Anatomy and physiology of the skin

Do you really know what is the skin?

How it grows?

What functions does it have?

Why we need to take care of it?

In this article we will discuss the skin composition, its structure and functions, so we can understand why it’s important to have some special care with this organ.

It’s common knowledge that the skin has three different layers with specific compounds and functions (see fig.1). Knowing how the epidermis, the dermis and the hypodermis works is specially useful to understand how to take care of our skin and what it’s needs are.

In this paper we will start with a more simplified approach of the theme and then we will look into it more closely. We want to make this knowledge approachable to everyone.

Fig. 1: Anatomy of the skin. From Stanford Medicine Children’s Health, consulted on 3, January, 2024, in https://www.stanfordchildrens.org/en/topic/default?id=anatomy-of-the-skin-85-P01336

We’re not used to think of the skin as an organ like the lungs or the heart for example, but it is in fact the largest organ of the body, generally accounting for about 15% of the total adult body weight. The skin performs many vital functions, including protection against external physical, chemical and biological dangers, has a fundamental role in thermoregulation, as well as prevention of excess water loss from the body, stores water and fat, is part of the excretion system helping eliminate toxins from our bodiesV, participates in homeostasis, acting as a barrier between the organism and its environment, helps to synthesize vitamin D when exposed to the sun and, we can’t forget that, the skin is a sensory organ, related to our nervous system.  In fact, “Current research characterizes skin as more than a barrier with sensory function, but as an intricate biological factory participating in cell signaling, metabolism, and protein synthesis, and as a vital component of the nervous, immune and endocrine systems” (Abdo, Sopko, & Milner, 2020).

As we can see in fig. 1, the three layers of the skin have different characteristics and components, as we will discuss in detail.

“Many organs have a series of layered interfaces; an avascular cellular epithelium that spontaneously regenerates, a basement  membrane zone (BMZ) an stroma or vascular supporting connective  tissue that does not regenerate. In skin (Fig. 1), these layers are  referred to as an epidermis of stratified squamous epithelium, a BMZ  and a fibrous neurovascular dermis which rests on a hypodermis or  subcutaneous fat” (Abdo, Sopko, & Milner, 2020).

  Epidermis

The top layer of the skin is called epidermis and is the thin outer layer of the skin, the one we can see and touch. The epidermis consists of 5 main layers described from deep to superficial: stratum basale, stratum spinosum, stratum granulosum, stratum lucidum and stratum corneum, as we can see in fig. 2.

Fig. 2: Skin layers and functions. Histology image provided by Dr. G M Reynolds PhD CSci FIBMS, Liver Unit of Queen Elizabeth Hospital Birmingham, UK. Image adapted by Britt ter Horst, with permission from G M Reynolds, obtained from Horst, Chouhan, Moiemen, Grover (2018).

The epidermis is mainly composed of sheets of keratinocytes,  but also contains non-epithelial cells, including antigen-presenting  dendritic Langerhans cells as well as melanocytes and Merkel cells. Next, we will discuss this types of cells due to its importance:

• Keratinocytes. The primary type of cells found in the epidermis In humans, they constitute 90% of epidermal skin cells. Responsible for synthesizing keratin, a protein with structural and protective functions.

• Squamous cells. The outermost layer is continuously shed is called the stratum corneum.” The stratum corneum is the outermost layer of the epidermis and marks the final stage of keratinocyte maturation and development. Keratinocytes at the basal layer of the epidermis are proliferative, and as the cells mature up the epidermis, they slowly lose proliferative potential and undergo programmed destruction. These finally differentiated, enucleated keratinocytes are termed corneocytes and retain only keratin filaments embedded in filaggrin matrix. Cornified lipid envelopes replace the plasma membranes of the previous keratinocytes, and the cells flatten, connecting to one another with corneodesmosomes and stacking as layers to form the stratum corneum” (Murphrey, Miao, Zito, 2022). We can see some of its features in fig. 3.

Fig. 3: Arrangements and structure of the stratum corneum. From Abdo, J. M., Sopko, N. A., & Milner, S. M. (2020).

• Basal cells. Basal cells are found just under the squamous cells, at the base of the epidermis. These cells constantly divide to form new cells to replace the squamous cells that wear off the skin's surface. As this cells move up in the epidermis, they get flatter, eventually becoming squamous cells.

• Melanocytes. Melanocytes are also found at the base of the epidermis and make melanin. The amount of melanin you have determines the color of your skin, hair and eyes. People who make more melanin have darker skin and may tan more quickly.

• Langerhans cells. This cells located in the epidermis are part of the body’s immune system. They help fight off germs and infections.

• Merkel cells. This are epithelial neuroendocrine cells. Their origin is unclear, they are rare and can be found in the basal layer of the epidermis and around the bulge region of hair follicles. Merkel cells function as mechano-receptors and can sense light touches. They are part of the tactile-end organs in the skin.

Dermal-Epidermal junction

The transition between de two top layers of the skin is assured by some particular anatomy - the dermal-epidermal junction. The interface between the epidermis and dermis is formed by a porous basement membrane zone that allows the exchange of cells and fluid and holds the two layers together.

The dermal-epidermal junction acts as support for the epidermis, establishes cell polarity and direction of growth, directs the organization of the cytoskeleton in basal cells, provides developmental signals, and functions as a semipermeable barrier between layers (Stepp, Spurr-Michaud, Tisdale, Elwell, & Gipson, 1990, cited by Paul A.J. Kolarsick, Maria Ann Kolarsick, Carolyn Goodwin).

