Endocrine Anatomy and Embryology

the podcast about preparing for the... I'm your host, Amanda Nicolley. Let's get started with our team timeout. Our patient today is the endocrine module from the general surgical curriculum. And we're going to kick off this module as always by talking about anatomy and embryology of the endocrine organs.

So the first one we're going to be talking about today is the thyroid gland. The thyroid gland is located in the anterior aspect of the neck and it consists of two thyroid lobes and a connecting band of tissue called the isthmus. The two symmetrical thyroid lobes are pyramidal in shape and have a number of important relations. Medially is the trachea and pharynx. Associated behind those include the recurrent laryngeal nerve and the trachea.

Lateral is the carotid sheath containing the internal jugular vein, carotid artery and the vagus nerve. And anteriorly are the strap muscles. The thyroid gland itself is invested by the pre-tracheal fascia and it's attached by a thicker condensation of this fascia to the trachea and this area is called the ligament of berry.

The isthmus is also attached to the trachea via condensations of the investing pre-tracheal fascia and these particular attachments is why the thyroid gland moves up and down with swallowing. The Isthmus is located over the second, third and fourth tracheal rings. A couple of other important features of the thyroid include the tubercle of Zucca Candle.

The tubercle of Zucca Candle is a posterolateral projection of thyroid tissue thought to be formed due to the contribution of the C-cells of the thyroid from the fourth brachial pouch. The tubercle of zucca candle is often affectionately termed the zuc. and is important because it has a relatively fixed relationship to the recurrent laryngeal nerve, so it can be used as a landmark during thyroidectomy for the nerve.

The other important feature of the thyroid dimension is the pyramidal lobe. And this is often a little tongue of pyramid-shaped tissue, as the name would suggest, that ascends from the thyroid isthmus on the left-hand side most commonly. And this is a remnant of the embryological descent of the thyroid down the thyroglossal tract and is often attached to the hyoid with a fibrous band of tissue, which is a remnant of this tract.

Given I've mentioned it, I'll briefly go into the embryology of the thyroid. The thyroid develops as an outpouching from the floor of the embryonic pharynx and this point of origin is later to become the foramen cecum at the base of the tongue. From this point the thyroid gland descends in the midline and the tract of this development is called the thyroglossal duct.

This descends from the base of the tongue, typically through the substance of the hyoid bone and down through the neck to the eventual resting place of the thyroid in the neck. There can be continuations of thyroid tissue down the thyrothymic tract and into the thymus, which can be considered thyroid tissue rests, although I haven't seen these in clinical practice. Importantly, the C cells of the thyroid are actually contributed to by the fourth branchial pouch.

This area frequently persists as the tubercle of zucca candle which we have already talked about.

So let's talk about the blood supply of the thyroid. There's two main vessels that supply the thyroid, the superior and inferior thyroid arteries. The superior thyroid artery is a branch of the external carotid artery and this travels down the inferior constrictor to enter into the superior pole of the thyroid. The superior thyroid artery has a relatively constant relationship to the external branch of the superior laryngeal nerve which is often located medial to the artery.

However, there is a sinea classification which talks about three possible locations of the nerve in relation to the artery. either more than one centimeter above the insertion into the top of the thyroid within a centimetre or more than a centimetre below and they split it into 1a and 2b. So it's good to be mindful that you may encounter the nerve in close relationship to this artery.

The other artery is the inferior thyroid artery which is a branch from the thyrocervical trunk which comes directly off the subclavian artery. This vessel often branches into a number of branches prior to entering into the thyroid itself and can have a variable relationship with the recurrent laryngeal nerve.

The recurrent laryngeal nerve is often found mixed within these small branches of the thyroid artery but can be found anterior, travel straight through the branches or posterior to the artery. In a small number of people there'll be an additional vessel called the thyroid ema artery which comes directly off the brachiocephalic vessel or can actually come off the aorta itself.

This vessel often runs on the trachea to enter into the thyroid isthmus and can be encountered during tracheostomy and cause bleeding. The venous drainage of the thyroid is variable but in general there is a drainage that mirrors the arterial supply. So there's a superior thyroid vein which will often drain into the internal jugular or the facial vein.

