Our skin falls under what is known as the integumentary system which comprises of the skin (technical term: the cutaneous membrane) and its appendages - the hair, nails and glands. It encompasses all the tissue and fibers that cover the body to protect it or ‘envelope’ it from the outside world.

The skin is the largest organ of our body, if peeled it off and laid it flat it would span about 1.5 to 2 meters and accounts for about 15% of an adults body weight. Why is it an organ? Organs are a collection of tissues that work together to perform a certain function; in the case of the skin, it’s function is to protect the internal organs and cells in the body from the outside world. In this sense, it is no different than the lungs, kidneys, heart and stomach. The tissues of the skin are made up of the epidermis, dermis and hypodermis.

The average square inch of skin has 20 blood vessels, 650 sweat glands, and more than a thousand nerve endings. It also has an incredible 60,000 pigment-producing cells (melanocytes.) All of these structures are packed into a stack of cells that are just 2 mm thick.

What is the skin’s purpose?

The skin has a variety of important functions; it protects the internal organs from the outside world. It is the first line of defense from heat, foreign bodies and chemicals. It is impermeable to water and other substances, as well as pathogens such as bacteria and viruses.

It functions as part of the immune system - The skin contains anti-microbial’s and antigens that help protect the body from external pathogens, many of which live on the skin. If the skin is punctured or grazed, those pathogens can enter the body. Langerhan cells help protect the skin by producing anti-microbial peptides called defensins. Anti-microbial peptides are one of the primary mechanisms used by the skin in the early stages of immune defense. They have broad antibacterial activity against gram-positive, gram-negative bacteria and viruses. The skin also produces lysosomes that attach to and kill pathogens.
Temperature regulation/thermoregulation. There are two mechanisms that help us maintain a normal bodily temperature: sweat glands and blood vessels. When the bodies temperature increases this causes vasodilation or for your capillaries to dilate/expand which allows heat to radiate from the surface. Heat also stimulates the sweat glands to release moisture from the body and as it evaporates, cools us down.
It has the ability to perceive our environment, it conducts sensation and has the ability to tell our brain that something is painful or pleasurable. It can detect levels of pressure exerted onto it.
Vitamin D synthesis - The skin contains a zoosterol (sterol produced by animals) called 7-dehydrocholesterol (7-DHC). When UV light reacts with 7-DHC it undergoes a chemical reaction that converts it to pre-vitamin D3 and then into cholecalciferol or vitamin D3. Vitamin D3 has a very important role for calcium and phosphate regulation in the body which is vital for healthy bones, teeth and muscle health. It is thought that vitamin D deficiency can also cause auto-immune disease, heart disease and some cancers.
Protection from UV light - melanin (produced by melanocytes) in our epidermis absorbs harmful UV rays and protects our body from radiation.

Skin Structure

The Skin is made of three distinct but connected layers; the epidermis (or keratinocytes), the dermis, and the subcutaneous layers. The epidermis contains 4-5 layers, the dermis contains 2 layers and the subcutaneous layer contains just one layer of fat that helps keep its structure and anchors the other two layers. There are varying layers of cells within each layer and as such, there is a great deal of variation of skin thickness depending on the geographical location of the skin. The eyelid, for instance, has the thinnest layer of the epidermis, measuring less than 0.1 mm whereas the palms and the soles of the feet have the thickest epidermal layer, measuring approximately 1.5 mm (Kolarsick et al, 2011) The skin is fed via the hypodermis (subcutaneous layer) to the dermis where there are a series of blood vessels bringing oxygen and nutrients to the skin. There are no blood vessels in the epidermis, instead, all nutrients are ‘fed’ via diffusion from the dermis to the epidermis.

The Outer Layer (Epidermis)

The epidermis is the outer-most layer of the skin and is mainly made up of nucleated and denucleated (dead) keratinocytes, or cells that make keratin, a fibrous, protective protein. The epidermis is made of 4-5 layers or strata. Cell division or mitosis is continually happening in the lower epidermal layers; as older cells get pushed to the surface they gradually die to create a protective outer shield before they are then shed. In this sense, the epidermis is in a constant state of renewal. The epidermis has embedded sebaceous glands (excretes sebum/skin oils), hair follicles and sweat glands.

The Inner Layer (Dermis)

The basement membrane connects the epidermis to the dermis. it is comprised of type IV collagen and laminin separates these two layers and serves as an anchor for dermal papillae and the smooth muscles that control hair follicles. (Watt and Fujiwara, 2011). The dermis ( a combination of loose and dense connective tissue) that houses blood vessels as well as sweat (water) and sebaceous (oil) glands, the hair follicle, nerves and Pacinian corpuscles (nerve endings.)

The dermis is connected to the subcutaneous layer; this fatty layer helps with thermoregulation (regulates body temperature) and anchors the skin to muscle and bone.

The Hypodermis

The hypodermis is the deepest of the three layers and lies underneath the dermis. Consisting primarily of fat and connective tissue it contains mostly large blood vessels and nerves. The hypodermis is much thicker than the epidermis and dermis. Its thickness depends where it is situated on the body and the fat content of the individual. This layer is important for regulating skin and body temperature.

References

Michalun, M. V., & DiNardo, J. C. (2015). Milady skin care and cosmetic ingredients dictionary.

Skin - Introduction to Structure and Function