Questions about glands

Here are some great questions about glandular cells!

Q. I wanted to clear up some confusion I have about exocrine glands and glandular cells types. I understand that there are 3 different glands; merocrine, holocrine, and apocrine. Within these glands with specific processes of secretion, there are different kinds of glandular epithelial cells; ion-transporting, serous secretory, mucous secretory, neuroendocrine, and myoepithelial cells. Is this correct?

A. Not quite! You’re correct in saying that based on their method of secretion, there are three types of exocrine glands (merocrine, holocrine, and apocrine). And you’re also correct that there are different types of glandular cells (ion-transporting, serous secretory, etc.).

However, not every exocrine gland can be neatly classified as merocrine/holocrine/apocrine! In fact, that’s just one way of looking at/classifying glands – there are actually three different classification schemes, and they overlap with each other sometimes! So it can be kind of confusing at first.

So let’s look at the three ways glands are classified, and then we’ll talk through some examples. Here are the three ways you can classify glands:

1. By the presence or absence of ducts

  • Exocrine glands (have ducts)
  • Endocrine glands (do not have ducts)

2. In exocrine glands, by the method of secretion 

  • Merocrine glands (secrete using exocytosis)
  • Apocrine glands (a portion of the cell is lost during secretion)
  • Holocrine glands (the entire cell is lost during secretion)

3. In exocrine glands, by the nature of the secreted substance

  • Serous glands (secrete a thin, watery substance)
  • Mucous glands (secrete a thick, viscous substance)
  • Mixed serous and mucous glands (secrete both substances)

Sometimes we use all three classification systems when describing a particular gland – but sometimes we just use one or two of them, and leave out the others. For example, some exocrine glands have both merocrine cells and apocrine cells – which means they can’t really be labeled as either merocrine or apocrine.

So you’ll notice as we talk about glands in different organ systems that we just classify them according to which of the three systems makes the most sense. In the pancreas, for example, we use the labels “exocrine” and “endocrine,” and for the exocrine glands, we also use the labels “serous” and “mucous” – but we don’t use the merocrine/apocrine/holocrine labels.

So the bottom line is that for now, all you need to do is understand the three classification systems so that when you encounter these descriptions in the future, you’ll know what they mean. But don’t worry about fitting all three classification systems together, because they are used independently of each other.

Q. Can multiple kinds of these cell types exist within a specific gland? For example, can neuroendocrine and mucous secretory cells compose the same apocrine gland?

A. Yes! You can have neuroendocrine cells and mucous secretory cells in the same exocrine gland.

However, as mentioned above, note that these cell types may have different secretion methods (e.g., the neuroendocrine cell may use merocrine secretion, and the mucous secretory cell may use apocrine secretion). If this is the case, then you wouldn’t label the gland “apocrine” – you’d just not use any of the merocrine/holocrine/apocrine labels.

Q. Does the specific glandular cell “take on” the mode of secretion of the gland it composes? For example, would a neuroendocrine cell of a merocrine gland exocytosis its product at the apical end of the cell?

A. Great question! The short answer is no 🙂 If you had a gland composed mostly of cells using merocrine secretion, but it also had some cells using apocrine secretion, those apocrine-secreting cells would not “take on” the merocrine secretion method! They’d just continue to use their preferred apocrine method – and you wouldn’t be able to slap a merocrine, apocrine, or holocrine label on that gland 🙂

The mode of secretion of a gland is determined by the type of cells that are in that gland. So if a gland is made up of cells that use merocrine secretion, it’s called a merocrine gland. If the cells use holocrine secretion, you’d call it a holocrine gland.

Q. Do myoepithelial cells technically secrete any kind of product?

A. No – they typically don’t secrete anything. They’re located around the perimeter of the gland, “hugging” the glandular cells, and their job is to contract and help the glandular cells secrete their products.

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