The epidermis is the outermost layer of the skin that is visible to the eye. It is made up of stratified squamous epithelial cells.
The epidermis and the dermis together form what is called as the cutis. The adjective cutaneous is used to describe anything connected to the epidermis and dermis.
The epidermis of the human skin comprises 4 to 5 layers, each with its own distinct function. At the cellular level, it is made up of four different types of cells, each again with its own role to play.
The epidermis acts as a protective and waterproofing layer of the body. It also helps the body to regulate body temperature. It contains no blood vessels and derives its nourishment from the blood vessels, which are present in the uppermost layer of the dermis.
The epidermis thickness varies from 0.3 mm (over the eyelids) to 1.5 mm (over the palms and soles of the feet). All over the body, it is separated from the dermis, the skin’s lower layer, by a basement membrane.
5 Layers of the epidermis
The structure of the epidermis of the skin is made up of 4 to 5 layers, depending on the region of the skin on the body.
For example, thick skin, found on the palms of the hands and soles of the feet, consists of five layers while thin skin found on most of the body consists of only four layers. Below is a list of all five layers:
The epidermal layers are explained starting from below upwards.
Stratum basale (germinal layer)
This lowermost layer of the epidermis consists of cuboidal or columnar cells. Epidermal cells are continuously being produced in this layer.
The newly formed cells in this layer push the previously formed cells upwards to the upper layer. It is called the germinal layer because it is germinating new cells.
The basal layer contains melanocytes, which are cells that produce melanin. Melanin is a pigment that gives the skin its brown color.
The basal layer also contains the Merkel cells, which are tactile cells originating from the ectoderm.
This epidermal layer is situated between the stratum basale and stratum granulosum. It contains eight to ten layers of prickle-shaped or polyhedral cells connected by desmosomes.
This layer is also referred to as “spinous” or “prickle-cell layer”. Keratinization of cells begins in this layer. These cells are also active in mitosis.
The main function of the stratum spinosum is to allow keratinocytes (cells that produce keratin) to mature.
This layer consists of two to four layers of cells connected by desmosomes. The cells of this layer are well defined and contain a nucleus. The cells contain granules and keratohyalin, which are later converted into keratin.
The granules contain a rich secretion, which is rich in lipids and acts as a waterproofing agent. In this layer, the keratinocytes are flatter and more irregular in shape than those in the stratum spinosum.
The stratum lucidum (Latin for the clear layer) of the epidermis contains flat dead cells without any definite outline and they have no nuclei. It is so named because of its translucent appearance under the microscope.
This layer is found only in the thick skin of the palms of the hands and the soles of the feet. The function of the stratum lucidum is to protect the skin in areas most vulnerable to friction and injury such as the palms of the hands and the soles of the feet.
The stratum lucidum is also responsible for the flexibility of the skin and its ability to extend.
The stratum corneum is the outermost layer of the epidermis made up of 10 to 30 thin layers of dead keratinocytes that are constantly being shed.
It is called a cornified layer or horny layer and consists of dead flattened epidermal cells filled with keratin corneocytes. They are continuously cast off.
The keratinocytes are called corneocytes when they arrive on this outermost layer. Keratin is an insoluble protein, which helps to protect the body from outside factors like heat, chemicals, light, and bacteria/viruses. Corneocytes are flattened cells without a nucleus.
As the outermost cells age, wear down and are shed off, they are replaced by new layers of cells.
Complete turnover of the cells takes 28 to 30 days in young adults, while the same process takes 45 to 50 days in the elderly.
Types of epidermis cells and their function
The epidermis consists of stratified squamous epithelium with four different types of cells.
In the epidermis, the skin cells are formed by a process called mitosis. The cells form in the lowermost epidermal layer.
They change their shape and composition when moving from below upwards and accordingly mature, differentiate and become keratinized (filled with a protein called keratin).
These keratinized skin cells are called keratinocytes. This process of moving from below upwards takes about 4 weeks to 6 weeks.
As the cells in the lower layers start moving upwards, the cells on the uppermost layer (stratum corneum) continue to die. What is visible to the eye are in fact the dead cells of the outermost layer of the epidermis.
These are strong cells and stick around for some time before they flake off. This flaking is not visible though you are losing about 50,000 dead skin cells every minute. Simultaneously, new cells are produced to replace the dead cells.
5% of the cells of the epidermis are engaged in producing melanin, which gives the skin its color. These cells are called melanocytes. The more melanin in your epidermal cells, the darker is the color of your skin.
Darkening of the skin by exposure to the sun is due to more melanin being produced by the melanocytes to protect the body from the ultraviolet effect of the sun rays.
The epidermis consists of four specialized types of skin cells. They are called epidermal cells.
Keratinocytes constitute 95% of the epidermal cells and are responsible for the synthesis of keratin, a protein that hardens and waterproofs the skin. They originate in the deepest layer of the epidermis and progress upwards in various stages towards the outer layer of the epidermis. Here, they are shed and replaced by new maturing cells.
Melanocytes produce melanin, the pigment, which gives color to the skin of the individual. They are present in the lowermost layer of the epidermis. They protect the body from the ultraviolet radiation of the sunlight that can cause damage to the DNA of the skin cells. Melanin synthesized in the melanocytes is transferred to the keratinocytes.
Langerhans cells in the epidermis are in fact phagocytic macrophages that are defensive in nature. They alert the immune system of any invasive attack by infectious agents like the virus or bacteria. They are believed to be responsible for the development of skin allergies.
Merkel cells are found deep in the epidermis at the epidermal-dermal border. Their function is not yet known but they are believed to be mechanoreceptors involved with the sensation of touch.
The function and role of Epidermis
The functions of the epidermis are quite a few considering its anatomical position.
- Being the outermost covering of the skin and body, the epidermis acts as a physical and chemical barrier, protecting the body from external factors of the environment such as trauma, viruses, bacteria, chemicals, heat, ultraviolet rays, and light.
- The epidermis helps to give color to the skin thus contributing to a person’s appearance. This is contributed by the melanocytes, which contain a brown pigment called melanin.
- The skin helps to preserve necessary ingredients like moisture, and nutrients such as minerals, vitamins, and proteins.
- New cell formation, as explained above is also an important function of the epidermis.
- The epidermis harbors the Langerhans cells, which are defensive in nature and help in a person’s immunity by alerting the immune system of the body in case of an attack by viruses and bacteria.
- The epidermis contains the Merkel cells, which are believed to be mechanoreceptors involved with the sensation of touch.
- The epidermis acts as a waterproof coat preventing water from entering the insides of the body.
- The epidermis along with the dermis contribute to the physiology of the sweating process, which helps to maintain body temperature and excretion of the waste.