Chances are, at this point in your courses you're oh-so-familiar with the structure of eukaryotic cells – and if not, here's a fantastic primer for you.
What you may have noticed, though, is that most cell-structure diagrams look pretty basic. You've got your circular animal cells, your more angular plant cells and all the organelles within the cell membrane.
Well, not surprisingly, those diagrams – while accurate! – don't tell the whole story. The truth is that cells come in all different shapes and sizes. And, especially in multicellular organisms like animals and plants, cells can look (and act) drastically different from each other.
Makes sense, right? You wouldn't expect the cells that make up a flower petal, for instance, to look and act the same as the cells that make up the plant's roots. Similarly, your skin cells, for example, would look drastically different than, say, your liver cells – because those two cells have very different functions in the human body.
That's where cell specialization comes in. Cell specialization allows new cells to develop into a range of different tissues, all of which work together to make living organisms function as a whole.
The process of cell specialization – exactly how cells develop into their diverse forms – is complex. There are hundreds of specific cell types in the body that arise from the very basic and general type cells called stem cells.
Stem Cells and Specialized Cell Types
All of the specialized cells in the body come from the same originating tissue: the group of stem cells that make up the earliest stages of an embryo. Stem cells are a unique type of cell, because, while they're immature cells without any specialization, they can follow a developmental "blueprint" to develop into the thousands of unique cell types found throughout your body.
There are different types of stem cells, separated by how many tissues they can develop into. The stem cells found in an embryo, for instance, can develop into any tissue type – which is how you go from a single stem cell to a fully formed human baby.
Adult stem cells, like the stem cells found in your bone marrow, can only develop into a handful of mature cell types. But the bottom line is that all stem cells are non-specialized "precursor" cells that can develop into at least one mature cell type.
How Stem Cells Become Specialized Tissues
Stem cells develop into mature tissues through a process called differentiation. To understand how differentiation works, think back to the cell-communication concepts you learned in your biology classes.
Cell communication works in three stages. A reception phase, in which special receptors on the cell's surface receive some kind of signal from the environment; a transduction phase, which relays that message from the cell surface to the inside of the cell; and a response phase, where the cell changes its behavior based on that signal.
So how does that work in cell differentiation? Well, let's say your body needs more red blood cells. It sends a signal to your blood stem cells that you need more red blood cells. This signal is received on the cell's surface.
The stem cell transmits (or transduces) that message to the nucleus, so the cell knows your body needs more red blood cells. Then the stem cells respond by activating the genes that'll help it develop into a red blood cell, and voilà – the cell becomes a red blood cell.
What Kind of Specialized Tissues Are There in the Body?
While scientists know the human body contains trillions of cells, exactly how many cell types make up the body is still an active field of study. The most recent estimate notes that there are at least 200 unique cell types in the human body, at least based on appearance. Some scientists think that estimate is low, though, and new cell types are still being discovered regularly.
The bottom line? You're looking at hundreds of different cell specialization pathways that your stem cells can take.
However, human cells all belong to one of four overall categories:
- Epithelial tissue: Epithelial cells line your tissues, and they're important for protecting the underlying tissues as well as helping with absorption. You'll find epithelial tissues in your skin, glandular tissue and more.
- Connective tissue: Connective tissue, well, connects and secures your tissues. It provides structural support to your body. This tissue type includes bones, cartilage, tendons, ligaments and fascia.
- Nervous tissue: Your nervous system helps transmit information throughout your body. It's composed of your central nervous system (or CNS), which includes your brain and spinal cord, and your peripheral nervous system (PNS), which includes the nerves throughout the rest of your body.
- Muscle tissue: This type is probably the easiest to picture – you know what muscles are! But you'll also find special types of muscle cells in your blood vessels, as well as your heart.
All of the 200 (or more) types of cells that make up the human body are found in one of those four tissue types – a lot more manageable to learn than memorizing hundreds of cell types, right?
