- Derived from epithelium
- begin at fetus/embryo
- Proliferation by mitosis
- Acquire small substances from ECF
- Anabolism → build secretory products
- Gland must be stimulated to secrete
- Number of cells present
- Goblet cells only
- All other glands
- Secretion → duct → actual free surface
- Secretion → bloodstream
- Number of cells present
- Morphological character → exocrine glands
- # of ducts
- One = simple
- Multiple = compound
- Many internal, one opening
- Liver, pancreas, salivary
- Shape of secretory portion
- Sac-like (alveolar, acinar)
- Simple tubular
- Small intestine
- Simple branched tubular
- Stomach, uterus
- Simple coiled tubular
- Sweat glands
- Simple alveolar
- Sebaceous glands
- Simple branched alveolar
- Sebaceous glands
- Compound tubular
- Compound alveolar
- Pancreas, mammary, salivary
- Compound tubuloalveolar
- Simple tubular
- # of ducts
- Manner of secretion
- Entire cell disintegrates and releases contents and secretion
- Cells replaced by mitosis
- Ex: Sebaceous glands, goblet cells
- Parts of the cell disintegrate
- Cell repairs itself
- Ex: Mammary glands, Axillary sweat glands (armpits)
- All endocrine
- Most exocrine
- Except those mentioned above
- Secretion by exocytosis
- Secretory vesicles → duct → actual free surface
Betts, J.G., et al. (2017). Anatomy and physiology. Houston, TX: OpenStax, Rice University. Retrieved from https://openstax.org/details/books/anatomy-and-physiology?Book%20details
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In this lesson, we’re going to talk about glands and their structure and function.
If you have watched the epithelial tissue lessons, you’ll remember that glands are derived from epithelium and the development of glands begins in the fetal and even the embryonic stages of development. The main functions of glands are to anabolize the secretory products and to secrete whatever that substance is. Anabolism means to build or create – so essentially the glands will take in small particles from the extracellular fluid, create the secretion – whatever it is – and then secrete it. Just know that glands must be stimulated in order to secrete. This could be a temperature regulation mechanism like with sweating, or another chemical reaction like with the pancreas.
We classify glands in a couple of ways – first is by the number of cells. They’re either unicellular, meaning a single cell, or multicellular, meaning many cells. There’s only one unicellular gland and that is the goblet cell. If you see it through a microscope, it literally looks like a little goblet – and there’s mucus in there! Everything else is a multicellular gland. We also classify by general function – glands are either exocrine or endocrine. Exo means outside, so exocrine glands secrete their substance through a duct and out to a free surface of some sort. Think sweat glands, salivary glands, mucus glands, etc. Endo means inside, so endocrine glands actual secrete their substances directly into the bloodstream (so it stays inside). The best example of endocrine glands would be things like the thyroid or adrenal glands. Fun fact, the pancreas actually has BOTH endocrine AND exocrine secretions! We will also classify glands by manner of secretion, which we’ll talk about a little bit later.
Now, specifically when we’re talking about exocrine glands, we have a way to describe them based on their shape and structure. First is by number of ducts. If there’s one duct it’s simple, if there is more than one duct, it’s compound – I’ll show you what this looks like in just a minute. Then we look at the actual secretory portion of the gland – if it’s tube-like it’s calle tubular – pretty easy, right? And if it’s sac-like it’s called alveolar. Another term you might see is “acinar”, but we’re going to use alveolar. So let’s look at the different combinations of these and what the structure looks like as well as an example of where it’s found.
First is simple tubular – this is pretty easy. First, remember that exocrine glands secrete to an actual free surface through a duct. So here’s the surface, and here’s the duct. With simple tubular, the secretory portion is tube-like – just like this. That’s it – simple tubular. These are found in the small intestine. Simple alveolar is the same in that it just has the one duct, but this time the secretory portion is sac-like – just like this. This is our sebaceous glands (or oil glands in the skin).
You can also see some branches – now this is where we differentiate between branches and multiple ducts. Branched means one duct has many secretory portions on it. Still only one duct though. So simple branched tubular looks like this. And simple branched alveolar is the same except the secretory portions are sac-like. These are in the stomach and uterus and these are also a form of sebaceous glands.
Now, let’s look at what compound looks like. Again – this is multiple ducts. So here’s the surface and the single external duct. Then we have multiple internal ducts, that lead to tube-like secretory portions – which may or may not be branched as well. Same thing here: surface, single external duct, multiple internal ducts with sac-like secretory portions – again may or may not be branched. We find compound tubular ducts in the liver and compound alveolar ducts in the pancreas, mammary glands, and salivary glands. Now, this may get confusing to some people – remember that exocrine secrete to an actual free surface – that doesn’t mean they secrete outside the body – just to a free surface. So in the pancreas, it secretes enzymes into the small intestine – the inside of the small intestine isn’t touching another tissue, so it is a free surface still. Make sense?
The last two types we’ll talk about are a bit specialized. Simple coiled tubular. It has one duct and a tube-like secretory portion – except it is coiled up. These are also found in our sweat glands. The last one is compound – so multiple ducts – tubuloalveolar. That means it has some tubular and some alveolar portions. This is the majority of our salivary glands.
So that was a basic rundown of the different shapes of exocrine glands.
Let’s talk about the other way we classify glands which is by the manner of secretion. Holocrine, apocrine, and merocrine. So if this is our secretory portion of our duct – remember we usually have a cuboidal epithelium of some sort. The cells in here are busy making whatever the secretory product is for that gland. In holocrine secretion – when it’s ready to secrete, the entire cell just disintegrates and releases that secretion into the duct to be secreted. If you think “hollow” because it just disappears and leaves a space here that gets replaced by a new cell. That’s holocrine. In Apocrine, just part of the cell disintegrates. So the cell will kind of pinch off part of it and it bursts to release the secretion, then just repairs itself. I like to think of the word “apex” which is usually like the top or a point like this, that’s how I remember it. And lastly is merocrine which is secretion by exocytosis. That means the cell has little vesicles that carry the secretion up and out of the cell, without any destruction of the cell wall itself. This is the majority of our exocrine glands and ALL of our endocrine glands. We see apocrine secretion in mammary and sweat glands, and we see holocrine in sebaceous glands and goblet cells.
So, remember that glands will create and secrete a certain substance depending on the function of the gland. It’s either endocrine, which is straight into the bloodstream, or exocrine which gets secreted through a duct to a free surface. We classify exocrine glands by their shape, so they’re either simple or compound and either tubular or alveolar. And finally we know there are three main ways that the secretion actually happens – holocrine, think “hollow”, apocrine, think “apex”, and merocrine – which uses exocytosis and makes up the majority of our glands.
Okay, so that’s it for our lesson on glands – make sure you check out the resources and images attached to this lesson for a great summary of what we just talked about. Now, go out and be your best self today. And, as always, happy nursing!