Epidermis

Skin, is human largest living organ, unlike nails and hair.

Skin is composed of three primary layers: epidermis and dermis. Each layer possesses specific characteristics and functions. Let's take a closer look into each layer so you understand your skin mechanisms.

EPIDERMIS

This is your outermost superficial layer of the skin. It is very important from a cosmetic standpoint because this layer gives the skin it's texture and moisture, and contributes to skin colour. Knowledge of the basic skin structure of the epidermis, best enables for you to improve the appearance of your skin.

Cells of the epidermis are keratinocyte, melanocyte, merkel and langerhans. Keratinocytes are the most abundant cells in the epidermis and understanding the maturation process of the keratinocytes (also known as the cell cycle) will help you to start doing prevention. The fact is many people only start focusing on the skin once there is an abnormality or at least a perceived problem. The skin is not only the largest living organ but also our first line defence from both internal and external aggressions (Matsui, 2023).

Epithelial tissue within the epidermis has a free surface (on the top) and a basal surface (on the bottom) which sits on a basement membrane and often sits on top of a connective tissue (Raj et al., 2023). Epithelial tissue is avascular meaning does not contain blood vessels, therefore, nutrients such as oxygen must diffuse through the basement membrane from capillaries in the underlying connective tissue . Other characteristics of the epithelial tissue is high rate of regeneration.

In the epidermis, there are 4 sub-layers on thin skin (mostly everywhere in the body) and 5 sublayers on thick skin (only on the palm of your hand and the sole of your feet). These sub-layers of the epidermis from the bottom to the top layers are:

Stratum Basale (Basal Layer)

The basal layer consist only ONE SINGLE LAYER and it has important role in cell cycle where the basal cells are responsible for maintaining the epidermis by continuously renewing the cell population. In the basal layer can be found keratinocytes, melanocytes and merkel cells, all from the stem cells. Keratinocytes start in the basal layer and move upwards - 'pushed' by the newborn keratinocytes.

The stratum basale (also called the stratum germinativum) is the deepest epidermal layer and attaches the epidermis to the basal lamina, below which lie the layers of the dermis. The cells in the stratum basale bond to the dermis via intertwining collagen fibers, referred to as the basement membrane. A finger-like projection, or fold, known as the dermal papilla (plural = dermal papillae) is found in the superficial portion of the dermis. Dermal papillae increase the strength of the connection between the epidermis and dermis; the greater the folding, the stronger the connections made.

As new cells are formed, the existing cells are pushed upward superficially away from the stratum basale. Two other cell types are found dispersed among the basal cells in the stratum basale. The first is Merkel cells, which functions as receptors and is responsible for stimulating sensory nerves that the brain perceives as touch. These cells are especially abundant on the surfaces of the hands and feet. The second is a melanocytes, cells that produces the pigment melanin (Bento-Lopes et al., 2023). Melanin gives hair and skin its color, and also helps protect the living cells of the epidermis from ultraviolet (UV) radiation damage (Castellano-Pellicena et al., 2021).

In a growing fetus, fingerprints form where the cells of the stratum basale meet the papillae of the underlying dermal layer (papillary layer), resulting in the formation of the ridges on your fingers that you recognize as fingerprints. Fingerprints are unique to each individual and are used for forensic analyses because the patterns do not change with the growth and aging processes.

Stratum Spinosum

As the name suggests, the stratum spinosum is spiny in appearance due to the protruding cell processes that join the cells via protein structure called desmosome. The desmosomes interlock with each other and strengthen the bond between the cells. It is interesting to note that the “spiny” nature of this layer is an artifact of the staining process . Unstained epidermis samples do not exhibit this characteristic appearance. The stratum spinosum is composed of eight to ten layers of keratinocytes, formed as a result of cell division in the stratum basale. Interspersed among the keratinocytes of this layer is a type of dendritic cell called the Langerhans cell (Zhou et al., 2022), which functions as a macrophage by engulfing bacteria, foreign particles, and damaged cells that occur in this layer (Clayton et al., 2017).