There is frequently a middle thyroid vein, which is an important anatomical landmark and often has to be identified and divided as you're coming laterally around the thyroid gland when you're doing a thyroidectomy. and this typically will drain directly as a short wide trunk into the internal jugular vein. And there can be a number of small inferior thyroid veins which drain into the internal jugular vein or brachiocephalic veins.

The lymph drainage of the thyroid is typically firstly to the central nodes, so the level 6 nodes. and then secondarily to the jugular chain nodes, so two, three, and four, and then to five and seven.

So given I have mentioned it, let's take a little segue into the levels of the neck and upper mediastinum in regards to the lymph node levels. So lymph node levels can be divided up anatomically into specific areas. Level 1 is split into 1a and 1b and this is pretty much the submental and submandibular areas. So the lymph nodes in the submental triangle bordered by the midline, the hyoid bone inferiorly and the anterior belly of digastrix are considered the 1A level lymph nodes.

And the submandibular group of lymph nodes bounded by the anterior belly of digastrix, the posterior belly of digastrix and the inferior border of the mandible are considered the level 1b lymph nodes. Levels 2, 3 and 4 are essentially the jugular chain lymph nodes. Level 2 is the upper jugular group and these are divided up into 2a and 2b. The superior border of the level 2 nodes is the base of the skull and the inferior border is a line drawn from the hyoid bone.

Anteriorly the boundary is the lateral border of the sternohyoid muscle and posteriorly is the posterior border of the sternocleidomastoid muscle. 2a and 2b is divided by the spinal accessory nerve which you won't necessarily be able to see because it runs through the substance of the sternocleidomastoid muscle but that is the anatomical division of 2a and 2b.

The level 3 lymph nodes are considered the middle jugular group and these anatomically are divided as the superior border being the hyoid bone, the inferior border being the level of the cricoid cartilage. Anterior is the anterior border of the sternocleidomastoid muscle and posteriorly is the posterior border of the sternocleidomastoid muscle. And then level four, as you probably could have guessed, is the lower jugular group.

These are defined with the superior aspect being the level of the cricoid cartilage, the inferior being the clavicle. and the anterior boundary is the medial border of the sternocleidomastoid muscle, and the posterior is the lateral border of the sternocleidomastoid muscle. The next group are the level 5 lymph nodes and these are the lymph nodes of the posterior triangle of the neck.

The posterior triangle is bordered anteriorly by the lateral border of the sternocleidomastoid muscle, posteriorly by the anterior border of the trapezius muscle and inferiorly by the clavicle. The next level are the level 6 lymph nodes and these are considered the anterior or often referred to in thyroid surgery as the central compartment. These lymph nodes essentially are all of the nodes in the anterior compartment around the thyroid and they include the pre and paratracheal nodes.

the precricoid node which has a name called the delphian node and the perithyroidal nodes which include all the lymph nodes along the recurrent laryngeal nerve. The boundary superiorly is the hyoid bone and inferiorly is the suprasternal notch. The lateral boundaries are the common carotid arteries. and basically all of the tissue between this is considered the central compartment. The last one is the level 7 nodes, which are the nodes of the superior mediastinum.

Let's get back on track and talk about the recurrent laryngeal nerve. The recurrent laryngeal nerve is a branch from the vagus nerve and it supplies motor supply to the intrinsic muscles of the larynx. This includes the lateral cricoaretenoid, posterior cricoaretenoid, the transverse and oblique interaretenoid and thyroaretenoid muscle. but not the cricothyroid muscle. So damage to the recurrent laryngeal nerve leads to unopposed adduction of the vocal cord.

The right recurrent laryngeal nerve arises from the right vagus nerve. It passes around the right subclavian artery to travel back up in the tracheoesophageal groove to then enter into the larynx just at the inferior border of the lower constrictor. The left recurrent laryngeal nerve is a branch of the left vagus nerve and it passes from lateral to medial around the arch of the aorta just adjacent to the obliterated ductus arteriosus.

