Thyroid embryology

veterinary medicine

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

2. Initial embryologyInitial embryology
Thyroid:Thyroid:
11
stst
endocrine gland endocrine gland
Day ~24th gestationDay ~24th gestation
Proliferation of Proliferation of endodermalendodermal cells on cells 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 the Initially develops caudal to the
“tuberculum impar”“tuberculum impar” (median tongue bud) (median tongue bud)
From the 1From the 1
stst
pharyngeal arch pharyngeal arch
Midline on floor of developing pharynxMidline on floor of developing pharynx
Eventually helps form the tongue as the 2 Eventually 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 = mesoderm Median embryologic swelling = mesoderm
that arises from the that arises from the second second pharyngeal pharyngeal
pouch (although the third and fourth pouch (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 1 1
stst
and 2 and 2
ndnd
pharyngeal pouches pharyngeal pouches

6. Initial embryologyInitial embryology
““thyroid primordium”thyroid primordium”
initial thyroid precursorinitial thyroid precursor
starts as a simple midline thickening starts as a simple midline thickening
 forms thyroid diverticulum forms 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 the Thyroid still connected to the tongue via the
thyroglossal ductthyroglossal duct
Later - tubular duct solidifies then obliterates Later - tubular duct solidifies then obliterates
entirely (during gestational weeks entirely (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 the persistence of the inferior end of the
thyroglossal duct (fx to obliterate)thyroglossal duct (fx to obliterate)
may be attached to the hyoid bone may be attached to the hyoid bone
(~thryoglossal duct cyst) OR may be (~thryoglossal duct cyst) OR may be
incorporated incorporated intointo a thyroglossal duct a thyroglossal duct
cystcyst

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

11. Parafollicular cellsParafollicular cells
Aka Aka C cellsC cells
Secrete calcitonin (Ca regulation)Secrete calcitonin (Ca regulation)
From From ultimobranchial bodyultimobranchial body (last (last
structure derived from the branchial structure derived from the branchial
pouches)pouches)
Ultimobranchial body – fromUltimobranchial body – from 5 5
t ht h

pharyngeal pouchpharyngeal pouch

12. Parafollicular cellsParafollicular cells
Migrating cells from the neural crest region Migrating cells from the neural crest region
infiltrate infiltrate  ultimobranchial body (UBB) ultimobranchial body (UBB)
This struct (neural crest + UBB) = This struct (neural crest + UBB) =
incorporated incorporated  thyroid gland thyroid gland
UBB UBB fusesfuses with the thyroid gland and with the thyroid gland and
disseminates disseminates 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 cyst Thyroglossal duct cyst
duct fx to atrophy duct fx to atrophy
can form a sinuscan form a sinus
Rx – “Sistrunk” procedure (includes resection part of Rx – “Sistrunk” procedure (includes resection part of
hyoid bone)hyoid bone)
2) Ectopic thyroid 2) 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 ‘lingual lingual
thyroidthyroid’’

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