Video: Thyroid gland histology

The thyroid gland – doesn’t look very exciting, does it? I mean, I supposed its shape is kind of cool, sort of butterfly-like, but, otherwise, it’s just a small, brown squishy piece of endocrine tissue in our necks. So, why do we care about it so much? Well, it affects loads of things, such as metabolism, development, and cardiovascular function. The cool things the thyroid gland can do are better understood when you know the microscopic structure of it. So, it’s a good thing you’ve come to join us in this tutorial on the histology of the thyroid gland.

In this tutorial, we’ll get down to the microscopic structures that make the thyroid gland function the way it does. Before we get started, here’s a quick overview of what we’ll cover today. We’ll start by reviewing what the thyroid gland is, what it does, and where it is in our body – the thyroid gland basics. Then we’ll dive down into the microscopic details starting with the gland-wide structures and then looking at the functional units in more detail. To wrap up, we’ll add in a clinical angle just to keep you on your toes.

Are you ready? Here we go.

Let’s start with our thyroid gland basics, looking at it from a macroscopic view. The thyroid gland is an organ of the endocrine system found in the neck anterior to the trachea just inferior to the larynx. It has a right lobe and a left lobe, connected by a central isthmus in the midline with an additional pyramidal lobe in forty percent of the population. It is brownish-red in color due to its rich blood supply.

The primary role of the thyroid gland, as an endocrine organ, is to produce hormones. Three different hormones are produced and secreted by the thyroid gland. Two are iodine-based hormones called triiodothyronine and thyroxine, also called tetraiodothyronine. These hormones increase the basal metabolic rate of cells throughout the body causing them to work harder. This will have some knock-on effects such as increased body temperature and heart rate. In children, it stimulates growth and central nervous system development. The third hormone produced by the thyroid gland is calcitonin. This hormone acts to lower calcium concentration in the blood.

Alright, now that we’re all caught up with our thyroid gland basics, what do you say we get to the good stuff. Time for histology. Most of the slides we’ll be looking at throughout this tutorial have been prepared with haematoxylin and eosin stain often called H&E stain. We’ll let you know if any images have bene prepared using a different stain. H&E is the most commonly used histological stain due to its simplicity and ability to clearly define the basic morphology of a tissue by staining nuclei and cytoplasm different colors. The haematoxylin component is applied first and stains nuclei blue in addition to parts of the cytoplasm that contain RNA. The sample is then counterstained with eosin, which stains proteins and cytoplasm varying shades of pink.

This is a histological slide of the thyroid gland. One of the most notable things from this more zoomed outview are the large fluid-filled spaces we can see throughout the gland. One of the roles of the thyroid gland is to store some of the hormone it has produced until it is needed. These spaces allow for that storage. We’ll take a closer look at them later on.

Just as most organs, the thyroid gland is surrounded by a thin capsule composed of fibrous connective tissue. This connective tissue extends into the gland’s parenchyma to help divide the lobes of the thyroid gland into smaller, irregularly-shaped lobules. In the image on the right, we can see part of one of these connective tissue extensions highlighted in green, and they are called trabeculae. In the image on the left, we can see the same trabecula highlighted in green, but can now appreciate the subdivision it creates within the thyroid gland.

And here’s an example of one of those lobules that the trabeculae help create. Remember, a lobule is a subdivision of the thyroid gland lobes and is separated from other lobules by trabeculae. Within each lobule is a collection of follicles which we’ll look at next.

The thyroid follicles, three of which we can see now highlighted in green, are the functional units of the thyroid gland. Each lobule contains a number of follicles of varying sizes. We’ll have a closer look at the parts of these follicles later in the tutorial.

Surrounding the follicles is the stroma of the thyroid gland. The stroma is the part of the thyroid gland that supports the functional units – the follicles. The stroma is made up of reticular connective tissue, part of which we can now see highlighted in green. Also, within the stroma is where we’ll find the fenestrated capillaries of the thyroid gland. We have indicated a few of these capillaries located between the connective tissue fibers. But how do we know that these are capillaries?

Well, they possess the typical thin wall composed of a single layer of squamous or flattened endothelial cells which allow easy passage for the liquids from the surrounding interstitium, especially important in an endocrine gland where the hormones they produce go straight into the bloodstream. And as luck would have it, in this section we also see some red blood cells stained bright red within the capillaries.

So, this stroma, composed of reticular connective tissue, contains not only capillaries, but is also where we would find lymphatic structures draining the thyroid gland and sympathetic nerve fibers that will act on the arterial supplying the thyroid gland.

Now that we have a good idea of the general structure of the thyroid gland, let’s take a closer look at the thyroid follicles.

Thyroid follicles are spherical structures, usually between 0.02 to 0.9 millimeters in diameter. Remember, the follicles are the functional units of the thyroid gland, with each part of them being involved in aspects of hormone production and of storage. We’ll now quickly identify each of these parts before jumping into the detail of them later on.