It again travels up the tracheoesophageal groove to enter into the larynx at the inferior border of the inferior constrictor. There is a situation where the recurrent laryngeal nerve is not in fact recurrent and this is most often associated with an arterial abnormality called the arteria lusoria.

which is where the right subclavian artery doesn't come off the right brachiocephalic trunk but instead comes off directly off the arch of the aorta usually distal to the left subclavian artery and in doing so passes posterior to the esophagus and in these situations the recurrent laryngeal nerve can rather than travel in an inferior to superior direction

come directly off the vagus and pass from lateral to medial and in this situation can potentially be damaged or mistaken for another structure during thyroidectomy. In terms of identifying the recurrent laryngeal nerve at thyroidectomy, the tubicle of Zucca Candle often has a pretty Close relationship with the nerve.

And the nerve is often found medial to this tubercle, which means as you're rolling the thyroid medially and you're looking at the thyroid gland, you'll see the tubercle coming up and moving anteriorly and you'll find the nerve just behind this. The other relationship includes the relationship to the inferior thyroid artery and the nerve is often closely associated with this artery and typically found between the branches of this artery.

The last place that it's commonly found is in close proximity to Barry's ligament. so this is that condensation of fascia just related to the superior medial aspect of the thyroid gland and the trachea and the nerve can actually be incorporated into this ligament even or very closely adherent to it. So it's really important during dissection of Berry's ligament that the nerve is identified and protected and the layers are individually divided.

The last thing to mention is that the recurrent laryngeal nerve can divide into two branches prior to its entrance into the larynx. And in this situation, often the anterior branch is the motor branch to the muscles of the larynx. And the posterior branch is the sensory branch supplying sensory supply to the larynx.

So our next topic is parathyroid glands. There are four parathyroid glands that are located behind the superior and inferior lobes of the thyroid. They are small glands, usually around 25 to 50 milligrams in weight. They are a London tan color, so a dark brown tan color and typically described as the size and shape of a red lentil.

In addition to their colour being a key factor in differentiating a parathyroid from the surrounding fat, Using the closed tips of a debakey forcep, you can gently push around a structure you think might be the parathyroid gland. you'll see that the parathyroid gland kind of wobbles within its capsule which is another feature that can help you determine that this is actually the parathyroid gland.

Interestingly, about 10-15% of people will have additional parathyroid glands and 1-3% of people will have less than 4 parathyroid. The function of the parathyroid glands is to secrete parathyroid hormone which is secreted by the chief cells in the parathyroid. The location of parathyroid glands is important to know because you may need to find these in the setting of primary hyperparathyroidism.

The location of the glands can be variable and understanding the embryology helps you determine where you might find them in a normal position and also what the potential ectopic locations of a parathyroid gland may be. So the superior parathyroid glands originate from the fourth pharyngeal power. It descends from the fourth pharyngeal pouch to its final resting place as the superior parathyroid gland.

And the relationship of the fourth pharyngeal pouch to the thyroid gland itself means that the superior parathyroid is found in a much more constant position than the inferior parathyroid gland. The inferior parathyroid gland originates from the third pharyngeal pouch, so starts higher but has to descend further to become the inferior parathyroid. The third pharyngeal pouch also is the origin for the thymus gland.

And so the thymus gland's descent into the anteromediastinum pulls the inferior parathyroid down with it. And so the location is a lot less constant than the superior gland. So the common position described for the superior parathyroid is that it's found posterior to the recurrent laryngeal nerve. on the posterior medial surface of the superior thyroid lobe near the tracheoesophageal groove.

It's often found at the level of the thyroid cartilage and it's usually one to two centimeters from where the recurrent laryngeal nerve crosses the inferior thyroid artery. It's typically also found posterior to the tubercle of Zucker candle. If the superior gland cannot be found, then it could be an undescended superior gland. So it can be found anywhere within the fourth arch derivatives, which includes the pharyngeal wall, the parapharyngeal space.

and neurovascular structures such as the common carotid sheath. So you want to have a look in the retroesophageal, retropharyngeal spaces and also in the carotid sheath if you can't find a superior gland. The superior gland can also be found within the capsule of the thyroid. So as a last resort, a hemothyroidectomy on the side of the gland you can't find may be required. As I mentioned before, the location of the inferior gland is more variable.

