ERS01 Embryology, Anatomy Of Thyroid And Parathyroids Flashcards
Development of thyroid gland
1st endocrine gland to develop
4th-7th week
Midline caudal to median tongue bud
—>**Endodermal thickening / outgrowth (Thyroid diverticulum) at **Foramen caecum (from pharynx)
—> Bi-lobed structure
—> remains connected to tongue by ***Thyroglossal duct during migration
—> duct later disappear, Thyroid becomes solid mass of cells
—> Thyroid gland continue to descend (pass through 2nd, 3rd tracheal ring)
—> migrate ventral to Laryngotracheal tube, Hyoid bone to reach definitive position (7th week)
Thyroid dysgenesis
- Thyroglossal duct cyst
- regions failed to close found along migration path
- lie at any point along migratory pathway of thyroid gland
- always in midline of neck
- may be connected to outside by a canal
—> Thyroglossal fistula (discharge orifice)
—> ***never congenital but usually follows infection / inadequate removal of Thyroglossal cysts
- can form Sinus
- present as midline neck lumps in region of Hyoid bone (painless, smooth, cystic)
- observed at age of 5 - Aberrant / Ectopic thyroid tissue (aka Accessory thyroid gland)
- lie at any point along migratory pathway of thyroid gland
- may be functional (but not sufficient for normal function)
- entire / part of thyroid located in another part of body
—> **Base of tongue (Lingual) / with **Lateral cervical LN
- remnant of Thyroglossal duct (remains as functional thyroid unit)
Development of parathyroid glands
Start at 5th week
Derived from Dorsal **Endodermal lining of 3rd and 4th **Pharyngeal pouch (4 in total)
—> lose connection with pharyngeal wall
—> Inferior + Superior parathyroid gland respectively
—> incorporated into posterior side of Thyroid
Pharyngeal pouches:
- Dorsal 4th pharyngeal pouch —> ***Superior parathyroid gland
- Ventral 4th pharyngeal pouch —> **Ultimobranchial body —> incorporate into Thyroid —> **Parafollicular cell (C cell) in Thyroid gland
- Dorsal 3rd pharyngeal pouch —> ***Inferior parathyroid gland
- Ventral 3rd pharyngeal pouch —> ***Thymus —> moves in caudal, medial direction, pulling Inferior parathyroid gland with it, later descend without Inferior parathyroid gland
Parathyroid imaging
- Parathyroid glands usually embedded between posterior border and capsule of Thyroid
- Most have 4 glands (from 2-6)
Parathyroid scintigraphy dual phase planar imaging
- inject tracer (Tc99m)
- imaged at 5 min, 2 hrs
- radiotracer taken up by Thyroid
- cleared from Thyroid with t1/2 30 mins
- retained by parathyroid tissue
—> detect ***Ectopic parathyroid glands (found along migration path)
Anatomy of Thyroid gland
General features:
- Butterfly-shaped
- Highly vascular
- Only gland stores hormone (Thyroglobulin)
Anatomy:
- Front of neck, surrounding tracheal cartilage
- Below Thyroid cartilage
- Bi-lobed, on either side of trachea, larynx
- 2 Lobes connected by Isthmus at level of 2nd-4th tracheal cartilage
- Pyramidal lobe (appendix) (in 50%, remnant of thyroglossal duct) extends from Isthmus towards Hyoid cartilage
- surrounded by **Sternothyroid + **Sternohyoid muscles + Cervical fascia
- encapsulated by ***Pretracheal fascia (tough fibrous capsule)
—> binds to gland in front and at back of trachea
—> Parathyroid glands: between gland capsule and fibrous capsule posteriorly
Histology:
- Composed of follicles —> Follicular cells surrounding Thyroglobulin / thyroid hormones
- Parafollicular cells (between follicles)
Physiology:
- Follicular cells: secrete ***Thyroglobulin into lumen of follicle
- Parafollicular (C) cells: secrete ***Calcitonin
