Close your eyes for a minute and pretend you've just bought a gorgeous property. This lovely land that you now own is set smack in the middle of a public park. The park itself is beautiful as well, and there is no way of telling when you're on park land vs. private property at the time of purchase.
So what's the first thing you do after you've invested this spot? Build a fence, right? Create something to keep the public (well meaning as they might be) out of your private space. You want something that tells people they are about to cross onto private property, not public land.
What kind of a fence do you build? Well, that depends - If you want to keep everything out, you build one kind of fence. If you want to let people see in, but not get in, it's another kind. Do you want sunlight, wind, fresh air, rain to get to the landscaping? If you want to completely isolate your property, maybe you build metal dome over it all. Obviously, the type of fence or barrier you create will depend on how it is used. If you're planning on building structures or a home on your new property, you'll have to go through the same considerations – what type of walls and barriers do you want to have around each individual part of your property.
Do you want to keep everyone out of everything? Maybe there are just certain areas of the house that are more private than others. Maybe you have cats that are indoor cats, they can't go outside. The dogs, however, can go inside or outside, as long as they are within the property limits. And what about guests? Do you want guests to visit? Long term or short term? Where can they go and stay in the house? Is there any areas of the house that are for you alone?
How are you going to decide who can come in and who can't? What securities have to be in place to make those choices a reality? Will there be keys that only you have? How about workers, and people coming to help out at the house – will they have access? And maybe you want to have extra security measures to deter those you don't want to have access – moats, walls, boiling hot oil to pour on invaders.
Obviously, I'm using the house and property analogy to lead into talking about how our bodies maintain boundaries. Just like the property, the first thing that we do is build a wall if we want a separation of space. Every cell in your body is built with a fence around it – the cell membrane (aka the plasma membrane) is that outer layer. That membrane keeps the outside out and the inside in. The type of doors and locks and security that barrier contains depends upon the type of cell we are talking about. Muscle cells are built differently from the cells that line the small intestine, which are in turn different from the cells that are found in your bones. We'll talk more about how the cell membrane is made up and why it matters when we talk about responsiveness as well as metabolism, and reproduction. But for now, know that the make up of the cell membrane is the primary thing that dictates how that cell behaves and who and what it communicates with.
But let's zoom out a little bit and look at the big picture – the boundary of the entire organism, in this case, humans is primarily the integument, or the skin. The respiratory system, digestive system, circulatory system, immune system and nervous systems all play roles, too.
The many layers of the integument form a relatively solid barrier which must be crossed if something is to enter the interior of the body. Look at the structure of our skin for a moment. There are two parts to the skin, and there is another part that is typically included with the skin structure, even though it is not officially considered a part of the integument. The two parts are the dermis (internal) and epidermis (superficial). We'll talk briefly about the structure of each of them.
The epidermis has many layers, or strata, of cells – there are 4 or 5 strata, depending upon which area of the body you're looking at. Over most of the body, there are four layers, and where we have thick skin, there are five layers. (That thick skin is found on the palms your hands and the soles of your feet – areas where you need extra protection)
Remember back in module 1 when I said that even at a microscopic level, form follows function? (Wolff's law). Under a microscope, the outermost strata contain flat cells, which are dead bags of a protein called keratin. The innermost strata (stratum germinativum or stratum basale) is where these cells form (germinate), and as they move through their lives they are pushed more and more superficially, changing from cells that are more cubed (cuboidal) into flat bags of keratin that resemble scales. There are a few cell types found here – the keratinocytes are the cells whose journey we will follow from deep to superficial as we move towards the surface of the skin. Melanocytes – are the cells that secrete melanin – a dark pigment that gives color to the skin – the more melanocytes, and thus the more melanin your skin has, the darker it will be. Epidermal dedritic cells help to activate immunity when necessary, and merkel's discs are connected to nerves and bring sensations of light touch into the central nervous system.
Moving superficially, the next layer is called the stratum spinosum. Just like it sounds, stratum spinosum means spiny layer. It's called that because of how it looks under a microscope. When cells are fixed for viewing on a slide, they lose water, which causes them to shrink slightly. However, the cells in this layer have gap junctions which cause parts of them to stay stuck together. When the areas between the stuck parts shrink, we're left with cells that have a spiny appearance. (How well you can see this phenomenon will depend on the quality of your microscope. Don't worry if you can't see it, just know that's how it got named!) In this layer, the keratinocytes are starting to produce a substance called cytokeratin, which will eventually become that protective substance called keratin. Cells in this layer are still mostly cuboidal in shape, and are still living.
