Content Reviewers:Rishi Desai, MD, MPH
The skin covers and helps protect the body from its environment.
It’s also known as the integument, which is part of the integumentary system.
The integumentary system also consists of structures that are derivatives of the skin, such as nails, hair, and glands within the skin, like sweat glands.
The skin can be split into two general types: thick skin and thin skin.
Thick skin has a much thicker outer layer of epidermis that helps to protect the body in places where it’s regularly exposed to slightly harsher physical conditions.
That’s why it’s typically found in places such as the palms of the hands and soles of the feet.
Thick skin can also be differentiated from thin skin because it doesn’t have hair follicles or oil-producing glands called sebaceous glands.
It does still contain sweat glands though.
The overall thickness is often more than 5mm, whereas thin skin is typically only between 1-2mm thick.
Thin skin has all the same structures as thick skin except it has a thinner epidermis and additional structures such as hair follicles and sebaceous glands.
If we compare histologic images of thick skin and thin skin at low magnification, we can see the three main layers that make up the skin.
At the top of the image is the surface epidermal layer or epidermis that’s significantly thicker in thick skin.
Hair follicles are only found in thin skin, and as we can see in this image, they’re located in the middle layer called the dermis or dermal layer.
The third and deepest layer is called the hypodermis, which consists mainly of adipose and loose connective tissue, although this image of thick skin doesn’t include the hypodermis.
Now, if we look at just the epidermis, there are sub-layers that make up the epidermis.
Starting from the base of the skin is the stratum basalis, spinosum, granulosum, lucidum, and corneum at the surface.
The Stratum Lucidum can only be seen histologically in very thick skin.
If we take a closer look at the epidermis of both thick skin and thin skin, we can see that the stratum basalis is only a single-layer of cuboidal or columnar germ cells.
These cells are attached to a basement membrane that’s normally too thin to see under the microscope.
These cells have a high level of mitotic activity, dividing to produce all the cells that migrate up to the surface of the skin.
In both thick and thin skin, the cells within the stratum spinosum synthesize a fibrous protein called keratin, which is why the cells themselves are called keratinocytes, even though they also produce other proteins as well.
This image of thick skin was taken while using a 60x oil immersion objective.
Oil immersion objectives allow for higher resolution images to be seen through the microscope.
All the skin cells of the epidermis are keratinocytes except for the germ cells, which are found in the stratum basalis.
The keratinocytes are attached to each other by strong cell-to-cell structures called desmosomes, which are more easily seen in this layer compared to the layers above the stratum spinosum.
The desmosomes are found in between the cells and look like tiny needles or spines.
As the keratinocytes mature, they’re pushed up closer to the surface by newly formed cells below and gradually become flatter in appearance as well.
Above the stratum spinosum, is a layer of 3-5 flattened cells that contain dense basophilic kerato-hyalin granules within their cytoplasm, which is why this layer is called the stratum granulosum.
In thick skin, the lighter or more lucid layer of cells seen above the stratum granulosum is the stratum lucidum, which is typically only 2-3 cells thick.
The keratinocytes in this layer will have lost their nuclei and organelles, although they’re still attached to one another by desmosomes.
This layer won’t be seen above the granulosum in tissue sections taken from thin skin.
Now, the stratum corneum is the outermost layer of dead keratinocytes from the underlying stratum lucidum.
So these cells also don’t have nuclei or organelles, but in this layer, the cells are almost completely filled with keratin filaments.
This layer consists of approximately 15-30 layers of the squamous keratinocytes.
Over time, the desmosomes in this layer gradually break down, which causes the cells to shed or desquamate from the surface of the skin.
In thick skin, the stratum corneum is often thicker than all of the other epidermal layers combined, whereas in thin skin, this layer is much thinner when compared to the rest of the epidermis.
Alright, let's move onto the dermis or dermal layer that sits below the epidermis.
The junction between the two layers is irregular, forming many projections of the dermal layer called dermal papillae.
The corresponding invaginations of the epidermal layer are called epidermal ridges, and the combination of the papillae and ridges is referred to as the papillary layer of the dermis.