Skin 101: Anatomy |

Skin 101: Anatomy

There’s nothing more beautiful than a baby’s skin…so soft and smooth and not a blemish to be seen. So what happens through the years? What causes all the changes that broadcast to the world “this one’s been around a while?”

We’re going to examine that by starting at the beginning.

What is skin?

Skin separates oneself from the outside world. It has multiple purposes and serves them extraordinarily well. It protects the body against invasion from microorganisms and against losing fluid and drying out. Yet it is open and permeable enough to allow the exchange of warmth, air and fluids. The skin regulates body temperature by evaporating water and through (sometimes dramatic) changes in blood flow. It is an exquisite sensory organ for touching and being touched. We sense the outside world primarily with our skin

The skin is busy… very, very busy

The skin replaces itself about every four weeks, all the while providing protection against environmental onslaught. The surface layer is our horny protective cover. It is made primarily of dead flattened and hardened skin cells that have slowly migrated to the surface from the deeper layers and which are constantly being shed or exfoliated. The cells are composed of proteins (primarily keratin, also the protein in hair, fingernails, animal hoofs and horns) within a matrix comprised of lipids (fats and oils). While the majority of our body is water on the inside, our outside skin ensures we won’t melt or dissolve in the rain or bathtub.

Skin facts:

Your skin is the largest organ of the body, accounting for 12% to 16% body weight, and has a surface area of 12 to 20 square feet. Its chemical composition is:

  • Water 70.0%
  • Protein 25.5%
  • Lipids 2.0%
  • Trace Minerals 0.5%
  • Other 2.0%

The skin has three major layers: epidermis, dermis, and subcutis (or hypodermis).



The epidermis is the outermost layer and varies in thickness from .05 millimeter on the eyelid to 1.5 millimeter on the palms and soles of the feet. It is the layer that contains the pigment producing melanocyte cells and the sensory nerve organs. The epidermis has five sub-layers, constantly working together to rebuild the skin surface.

Basal Cell Layer

The basal layer is the innermost layer of the epidermis and contains small round cells called basal cells which are constantly dividing, pushing older cells to the surface to be eventually shed. The basal layer also is where melanocytes reside, the cells that color the skin either naturally or through stimulation e.g. sun or tanning bed. It is patches of melanin that cause freckles, birthmarks and age spots. Melanocytes are also the source of melanoma, a particularly virulent and often deadly form or skin cancer. Basal cells are involved in basal cell carcinoma, another form of skin cancer.

Squamous Cell Layer

Basal cells pushed outward become squamous (or flat) cells. In this layer, cells are synthesizing keratin. This layer is the thickest part of the epidermis and is involved in the transfer of substances in and out of the body. Cells with keratin are call keratinocytes. Squamous cells may become cancerous, a condition called squamous cell carcinoma.

Stratum Granulosum and Stratum Lucidum

Keratinoctes are pushed up through these two thin epidermal layers towards the skin surface. In the process, they become bigger and flatter and more adherent to one another. Eventually, they dehydrate and die, becoming the tough durable material that forms the outmost layer of epeidermis, the stratum corneum.

Stratum Corneum

This layer is made up of 10 to 30 thin layers of dead keratinocytes. These cells are constantly being replaced and exfoliated, a process that slows dramatically as one ages. While young adults have complete cell turnover every 28 to 30 days, in elderly adults the process takes 45 to 50 days.


The dermis, just below the epidermis, is the thickest layer of the skin and can vary in thickness from 1.5 to 4 millimeters thick. It makes up about 90% of the skin. Its main functions are temperature regulation and supplying the epidermis with nutrient-saturated blood. The dermis is also where most of the skin’s specialized cells and structures are located. These include blood and lymph vessels, hair follicles, nerve endings, and sebaceous and sweat glands. There are approximately 3 million sweat glands which can produce up to two liters of sweat per hour! 

Dermis is held together primarily by a protein called collagen that is synthesized by cells called fibroblasts. These are the cells that give skin its strength and resilience. Collagen is a tough, insoluble protein found in all connective tissue in the body (ligaments, tendons, fascia, etc.) Collagen supports the epidermis, giving it its durability. Elastin, a very similar protein, is also produced. Collagen and elastin are primary components of matrix, the substance surrounding and in which fibroblast reside. Elastin allows skin to spring back into place when stretched and keeps it flexible.

