Thyroid anatomy/embryology Flashcards
What is the embryology of the thyroid gland?
The thyroid gland originates from the first pharyngeal pouch and begins as an out-pouching of the endodermal tissue from the base of the tongue, called the foramen ceacum, as early as the fourth week of gestation. Descends over next weeks anterior to the airway structures, stops at the level of the 2-4th tracheal cartilages by week 7. Route of descent is the embryological origin of the thyroglossal tract, duct or cyst, and the distal part portion constitutes the pyramid of the thyroid. The parathyroid cell masses and the ultimobranchial bodies also descend with the rudimentary thyroid over this period of development. Bilateral thyroid buds then develop with their associated art/venous and lymphatic drainage.
What is the tubercle of Zuckerkandl?
The posterolateral aspect of each lobe features a variable sized protuberance of thyroid tissue known as the tubercle of Zuckerkandl - this is the site of the fusion of the ultimobranchial body and the median thyroid process.
What is the ligament of Berry?
The area medial to the tubercle of zuckerkandl - area of thickened pretracheal fascia - this tethers the thyroid to the trachea.
What is the ultimobranchial body?
Embryological structure which gives rise to the parafollicular cells of the thyroid gland. A derivative of the ventral recess of the fourth pharyngeal pouch. Endodermal origin Precursors of the parafollicular (C) cells of the thyroid (secrete calcitonin)
What are the C cells of the thyroid?
Endodermal origin from the ultimobranchial body (derivative of the fourth pharyngeal pouch). Previously thought to be derived from the neural crest. They are pale staining, large cells. Small numbers in the thyroid gland, in the basement membrane which surrounds the follicle. They are not in contact with the follicular lumen. They secrete calcitonin (lowers calcium by inhibiting the resorption of bone by osteoclasts. Malignant change leads to medullary thyroid cancer (3% of all thyroid cancers)
Bethesda Criteria in Thyroid
FNA results, category with (risk of malignancy) 1 - non diagnositc/insufficient 2 - benign (0-3%) 3 - atypia of uncertain significance or follicular lesion of uncertain significance (5-10%) 4 - follicular nodule or suspicious or follicular neoplasm (15-30%) 5 - Suspicious for malignancy (60-70%) 6 - Malignant (>97%)
Blood supply of the thyroid gland
External carotid artery - superior thyroid artery Thyrocervical trunk - inferior thyroid artery (more variable in its course)
Describe the course of the recurrent laryngeal nerve
The RLN is a branch of the vagus nerve. It supplies the intrinsic muscles of the larynx (except cricothyroid). It is the nerve of the 6th pharyngeal arch The vagus nerves exit the skull via the jugiular foramen and run within the carotid sheath. The RLN branches and runs asymetrically. The right passes under the right subclavian artery and has a more oblique course. The left passes in front of the arch of the aorta before running under it (at the ligamentum arteriosum) in a recurrent fashion. More vertical. They run superomedially in the tracheo-oesophageal groove. The posteromedial aspect of the thyroid gland is in close approximatoin to the last segment of the extralaryngeal nerve. Lateral side of Berrys ligament and the medial side of ttubercle of Zuckerkandl - plastered in place by the overlying fascia which contain the terminal branches othe inferior throid artery. Here is = most constant and most prone to injury 72% of RLNS divide into two branches - anterior = motor fibres to laryngeal muscles Enters larynx under cover of cricopharyngeus It can be non-recurrent (1 in 200 on the right, extremely rare on the left and usually associated with arterial variation.
What are the rates of nerve damage in thyroidectomy?
RLN - Injury to RLN is <1% (vocal cord paralysis) - Up to 7% will suffer transient vocal cord palsy
What is the significance of the EBSLN in thyroid surgery? How do you define its location?
The external branch of the superior laryngeal nerve is also a branch of the vagus nerve. It is the motor supply of the cricothyroid muscle. It branches from the vagus in the carotid sheath and courses close to the upper pole of the thyroid. The Cernea classification describes its variations: - Type 1 - nerve crosses the superior thyroid vessels >1cm above the superior pole - Type 2a - nerve crosses the superior thyroid vessels <1cm above the superior pole - Type 2b - nerve crosses the superior thyroid vessels below the superior pole
