1 Functions of tissues
The structure of each tissue of the body is organised to carry out its own specific functions, and this is reflected in the arrangement of the cells and its histological appearance.
The week focuses on five functions of tissues, namely:
As you should already be aware, different tissues are capable of different functions. However, these functions may be interrelated. For example, the cells of the endocrine system secrete hormones, which are involved in communication between different tissues.
Secretion is an example of a function that is particularly specialised. Many cell types release molecules into the extracellular environment, but secretion is a specific function carried out by epithelial cells. Some examples are given in Table 1.
Table 1 Tissues and secretion
|Exocrine pancreas||digestive enzymes|
|Islets of Langerhans||insulin, glucagon|
|Stomach epithelium||acid, intrinsic factor|
Cells that carry out secretion generally store the secreted proteins either inside the cell, in secretory vesicles, or in extracellular depots (e.g. the thyroid gland). The secreted material is released when the cell or tissue receives an appropriate signal, as seen in the Figure 1 below.
Tissues respond to a variety of stimuli. For example, the thyroid responds to thyroid-stimulating hormone (TSH) released by the pituitary gland, which promotes the division of thyroid epithelial cells and the release of thyroid hormones.
It is important when viewing a tissue to consider how it may have adapted to physiological changes or signals from other cells. For example, what factors can you think of that might cause the expansion (hypertrophy) of muscle cells in the heart (cardiac myocytes)?
There are a number of factors you could identify to explain this observation, including: a programme of heavy exercise or training; high blood pressure, requiring greater effort from the heart to pump blood; and heart valve regurgitation, causing inefficient pumping of blood into the circulation. Understanding the likely cause of things that you observe in tissue sections is key, as it allows decisions to be made as to what medical intervention (if any) is required.