The first thing to note is that each follicle is lined by follicular epithelium. This single layer of epithelial cells rests on an external basal lamina, which we can now see highlighted in green. Within the follicular epithelium is the lumen. The lumen is filled with colloid, which is the semisolid substance now highlighted in green. We’ll chat a bit more about this a little bit later.

Alright, back to the follicular epithelium. The follicular epithelium is a simple epithelium, meaning it is one-cell layer thick. Each cell, however, can vary in shape from columnar to cuboidal or even squamous depending on the level of activity of the follicle. Active epithelial cells appear cuboidal to low columnar, whereas inactive cells are squamous. The cytoplasm appears to have purple and somewhat fuzzy appearance due to the presence of an extensive rough endoplasmic reticulum which synthesizes the precursors of the thyroid hormones. On their apical ends, these cells possess microvilli of varying lengths. These cells are responsible for producing T3 and T4 thyroid hormones.

And now for a closer look at the colloid. Colloid is a semisolid protein-rich substance filling the lumen of follicular cells. In an H&E stain section, colloid appear bright pink and is often seen with lighter lines running through it. These are just artifacts introduced when preparing dissection. In real life, colloid would uniformly fill the whole follicle. It is made of thyroglobulin, which is an inactive stored form of the thyroid hormones. There’s enough stored thyroid hormone in the follicles to regulate the metabolism of the body for up to three months.

In response to a hormone released by the pituitary gland known as TSH or thyroid-stimulating hormone, colloid is reabsorbed by the follicular cells by endocytosis.

We’re actually really lucky because we can see active resorption areas on our slide, which appears these rounded clear bubbles in the colloid. The follicular cells then release T3 and T4 from its stored form and send them into circulation. Within the colloid of some follicles, oxalate crystals can be found. These are deposits of calcium oxalate and are commonly found in inactive follicles of all the individuals.

The final cell type of the thyroid gland that we’ll identify are the parafollicular cells, which we can now see highlighted in green. As their name suggests, they are alongside of the follicles rather than part of them. They’re never in contact with colloid. This group of parafollicular cells is found near few different follicles. Parafollicular cells are also called C cells. They produce the third hormone of the thyroid gland, calcitonin. This hormone helps to regulate levels of calcium in the blood specifically it decreases calcium levels by repressing bone resorption. It is an antagonist to the parathyroid hormone, which is released from neighboring parathyroid glands.

Now that we’ve identified all the components of the thyroid gland on an H&E stain, do you think we can still identify them on a slide with a different preparation? A trichrome stain has been used to create the slide, so the colors are certainly different. We can, however, still see the distinct functional units of the thyroid gland – the thyroid follicles.

The follicular epithelium can be seen here surrounding the follicle with parafollicular cells also visible between the follicles, and the colloid is seen in a very different color ranging from lilac to deep red in the lumen of the follicles. If we look even closer, we can also see a capillary in the longitudinal section here with red blood cells contained within.

Now what if we had something go wrong with our thyroid gland, like an autoimmune attacking of its tissue? What do you think we’d see histologically?

Let’s look at the condition of the thyroid gland that occurs as a result of autoimmune attack – Hashimoto’s thyroiditis. This condition involves the immune system attacking and destroying follicular epithelium of the thyroid gland, which gradually decreases its ability to produce thyroid hormones. There are no specific symptoms of Hashimoto’s thyroiditis. Its onset is very gradual, and when symptoms do start to show, they are the same as hypothyroidism. This should make sense since what’s happening is resulting in a decrease of thyroid hormones. For those individuals with hypothyroidism, hormone replacement therapy is used to improve symptoms.

Alright, and now you’re an expert on the thyroid gland histology. Before I let you go, here’s a quick recap of what we did today.

We started by looking at the overall structure of the thyroid gland, noting the many spherical-shaped units within the tissue. We saw that the gland is surrounded by a thin capsule and that this capsule projects inwards as trabeculae, dividing the gland into smaller sections. These smaller divisions are called thyroid lobules. Within the thyroid lobules, we discovered the functional units of the gland called the thyroid follicles. Surrounding and supporting the thyroid follicles is the stroma of the thyroid gland, which is composed of reticular connective tissue. The stroma is also where we found the capillaries that supplied the thyroid gland.

Next, we took a closer look at the thyroid follicles and identified the single layer of follicular epithelium that surrounds the spherical units. We saw these epithelial cells sitting on the basal lamina and surrounding the lumen of the follicle. Within the lumen, we identified the semisolid protein-rich substance called colloid, which sometimes contain oxalate crystals when inactive.

The final structures we looked at where the parafollicular cells found between the follicles which produce calcitonin. Finally, we talked about the autoimmune disease, Hashimoto’s thyroiditis, which leads to symptoms of hypothyroidism.

And that brings us to the end of our tutorial. Hope you enjoyed it and thanks for joining me.