TBL24 - Thyroid and Parathyroid Glands

Question 1

Where does the thyroid gland originate embryologically and what does it originate as?

Answer 1

At the foramen cecum in the posterior portion of the developing tongue, the thyroid gland originates as an endodermal bud designated the thyroglossal duct

Question 2

During midline descension of the thyroid, what does the thyroglossal duct retain its attachment to? After the thyroid reaches its final position anterior to the trachea, what happens to the duct?

Answer 2

1) During midline descension of the thyroid, the thyroglossal duct retains its attachment to the posterior tongue
2) After the thyroid reaches its final position anterior to the trachea, the duct normally degenerates and disappears

Question 3

How do thyroglossal duct cysts develop and where are they typically located?

Answer 3

1) Development of the thyroid gland begins in the floor of the embryonic pharynx at the site indicated by a small pit, the foramen cecum, in the dorsum of the postnatal tongue
2) Subsequently, the developing gland relocates from the tongue into the neck, passing anterior to the hyoid and thyroid cartilages to reach its final position anterolateral to the superior part of the trachea
3) During this relocation, the thyroid gland is attached to the foramen cecum by the thyroglossal duct
4) This duct normally disappears but remnants of epithelium may remain and form a thyroglossal duct cyst at any point along the path of its descent
5) The cyst is usually in the neck, close or just inferior to the hyoid, and forms a swelling in the anterior part of the neck. Surgical excision of the cyst may be necessary
6) Most thyroglossal duct cysts are in the neck, close or just inferior to the body of the hyoid

Question 4

Where do the aortic arches arise from, course through, and terminate?

Answer 4

The aortic arches arise from the aortic sac and course through mesenchyme of the corresponding pharyngeal arches to terminate in the dorsal aortae

Question 5

What comprises the wall of the pharyngeal gut (cranial part of foregut)? What do endodermal outpocketings from the lumen of the pharyngeal gut create between the pharyngeal arches?

Answer 5

1) The pharyngeal arches comprise the wall of the pharyngeal gut (cranial part of foregut)
2) Endodermal outpocketings from the lumen of the pharyngeal gut create pharyngeal pouches between the pharyngeal arches

Question 6

What forms the ultimobranchial body? What form parafollicular cells and where do they become incorporated into?

Answer 6

1) Endoderm of the 4th pharyngeal pouch forms the ultimobranchial body (ignore term)
2) Neural crest cells that migrate into the body form parafollicular cells, which become incorporated into the developing thyroid gland

Question 7

What do the thymus and inferior pair of parathyroid glands differentiate from? After the primordia lose their connection with the wall of the 3rd pouch, where are the inferior parathyroids pulled?

Answer 7

1) The thymus and inferior pair of parathyroid glands differentiate from endoderm of the 3rd pharyngeal pouch
2) After the primordia lose their connection with the wall of the 3rd pouch, the inferior parathyroids are pulled caudally by the migrating thymus to the dorsal surface of the thyroid gland

Question 8

What is the superior pair of parathyroid glands a derivative of? Where do the primordia attach to after losing contact with the pouch wall?

Answer 8

1) The superior pair of parathyroid glands is an endodermal derivative of the 4th pharyngeal pouch
2) After the primordia lose contact with the pouch wall, they attach to the dorsal surface of the migrating thyroid gland

Question 9

Where is aberrant thyroid glandular tissue typically located?

Answer 9

1) Aberrant thyroid glandular tissue may be found anywhere along the path of the embryonic thyroglossal duct
2) Aberrant thyroid tissue may be in the root of the tongue, just posterior to the foramen cecum, resulting in a lingual thyroid gland, or in the neck, at or just inferior to the hyoid

Question 10

Why are the inferior parathyroids susceptible to ectopic location in the thymus?

Answer 10

Because both the thymus and the inferior paraythyroid glands migrate from the same 3rd pharyngeal pouch, they are susceptible to ectopic location in the thymus

Question 11

What creates pharyngeal clefts between the pharyngeal arches? As the second arch grows over the third and fourth arches, what forms the cervical sinuses?

Answer 11

1) Invaginations of the surface ectoderm create pharyngeal clefts between the pharyngeal arches
2) As the second arch grows over the third and fourth arches, remnants of the 2nd, 3rd and 4th pharyngeal clefts form the cervical sinuses (not labeled)

Question 12

When are the cervical sinuses normally obliterated?