There are two layers to the dermis, the papillary layer and reticular layer. The former has less collagen and contains the vascular vessels so important in temperature control. The latter is a dense thick network of collagen fibers arranged parallel to the skin surface. It also supports other skin components such has hair follicles, sweat glands, and sebaceous glands.


The subcutis is the innermost layer of the skin and consists of a network of fat and collagen producing cells. It is also known as the hypodermis or subcutaneous layer. It functions as both a thermal insulator and shock-absorber. Skin blood and lymph vessels, nerves, and hair follicles pass through the subcutis.

Also see: Skin 102

Now that we understand the basic structure of the skin, we will next address the aging process and how those pesky sunspots, wrinkles, and sagging jowls come about. (Yuck!!)  The good news is there are ways for you to prevent, slow, and even reverse some of these changes.

go to skin 102


  1. thank you for your insight and knowledge. 3 years ago I was damaged by thermage, a Radio frequency, at mono polar level treatment. It’s sort of ruined my life in many ways. I looked 30, now 60. Loss of elastin and volume mostly as well as a bit of dermal scarring. Holding things up is my best bet and plumping. Too frightened of surgery. Would there be any science to help with restoration of fat, creams which would be most beneficial, and would micro needling or PRP help in your opinion. Thank you

    • drjohn says:

      Sally, we are deeply troubled your unfortunate experience. Would you be willing to share pictures of yourself before and after to help educate our audience in the dangers of monopolar RF? (we can disguise with eye blocks if you wish). If yes, please send to

      • Mary says:

        Somehow I am only just seeing this. What is the danger of monopolar RF? Is it reliant on expertise of user? I was thinking of getting Thermage next year. I had Exilis last spring, and had very good results, always following up each treatment immediately with the AnteAge MD duo. Thank you for this blog and thank you for the products!

        • drgeorge says:

          Both monopolar and bipolar radiofrequency are safe, or potentially dangerous, based on the amount of energy applied (voltage, volume of tissue through which the energy passes, and duration of electrical signal). Both methods are designed to apply current in a “controlled” fashion which, of course, depends on good engineering and good operator training. The energy is converted into heat which causes collagen shrinkage and tissue injury that leads to collagenesis, or fat ablation, depending on the indication for treatment. The depth of the energy penetration determines what tissues are affected, and current density (total energy delivered and the tissue mass through which it passes). Generally speaking, monopolar current travels deeper and bipolar more superficially. The image below is helpful in understanding the difference.

  2. How will High Molecular Weight HA penetrate the epidermis? I understand from your blog that LMW HA (sodium hyaluronate) is inflammatory, but this is the form most skin care products seem to use due to its ability to penetrate skin? Thanks!

    • drgeorge says:

      The only kind of hyaluronic acid that truly “penetrates” is found in facial fillers and penetration is accomplished through use of a syringe and needle. Otherwise, HA is best when it reamins on the surface of the skin. Only low molecular weight topical HA has any capacity to penetrate, and since it is recognized as a pro-inflammatory stimulus, certainly not recommended. The benefit of topical HA is because it is a powerful humectant that absorbs atmospheric water which provided the “hydration” effect and contributes to enhanced barrier function, albeit temporary. It is our opinion that manufacturers who tout that their HA is low molecular weight, hence making it more capable of penetration, are not fully informed about the differences that molecular weight makes.

    • drjohn says:

      Penetration is not the issue, and never has been. HA is present in all layers of skin. HA acts as a signaling molecule as well as a barrier constituent. It is present even in the superficial layers of healthy skin, where it forms a barrier against outside stresses and helps to minimize losses of moisture. LMW-HA signals increase skin thickness, but do so via inflammatory pathways, which leads to an aging, scar like skin phenotype and fails to facilitate barrier repair and maintenance. In contrast, LMW-HA promotes keratinocyte differentiation and improves permeability barrier function in the epidermis when applied topically.

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