Epidermal Biology

Written by: Jake Khalde

“Hey! Your epidermis is showing!” is a joke many people have heard from their peers or have seen in biology textbooks. The epidermis is magnificent, making up the top layer of the skin. The epidermis is responsible for many of the skin’s wonders: melanin production, UV protection, skin cell production, and hydration. The epidermis does not just work with the dermis and hypodermis; it has its own layers: the stratum basale, stratum spinosum, stratum granulosum, stratum lucidum, and the stratum corneum. But what do each of these layers do?

The Stratum Basale

The deepest layer of the epidermis, the stratum basale, produces new skin cells. This layer also holds keratinocyte stem cells and melanocytes. Keratinocyte stem cells are not entirely understood; however, research shows that they sustain cell turnover and produce keratin, the protein responsible for building structures like hair, the outer layer of the skin, and nails. Keratinocyte stem cells do not undergo apoptosis, programmed cell death, or asymmetric chromosome segregation, which protects the genome stability, thus making these cells the likely origin of skin cancer. Melanocytes are the skin cells that produce melanin, the pigment in the skin which determines skin color.

Stratum Spinosum

Keratinocytes and proteins make up the stratum spinosum layer of the skin. Keratinocytes produce keratin and, over about four weeks, rise to the top of the skin. The protein which holds these keratinocytes together is called desmosome. This protein is sticky, similar to glue, and allows for this layer of the skin to be a network of skin flexibility and support.

Stratum Granulosum

Layers of cells stacked on top of each other, the stratum granulosum is the water-proof, middle layer of the epidermis. It acts as a barrier between the live, active cells underneath this layer and the flattened, inactive cells above this layer. The cells in this layer are said to excrete lipids, which create a water sealant. This middle layer marks the shift between the live and dead skin cells.

Stratum Lucidum

Translucent under the microscope, the stratum lucidum is a layer of dead skin cells that is found mainly on areas of thick skin, like the soles of the feet. There are estimated three-to-five layers of flat, dead keratinocytes. This layer is filled with eleidin, an intercellular protein that gets converted into keratin in the stratum corneum. This layer functions by helping in the skin’s waterproof properties, reducing the friction between layers, degenerating skin cells, and allowing the skin to stretch.

Stratum Corneum

The layer of the skin visible to the human eye, the stratum corneum is where dead skin cells flake off. In this layer, the cells which produce keratin, keratinocytes, lose their nuclei and other organelles and form a strong network that blocks harmful things like pathogens from entering the body. Fat is found in this layer to prevent water from leaving the body easily. The corneocytes flake off over time as the stratum basale produces more keratinocytes which move up the layers of the skin as they mature.

Despite there being three layers of the skin, there are also five layers of the epidermis. Collectively, these layers work wonders to maintain homeostasis and protect the body. This system proves to be fascinating, making it important to continue to learn and understand its biology.

Labeled Epidermal Layers

Sources:

First Image: Rojgar Aur Nirman

Second Image: Medical News Today

Cleveland Clinic. “Epidermis (Outer Layer of Skin): Layers, Function & Structure.” Cleveland Clinic, 19 October 2021, https://my.clevelandclinic.org/health/body/21901-epidermis. Accessed 31 May 2022.

The Free Dictionary by Farlex. “Stratum lucidum | definition of stratum lucidum by Medical dictionary.” Medical Dictionary, N.D., https://medical-dictionary.thefreedictionary.com/stratum+lucidum. Accessed 31 May 2022.

Georgiev, Deyan. “What is the Stratum Granulosum? (with pictures).” The Health Board, 28 April 2022, https://www.thehealthboard.com/what-is-the-stratum-granulosum.htm. Accessed 31 May 2022.

The Health Board. “What is the Stratum Lucidum? (with pictures).” The Health Board, 21 May 2022, https://www.thehealthboard.com/what-is-the-stratum-lucidum.htm. Accessed 31 May 2022.

Moore, Sarah. “Keratinocyte Stem Cells: An Overview.” AZoLifeSciences, 26 February 2020, https://www.azolifesciences.com/article/Keratinocyte-Stem-Cells-An-Overview.aspx. Accessed 31 May 2022.

Uday, Linica. “What is the function of stratum lucidum? Where is this layer found?” Socratic, 29 July 2017, https://socratic.org/questions/what-is-the-function-of-stratum-lucidum-where-is-this-layer-found. Accessed 31 May 2022.

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