What is skin? (Epidermis) | Integumentary system physiology | NCLEX-RN | Khan Academy

Summary

This video delves into the fascinating layers of human skin, starting with the epidermis, which is composed of five distinct layers: the stratum basale for cell generation, stratum spinosum with desmosomes, stratum granulosum for keratin-handling proteins, the clear stratum lucidum, and the outermost stratum corneum, made up of dead keratinocytes. The script also touches on the role of melanocytes in determining skin color and the unique shedding process of reptiles, providing an engaging exploration of skin's complex structure.

Takeaways

• 💀 The outermost layer of the skin, the stratum corneum, is composed of dead cells.
• 🌈 The skin has three main layers: the epidermis, the dermis, and the subcutaneous tissue (hypodermis).
• 🌱 The epidermis is divided into five layers, known as strata, each with unique characteristics.
• 📚 The deepest layer of the epidermis, the stratum basale, is where new skin cells (keratinocytes) are generated.
• 🌞 Melanocytes in the stratum basale produce melanin, which determines an individual's skin color.
• 🔗 The stratum spinosum is named for the desmosomes that connect cells, giving a 'spiny' appearance when cells shrink.
• 🛡 Langerhans cells in the stratum spinosum are part of the immune system, helping to protect against pathogens.
• 🍂 The stratum granulosum contains keratohyalin granules that produce proteins to handle keratin.
• 💧 The stratum granulosum also releases lamellar bodies, which form a lipid layer to protect the skin from pathogens.
• 👻 The stratum lucidum contains dead keratinocytes that have lost their nuclei and organelles, appearing clear under a microscope.
• 🐍 Reptiles shed the entire stratum corneum in one piece, unlike humans where cells slough off individually.

Q & A

What is the outermost layer of the skin called?

The outermost layer of the skin is called the stratum corneum.

How many layers does the epidermis have, and what are they called?

The epidermis has five layers, known as strata. They are the stratum basale, stratum spinosum, stratum granulosum, stratum lucidum, and stratum corneum.

What type of cells are produced in the stratum basale and what is their function?

The stratum basale produces keratinocytes, which are involved in the production of the protein cytokeratin, contributing to the skin's tough outer layer.

What is the role of melanocytes in the stratum basale?

Melanocytes in the stratum basale produce melanin, a pigment that determines the skin color of an individual.

Why is the stratum spinosum also known as the spiny layer?

The stratum spinosum is called the spiny layer because of the presence of desmosomes that connect the keratinocytes, giving them a spiky appearance when cells lose water.

What are Langerhans cells and where are they located within the skin?

Langerhans cells are immune cells that reside in the stratum spinosum, where they help protect the body by identifying and consuming foreign bodies such as bacteria and fungi.

What are keratohyalin granules and what do they produce?

Keratohyalin granules, found in the stratum granulosum, produce keratin-handling proteins that assist in the management of cytokeratin within the cell.

What is the function of the lipid layer formed by lamellar bodies in the stratum granulosum?

The lipid layer formed by lamellar bodies in the stratum granulosum is impermeable and provides a water-tight barrier to prevent pathogens from penetrating deeper into the skin.

Why are the cells in the stratum lucidum considered dead?

The cells in the stratum lucidum are considered dead because they have lost their nuclei and other organelles, which typically give cells their color and functionality.

How does the stratum corneum differ from the other layers of the epidermis?

The stratum corneum differs from other layers as it is composed entirely of dead keratinocytes that continuously slough off, making way for new cells to rise to the surface.

What is unique about how reptiles shed their stratum corneum compared to human skin?

Reptiles shed their entire stratum corneum in one piece, unlike human skin where the dead keratinocytes of the stratum corneum slough off individually and continuously.

Outlines

00:00

🧬 The Epidermis: Our Protective Layer

The first paragraph delves into the structure of the epidermis, the outermost layer of human skin. It is highlighted that the epidermis is composed of five layers or strata, each with distinct characteristics. The stratum basale, the deepest layer, is where keratinocytes are generated, which produce cytokeratin, a protein that contributes to the skin's toughness. Melanocytes in this layer produce melanin, determining skin color. The stratum spinosum, above the basale, is characterized by desmosomes that give it a 'spiny' appearance under certain conditions. This layer also contains Langerhans cells, which are part of the immune system. The discussion sets the stage for understanding the complex and vital role of the epidermis in protection and appearance.

