L01 - Embryology and Anatomy of the Thyroid and Parathyroid glands

Question 1

Describe the origin, migration of thyroid glnad.

Answer 1

3rd week: thyroid diverticulum appears

• Endoderm thickens at foramen caecum (at midline of neck caudal to median tongue bud)&raquo_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)

Question 2

Name 2 developmental defects of the thyroid gland?

Answer 2

Ectopic/aberrant thyroid tissue

Thyroglossal duct cysts/ fistula

Question 3

Where can aberrant/ectopic thyroid tissue be found?

Answer 3

anywhere along path of descent of thyroid gland:

Common:

• base of tongue
• With lateral cervical lymph nodes

Question 4

Pathogenesis of thyroglossal cyst/ fistula

Answer 4

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)

Question 5

Describe the location and gross structure of thyroid gland?

Answer 5

• 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)

Question 6

What connects the thyroid gland with the trachea?

Answer 6

 Encapsulated by a tough fibrous capsule

 Pretracheal fascia (false capsule) binds the gland in front and at the back of trachea

Question 7

Structures in front of the thyroid gland?

Answer 7

Deep to superficial
 Sternothyroid muscle (covers, protects thyroid gland)
 Sternohyoid muscle
 Cervical fascia

Question 8

Describe arterial supply of thyroid?

Answer 8

1) External carotid artery&raquo_space; superior thyroid artery&raquo_space; superior 1/3 thyroid gland + isthmus
2) Subclavian artery&raquo_space; thyrocervical trunk&raquo_space; inferior thyroid artery&raquo_space; inferior 2/3 thyroid gland + lower isthmus

Question 9

Venous drainage of thyroid?

Answer 9

1. Superior thyroid veins&raquo_space; internal jugular veins&raquo_space; BCV
2. Middle thyroid veins&raquo_space; internal jugular veins&raquo_space; BCV
3. Inferior thyroid veins&raquo_space; brachiocephalic veins&raquo_space; superior vena cava

Question 10

Nerve supply of larynx?

Answer 10

1) Vagus&raquo_space; Superior laryngeal nerve&raquo_space; External laryngeal nerve&raquo_space; Cricothyroid muscle (tensor vocal cord)
2) Vagus&raquo_space; L/R recurrent laryngeal nerves&raquo_space; all other laryngeal muscles

Question 11

Describe the course of the L/R recurrent laryngeal nerves?

Answer 11

Lies in tracheooesophageal groove

Posterior to inferior thyroid artery

~50% cases: embedded with pretracheal fascia

Question 12

Histology and cell types in the thyroid gland?

Answer 12

vesicular follicles

• Follicular cells
• Parafollicular cells

Question 13

Epithelium type, arrangement of follicular and parafollicular cells?

Answer 13

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

Question 14

5 Function of the follicular cells in thyroid gland?

Answer 14

1. Secrete thyroglobulin into lumen of follicle (stored in colloid)
2. Uptake and transport iodine from basal membrane into lumen of follicles
3. Iodinate thyroglobulin
4. Form Tri-iodothyronine T3, Tetraiodothyronine/thyroxine T4
5. Stimulation of TSH&raquo_space; resorption of colloid

Question 15

Describe the transport and function of T3, T4?

Answer 15

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

Question 16

Describe the reactions that form T3?

Answer 16

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

Question 17

Describe the reactions that form T4?

Answer 17

4 iodine molecules

Condensation of 2 molecules of DIT and elimination of alanine residue

2DIT - Alanine = T4

Question 18

Describe the uptake and transport of iodine through follicular cells?

Answer 18

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)

Question 19

Describe the 2 resorption pathways of colloid by follicular cells?

Answer 19

1. Lysosomal pathway (= major pathway): endocytosis T3, T4&raquo_space; form endosomes to transport to basal membrane > exocytosis into capillary lumen
2. Megalin receptor-mediated transepithelial pathways

Question 20

Describe the major function of Parafollicular cells?

Answer 20

secrete calcitonin

↓ blood levels of calcium (Ca2+), phosphates (HPO42-) by

1) Inhibiting bone resorption
2) Accelerating uptake into bone matrix

Question 21

Describe the origin of parathyroid glands?

Answer 21

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

Question 22

Describe the migration of parathyroid glands?

Answer 22

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

Question 23

How to investigate ectopic parathyroid tissue?

Answer 23

Technetium -99m (Tc99m) radiotracer:

Taken up by thyroid gland and cleared in 30 mins, but retained by abnormal parathyroid tissue

Question 24

Define the anatomical position of the parathyroid glands?

Answer 24

Usually embedded between the posterior border of fibrous capsule and capsule of thyroid

Most have 4 glands (number varies from 2 to 6)

Question 25

Define the 2 types of epithelial cells in parathyroid glands?

Answer 25

At the top and bottom of each gland:
Principal/ Chief cells

At the center of each gland:
Oxyphil cells

Question 26

Compare the staining of cell types in parathyroid cells?

Answer 26

Principal/ chief cells = more condensed, darker

Oxyphil cells = eosinophilic, lighter

Question 27

Compare function of parathyroid gland cells?

Answer 27

Principal/ chief = major source of parathyroid hormone (PTH)

Oxyphil cells = Unknown function

Question 28

Main actions of parathyroid hormones?

Answer 28

↑ blood Ca2+, Mg2+ (antagonized by calcitonin released by parafollicular cells in thyroid gland)

↓ blood phosphate

Promotes formation of calcitriol