Answer 12

After overgrowth by the second pharyngeal arch is completed, the cervical sinuses are normally obliterated

Question 13

What are lateral cervical cysts?

Answer 13

Lateral cervical cysts, remnants of the cervical sinus, are most often just below the angle of the jaw, although they may be found anywhere along the anterior border of the sternocleidomastoid muscle

Question 14

How are lateral cervical cysts and thyroglossal cysts distinguished during physical examination? How are thyroglossal cysts and cancerous thyroid nodules distinguished?

Answer 14

1) Lateral cervical cysts will be anywhere along the border of the SCM and commonly in the mandible region (just below the angle of the jaw). Thyroglossal cysts are found along the midline of the neck.
2) Thyroglossal cysts are remnants of the thyroglossal duct, which is attached to the posterior tongue. Therefore, when a patient sticks out their tongue, the thyroglossal cyst will most upwards. Although thyroid nodules are located in the same region as thyroglossal cysts, they are not attached to the posterior tongue and will not move when the patient sticks out his or her tongue

Question 15

Where does the bi-lobed thyroid gland lie in relation to the sternohyoid and sternothyroid muscles and in relation to the trachea? What are the lobes of the thyroid gland interconnected across and by?

Answer 15

1) The bi-lobed thyroid gland lies deep to the sternohyoid and sternothyroid muscles, anterolateral to the trachea
2) The lobes are interconnected across the 2nd-3rd tracheal rings by the isthmus

Question 16

What does thyroid parenchyma consist of? What is the precursor of the thyroid hormones? Cite the two thyroid hormones and their primary functions.

Answer 16

1) Thyroid parenchyma consists of spherical follicles filled with thyroglobulin and lined by simple cuboidal epithelium
2) This iodinated glycoprotein is the temporary storage form and precursor (aka colloid) to main thyroid hormones before release as triiodothyronine (T3) and tetraiodothyronine (thyroxine; T4)
3) They increase oxygen consumption and metabolic rates of most body tissues and are essential for normal growth, maturation, and mental activity

Question 17

What is located in the loose connective tissue between the follicles of thyroid parenchyma? Cite the cells’ hormonal secretion and its function.

Answer 17

1) A small clump of parafollicular cells in the loose connective tissue between the follicles
2) These neural crest–derived parenchymal cells secrete calcitonin, which lowers blood calcium levels and counterbalances actions of parathyroid hormone

Question 18

What is a goiter? Why does hypertrophy of the follicular epithelium occur with Graves disease?

Answer 18

1) Goiter, a nonspecific term for chronic enlargement of the thyroid, may occur in various disorders of this organ
2) Hyperthyroidism leads to many thyroid diseases, the most common being exophthalmic goiter (Graves disease)
3) This autoimmune disorder is caused by antibodies
to the TSH receptor on follicular cells
4) Lymphocyte infiltration of surrounding stroma accompanies lymphoid follicles with germinal centers

Question 19

Where are the pairs of superior and inferior parathyroid glands embedded?

Answer 19

The pairs of superior and inferior parathyroid glands (each the size of an apple seed) are embedded in the posterior surface of the thyroid gland

Question 20

Cite the function of parathyroid hormone (PTH).

Answer 20

The parathyroid synthesizes and secretes parathyroid hormone (PTH), which maintains blood calcium levels by increasing the rate of osteoclastic activity, thus mobilizing calcium from bone

Question 21

What is the usual cause of primary hyperparathyroidism and why do kidney stones result?

Answer 21

1) Primary hyperparathyroidism is usually due to an adenoma of one or more parathyroid glands
2) Excessive production of PTH in this disorder leads to hypercalcemia (high serum calcium levels) because of increased osteoclastic activity of bone
3) Enhanced reabsorption of calcium in renal tubules may lead to nephrolithiasis, or formation of renal stones, rich in calcium oxalate and calcium phosphate

Question 22

Why can thyroidectomy be fatal?

Answer 22

1) Atrophy or inadvertent surgical removal of all the parathyroid glands results in tetany, a severe neurologic syndrome characterized by muscle twitches and cramps
2) The generalized spasms are caused by decreased serum calcium levels
3) Because laryngeal and respiratory muscles are involved, failure to respond immediately with appropriate therapy can result in death