Skin anatomy and physiology

Definitions & Key takeaways

The skin or the integumentary system is the largest organ of the body that has many important functions in physiology. It protects the body from infections, helps in thermoregulation, and contains nerve receptors that detect pain, sensation, and pressure.

The integumentary system is divided into three major components including the epidermis, dermis, and hypodermis. The epidermis is the most superficial layer and it's responsible for protection from pathogens, and the environment, for vitamin D production, and for giving the skin its color. The dermis lies below the epidermis and controls temperature regulation and helps with sensation. The hypodermis lies below the dermis and provides a point of attachment for the skin to the deeper muscles.

The skin also contains several accessory structures, including hair, and nails. Hair provides protection and insulation, while nails provide support and protection for the tips of the fingers and toes.

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Introduction0:00–0:55

The skin makes up around 16% of total body weight, making it the largest organ in the body. Although it's hard to imagine it as a single organ.
The skin along with its accessory structures like oil and sweat glands makes up the integumentary system. The integumentary system protects the body from infections, helps regulate body temperature and contains nerve receptors that detect pain, sensation and pressure.
Now, the skin is divided into three layers. The epidermis, dermis and hypodermis.
The epidermis forms the thin outermost layer of skin underneath is the thicker dermis layer that contains the nerves and blood vessels.
And finally, there's the hypodermis, which is made of fat and connective tissue that anchors the skin to the underlying muscle.

Epidermis0:55–5:49

The epidermis itself is made up of multiple layers of developing keratinocytes which are flat pancake shaped cells that are named for the keratin protein that they're filled with.
Keratin is a fibrous protein that allows keratinocytes to protect themselves from getting destroyed when you rub your hands through the sand at the beach.
Keratinocytes also make and secrete glycolipids, glyco, meaning part sugar and lipid, meaning part fat glycolipids help to prevent water from easily seeping into and out of the body.
Keratinocytes start their life at the lowest layer of the epidermis, called the stratum basale or basal layer, which is made of a single layer of stem cells that continually divide and produce new keratinocytes.
These new keratinocytes then migrate upwards to form the other layers of the epidermis. The stratum basale also contains another group of cells, melanocytes which secrete a protein pigment or coloring substance called melanin.
Melanin is actually a broad term that constitutes several types of melanin found in people of differing skin color. These subtypes of melanin range in color from black to reddish yellow and their relative quantity define a person's skin color.
When keratinocytes are exposed to the sun, they send a chemical signal to the melanocytes which stimulates the melanocytes into making more melanin.
The melanocytes move the melanin into small sacs called melanosomes. And these get taken up by the newly formed keratinocytes.
Melanin then acts as a natural sunscreen because its protein structure dissipates or scatters UVB light which if left unchecked can damage the DNA in the skin cells and lead to skin cancer.
Darker types of melanin and greater quantities of this kind of melanin are produced by individuals living close to the equator because they typically get more sun exposure.
However, it's a fine balance because UVB light helps us generate Vitamin D which is an important regulator of calcium absorption.
Keratinocytes contain cholesterol derived molecules that are activated by UVB into Vitamin D As keratinocytes in the stratum basale mature and lose the ability to divide.
They migrate into the next layer called the stratum spinosum, which is about 8 to 10 cell layers. Thick keratinocytes in the stratum spinosum layer have tiny proteins on the membrane that look like tiny spines.
These help the cells adhere to one another. The stratum spinosum layer also has dendritic cells lurking around and these are star shaped immune cells that are constantly patrolling, looking for invading microbes.
The next layer up is the stratum granulosum, which is 3 to 5 cell layers. Thick keratinocytes in this layer begin the process of keratinization which is where the keratinocytes flatten out and die.
And in this process, they create the epidermal skin barrier to do this keratinocytes. In the stratum granulosum layer, produce large amounts of keratin precursor proteins and glycolipids which remain within Granules called keratohyalin Granules and lamellar Granules respectively.
Keratohyalin Granules eventually start to aggregate and crosslink forming enormous bundles of keratin within the keratinocyte lamellar Granules.
On the other hand, get secreted and stick to the outer cell surface. It forms a sort of cement between the cells making them more resistant to external forces and water loss over time.
The intracellular organelles disintegrate. So the cells flatten out and die.
Keratinization leads to development of the stratum lucidum layer which is 2 to 3 cell layers, thick of translucent dead keratinocytes that have secreted most of their lamellar Granules.
The stratum lucidum is only found in thick skin like on the palms and soles of the feet because those are the areas that need extra protection.
The stratum lucidum is absent in thin skin which covers the rest of the body and the other layers are thinner. Finally, there's the stratum corneum or the uppermost and thickest layer of the epidermis, which is like a wall of 20 to 30 layers where the glycolipid acts like the cement and the dead keratinized cells of the bricks.
The dead keratinocytes in this layer secrete natural antibiotics called defensins which poke holes in bacteria as new keratinocytes push up into the stratum corneum.
Older dead cells are shed forming skin flakes or dandruff. Now, the dermis lies below the stratum basale of the epidermis and it's much thicker than the epidermis.