Dermis

The dermis is the middle layer of the skin (fig 1), it consists in an integrated system of fibrous, filamentous, and amorphous connective tissue and contains many structures and differentiated cells, like: blood vessels, lymph vessels, hair follicles, sweat glands, collagen bundles, fibroblasts, nerves, sebaceous glands, mast cells, macrophages, lymphocytes, The dermis also contains pain and touch receptors. And is held together by a protein called collagen. This layer gives skin pliability, elasticity, flexibility and strength.

In the words of Paul A.J. Kolarsick, Maria Ann Kolarsick, Carolyn Goodwin, “ The dermis interacts with the epidermis in maintaining the properties of both tissues. The two regions collaborate during development in the morphogenesis of the dermal-epidermal junction and epidermal appendages and interact in repairing and remodeling the skin as wounds are healed.

The dermis has many different functions related with its anatomy. We will discuss some of the most significant as we relate them with dermis structures and cell’s mentioned above. The dermis:

• has collagen and elastin: the principal component of the dermis is collagen, a fibrous family of proteins with at least 15 genetically distinct types in human skin. Collagen is a major structural protein for the entire body and can be found in tendons, ligaments, the lining of bones, and the dermis. It is a protein that makes skin cells strong and resilient, a major stress-resistant material of the skin. Elastic fibers, on the other hand, play a role in maintaining elasticity but do very little to resist deformation and tearing of the skin.  In fact, elastin keeps skin flexible and also helps stretched skin regain its shape.

• grows hair: the roots of hair follicles attach to the skin.

• keeps us in touch: nerves in the dermis signal when something is too hot to touch, itchy or soft and help us feel pain.

• makes oil: sebaceous glands in the dermis help keep the skin soft and smooth. Oil also prevents the skin from absorbing too much water when we swim or get caught in a rainstorm

• produces sweat: sweat glands in the dermis release sweat through skin pores. Sweat helps regulate our body temperature.

• Supplies blood: blood vessels in the dermis provide nutrients to the epidermis, keeping the skin layers healthy.

Hypodermis

 The hypodermis or subcutaneous fat layer is the deepest layer of the skin. It consists of a network of collagen and fat cells. It helps conserve the body's heat and protects the body from injury by acting as a shock absorver.

The hypodermis has multiple functions:

• cushions muscles and bones: fat in the hypodermis protects muscles and bones from injuries when you fall or are in an accident.

• has connective tissue: this tissue connects layers of skin to muscles and bones.

• helps the nerves and blood vessels: nerves and blood vessels in the dermis get larger in the hypodermis. These nerves and blood vessels branch out to connect the hypodermis to the rest of the body.

• regulates body temperature: fat in the hypodermis keeps us from getting too cold or hot.

Conclusion

“Skin is an intricate, self-renewing organ that is our primary defense  barrier against a hostile environment. It protects against harmful anti-gens and chemicals, dehydration and overhydration, and ultraviolet  radiation. It provides structural integrity and resilience, allows selective  absorption, antioxidant storage, controls thermoregulation through  fluctuations in cutaneous blood supply and perspiration and can stimulate epidermal regeneration when injured” (Abdo, Sopko, & Milner, 2020).

Studying skin anatomy and physiology we conclude that the three layers of the skin form an effective barrier to the external environment, allow the transmission of sensory information, and serve a significant role in maintaining homeostasis. The dynamic epidermis continually produces a protective outer layer of corneocytes as cells undergo the process of keratinization and terminal differentiation. Collagen and elastic filaments of the dermal layer provide the underlying tensile strength of the skin, whereas the layer of subcutaneous fat provides a store of energy for the body (Paul A.J. Kolarsick, Maria Ann Kolarsick, Carolyn Goodwin).

As the skin is such an important organ we must protect it and help maintain its integrity and functionality.

References

Abdo, J. M., Sopko, N. A., & Milner, S. M. (2020). The applied anatomy of human skin: A model for regeneration. ScienceDirect. Retrieved from https://www.sciencedirect.com/science/article/pii/S2213909520300033

Britt ter Horst, Gurpreet Chouhan, Naiem S. Moiemen, Liam M. Grover (2018). Advances in keratinocyte delivery in burn wound care.  ScienceDirect.  Retrieved from https://www.sciencedirect.com/science/article/pii/S0169409X17300960?via%3Dihub

Cleveland Clinic, consulted on 3, January, 2024, in

https://my.clevelandclinic.org/health/body/10978-skin

Morgan B. Murphrey; Julia H. Miao; Patrick M. Zito (2022). Histology, Stratum Corneum. National Library of Medicine, consulted on 3, January, 2024, in https://www.ncbi.nlm.nih.gov/books/NBK513299/

Paul A.J. Kolarsick, Maria Ann Kolarsick, Carolyn Goodwin. Anatomy and Physiology of the Skin. In https://www.ons.org/sites/default/files/publication_pdfs/1%20SS%20Skin%20Cancer_chapter%201.pdf

Stanford Medicine Children’s Health, consulted on 3, January, 2024, in https://www.stanfordchildrens.org/en/topic/default?id=anatomy-of-the-skin-85-P01336