05:03

🔬 The Journey Through the Epidermal Layers

Paragraph two continues the exploration of the epidermis, focusing on the stratum granulosum, known for its keratohyalin granules that aid in keratin handling. This layer also releases lamellar bodies, which form a lipid barrier to protect against pathogens. The stratum lucidum is described as a layer of dead keratinocytes, appearing clear due to the absence of organelles. The outermost layer, the stratum corneum, is composed of dead, flat keratinocytes that continuously shed, allowing new cells to rise to the surface. The paragraph also draws an interesting comparison to reptiles, which shed this layer in one piece, unlike the human skin's gradual cell turnover.

10:03

🐍 The Epidermal Life Cycle and Transition to Dermis

The final paragraph of the script wraps up the discussion on the epidermis by emphasizing the life cycle of its cells, from generation in the stratum basale to eventual shedding in the stratum corneum. It contrasts the living cells in the deeper layers of the epidermis with the dead cells in the top layers. The transition to the dermis is hinted at, suggesting a deeper dive into the living cells and vascular structures that support the skin's functions. This paragraph encapsulates the dynamic nature of the epidermis and paves the way for understanding the dermis's role in skin health and maintenance.

Keywords

💡Epidermis

The epidermis is the outermost layer of the skin, composed of five sub-layers or strata. In the video, it is described as the 'topmost layer of skin' and is crucial for understanding the skin's structure and function. The epidermis serves as the body's first line of defense against environmental factors and plays a significant role in the skin's appearance.

💡Stratum Basale

The stratum basale, also known as the basal layer, is the deepest layer of the epidermis. It is where cell division occurs, generating keratinocytes, which are essential for skin's protective function. The video explains that this layer is vital for the production of keratin, a protein that contributes to the skin's toughness, and melanocytes, which produce melanin and determine skin color.

💡Keratinocytes

Keratinocytes are the primary cells found in the epidermis, particularly in the stratum basale. They produce keratin, a fibrous protein that provides strength and resilience to the skin. The video emphasizes their role in generating the tough outer layer of the skin and their journey from the stratum basale to the surface.

💡Cytokine

Cytokine, derived from 'keratin' and 'cyte' meaning cell, refers to a protein called cytokeratin. This intermediate filament is crucial in the video's discussion of the skin's structure, as it is the main component of keratin, which gives the skin its protective outer layer.

💡Melanocytes

Melanocytes are cells in the stratum basale that produce melanin, the pigment responsible for skin color. The video clarifies that the number of melanocytes is consistent across different skin tones; it is the amount of melanin that varies, leading to differences in skin color.

💡Stratum Spinosum

The stratum spinosum, or 'spiny layer,' is the layer above the stratum basale. It is characterized by the presence of desmosomes, which are cell connections that give the layer its 'spiny' appearance when viewed under a microscope. This layer also contains Langerhans cells, part of the immune system, which help protect the skin from pathogens.

💡Desmosome

Desmosomes are specialized cell connections found between keratinocytes in the stratum spinosum. They help to anchor cells together, providing structural integrity to the skin. The video uses the term to illustrate how cells in the skin are interconnected, contributing to its overall strength and resistance.

💡Stratum Granulosum

The stratum granulosum, or 'granular layer,' is known for its keratohyalin granules, which are involved in the processing of keratin. The video explains that this layer is where keratinocytes begin to die, losing their nuclei and organelles, and preparing the skin for its protective role.

💡Lamellar Bodies

Lamellar bodies are lipid-rich structures released by cells in the stratum granulosum. They contribute to the formation of a lipid layer at the skin's surface, which acts as a barrier against water loss and the entry of foreign substances. The video highlights their importance in maintaining the skin's barrier function.

💡Stratum Lucidum

The stratum lucidum, or 'clear layer,' is a thin layer of dead keratinocytes that have lost their nuclei and organelles. As described in the video, this layer is characterized by its transparency and is a transitional zone between the living cells of the lower epidermis and the fully keratinized cells of the stratum corneum.

💡Stratum Corneum

The stratum corneum is the outermost and dead layer of the epidermis, composed of stacked layers of keratinocytes. The video describes it as the 'coroner's layer,' emphasizing its role in the skin's barrier function. It is also noted for its continuous shedding of dead skin cells, which is a natural process of skin renewal.

Highlights

Beauty is often said to be skin deep, but the outermost layer of skin is composed of dead cells.

The skin has three main layers: the epidermis, dermis, and subcutaneous tissue (hypodermis).

The epidermis is the topmost layer and is made up of five layers or strata.

