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Ref No: TN773 | Published On: 15 โDecโ 2014 | Status: Current
Clinical update: Skin
The skin is a functional system of tissues and cells that provides protection from the external environment. The skin is comprised of two main layers โ the epidermis and dermis โ with subcutaneous tissue beneath. Figure 1 โ Structure of the skin
Epidermis
The epidermis is the thin outer layer that is composed of stratified squamous epithelium. There are four different types of cells found in the epidermis:
- keratinocytes
- melanocytes
- Langerhans cells
- Merkel cells The epidermis is organised into four sublayers or strata:
- stratum basale (basal layer)
- stratum spinosum (spinous layer)
- stratum granulosum (granular layer)
- stratum corneum (keratinised or horny layer) Newly formed cells in the stratum basale move up towards the surface of the skin pushing old cells upwards. The old cells rise to the surface accumulating keratin as they move. The old cells die, flatten out
and overlap to form a tough membrane on the outer surface of the epidermis. Eventually these cells are shed off as calluses or collections of dead skin and are replaced by underlying cells that also become filled with keratin. This process is known as keratinisation and takes between two and four weeks to complete.
Dermis
The dermis, located beneath the epidermis, is considerably thicker because it is composed of connective tissue containing elastic fibres (elastin) and protein fibres (collagen). The elastin and collagen fibres give the skin pliability but are resistant to stretching. The dermis contains hair follicles, nails, sweat glands, sebaceous glands, blood vessels and nerves. The two sublayers of the dermis are:
- Papillary layer โ a thin layer of loose connective tissue that lies beneath the epidermis. It contains capillaries that nourish the epidermis.
- Reticular layer โ a dense layer of connective tissue that consists of elastin and collagen fibres. Elastin and collagen fibres give the skin pliability. Ageing, hormones and ultraviolet rays cause degeneration of elastin and collagen fibres, resulting in wrinkles and sagging of the skin.
Subcutaneous tissue
The subcutaneous tissue, also called the superficial fascia or hypodermis, is found beneath the dermis. Subcutaneous tissue consists of adipose (fat) and connective tissue and accommodates large blood vessels and nerves. Fibres in the dermis extend downwards into the subcutaneous tissue connecting the skin to it. In turn, the subcutaneous tissue connects to underlying muscles, bones and tissue.
Skin functions
The primary functions of the skin are:
- protection
- regulation of body temperature
- excretion
- detection of stimuli
- synthesis of vitamin D
- blood reservoir
Protection
The skin, as a physical barrier to the external environment, protects the body from injury, infection, loss or gain of bodily moisture and UV radiation. The skin's layers of cells provide a protective barrier to underlying body tissues and organs against abrasion and other injuries. Lipid secretions produced by the sebaceous glands assists in preventing loss and gain of bodily moisture. Sebaceous glands in the dermis secrete sebum to lubricate the hair and repel water from the skin. Protection against UV radiation is provided by melanocytes. These pigmentโforming cells located at the base of the epidermis produce melanin. Melanin absorbs UV light to protect the epidermis and dermis from the harmful effects of UV light. Exposure to ultraviolet rays stimulates the melanocytes to produce extra melanin resulting in tanned skin.
Regulation of body temperature
The skin plays a significant role in maintaining body temperature. Sweat glands and blood vessels act as temperature regulators of the skin. Changes in body temperature are detected by receptors in the skin that send nerve impulses to the brain, which respond by sending output nerve impulses back to the sweat glands and the blood vessels. Perspiration is constantly produced by sweat glands. The amount of perspiration sweat glands release is determined by changes in body temperature. An increase in body temperature causes sweat glands to produce perspiration more rapidly. A decrease in body temperature causes sweat glands to produce perspiration less rapidly. Blood vessels in the skin dilate or constrict to assist in maintaining body temperature. When body temperature rises, blood vessels dilate increasing blood flow through the skin, allowing heat to radiate into the external environment. A lowered body
SunSmart UV Alert
The SunSmart UV Alert is a tool you can use to protect yourself from ultraviolet (UV) radiation. It lets you know the time during that day you need to be SunSmart. The Alert is issued by the Bureau of Meteorology when the UV index is forecast to reach 3 or above. At that level, it can result in damage to your skin and lead to skin cancer. Figure 2 โ How to read the alert References Better Health Channel (March 2012) Skin Layers Explained Retrieved from http://www.betterhealth.vic.gov.au Brain, M (2001, April 1) How Sunburns and Sun Tans Work Retrieved from http://health.howstuffworks.com/skinโcare/beauty/sunโ care/sunscreen.htm Brannon, H (2014, October 23) Skin Anatomy Retrieved from http://dermatology.about.com/cs/skinanatomy/a/anatomy.htm Cancer Council Australia (2014, January 21) UV Alert Retrieved from http://www.cancer.org.au/preventingโcancer/sunโ protection/uvโalert/ Marieb, E (1998) Human Anatomy and Physiology California, United States of America: The Benjamin/Cummings Publishing Company Martini FH (2001) Fundamentals of Anatomy and Physiology (Fifth Ed) New Jersey: Prentice Hall MyDr (2009, September 30) Skin Biology and Structure Retrieved from http://www.mydr.com.au/skinโhair/skinโbiologyโandโ structure National Geographic Society, Skin (1996โ2014) Retrieved from http://science.nationalgeographic.com/science/healthโandโhumanโ body/humanโbody/skinโarticle/ Nozza, J and Rodda, C (2001) Vitamin D Deficiency in Mothers of Infants with Rickets. The Medical Journal of Australia, 175 (5): 253 โ 255 Retrieved from https://www.mja.com.au/journal/2001/175/5/vitaminโdโdeficiencyโmothersโinfantsโrickets Swerdlow, J (n.d.) Unmasking Skin Retrieved from http://science.nationalgeographic.com/science/healthโandโhumanโbody/humanโ body/unmaskingโskin/#page= Tortora GJ and Grabowski SR (1993) Principles of Anatomy and Physiology New York: Harper Collins