The next layer out (or superfically) is the stratum granulosum (granular layer) – called this because when skin is stained under a microscope, dots (graininess) can be seen which are what hold the keratin molecules together. It is in this layer of the skin that the cells are dying – they lose their organelles here, as well as produce what are called lamellar granules, which eventually fuse with the cell membrane, release their contents into the extracellular space, and helps to form a waterproof barrier within the skin.
Thick skin has an extra layer which is found next when present – just underneath the outermost layer – called the stratum lucidum. And it serves to provide an extra layer of protection for the areas that are most in need of it. Thick skin is found on the soles of our feet, and the palms of our hands. Two “high traffic” areas in need of as much protection as possible as we move through life. The stratum lucidum is composed of 3-5 layers of dead cells which contain a form of keratin called eleidin. It's name (clear layer) comes from the fact that it appears translucent under a microscope.
The outermost strata, the stratum corneum, is several cell layers thick (can be up to 25 cell layers), and serves to protect the body physically and chemically (we'll talk more about those different ways in a bit). If your arm rubs against a wall as you're walking down the hallway, most of the time, you may only lose a few layers of already dead cells. Most of the time, scraping off a few layers of those “bags of keratin” isn't even noticeable. Stratum corneum literally translates to the “horned layer.”
So what are some of the ways that skin provides a barrier to the outside – it's fairly obvious with all those layers that there is a physical barrier formed, and the close connections between cells don't allow a lot of things passing through them. But there are other ways that the skin provides a barrier. The skin produces something called the “acid mantle”, which is an acidic mixture that has bactericidal and germicidal properties, protecting us from stuff that gets on the skin. I mentioned melanocytes before – melanocytes produce melanin, which helps protect us from the sun (this is why you get tan when you're in the sun – you cause the melanocytes to jump into high gear). Even your DNA within the skin cells enters the picture when it comes to protection – when UV rays get to the DNA, it vibrates, creating heat, and transforming the potentially damaging energy of the UV rays into relatively harmless heat energy that can be dissipated easily.
The skin is not impenetrable, however. Some things can get through the skin – both in and out. Water can go both ways through the skin – we can lose moisture (primarily through sweat, as the keratin creates a mostly waterproof barrier) out, but we can also absorb water – when you sit in a bathtub, and look at your wrinkled fingers and toes, that wrinkling is a result of water being absorbed into the skin. It also has the added bonus of improving your grip when your hands are wet, how cool is that??
Lipid soluble substances can be absorbed through the skin – this is because they can travel relatively easily through the cell membranes of skin cells, and travel in that route. This is why some drugs can be administered transcutaneously – across the skin. (Think anything worn in a patch form is being administered by this route.)
Some plant secretions may be taken in through the skin. You may have noticed that some folks are susceptible to poison ivy. The resin secreted by the plant doesn't cause a problem unless it crosses through the skin barrier, and creates the inflammatory response that we know as a poison ivy rash. If the resin is unable to cross that barrier, there is no response.
The type of tissue that forms the epidermis is stratified (many layers) squamous (flat cells) epithelial tissue. Remember that epithelial tissue always has a free surface, which lines the center of a hollow organ or the outside of the body. Other places where stratified squamous ET is found are in the inside of the mouth, the esophagus, the anus, and the vaginal canal. The layers that are found in skin are different in these areas (why?), but can you think of some reasons why might this be a good choice for a “fence” or a barrier in these areas?
Underneath the epidermis, the dermis is found. Within the dermis are blood vessels and nerves and sweat glands and connective tissue that holds the party together. Except for the merkel disks, all sensations received through the skin are transmitted to the CNS through receptors found in this layer. We'll go more into detail with this when we talk about responsiveness and irritability.
The blood vessels that are here give our skin two more functions – heat, or temperature regulation, and creating a blood reservoir. Heat regulation is fairly straight forward, but let me explain the blood reservoir part. At any given time, about 5% of your blood volume is found in the dermis. Most of the time, this is more than what is needed for actual dermal function. This allows for blood availability during certain activities - blood can be shunted to the muscles when you exercise, for example, or to the digestive tract after you eat. And doing either of those activities doesn't rob anywhere else of enough blood to survive.