Human skin is incredible. It is the body's largest organ, covering just about every nook and cranny of our entire body. The skin has many functions, it is made up of many different parts and also has the amazing ability to regenerate and heal itself. Along with the hair, nails, sweat and sebaceous glands, it forms the integumentary system. This comes from the Latin for covering, integumentum.
There are five main functions of the integumentary system. The first, and possibly the most obvious, is protection. It covers up all of our internal organs, binding and protecting them from injury, chemical agents and ultraviolet light. It can help prevent harmful microorganisms from entering the body as well as preventing water loss- leading to dehydration.
The next function is sensation. This allows us to protect ourselves from our immediate environment. Numerous sensory nerve endings are contained within the skin and they respond to touch, temperature, pain and pressure.
The integumentary system regulates body temperature. It acts as a thermostat. The body must remain at 36.8 degrees Celsius C. Changes in this temperature can result in illness such as hypothermia (where the body temperature falls below 35C) and fever (where the body temperature can rise up to 39C).
Body heat is generated by muscle contraction and also from digestive organs, particularly the liver. Body heat can be lost through faeces, urine and expired air, but mainly it occurs through sweating.
Shivering and contraction of the arrector pili muscle (Goosebumps) are also ways the body can generate and keep in heat. Also, if the body is too hot, it looses heat from the body by radiation. The capillaries in the skin expand and the blood flow increases. This is called vasodilatation. If the body is too cold, vasoconstriction occurs. Blood vessels in the skin constrict. Less blood passes through them keeping the blood closer to the internal organs and preventing heat loss from the skin's surface.
Skin can eliminate waste. Sweat will carry salts, lactic acid, urea and other substances out from the body. Sebum is also secreted onto the skin, keeping it supple and helping to waterproof it. Although the skin is mostly waterproof, some substances can pass through it into the basal layer. Fat-soluble substances such as steroids and vitamins A, D and E along with carbon dioxide and oxygen can be absorbed. Transdermal patches, topical medications, essential oils and ultraviolet light are some more examples of agents passing into the skin through absorption.
A chemical called ergosterol, which is located in the skin, can be turned into vitamin D when activated by ultraviolet rays from the sun. Our bodies need vitamin D to absorb calcium and phosphorus from food, leading to healthy bones.
There are numerous components involved in building the integumentary system. It is necessary to look at these separately in order to build up a bigger picture. The skin can be spilt up into three very separate layers, the epidermis, the dermis and the subcutaneous layer.
Firstly, lets look at the epidermis. This is the top section of the skin. It is made up of stratified squamous epithelium and has a protective function. Keratinocytes make up 90% of epidermal cells. The epidermis houses five layers and is replaced on average every forty days. The deepest layer is the stratum germinativum, or the basal layer, rising through to the stratum spinosum, stratum granulosum, stratum lucidum and finally the most superficial layer being the stratum corneum. There is no blood or nerve supply in the epidermis.
Nutrients are passed through by diffusion from the dermis. The most superficial of the cells die off, as they are too far away to receive the benefits of this diffusion.
There are 25-30 layers of columnar epithelial cells lying on a basement membrane in the stratum germinativum. The cells in this layer contain a nucleus and divide by a process called mitosis. The job of these new cells is to push the old ones up in the direction of the stratum corneum. This layer contains melanocytes, along with Merkel's discs (or tactile discs) that give the skin it's sense of touch.
One in ten of these basal cells is a melanocyte. Granules of melanin are produced through a process called melanogenesis. Melanin is the chemical responsible for pigment in our skin and can also be stimulated by ultraviolet rays. Melanin is necessary to protect the deeper layers of skin from sun damage. The number of melanocytes is usually the same in everybody, but they will produce different amounts of melanin, producing different depths of skin colour. Albinism is a condition where an individual cannot produce melanin.
Attached above the stratum germinativum is the stratum spinosum. Nutrients and oxygen in the interstitial fluid are generally thought to be exhausted by the time they reach this layer. Sometimes known as the prickly layer, it has a pattern of interlocking projections, anchoring itself to the stratum germinativum below and the stratum granulosum above.
Cells in the next layer, the stratum granulosum, contain between one and three rows of dead and living squamous cells. Many small granules called keratohyalin can be found in their cytoplasm as the cells start to die off and lose their nucleus, and this is why it is sometimes called the granular layer. The granules form a waterproofing device and so stop nutrient transport getting any further. This leads to the death of the cells. Keratinisation takes place.
Keratinisation is the process by which cytoplasm (the intracellular gel-like fluid) is replaced by a tougher protein called keratin. Keratinocytes begin their life in the basal layer, coming from keratinocyte stem cells gradually becoming more keratinised as they push up through the layers of the epidermis. Eventually they reach the outermost layer, the stratum corneum. The process is necessary to prevent foreign matter and infectious agents entering the skin and to prevent moisture from escaping the skin. They also protect the body from minor injury.
Above the stratum granulosum lays the stratum lucidum (Latin for clear layer). The cells are flat and translucent. The clear substance contained within the cells is called eleidin, which will become keratin. The transparency of the cells allows sunlight to filter into the deeper layers. Stratum lucidum is usually only found on thicker layers of the skin such as the fingertips, the palms of the hands and the soles of the feet. They contain no nucleus but instead contain an oily, waterproofing substance. This is also called the barrier layer of the skin.
