Anatomy and Embryology of the Parathyroid Gland

INTRODUCTION (1 mark)

EMBRYOLOGY (6 marks)

Pharyngeal Pouch Origin

Key paradox (important exam point): The inferior parathyroid glands arise from the superiorly positioned 3rd pharyngeal pouch, while the superior parathyroids arise from the inferiorly positioned 4th pharyngeal pouch. The inferior glands travel a longer distance (with the thymus), so end up below - hence the name "inferior" despite arising from a higher pouch.

Migration of Inferior Parathyroid (PIII)

• The inferior glands migrate caudally alongside the thymus as it descends into the thorax
• The inferior parathyroid normally detaches from the thymus and settles at the lower pole of the thyroid
• If it fails to detach, it may descend further into the mediastinum - a common source of ectopic parathyroid
• Due to this longer migratory path, inferior glands are more variable in final position

Migration of Superior Parathyroid (PIV)

• The superior glands have a shorter, more predictable migratory path
• They remain in close relation to the posterior surface of the upper thyroid lobe
• Consequently, they are more constant in anatomical position (>80% in a predictable 2 cm zone)

Clinical Significance of Embryology

GROSS ANATOMY (5 marks)

Number and General Features

• Usually four in number (two superior, two inferior), but supernumerary glands occur in ~5% of the population
• ~13% of people have some abnormality in parathyroid tissue configuration
• Shape: Oval or lenticular ("Portland brick" shaped)
• Size: 5 × 3 × 1 mm
• Weight: ~30 mg each (combined ~120-140 mg total)
• Colour: Yellow-brown (similar color to fat; described as "Portland brick" color)
• Each gland lies within a halo of fat and is freely mobile over the thyroid capsule - this distinguishes parathyroid glands from thyroid nodules, which cannot move freely

Position of Superior Parathyroid Glands

• Located at the posterior aspect of the thyroid lobe, in an area 2 cm in diameter
• Centered ~1 cm around the junction of the inferior thyroid artery and the recurrent laryngeal nerve (RLN), close to the cricothyroid junction
• Position is dorsal (deep) to the coronal plane of the RLN
• Covered by pretracheal fascia that envelopes the thyroid and connects to hypopharynx, esophagus, and carotid sheath
• Lies at the level of the cricothyroid junction / upper 2/3 of thyroid lobe

Position of Inferior Parathyroid Glands

• More variable in position (due to longer embryological migration)
• In >50% of cases: at the inferior pole of the thyroid gland (anterior, lateral, or posterior surface)
• Position is ventral (superficial) to the coronal plane of the RLN
• Located ~2.5 cm below the intersection of RLN and inferior thyroid artery

Relationship to Recurrent Laryngeal Nerve (RLN)

• Superior PTG - dorsal/posterior to RLN
• Inferior PTG - ventral/anterior to RLN

ECTOPIC POSITIONS (2 marks)

Inferior Parathyroid Ectopia (more common)

• As many as 28% of inferior glands are found within the thymus or thyrothymic ligament
• Intrathyroidal glands are reported in 1-3% and are more common with superior glands

Superior Parathyroid Ectopia (less common)

• Retroesophageal or paraesophageal space (most common ectopic site when enlarged)
• May descend into tracheoesophageal groove at or below mid/inferior thyroid pole
• Color similar to esophagus - non-bloody field essential for identification

BLOOD SUPPLY (2 marks)

Arterial Supply

• Both superior and inferior parathyroid glands are supplied by the inferior thyroid artery, which arises from the thyrocervical trunk
• Abundant anastomoses also exist with:
  • Superior thyroid artery
  • Laryngeal arteries
  • Pharyngeal arteries
  • Esophageal arteries
  • Tracheal arteries
• 10% of inferior glands derive their dominant arterial supply from a branch of the superior thyroid artery
• These anastomoses explain why ligation of the inferior thyroid artery during thyroid surgery does not always compromise parathyroid blood supply

Clinical Note - Hypoparathyroidism after Thyroidectomy

Venous and Lymphatic Drainage

• Venous drainage via the thyroid venous plexus into the internal jugular veins
• Lymphatics drain to the deep cervical nodes and mediastinal nodes

HISTOLOGY (2 marks)

Cell Types

• Chief cells are the major parenchymal cells and the primary source of PTH
• Oxyphil cells are fewer and larger, with abundant mitochondria - they concentrate technetium-99m more than other tissues, forming the basis of sestamibi scanning
• Fat content of normal gland increases with age (~50% fat in adults)

NERVE SUPPLY (1 mark)

• Derived from the cervical sympathetic ganglia (vasomotor function only)
• The parathyroid glands have no secretomotor nerve supply - PTH secretion is regulated entirely by serum calcium levels via calcium-sensing receptors (CaSR)

APPLIED ANATOMY / SURGICAL SIGNIFICANCE (1 mark)

1. Bilateral neck exploration - knowledge of anatomy guides the surgeon to find all four glands and prevent inadvertent devascularization
2. RLN landmark - the plane of the RLN separates superior (dorsal) from inferior (ventral) glands
3. Inferior thyroid artery - key surgical landmark; superior PTG is 2.5 cm above and inferior PTG is 2.5 cm below its intersection with the RLN
4. Pretracheal fascia relationship - superior glands can "move freely" under this fascia; this mobility distinguishes them from fixed thyroid nodules
5. Missing gland protocol - if a gland is not found in its expected location, knowledge of ectopic sites from embryology (mediastinum, carotid sheath, retroesophageal, submandibular) directs further search
6. Symmetry - there is a degree of bilateral symmetry in parathyroid position; if one side is difficult, finding the gland on the opposite side first helps guide exploration

SUMMARY TABLE

• Cummings Otolaryngology Head & Neck Surgery, Chapter 123
• Bailey and Love's Short Practice of Surgery, 28th Ed., Chapter 56
• Scott-Brown's Otorhinolaryngology Head & Neck Surgery, Vol 1, Chapter 77
• Schwartz's Principles of Surgery, 11th Ed.