#6 The Integumentary System, Part 1 - Skin Deep

Your skin protects your body against infection and extreme temperatures, maintains your balance of fluids, and even synthesizes vitamin D for your own personal use.
Together with your hair, nails, and sweat and oil glands, your skin forms your integumentary system.

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<Skin Layers: Epidermis, Dermis, & Hypodermis>
Like an everlasting gobstopper, the key to your integumentary system is layers.
The epidermis is the only layer you can actually see, assuming that your skin is intact, which is why it's what you think of, when you think of "skin." It's made of stratified squamous epithelial tissue.
The dermis just below it is where most of the work that skin does gets done, like sweating and circulating blood, and feeling everything everywhere all the time.
And the bottom there's the subcutis, or hypodermis, composed mostly of adipose or fatty tissue.

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<Types of Epidermal Cells: Keratinocytes, Melanocytes, Langerhans Cells, and Merkel Cells>

Keratinocytes:
the building blocks of the tough, fibrous protein keratin

These cells are constantly dying and being replaced - you lose millions of them every day, enough to completely replace your epidermis every 4 to 6 weeks.

Melanocyte:
synthesizes melanin

These are your dendritic, or Langerhans cells, which are kinda star-shaped, and like white blood cells and platelets, they actually originate in your bone marrow.

Merkel Cells:
combine with nerve endings to create a sensory receptor for touch

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<Layers of Skin>
First it pierced your stratum conrneum, which means - pardon my Latin - "horny layer."
This is the ourtermost layer and also the roughest, made up of about 20 or 30 sheets of dead keratinocyte cells.
The nail drives through your stratum lucium, or "clear layer."
This holds two or three rows of clear, flat, dead keratinocytes that are only found in the thick skin of your palms and foot soles.
The deeper you go through the layers of the epidermis, the younger the cells get.
This whole process is due in part to the fact that the epidermis is epithelial, so it's avascular.
That means that all the oxygen and nutrients that its cells need have to come from the dermis below it.
When that nail cuts through the fourth layer - the stratum spinosum, or "spiny layer" - it's getting closer to the point where cell regeneration, or mitosis, is active.
These cells look prickly when they're dehydrated for microscope slide preparation - hence the name - and that's because they contain filaments that help them hold to each other.
Finally, that dang nail touches down on your deepest, thinnest epidermal level - the "basal layer" or stratum basale.
It's just a single layer of columnar cells, but it's like a cell factory where most of that new-cell production happens.
This stratum is also what connects the epidermis to the layer of skin below it, the dermis.
The ultraviolet radiation in sunlight can damage the epidermis, causing elastic fibers to climp up.

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<Layers of the Dermis>
The dermis is where most of the skin's work is done, and it does it in just three layers.
The upper, papillary layer is composed of a thin sheet of areolar connective tissue that's riddled with little peg-like projections called dermal papillae.
These papillae are pretty neat because in the thick skin of your hands and feet, these tiny protrusions from unique friction ridges that press up through the epidermis to help our fingers and feet grip surfaces.