The stratum basale is the deepest layer of the epidermis where keratinocytes are generated.

Keratinocytes produce cytokeratin, a protein that helps give skin its tough outer layer.

Melanocytes in the stratum basale produce melanin, the pigment that determines skin color.

The number of melanocytes does not determine skin color; it's the amount of melanin.

The stratum spinosum is characterized by desmosomes, which give the layer its 'spiny' appearance.

Langerhans cells in the stratum spinosum are part of the immune system, looking for foreign bodies.

The stratum granulosum contains keratohyalin granules that produce keratin-handling proteins.

Lamellar bodies in the stratum granulosum release lipids to form a protective lipid layer on the skin's surface.

The stratum lucidum consists of dead keratinocytes that have lost their nuclei and organelles.

The stratum corneum is the topmost layer, composed of dead, flat keratinocytes.

The stratum corneum cells continuously slough off, making way for new cells to rise to the surface.

Reptiles shed the entire stratum corneum in one piece, unlike humans.

The epidermis lacks blood vessels; nutrients and oxygen come from below and move upwards.

Transcripts

- They say that beauty is only skin deep,

but did you know that the outermost layer of your skin

is actually composed of entirely dead cells?

I have no idea what's beautiful about dead tissue,

but in this video, we'll seek to figure that out.

We'll talk about the different layers of skin.

As you can see here, there are many layers to the skin,

and there are three different sets of layers

that we can talk about.

There's the epidermis, which is the first five layers,

the dermis, which is the next two layers,

and then the last part, the subcutaneous tissue

or the hypodermis.

In this video, we'll start by talking about

the most superficial part of your skin,

and that is the epidermis,

and I'm sure your friends have told you before

that your epidermis is showing.

The epidermis is the topmost layer of skin,

and itself is comprised of five layers

or as we call them, strata.

So, five layers or strata,

and each strata or stratum has its own

important characteristics.

So, we'll start from the bottom and talk about

the deepest or bottommost layer of the epidermis,

and that's known as the stratum basale or the basal layer.

The stratum basale sits right above the dermis

and is the place where we first generate

what are called keratinocytes.

And so, I'm drawing a whole bunch of

keratinocytes right here.

These are keratinocytes, and, in fact,

the stratum basale has multiple layers of keratinocytes.

And as you might recall, cyte just means cell.

So, what does keratino mean?

Well, it comes from a protein or an intermediate filament

that's called cytokeratin, cytokeratin,

which is something we first start making

in the stratum basale.

Sometimes it's just called keratin,

and it helps give our skin it's tough outer layer

when we get to the top, as we'll talk about in a minute,

and in other animals, keratin

actually is the main ingredient for things like horns,

so horns or even hooves.

So, it's definitely the tough stuff

that help protect our skin.

And another very important thing to know

about the stratum basale is this is where we have

very rapid cell division.

Rapid cell division because this is the bottom layer

of our epidermis.

So, cells are made here, and slowly they move upwards.

Another thing that's important about the stratum basale

is this is where we get our skin color from.

So, there are components of the stratum basale

that determine what our skin color is going to be,

and the cells that confer skin color for us

sit here as well, and these guys,

you may have heard about them before,

are called melanocytes, melanocytes.

So, melanocytes, and as you know,

cytes again just means cell,

but melano just indicates that there's a special pigment

that's made in these cells

that helps determine our skin color,

and that pigment, as you may have guessed,

is called melanin, melanin,

and the interesting thing about these cells or this pigment

is that it's not the number of melanocytes

that determine how dark your skin is.

In fact, people with dark skin color and light skin color

have the same number of melanocytes.

It's actually melanin, the amount of melanin that you have,

that determines how dark your skin color is.

Meaning that darker skin people have more melanin.

All right, so as we move up to our next layer over here,

we see that our keratinocytes make it up here too,

and we might start to notice some of the cytokeratin

that's being produced here a little bit,

and one of the unique features about this next layer

is that we have this little connection that's starting to be

generated in between our keratinocytes.

This guy is called a desmosome, a desmosome,

and this is involved in what gives this layer its name.

So, this layer that sits above the stratum basale

is called the stratum spinosum, the stratum spinosum,

which, as you might guess from the fact

that we call this our basal layer,

this stratum is our spiny layer,

and the reason why we call it that

is because of the desmosome.

When we look at this layer of skin,

sometimes underneath some microscopes,

where we have lost moisture

or the water that is in our cells,

we'll have these shrunken cells

that actually look like stars or kind of like spiny cells,

very pointy shapes,

and the reason why they're pointy is because

they're still connected by these desmosomes,

but they've lost the water.

