Skin - Epidermis
Epidermis
The epidermis is the outermost layer of the skin and its main function is to protect the body from the external environment. It is made up of five sublayers that work together to rebuild the surface of the skin, because it renews itself every 28 days. The skin constantly sheds dead cells, which may amount to 30,000 to 40,000 cells per minute.
a. The Basal Layer of the skin (Stratum Basale)
This is the inner most layer of the epidermis and contains small, round cells, which undergo mitosis and continuously divide into new cells. These cells push the older, cells to the surface of the skin, where they are eventually shed.
This layer is also called Stratum germinatiivum as it constantly regenerates to produce fresh skin cells. A mutation or disruption in the normal division of basal cells can result in rapid multiplication and accumulation of these cells resulting in Basal Cell Carcinoma. An estimated 4.3 million cases of BCC are diagnosed in the U.S. each year
Melanocytes are found in the basal layer, which produce a pigment called melanin. In addition to providing the skin with its colour, melanin helps in skin protection when exposed to the harmful UV rays from the sun. The most common evidence of this is the development of a tan, when exposed to sunlight for extended periods of time, where melanin production is increased transiently, in response. If melanocytes aggregate together to form a patch, it may result in visible birthmarks, freckles or age spots on the skin.
If melanocytes undergo a malignant transformation, that is, if they undergo a change from normal benign cells to atypical, neoplastic cells, with hyperplasia and hypertrophy, they form melanomas, which is a type of skin cancer. This may occur on the surface of the skin, in the irises of the eyes and very rarely inside the nose or throat. The transformation is largely attributed to prolonged exposure to UV radiation and can be treated successfully if caught early. In the past decade (2011 – 2021), the number of new invasive melanoma cases diagnosed yearly, showed an increase of 44 %.
The basal layer is also home to the Merkel cells, nerve cells that function as sensory receptors and are in contact with sensory neurons in the dermis. It is assumed that its function is mechanoreception or tactile (touch) perception. They are neuro-endocrine cells and contain markers like chromogranin A and neuropeptides, which function and neurotransmitters that also help with the differentiation and growth of cutaneous cells.
b. The Squamous cell layer (Stratum Spinosum)
This is the layer directly above the basal layer and is also called the “spiny layer” because the cells here are held together by spine like projections called desmosomes. These maturing cells that are pushed upwards from the basal layer are called Keratinocytes or Squamous cells. They produce keratin, a tough protein that constitutes a large part of the skin, hair and nails and is responsible for protection. Keratin releases a glycolipid that helps to prevent water loss from the body, adding to its water resistant properties. Keratinocytes regulate the production and metabolism of vitamin D3, after exposure to adequate amount of sunlight. Vitamin D3 is an essential hormone responsible for various body functions including the maintenance of calcium levels, bone strength, immunity and hair health.
The squamous layer is the thickest layer in the epidermis. It contains the Langerhans cells which function as immune- protective macrophages, engulfing foreign materials, other damaged or dead cells or invading bacteria.
An overgrowth of the keratinocytes may lead to Bowen’s disease, which is the earliest form of squamous cell carcinomas. Squamous cell carcinoma (SCC) is the second most common form of skin cancer. More than 1 million cases of SCC are diagnosed in the U.S. each year.
c. Stratum Granulosum
The next layer of skin cells have a grainy appearance, as the keratinocytes that are pushed upwards slowly disintegrate through dehydration, losing their nuclei and organelles, while leaving behind keratin and keratohyalin proteins, which accumulates as lamellar granules. These contribute to the water sealing properties of the skin. The cells are usually three to five layers deep and are flattened.
d. Stratum Lucidum
This is a thin, smooth, transparent layer that is usually found in areas with thick skin, like the soles of the feet or fingertips. These cells are densely packed and held together by a derivative of keratohyalin protein called eleidin, which acts as a barrier to water and gives rise to its transparent or lucid properties.
e. Stratum Corneum
As the most superficial layer of the epidermis, the stratum corneum is exposed to the outside environment, giving rise to increase keratinization or cornification. It is sometimes called the horny layer because its cells are toughened like the horns of an animal. The thickness of the stratum corneum varies throughout the body. In the palms of the hands and the soles of the feet, knees, elbows, knuckles, which is stabilized by the stratum lucidum as well.
The cells in this area, which is usually 15-30 layers thick, are called keratinised squames or squamous cells, embedded in a lipid matrix composed of ceramides and fatty acids. They are hexagonal, flattened scales of dead cells, which flake off or shed regularly to reveal the underlying layer, helping in skin regeneration. The entire layer can be replaced in a period of around 4 weeks in young adults, but the process slows down with age, taking 45-50 days in the elderly.
This layer of keratinised or cornified cells helps to prevent microbes from penetrating the skin to access underlying tissues, maintains water flux and hydration, initiates protective inflammation through cytokine activation and dendritic cell activity. The tough stratum also acts as mechanical barrier that prevents the abrasion of the delicate inner layers and reduces the permeability of the skin to toxins, irritants or allergens. As the dead squames flake off, they add to the dust found in homes or surroundings.
If the layers of the stratum corneum increase, it is referred to as hyperkeratosis and is seen in various skin conditions. It is believed that prolonged exposure to the sun or radiation, causes increased keratinization and may lead to the formation of cutaneous horns, which are spine like, curved and hard yellow brown projections from the surface of the skin, resembling animal horns. Giant horns are also known to occur. The largest described is a 76-year-old Parisian woman named Madame Dimanche (Widow Sunday) in the 19th century, who grew a horn from her forehead that was nearly 25-cm long.
Microdermabrasion (MDA) is a minimally invasive epidermal resurfacing procedure used to treat uneven skin tone or texture and acne scars. gentle mechanical abrasion is applied to the skin, which ultimately removes the stratum corneum layer, revealing new epidermis as part of the healing process.
References
1. Yousef H, Alhajj M, Sharma S. Anatomy, Skin (Integument), Epidermis. [Updated 2020 Jul 27]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2021 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK470464/
2. https://www.everydayhealth.com/news/10-amazing-facts-about-skin/
3. Masland et al. The Senses: A comprehensive Reference, 2008
4. https://open.oregonstate.education/aandp/chapter/5-1-layers-of-the-skin/#navigation
5. https://www.skincancer.org/skin-cancer-information/skin-cancer-facts/#nonmelanoma
7. Del Rosso, James Q.; Levin, Jacqueline (2011). "The Clinical Relevance of Maintaining the Functional Integrity of the Stratum Corneum in both Healthy and Disease-affected Skin". The Journal of Clinical and Aesthetic Dermatology. 4 (9): 22–42. ISSN 1941-2789. PMC 3175800. PMID 21938268.
8. https://dermnetnz.org/topics/cutaneous-horn/
9. Shah M, Crane JS. Microdermabrasion. [Updated 2020 May 4]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2021 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK535383/