1300 dead cells per cm are shed per hour from the final, and most superficial layer of the epidermis. This is known as the stratum corneum, or the horny layer. Around twenty layers of tightly packed, fully keratinised flat cells exist here. Cells can absorb water to aid hydration. This is one of the reasons humans experience wrinkling' on fingers that have been in water for some time. Friction causes these cells to rub off through a process called desquamation, allowing them to be replaced by the cells underneath. Stratum corneum can vary in thickness. This is because some parts of the body require more grip and protection.
The middle section of the skin is called the dermis. It is made up of strong connective tissue. It's good blood and lymphatic supply and contains many nerve endings. It is split into two definable layers, the papillary region and the reticular layer.
The papillary region is so called because it has papillae, or ridges which connect it to the epidermis. These papillae form visible contours, which we know as fingerprints. They increase surface friction and help us to grasp and hold objects. There are capillaries that supply the nutrients to the first layer of the epidermis and also nerve endings.
The reticular layer is thicker and made up of dense bundles of collagen and elastin fibres. This allows the skin to be pulled in many directions, whilst returning to its original shape. Stretch marks, or straie are the overstretching of these fibres.
Nerves can be found in the dermis. Sensory nerves such as Messiner's and Pacini's corpuscles can be found in the papillary region, which identify touch and sensation by sending signals to the brain. Nerve endings that detect pain, and temperature are called nociceptors. Motor nerves send messages from the brain to the arrector pili muscle and the sweat and sebaceous glands.
Blood vessels enable the skin to be nourished. Oxygen and nutrients can enter cells and waste products can be carried away.
Hair has the same keratinised properties of the epidermis and has a protective function. There are three concentric layers, the medulla, cortex and outer cuticle. The bulb is the base of the hair, and the matrix is where mitosis takes place. When they reach the upper part of the matrix they become keratinised. Melanocytes are found in the matrix. The dermal papilla enters the hair bulb at the bottom. This has a capillary loop, which enables nutrients to enter the bulb. The medulla is the inner part of the hair. It reflects light but is not always present. The cortex gives the hair strength, and contains melanin and the cuticle is the outermost part of the hair, containing one layer of flat, pigment-free cells.
The arrector pilus is a small, involuntary muscle attached to the hair follicle. It is controlled by the sympathetic nervous system. During cold conditions the muscle contracts, so the hair stands on end. Air is then trapped between the hair and the skin, forming an insulating barrier. Sometimes this can happen in response to an emotion. This function is known as vestigial, meaning it has lost some or most of its original function through evolution. Human beings generally do not have enough hair to make this effective.
A small, sac-like exocrine gland can be found near the root of each hair. This is known as a sebaceous gland and secretes a substance made up of fats, cholesterol, proteins and inorganic salts named sebum (Latin for fat or tallow). The sebum is produced in special cells and as they burst, the sebum is released. The sebaceous glands have many purposes; to stop the hair from becoming brittle by lubricating it, to soften the skin, to prevent evaporation of water from the skin and also to prevent infection, as it is mildly antiseptic. One problem that sebum has is that millions of microorganisms adhere to it.
There are more sebaceous glands found on the scalp, face, nose, forehead and chin. Hormones control the activity of the sebaceous glands. Our skin gets drier as we age because sebum production levels decrease.
Glabrous (non-haired) areas also contain sebaceous glands. These include the eyelids, penis, labia minora and the nipples. The sebum in these areas travels through ducts that terminate in the sweat pores on the skin's surface. Meibomian glands are a type of sebaceous gland found on the eyelids. They secrete sebum into the tears to prevent evaporation and coat the eye. Earwax also contains sebum.
Sudoriferous glands, or sweat glands can be found on most parts of the skin. They are coiled and tubular in shape and their ducts open onto the surface of the skin- known as pores.
Sweat is essentially a watery fluid, made up of electrolytes and waste products. Our bodies are constantly excreting it. It exists as a method of excretion, removing waste substances and also for thermoregulation. A cooling effect happens as sweat evaporates. Sweat production is increased when our muscles heat up after exercise, or during hot weather. Nausea and nervousness can also increase sweat production. It follows then that sweat production is decreased by cold.
There are two types of sweat glands in the human body - Eccrine and apocrine.
Eccrine glands can be found in most places on the body. The sweat produced is a mixture of water (99%), salt and toxins and is produced during physical exertion or emotional stress.
Apocrine glands are less numerous and are restricted to the armpit, nipple and anogenital areas. Apocrine glands are attached to the hair follicle and secrete a milky, fatty fluid. Pheromones are also present in apocrine sweat and this has links to sexual attraction and arousal, and the recognition of mothers by their babies.
Body odour is produced when bacteria starts to break down apocrine sweat.
Finally, we move on to the sub-cutaneous layer. This is the deepest layer of the skin. Its function is to insulate the body and protect it- to some extent- from injury. It is also a storage place for some nutrients. It is composed of adipose tissue, which provides the fatty insulation, and areolar tissue, which is comprised of collagen and stretchy elastin, connecting the skin to tissues and muscles via the fascia.