Dermis5:49–8:45

The dermis is divided into two layers, a thin papillary layer below the stratum basale and a deeper reticular layer. The papillary layer contains fibroblasts which produce a connective tissue protein called collagen.
The fibroblasts are arranged in finger like projections called papilla, each of which contains blood vessels and nerve endings.
One type of nerve ending found here is called a Meissner corpuscle. And this is a disc shaped structure that detects fine touch and it allows you to know exactly where, for example, a feather touches your arm.
Another type of nerve found in the papilla are called free nerve endings which are dendrites that detect pain the papillary layer also contains macrophages which capture pathogens that make it past the epidermis.
It's also the papillary layer that's responsible for fingerprints. These are necessary for gripping and sensing abilities of the fingers and feet and they make each of us as unique as snowflakes.
Next is the reticular layer of the dermis, which is even thicker than the papillary layer like the papillary layer. However, the reticular layer contains fibroblasts with scattered macrophages.
But the collagen in the reticular layer is packed very tightly together making it an excellent tissue support. In addition, fibroblasts in the reticular layer secrete elastin, which is a stretchy protein that gives skin its flexibility.
The reticular layer also contains the skin's accessory structures like oil and sweat glands, hair follicles, lymphatic vessels and nerves and all of the blood vessels that serve these tissues.
One type of nerve ending found here is called a pacinian corpuscle and this is an onion shaped structure that detects pressure or vibration and it allows you to feel when someone's grabbing your arm.
Since the reticular layer contains lots of blood vessels and sweat glands, it's also largely responsible for regulating temperature when body temperature rises.
Like during a workout. The nervous system makes these blood vessels dilate and makes the sweat glands secrete sweat, dilation of blood vessels, brings more blood closer to the skin surface and that allows heat to get lost to the outside environment.
As sweat coats the skin surface, it takes heat to evaporate. So, heat is slowly lost from the skin surface with every drop of sweat that evaporates away.
In the opposite situation. When it's cold outside blood vessels constrict.
And that diverts blood flow away from the skin and there's no sweat. So body heat is conserved.

Hypodermis8:45–9:12

Finally, there's the layer just below the dermis called the hypodermis or subcutaneous tissue. This layer contains fat cells called adipocytes which help store most of the fat in our body but also contains fibroblasts, macrophages, blood vessels, nerves and lymphatics.
The hypodermis helps insulate deeper tissues, provides padding to the body and anchors the skin to the muscle with connective tissue proteins like collagen.

Review9:12–9:52

All right, it's a quick recap. The skin or the integumentary system is the largest organ of the body that has many important functions in physiology.
The integumentary system is divided into three major components, including the epidermis, dermis and hypodermis. The epidermis is the most superficial layer and it's responsible for protection from pathogens and the environment for Vitamin D production and for giving the skin its color.
The dermis lies below the epidermis and controls temperature regulation and helps with sensation. The hypodermis lies below the dermis and provides a point of attachment for the skin to the deeper muscles.