Typically it's found below the inferior thyroid artery, lateral to the recurrent laryngeal nerve and behind the posterior aspect of the inferior lobe of the thyroid gland. If the inferior gland cannot be found then you want to be looking at its path of descent and other structures that it formed with such as the thymus. So the next most common location is in the thyrothymic tract, which is a little ligament that runs from the inferior lobe of the thyroid down towards the thymus.

gentle traction on that ligament can pull that ligament up and also often pull up the thymus tissue itself and you can inspect that tissue to see if you can find a gland. Other ectopic sites for the inferior parathyroid include in the thyroid capsule itself, in the anterior mediastinum, in the carotid sheath and even can be all the way down to the aortopulmonary window.

the other thing i forgot to mention is that with a parathyroid adenoma of the superior gland they often talk about a superior descended adenoma This is where the weight of the adenomatous gland actually causes the superior parathyroid to descend down through the back of the neck. And this can mean that that location can be variable. But if it does descend into the mediastinum, it doesn't descend into the anterior mediastinum like the inferior gland would with the thymus.

that it will descend into the posterior mediastinum.

So let's talk about the blood supply of the parathyroid gland. Both the superior and inferior parathyroid glands receive their blood supply from the inferior thyroid artery. from tiny branches typically that come from the medial aspect. So they come from the thyroid side of the blood vessel before they enter into the parathyroid gland.

Typically you can see the small blood vessel coming into the parathyroid, which is another way that you can identify that that is what the structure you're looking at actually is. So when you're doing your dissection, it's important to try to sweep the vessels away from the thyroid capsule and stick to your capsule dissection in order to preserve the blood supply to a parathyroid.

And if you're doing a frozen section to confirm that it is a parathyroid, you want to try to take the lateral aspect of the gland to send for frozen section so that you're not disrupting the blood supply. If you devascularise a parathyroid during an operation, you can perform a parathyroid.

gland transplantation where you basically chop the parathyroid gland up into very small pieces with a little bit of saline and you use a blunt tipped needle to transplant that into the ipsilateral sternocleidomastoid muscle. and it will basically start functioning again after four to six weeks.

to finish off this episode let's talk about the adrenal glands the adrenal glands are paired glands which sit superior to the kidneys The right is a pyramidal shape and the left is more chrysanteric which sits above the left kidney. The adrenal glands sit within gerodas fascia of the kidney but do have their own capsule. The adrenals comprise of a cortex and a medulla. The cortex comes from the mesodermal tissues and the medulla from the ectodermal tissue of the embryonic neural crest.

The cortex has three layers. The GFR, if you remember the mnemonic from medical school. So the zona glomerulosa, which makes aldosterone. The zona fasciculata, which makes glucocorticoid. and the zona reticularis which makes sex hormones and then the medulla has one layer and this comprises the chromaffin cells that make noradrenaline The relations of the adrenal glands varies on each side.

so the right adrenal gland abuts the superior medial aspect of the kidney medially is the ivc posteriorly is the diaphragm anteriorly it's half covered by the bare area of the liver and then the inferior leaf of the coronary ligament crosses it so that the lower aspect has a peritoneal covering The left adrenal gland medially abuts the aorta and the left cruce of the diaphragm. Posteriorly is the diaphragm.

Laterally it abuts the kidney where it sort of drapes over the superior pole and anteriorly is the tail of the pancreas and the splenic hilum. The arterial supply of the adrenal glands is with multiple small vessels that enter into the substance of the gland. The location or origin of these vessels include the inferior phrenic, the aorta itself and the renal arteries. The venous drainage is more interesting in that there usually is a single adrenal vein.

On the left hand side, this vein drains into the left renal vein, which obviously takes a longer course to cross the midline and get to the inferior vena cava. On the right hand side the adrenal vein drains directly into the IVC often with a very short trunk and that makes operating on the right adrenal much more difficult. The lymph drainage of the adrenal gland is to the para-aortic lymph nodes.

Because I've talked about the pancreas in the hepatobiliary module, I'm not going to go into further detail about that here, but that would be the last endocrine organ to talk about. I hope you enjoyed today's episode. I hope it's whetted your appetite for a fantastic series on endocrine surgery. And once again, please leave me a review. I absolutely love to read them. Subscribe to the program and leave a rating. It helps other people to find the podcast.

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