We have skin that covers our bodies but when does it stop being skin? Our tongues have skin but do our throats? What about our stomachs, lungs, intestines, and everything on the inside they don’t have skin. So where does it end and become whatever it is?
In: Biology
The outermost surface of skin is epithelium, and you have that in all kinds of places, in your nose, throat, blood vessels, kidneys, etc.
But underneath the epithelium, skin has dermal tissue. That contains nerves, hair follicles, muscle fibres, capillaries, sweat glands. That’s the tissue that makes skin skin, and you only find those things on the outside.
It’s also the largest organ in the body.
Those body regions are also composed of epithelial cells.
There are four main categories of human tissue:
Muscle- muscle ‘stuff’
Nervous – nerves and brain, signaling ‘stuff
Connective- bones, cartilage, things that hold you together. Fibrous ‘stuff’
And epithelial tissue. Barriers and membranes.
Although, once you’re inside the body, we tend to call them endothelial cells.
Anything that contacts the environment, like the things you listed, is in that category.
Also, membranes around body cavities and the interior surface of blood vessels.
Also also, body structures that produce things for you like glands.
I think what you are asking is where are the transitions to “insides”. At all of your openings your epithelial tissue transitions from “ cutaneous” or skin which has hair, sweat and oil glands etc, to “mucus membranes” which among other things secrete mucus to protect the body from pathogens. Your lips are the most obvious of the transition zones. Notice how they are smoother, heal differently, and have different textures than your chin, but then notice another change even from the top of your lip to the inside of your mouth.
Everything in your body should have an epithelium (basically an outer layer of sorts) and a membrane that creates that particular organ “space” with in your body IE : the pleura for your lungs or the meningeal layers that protect your brain, there’s even the periosteum that covers your bones hell even the skin itself has an epidermis.
On to your question the skin is an organ that functions as the outermost layer of what is you (among other stuff) therefore only the outer layer of you is skin like the palm of your hands or your face when on the inside say your mouth that outer layer within you is not skin just another epithelium , the tongue for example has an stratified epithelium.
Edit some sources : [on the skin](https://www.ncbi.nlm.nih.gov/books/NBK537325/) and [epithelium](http://www.histologyguide.com/slidebox/02-epithelium.html)
Also, the epithelium that makes up what you think of as your skin has a large quantity of keratin in it, which makes it harder, less porous, provides a barrier to your external environment. That same keratin makes up your nails and hair. It’s a feature of the epithelium in that location. But really, all linings that face the environment are made up of epithelial cells with different features. Your nasal passages, the lining of your throat, all of your intestines, even the alveoli in your lungs – they all “face” the outside world. (Think of it an a tube into your body and back out…)
“Skin” is just a word for your epithelium that is exposed to air and the radiant energy of the sun.
Latest Answers