L01 - Embryology and Anatomy of the Thyroid and Parathyroid glands Flashcards
Describe the origin, migration of thyroid glnad.
3rd week: thyroid diverticulum appears
- Endoderm thickens at foramen caecum (at midline of neck caudal to median tongue bud)»_space; forms bilobed structure
> > migrates ventral to laryngotracheal tube, passes anterior to the hyoid bone
> > Remains connected to the tongue by the thyroglossal duct
7th week: reaches its definitive position (ant. 2nd-6th tracheal rings)
Name 2 developmental defects of the thyroid gland?
Ectopic/aberrant thyroid tissue
Thyroglossal duct cysts/ fistula
Where can aberrant/ectopic thyroid tissue be found?
anywhere along path of descent of thyroid gland:
Common:
- base of tongue
- With lateral cervical lymph nodes
Pathogenesis of thyroglossal cyst/ fistula
Failure to obliterate lumen of thyroglossal duct
Cysts can form along midline migratory path of thyroid gland
» may remain connected to the outside via fistula
No symptom (unless infected = pain)
Describe the location and gross structure of thyroid gland?
- Below thyroid cartilage
- 2 lobes, butterfly shape on either side of trachea, connected by Isthmus at 2-4th tracheal cartilage
- 50% have Pyramidal lobe (from isthmus to hyoid cartilage)
What connects the thyroid gland with the trachea?
Encapsulated by a tough fibrous capsule
Pretracheal fascia (false capsule) binds the gland in front and at the back of trachea
Structures in front of the thyroid gland?
Deep to superficial
Sternothyroid muscle (covers, protects thyroid gland)
Sternohyoid muscle
Cervical fascia
Describe arterial supply of thyroid?
1) External carotid artery»_space; superior thyroid artery»_space; superior 1/3 thyroid gland + isthmus
2) Subclavian artery»_space; thyrocervical trunk»_space; inferior thyroid artery»_space; inferior 2/3 thyroid gland + lower isthmus
Venous drainage of thyroid?
- Superior thyroid veins»_space; internal jugular veins»_space; BCV
- Middle thyroid veins»_space; internal jugular veins»_space; BCV
- Inferior thyroid veins»_space; brachiocephalic veins»_space; superior vena cava
Nerve supply of larynx?
1) Vagus»_space; Superior laryngeal nerve»_space; External laryngeal nerve»_space; Cricothyroid muscle (tensor vocal cord)
2) Vagus»_space; L/R recurrent laryngeal nerves»_space; all other laryngeal muscles
Describe the course of the L/R recurrent laryngeal nerves?
Lies in tracheooesophageal groove
Posterior to inferior thyroid artery
~50% cases: embedded with pretracheal fascia
Histology and cell types in the thyroid gland?
vesicular follicles
- Follicular cells
- Parafollicular cells
Epithelium type, arrangement of follicular and parafollicular cells?
Follicular cells:
- Simple cuboidal (= majority) to squamous epithelium
- Surround thyroglobulin/ thyroid hormones filling homogenous colloids (pools)
Parafollicular cells:
- Lie between follicles or embedded within follicle
- Light-staining
5 Function of the follicular cells in thyroid gland?
- Secrete thyroglobulin into lumen of follicle (stored in colloid)
- Uptake and transport iodine from basal membrane into lumen of follicles
- Iodinate thyroglobulin
- Form Tri-iodothyronine T3, Tetraiodothyronine/thyroxine T4
- Stimulation of TSH»_space; resorption of colloid
Describe the transport and function of T3, T4?
T4:
Transported in blood (bound to thyroxine-binding globulin from liver)
> > 40% converted to T3 in cells
> > Bind with intracellular receptor molecules = protein synthesis
> > Increase rate of glucose, fat, protein metabolism for growth and development of CNS
Describe the reactions that form T3?
Condensation of 1 molecule of 3-monoiodotyrosine (MIT) + 1 molecule of 3,5-diiodotyrosine (DIT)
and elimination of 1 alanine residue
MIT + DIT - Alanine = T3
Describe the reactions that form T4?
4 iodine molecules
Condensation of 2 molecules of DIT and elimination of alanine residue
2DIT - Alanine = T4
Describe the uptake and transport of iodine through follicular cells?
Uptake of iodide through sodium/iodide symporter (NIS) at basal membrane into follicular cells
> > iodide/chloride transporter (pendrin) at apical membrane (iodide converted to iodine)
Describe the 2 resorption pathways of colloid by follicular cells?
- Lysosomal pathway (= major pathway): endocytosis T3, T4»_space; form endosomes to transport to basal membrane > exocytosis into capillary lumen
- Megalin receptor-mediated transepithelial pathways
Describe the major function of Parafollicular cells?
secrete calcitonin
↓ blood levels of calcium (Ca2+), phosphates (HPO42-) by
1) Inhibiting bone resorption
2) Accelerating uptake into bone matrix
Describe the origin of parathyroid glands?
Endodermal:5th week:
1) Dorsal recess of 3rd pharyngeal pouch proliferates to form inferior parathyroid glands
2) Dorsal recess of 4th pharyngeal pouch forms superior parathyroids
Ventral epithelium of 3rd and 4th pouches contribute to thyroid gland formation
Describe the migration of parathyroid glands?
Initially migrate together than detach:
1) Inferior parathyroid with thymus*** (not thyroid)
2) Superior parathyroid with ultimobranchial body (gives rise to C-cells / parafollicular cell in thyroid gland)
7th week: fixed at posterior side of thyroid gland
How to investigate ectopic parathyroid tissue?
Technetium -99m (Tc99m) radiotracer:
Taken up by thyroid gland and cleared in 30 mins, but retained by abnormal parathyroid tissue
Define the anatomical position of the parathyroid glands?
Usually embedded between the posterior border of fibrous capsule and capsule of thyroid
Most have 4 glands (number varies from 2 to 6)
Define the 2 types of epithelial cells in parathyroid glands?
At the top and bottom of each gland:
Principal/ Chief cells
At the center of each gland:
Oxyphil cells
Compare the staining of cell types in parathyroid cells?
Principal/ chief cells = more condensed, darker
Oxyphil cells = eosinophilic, lighter
Compare function of parathyroid gland cells?
Principal/ chief = major source of parathyroid hormone (PTH)
Oxyphil cells = Unknown function
Main actions of parathyroid hormones?
↑ blood Ca2+, Mg2+ (antagonized by calcitonin released by parafollicular cells in thyroid gland)
↓ blood phosphate
Promotes formation of calcitriol