Now, let's check in one some of the special cell types you're likely to come across in your biology classes – the ones you'll need to know a little more in-depth.
Specialized Blood Cells
Your circulatory system is one of the ones you're most likely to cover in biology class – so now's the time to get to know it! Your circulatory system is made up of a series of blood vessels – arteries, veins and capillaries – as well as a few specialized blood cell types:
- Red blood cells: These red, disc-shaped cells are the ones responsible for carrying oxygen throughout your body. They contain hemoglobin, a special protein that can bind to the oxygen from the air you breathe, and then release it back into the tissues that need it.
- White blood cells: Need help fighting off a cold or flu? Your white blood cells are there to help you! White blood cells make up a key component of your immune system. They help your body identify dangerous pathogens, and destroy them to keep you from getting too sick.
- Platelets: The smallest cell type within your blood, platelets play a key role in blood clot formation. Once platelets sense damage or torn tissue, they start to clump together, forming a blood clot to slow or stop the bleeding.
Your body constantly churns out fresh blood cells to replace older or damaged ones. And all your blood cells are "born" within your bone marrow, from a population of stem cells that specialize in creating blood cells.
Specialized Nerve Cells
You'll also likely come across the cells of the nervous system in your body. But don't worry – while the brain might seem complicated, learning about your nerves is likely easier than you think.
For one, there are only two major classifications of nerve cells: neurons and glia.
Neurons are nerves – the cells you're probably picturing when you think of your nervous system. They transmit information to control all the "thinking" in your brain, and also control muscle movement and other basic body functions.
Also, nerves throughout your body send signals back to your spinal cord and brain. Pain-sensing nerves, for instance, tell your brain when you're hurt, so you can avoid whatever caused the pain.
Glia are the supporting cells that help your nerves function properly. There are a few major types of glia, and all play a role in helping your brain, spinal cord and other nerves communicate efficiently. Some glial cells produce myelin, a waxy substance that "insulates" your neurons for better communication.
Others act as the immune cells of the brain, helping fight off infections that would otherwise harm your nerves. And still others help keep your neurons supplied with nutrients so that your nervous system has the energy to work properly.
Specialized Muscle Cells
The third major cell types you're likely to study are your muscle cells. And, thankfully, the three muscle cell types are easy to learn.
First, you've got skeletal muscle cells – the cells that make up virtually all of the muscles in your body. Skeletal muscle is the kind of muscle that – surprise – is anchored to your skeleton.
It contracts to move your bones. So, say, when you contract your bicep, you'll bend your elbow. Skeletal muscle cells are, in part, voluntarily controlled by your brain. That means you can decide to move your leg, for instance, and your brain will send a signal that corresponds with that movement.
Next, you've got cardiac muscle cells. These are the cells that make up your heart and contract to pump blood through your body. Cardiac muscle cell contraction is not voluntarily controlled – instead, your body maintains a steady heart rhythm without you having to think about it.
Finally, there are smooth muscle cells. Smooth muscle makes up the linings of certain blood vessels, as well as some organs, like your stomach. Smooth muscle is important for helping your organs move. For example, smooth muscle contraction helps move food through your digestive tract to allow for proper digestion.
Like cardiac muscle, smooth muscle contraction is not voluntarily controlled. So, for instance, you don't need to think about moving food from your stomach into your intestines because your body just does it for you.
The Bottom Line: Cell Specialization
Here's the gist of what you need to know about cell specialization:
- Cells develop from immature stem cells into mature, highly functional cells by a process called differentiation.
- Differentiation allows developing cells to take on unique structures, and it allows the cell to carry out specialized functions.
- The process of differentiation is triggered by signals from the environment and results in changes in gene expression that guide the cell's development.
- Differentiation allows cells to develop into the four major tissue types: epithelial tissue, nervous tissue, connective tissue and muscle tissue.
- There are at least 200 different cell types in the human body. Some that you'll need to know best include specialized blood cells, specialized nerve cells and specialized muscle cells.