Skin: A User’s Manual

In addition to being the body’s largest organ, the skin is an amazing entity with a number of functional roles, including:

  1. Protection: Your skin acts as a protective barrier that keeps bacteria, viruses, and foreign substances from entering.  It also protects your internal organs and blood vessels from damage caused by trauma.
  2. Regulation of body temperature: Through the dilation and constriction of small blood vessels, the evaporation of body sweat, and the subcutaneous fat layer, your skin keeps your body temperature at a constant, comfortable level.
  3. Communication: Via nerve endings, your skin provides important information about your environment, and via facial expression and aesthetic appearance, it facilitates your interactions with others.

And these are only a few of the amazing things your skin does on a daily basis!

The skin’s functionality is strongly linked to its structural composition.  At its most basic level, the skin is divided into three layers: the epidermis, the dermis, and the subcutaneous fat.

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THE EPIDERMIS = the thin, outermost, protective barrier

The epidermis itself is divided into four layers.  It is not important to know the names of these layers, but you should know that the cells at the very top of the epidermis began their life at the very bottom.  This “very bottom” layer is called the basal cell layer.

The basal cell layer is where cell division occurs.  As cells divide, they migrate upwards and differentiate (change shape and composition) until they reach the outermost layer of skin. Here, they are shed.  The entire process, from cell division to shedding, takes approximately 4 weeks.  By the time these cells reach the skin surface, they are flat, dead, and filled with keratin. Different parts of your body have a different amount of these outermost cells. For example, the palms and soles (which are quite thick) can have up to 100 layers, causing the epidermis to be almost 1 mm thick in the soles of the feet. By comparison, the epidermis may measure a mere 0.1 mm in other areas of your body.

Your epidermis is home to a number of different cells:

  1. Keratinocytes: These guys produce keratin, a fibrous protein that makes up the protective outer layer of your skin. Keratin is a great “shield” from the external environment, as well as a powerful waterproofing agent.
  2. Melanocytes: These cells produce melanin, which results in skin color, and provide UV protection.
  3. Langerhans cells: Langerhans cells are your skin’s immune system. They take up  foreign material that manages to enter the epidermis, and migrate out of the skin, and into the body, to mount an immune response.

THE DERMIS = the thick, functional component

While the epidermis provides the skin with protection, the dermis provides the skin with the structures necessary for appropriate function.  It is a living entity (whereas the epidermis has no blood vessels) and subsequently filled with a vibrant network of sensory and vascular components.  The dermis is brilliantly constructed because it is at once strong, tough, flexible, and elastic. Of note, it is also much thicker than the dermis, raning from 1 – 4 mm in size.

What components make up the dermis?

  1. Collagen, elastin, extrafibrillar matrix: These are the structural components that provide strength, flexibility, and elasticity to the skin.
  2. Nerves: They allow you to detect pain, temperature, and touch. Some nerves are simple structures (e.g. just free nerve endings) while others are more complex (corpuscular, to be precise). These more complex structures detect deep pressure, vibration, and light touch.  Certain regions of your body (such as your fingertips and toes) have more nerve endings, making them quite sensitive to touch.
  3. Blood vessels: These vessels move nutrients and oxygen throughout the skin. They also keep your body temperature steady. For example, when it is hot, your blood vessels dilate (widen) to bring blood up to the skin surface, thus releasing heat. When it is cold, your blood vessels constrict (narrow), to keep your blood close to your core, thus keeping you warm.
  4. Eccrine glands: These glands regulate body temperature by excreting sweat onto the surface of the skin, thus allowing for evaporation and cooling.
  5. Apocrine sweat glands: These glands are located in the armpits and the genital region, and they are dependent on androgens. Although they are not known to have a useful function, they are responsible for body odor, which occurs when bacteria on the skin’s surface feast on the sweat released from the apocrine glands.
  6. Hair follicles: This is where your hair “lives”. The hair shaft travels through the epidermis until it reaches the surface. Sebaceous glands are located within the hair follicle, and are responsible for producing an oily substance called sebum. This substance keeps the skin moist, soft, and protected against foreign agents.

THE SUBCUTANEOUS FAT

The name says it all!  This fat layer is located under the dermis, where it has three very important functions:

  1. insulation
  2. protection (it literally acts as a “cushion”)
  3. energy storage

REFERENCES:

Marks JG, Miller JJ. Structure and Function of Skin. In: Lookingbill & Marks’ Principles of Dermatology. 4th ed. Elsevier Inc; 2006:5-13.

Structure and Function: Biology of the Skin. The Merck Manual Home Edition. http://www.merck.com/mmhe/sec18/ch201/ch201b.html. Accessed on October 18th, 2009.

The Structure of Normal Skin. DermNet NZ. http://www.dermnetnz.org/pathology/skin-structure.html. Accessed on October 18th, 2009.