A few years ago, I got into a heated debate.

The topic was: what is the largest organ in the body?

My “opponent”, a very bright student studying human health and disease, insisted that the liver was the largest, while I contended that it was the skin.

I was sure I was right (although this young gentleman’s self-assured smirk was making me doubt myself more and more every minute).

He pontificated that the skin couldn’t even correctly be called an organ.

I countered that it was an organ and asked what evidence did he have that it wasn’t.

To this, he recited that an organ was a self-contained group of tissues that cooperated to perform a particular function in the body, like a kidney or the heart. He proclaimed that a tissue was a group of similar cells with similar functions and thus the skin was a tissue, not an organ.

Defiantly, and a little more defensively than the situation warranted, I replied that he was wrong and whether he liked it or not the skin was an organ. (Not the most persuasive argument I’ve ever used but it’s one that’s infuriating to counter!)

We ended up Googling the answer and to my pride’s relief, I had remembered my biology 101 course correctly, the skin is indeed an organ.

When people think of organs, they, like my sparring buddy, usually think of the brain, heart, liver, lungs, kidneys, and other internal organs. But the definition of an organ as a group of distinct tissues working together towards a common goal applies to more than just these. It also includes the nose, the anus, the thyroid, the head and neck, the eyes, and even the bronchi! In fact by this definition of an organ, we have 78 in total.

But how is the skin an organ?

The skin is an organ because it is composed of tissues working together. The main three tissues are:

  • The epidermis, the outermost layer of skin.
  • The dermis, under the epidermis.
  • The subcutaneous or fat layer, under the dermis.

Each layer has its own distinct form and functions and is composed of a unique profile of cells. Let’s look at each of the skin’s tissues in more detail.

Guarding Against Bad Guys: The Epidermis
Introduction:

The tough outermost layer of the skin that can be very thin, like on your eyelids, or thicker, like on the soles of your feet. Keratinocytes make up 90% of the cells of this layer. They originate in the bottom of this layer and are slowly pushed outward by newer cells until they’re shed off. In this way, the skin is constantly replenishing itself.

Main Roles:
  • Making new cells
  • Making melanin
  • Immunodefense
  • Regulating body temperature
  • Reducing sun damage
Components:
  1. Keratinocytes: physical separation of the organism from its environment, reduce loss of moisture and heat, act as the core structural unit of the epidermis, play as immunomodulatory role.
  2. Melanocytes: produce melanin, a pigment that protects cells against UV damage.
  3. Langerhans cells: key to immunodefense they are mucosa and skin-resident dendritic cells and act as antigen-presenting cells.
  4. Merkel cells: role is unclear but thought to play a role in our sense of touch as part of the neuroendocrine.
Sublayers:

The stratum basale (innermost sublayer, also called the stratum germinativum), stratum spinosum, stratum granulosum, stratum lucidum, stratum corneum (outermost sublayer).

Fun Fact:

This layer has no blood supply! This is why you can shave and, assuming you don’t cut yourself, not end up writhing in pain.

Where the Party Is: The Dermis
Introduction:

The strong and elastic layer that comprises comprises around 90% of the thickness of the skin.

Main Roles:
  • Producing sweat
  • Growing hair
  • Making oil
  • Supporting the skin’s blood supply
  • Enabling you to feel stimuli (pressure, temperature and pain)
  • Regulating temperature
  • Water storage
Components:
  1. Blood and lymph vessels
  2. Hair follicles
  3. Sweat glands: eccrine (or true sweat glands) and the specialized kind called, apocrine glands (produce a milky sweat responsible for the smells of sexual attraction and include mammary glands).
  4. Sebaceous glands: produce the oil-like substance, sebum
  5. Nerve endings
  6. Collagen (for durability) and Elastin (for stretchiness)
Sublayers:

Papillary (upper) and reticular (lower) layers.

Fun Fact:

The blood vessels in this layer transport the vitamin D made by this layer to other parts of the body!

Bringing Us Together: The Subcutaneous Tissue
Introduction:

The layer comprising a network of fat and collagen cells that protecting the body from excessive heat loss. It also acts as a shock absorber protecting the inner components of the body, particularly the inner organs.

Other names:

Subcutis (cutis = skin), hypodermis (hypo = beneath, dermis = skin), hypoderm, and superficial fascia (fibrous tissue).

Main Roles:
  • Attaching dermis to muscles and bones.
  • Physically supportive structure for many of the skin’s components.
  • Regulating of body temperature.
  • Fuel storage in the form of fat.
  • Protecting our inner organs and tissues by cushioning them.
  • Immunodefence.
Components:
  1. Fibroblasts: produce collagen and other fibers.
  2. Adipose cells: store fat as triglycerides
  3. Macrophages: antigen-presenting cells derived from monocytes.
  4. Blood and lymph vessels
  5. Mammary glands (sometimes!) and glandular part of sweat glands.
  6. Mast cells: key immune cells in the skin’s inflammatory and allergic response.
  7. Hair follicle roots
  8. Flat sheets of muscle (sometimes!)
  9. Bulbous and lamellar corpuscles: mechanoreceptors.
  10. Bursae: fluid filled sacs found in joint.
Fun Fact:

The thickness of this layer varies throughout the body and from person to person!

So there you have it! Conclusive proof that the skin is indeed an organ. You can now whip out this fact at parties but, unlike my younger self, be able to articulate that the skin is an organ because it is composed of distinct types of tissues working together to protect and support your body.

Diagram of human skin

CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=507467

Helpful Sources:
American Academy of Dermatology
Merck Manual
National Cancer Institute
National Geographic


Article by Olwen Reina. Contact Olwen at olwen@tempobioscience.com.