So, that's why sometimes these layers look shriveled,

so shriveled because they've lost water, as well as spiny.

So, that's how we remember the name of the stratum spinosum.

Another type of cell that sits in this layer

is an immune cell that is looking for foreign bodies

or pathogens or things that can be destructive to our body

like bacteria and fungi and eat them.

These are called Langerhans cells, Langerhans cells,

and as you may have guessed,

these are part of our immune system.

So, they hang out here but can definitely travel

to other layers, and we talk about them more

in our dedicated immune system tutorial.

All right, and so, it's onward and upward

to our next layer that's here.

This more superficial layer is called

the stratum granulosum, the stratum granulosum,

which just means the granular layer,

and the reason why we call it this is because

the keratinocytes that sit here now,

in addition to the desmosomes

that we talked about from the spiny layer,

have a tremendous amount of granules

that are being produced here,

and these granules are very apparent

when we look at them under a microscope.

These granules are called kerato, keratohyalin granules,

so keratin hyalin granules,

and I know what you might be thinking.

So, does this mean we're making

a whole bunch more keratin in this layer?

Well, we are, but these granules don't actually house

keratin or the cytokeratin we talked about.

They hold a whole bunch of other proteins

that help handle our keratin.

Just know that these granules make proteins

that are best remembered as your keratin-handling proteins.

So, your keratohyalin granules make

your keratin-handling proteins,

because they move your cytokeratin around in the cell.

The other thing the stratum granulosum does

is that it releases these things that are called lamellar,

lamellar bodies, and these lamellar bodies contain

a whole bunch of lipids that then come together

and form a strong lipid layer that sits

at the top of our skin.

This lipid layer is impermeable.

It gives the skin its water-tight capabilities

that prevent foreign pathogens from making their way

down deeper into our skin

and eventually into our bloodstream.

And so, from there we go to our next layer up,

to the stratum lucidum, the stratum lucidum,

and sure enough, this is our lucid layer

or our clear, I'll write that here, this is our clear layer.

And the way I think about it,

is that in the stratum granulosum,

our keratinocytes worked so hard

to make these keratohyalin granules

and release these lamellar bodies

that they actually died by the time they make it up

to the stratum lucidum.

So, these cells that we have here,

these keratinocytes, are actually dead.

These are zombie keratinocytes,

and they look that way on microscope

because they've lost their nuclei or other organelles

that usually gave them the color we saw

at our earlier, more alive layers.

And so, as a result,

these layers will not have these granules

or nuclei or organelles that we see in the previous layers.

Instead, they're going to be far clearer

or more see-through than what we saw earlier,

and that's an important note to make off to the side.

The stratum granulosum has lost their nuclei and organelles,

which are usually supposed to give our cells color

when we look at them under a microscope,

and this trend continues

when we move on to our topmost layer here.

This is the stratum corneum, the stratum corneum,

our topmost layer of the epidermis of our skin entirely,

and the way I think about this is

that this is the coroner's layer, the coroner,

and as you know, the coroner's office

is where dead bodies go for autopsies,

and so this is also a layer of dead skin.

And one of the key characteristics of the stratum corneum

is it will have stacked layers of our keratinocytes,

our dead keratinocytes, and we can have up to 15 to 20

stacked layers of these flat, simple,

squamous epithelial cells.

So, I'll write that off over here.

We can have about 15 to 20 layers of our keratinocytes.

Again, they're still dead,

because they died in the stratum lucidum,

and because this is our topmost layer,

we'll see that these cells will randomly,

as well as continuously, slough off.

So, they will randomly and continuously slough off

or leave, they'll fall off, and that makes way for

newer cells underneath or newer keratinocytes

to make their way up to the top.

And another interesting thing about this layer is that

reptiles, I'll write this in like a reptile green,

so reptiles will molt this layer or shed this layer off

in one piece, and so that's why you'll see

like snakeskin that comes off

as a single, intact covering to the snake.

So, they molt off the entire stratum corneum,

which is very interesting and also very gross.

All right, so those are all your layers of your epidermis,

all five of them, the top two being dead,

and these bottom three,

or deeper three layers of the epidermis, being alive,

and as we move deeper down

and start to talk about the dermis,

those cells are also alive and have vessels in them

to nourish them.

We don't have vessels anywhere in the epidermis

because our nutrients and our oxygen come from below

and make their way upwards.