Blood supply to Thyroid gland
Arterial supply:
- Superior thyroid artery
- branch of ECA
- run downward, forward
- run with External laryngeal nerve
- supply upper 1/3 + anterior of Thyroid lobe + Upper Isthmus - Inferior thyroid artery
- branch of Thyrocervical trunk (from Subclavian artery)
- supply lower 2/3 + posterior of Thyroid lobe + Lower Isthmus
Venous drainage:
- Superior thyroid vein
- upper pole of Thyroid lobe
- run upward and laterally
- into Internal jugular vein —> Brachiocephalic vein - Middle thyroid vein
- middle pole of Thyroid lobe
- into Internal jugular vein —> Brachiocephalic vein - Inferior thyroid vein
- lower border of Isthmus
- into Brachiocephalic vein
1-3 form venous plexus
Blood supply to Parathyroid gland
Inferior thyroid artery
Nerve closely associated with Thyroid gland
- External laryngeal nerve
- branch of Superior laryngeal nerve (branch of Vagus nerve)
- supply Cricothyroid muscle (Tensor of vocal cord) (in Upper Thyroid gland)
- importance in Thyroid / Parathyroid surgery: may accidentally cut nerve - Recurrent laryngeal nerve
- branch of Vagus nerve
- bend and hook around Aortic arch
- in Tracheo-oesophageal groove posterior to Inferior thyroid artery
- embedded in pre-tracheal fascia (50%)
- importance in Thyroid / Parathyroid surgery:
—> Recurrent laryngeal nerve run behind gland
—> Injury: Partial (Abductor) / Total paralysis of vocal cord
Histology of Thyroid gland
Consists of Vesicular follicles
- lined with simple cuboidal / squamous epithelium
—> secrete ***Thyroglobulin
- lumen filled with Thyroglobulin (homogenous colloid) (Eosinophilic)
Parafollicular cells (C cells):
- derived from Ultimobranchial body
- secrete ***Calcitonin
- between follicles / embedded within follicles
- light staining
Blood capillaries:
- in between follicles
- bring hormones into blood circulation
Thyroid hormones
- Triiodothyronine (T3)
- Thyroxine (Tetraiodothyronine T4)
- by ***Follicular cells
- present as Thyroglobulin (storage form)
- T4 transported in blood bound to Thyroxine-binding globulin from liver
- 33-40% of T4 converted to T3 in cells (T3 more potent)
- bind with ***Nuclear receptor molecules, initiate new protein synthesis
—> ↑ basal metabolic rate (↑ body temp)
—> ↑ use of glucose for ATP production
—> ↑ lipolysis
—> ↑ cholesterol excretion in bile
—> stimulate protein synthesis
—> accelerate body growth
—> development of nervous system
- Calcitonin (antagonist of PTH)
- by ***Parafollicular cells
—> ↓ blood Ca, PO4 by inhibiting bone resorption
—> accelerate uptake of Ca, PO4 into bone matrix
Synthesis and release of Thyroid hormones
T4:
- condensation of 2 DIT (Diiodotyrosine) (with elimination of an alanine residue)
T3:
- condensation of 1 MIT (Monoiodotyrosine) + 1 DIT (with elimination of an alanine residue)
Steps:
Secretion of Thyroglobulin by follicular cells into lumen
—> Uptake and transport of iodine from blood into follicle lumen (by **Sodium/Iodide symporter NIS)
—> **Iodination of Thyroglobulin
—> MIT, DIT formation
—> T3, T4 formation
—> Resorption of colloid
—> 1. **Lysosomal pathway (major) + 2. **Megalin receptor-mediated transepithelial pathway
—> Release of T3, T4
Megalin receptor: increased expression in pathological stage
Histology of Parathyroid gland
- separated from Thyroid gland by capsule
- surrounded by capsule
- 2 kinds of epithelial cells:
- ***Principal / Chief cells
- major source of PTH
—> ↑ Ca, Mg, ↓ PO4 in blood
—> ↑ Calcitriol (Active Vit D: 1,25-dihydroxyvitamin D3) formation - Oxyphil cells
- eosinophilic
- unknown function
