Untitled Flashcards Set

1. The Skin

The skin is a complex arrangement of structures with a range of different, but important, functions. The skin is composed of two main layers, the epidermis and dermis. The subcutaneous layer is found beneath the dermis and is not considered part of the skin.

HCPs need to be knowledgeable about what is considered to be healthy skin. This is important, as the condition of the skin may often be a sign of underlying disease. Changes in the skin may be one of the first indicators of an underlying health problem. An example is cyanosis where, because of poor delivery of oxygen to the tissues, the patient’s skin appears blue.

2. Structure of the skin

The skin is the largest organ in the body, accounting for approximately 16% of the total body weight of an adult. The skin weighs twice as much as the brain, approximately 3-5kg. Skin varies in thickness according to function and area of the body. On the eyelids, the skin is only 0.5mm thick, whereas it can be as much as 3-4mm thick on the soles of the feet. Skin is generally 1-2mm thick. The skin consists of thick outer layers, a widespread system of sweat glands sensitive to temperature changes and an extensive layer of fatty tissue under the surface of the skin. The skin also contains many cells that are sensitive to touch, pain, pressure, itching and temperature. 



3. Epidermis

The epidermis is composed of stratified keratinised squamous epithelium and is made up of many layers near the surface of the skin. The epidermis contains four main types of cells, most of which are keratinocytes, which make up 90% of the cells found in this layer. Melanocytes make up 8% of epidermal cells and are responsible for producing the pigment, melanin. Langerhans cells and Merkel cells are also found within the epidermis. Langerhans cells are involved in the immune response and Merkel cells function in the sensation of touch. 

The epidermis is avascular (without blood vessels) and is dependent on blood vessels of the dermis for oxygenation, metabolite provision and removal of metabolic waste products. The epidermis is made up of a number of layers, including: 

  • Stratum basale, the deepest layer (also called the stratum germinativum).

  • Stratum spinosum or prickle cell layer.

  • Stratum granulosum or granular layer.

  • Stratum lucidum (finger tips, palms and soles).

  • Stratum corneum, which is the top layer.

These layers represent the different stages of maturation of the cells and their movement from the stratum basale up to the stratum corneum, where they are shed. The epidermis renews itself through cell division in its deepest layer.


3.1. Stratum basale

The stratum basale is made up of a single row of columnar keratinocytes. Other types of cells found within this layer include melanocytes and Merkel cells.

The stratum basale is the only layer within the epidermis that consists of cells capable of division. Keratinocytes in the stratum basale undergo mitosis and two daughter cells are produced; one remains in the stratum basale, while the other migrates up through the other layers to the surface of the epidermis. This process takes approximately 28 days in an average epidermis of 0.1mm thickness.

The stratum basale is the nearest layer to the dermis and is located under the epidermis. The dermis contains a blood supply and provides nourishment to the stratum basale. However, as the daughter cells move further away from the stratum basale, they receive less nutrition. As a result the cells die. In addition, the cells become more keratinised – they accumulate more keratin, which is a fibrous protein involved in protecting the skin from heat, chemicals and microorganisms.

In healthy skin, a balance between the formation of new keratinocytes in the stratum basale and the shedding of dead keratinocytes from the stratum corneum is maintained.

The stratum basale also contains melanocytes, which produce melanin. Melanocytes account for one in every six cells in the stratum basale. Melanin is a pigment that protects the skin from the harmful effects of ultraviolet (UV) light. Melanocytes have long, slender projections that contain melanin granules. The projections extend between the keratinocytes and transfer the granules to them.

Once inside the keratinocytes, the granules gather to form a protective covering around the nucleus, protecting the keratinocytes from damage caused by UV light.

The colour of the skin is genetically determined and environmentally modified by UV light exposure. Skin colour is related to the amount of melanin in epidermal cells. The more melanin produced, the darker the skin. Nurses should be able to advise patients of the importance of avoiding too much UV light, whether from natural sunlight or sunbeds. They should also inform patients of the need to cover their skin when outdoors and to use appropriate UV filter creams, commonly known as sun blocks. 

Merkel cells are also found in the stratum basale scattered among the keratinocytes. Merkel cells make contact with the flattened process of a sensory neuron called a Merkel disc. Merkel cells and their discs detect the sensation of touch.