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Histology, microscopy, anatomy and disease
Histology, microscopy, anatomy and disease

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2 Tissue structure and functions

In the next sections you'll learn about specialist tissues for secretion, movement, strength, excretion and communication and examine them under the virtual microscope.

Secretion: Functions of the thyroid

The thyroid is a two-lobed endocrine (hormone-secreting) gland located in the neck, as can be seen in Figure 4 below.

Described image
Figure 4 Location of the thyroid gland (adapted from BTF, 2016).

The gland produces two related hormones: thyroxine (T4) and tri-iodothyronine (T3), which are involved in the control of metabolic rate. T4 contains four molecules of iodine and T3 contains three molecules of iodine. Deficiency of iodine leads to proliferation of the cells in the thyroid, in response to the unmet demand for the hormones.

Histologically, normal thyroid tissue consists of many follicles lined with a single layer of epithelial cells called thyrocytes. These cells synthesise a protein (thyroglobulin), which acts as a precursor and storage site for the thyroid hormones. The thyroglobulin is secreted and stored inside the thyroid follicles.

When stimulated by thyroid-stimulating hormone (TSH) from the pituitary, thyroglobulin is internalised (phagocytosed) by the thyrocytes and broken down to release the hormones, which are then secreted into the blood.

Thyroid histology is altered in conditions such as Graves’ disease (as a result of an overactive thyroid) and in Hashimoto’s thyroiditis, an autoimmune disease, where the body’s own immune system attacks and destroys the thyroid epithelium.

Activity 1

Having read a little about the structure and function of the thyroid you will now examine a sample of tissue using the virtual microscope.

Open the virtual microscope [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)] in a new window or tab. Find Slide 1 in the ‘Week 3’ category. Slide 1 shows a thyroid lobule containing about 50 follicles lined with thyroid epithelium, which produce the hormones thyroxine (T4) and tri-iodothyronine (T3).

Spend a few moments familiarising yourself with the structure of this sample, as guided by the slide’s interactive legend.