + UBB) =
incorporatedincorporated  thyroid glandthyroid gland
 UBBUBB fusesfuses with the thyroid gland andwith the thyroid gland and
disseminatesdisseminates its cells into itits cells into it
 SummarySummary: the C cells of the thyroid are of: the C cells of the thyroid are of
neural crestneural crest origin.origin.

13. Clinical correlationsClinical correlations
1)1) Thyroglossal duct cystThyroglossal duct cyst
 duct fx to atrophyduct fx to atrophy
 can form a sinuscan form a sinus
 Rx – “Sistrunk” procedure (includes resection part ofRx – “Sistrunk” procedure (includes resection part of
hyoid bone)hyoid bone)
2) Ectopic thyroid2) Ectopic thyroid
 can occur anywhere along the path of descentcan occur anywhere along the path of descent
 most common at the base of the tongue ‘most common at the base of the tongue ‘linguallingual
thyroidthyroid’’

15. Clinical correlationsClinical correlations
3)3) Accessory thyroidAccessory thyroid
 from remnants of thyroglossal ductfrom remnants of thyroglossal duct
 may be functional but insufficient formay be functional but insufficient for
normal function of thyroidnormal function of thyroid