Stratum Granulosum

The stratum granulosum has a grainy appearance due to further changes to the keratinocytes as they are pushed from the stratum spinosum. The cells (three to five layers) become flatter, their cell membranes thicken, and they generate large amounts of the proteins keratin, which is fibrous, and keratohyalin (Freeman & Sonthalia, 2023), which accumulates as lamellar granules within the cells. These two proteins make up the bulk of the keratinocyte mass in the stratum granulosum and give the layer its grainy appearance. The nuclei and other cell organelles disintegrate as the cells die, leaving behind the keratin, keratohyalin, and cell membranes that will form the stratum lucidum, the stratum corneum, and the accessory structures of hair and nails.

Stratum Lucidum

The stratum lucidum is a smooth, seemingly translucent layer of the epidermis located just above the stratum granulosum and below the stratum corneum. This thin layer of cells is found only in the thick skin of the palms, soles, and digits. The keratinocytes that compose the stratum lucidum are dead and flattened. These cells are densely packed with eleiden, a clear protein rich in lipids, derived from keratohyalin, which gives these cells their transparent (i.e., lucid) appearance and provides a barrier to water.

Stratum Corneum

The stratum corneum is the outermost superficial layer of the epidermis we can see with naked eyes and is the layer exposed to the outside environment. The increased keratinization (also called cornification) of the cells in this layer gives it its name. There are usually 30 to 70 layers of dead cells in the stratum corneum (Pham et al., 2023). These dry and compacted dead layer helps prevent the penetration of microbes and prevent dehydration of underlying tissues, and provides a mechanical protection against abrasion for the more delicate to the underlying layers (Ohno et al., 2023). Cells in this layer are shed periodically and are replaced by cells pushed up from the stratum granulosum underneath (or stratum lucidum in the case of the palms and soles of feet). The entire layer is replaced during a period of about 4 to 7 weeks. Cosmetic skin resurfacing procedures such as microdermabrasion or epiblading, help remove some of the dry, upper layer and aim to keep the skin looking “fresh” and healthy.

Although stratum corneum compacted with dead skin cells, it still play important role as the first line of defense (Rajkumar et al., 2023). It is crucial to keep this layer well hydrated otherwise the skin barrier function will not work properly and can lead to skin conditions such as dry skin, sensitised to any products such as stinging sensation (de Szalay & Wertz, 2023). Therefore, to have the ability to respond well to any treatments, the health of stratum corneum is paramount (Goleva et al., 2019).

REFERENCES

Bento-Lopes, L., Cabaço, L. C., Charneca, J., Neto, M. V., Seabra, M. C., & Barral, D. C. (2023). Melanin's Journey from Melanocytes to Keratinocytes: Uncovering the Molecular Mechanisms of Melanin Transfer and Processing. International journal of molecular sciences, 24(14), 11289. https://doi.org/10.3390/ijms241411289

Castellano-Pellicena, I., Morrison, C. G., Bell, M., O'Connor, C., & Tobin, D. J. (2021). Melanin Distribution in Human Skin: Influence of Cytoskeletal, Polarity, and Centrosome-Related Machinery of Stratum basale Keratinocytes. International journal of molecular sciences, 22(6), 3143. https://doi.org/10.3390/ijms22063143

Clayton, K., Vallejo, A. F., Davies, J., Sirvent, S., & Polak, M. E. (2017). Langerhans Cells-Programmed by the Epidermis. Frontiers in immunology, 8, 1676. https://doi.org/10.3389/fimmu.2017.01676

de Szalay, S., & Wertz, P. (2023). Protective Barriers Provided by the Epidermis. International journal of molecular sciences, 24(4), 3145. doi:https://doi.org/10.3390/ijms24043145

Freeman, S. C., & Sonthalia, S. (2023). Histology, Keratohyalin Granules. In StatPearls. StatPearls Publishing.

Goleva, E., Berdyshev, E., & Leung, D. Y. (2019). Epithelial barrier repair and prevention of allergy. The Journal of clinical investigation, 129(4), 1463–1474. https://doi.org/10.1172/JCI124608

Matsui T. (2023). Epidermal Barrier Development via Corneoptosis: A Unique Form of Cell Death in Stratum Granulosum Cells. Journal of developmental biology, 11(4), 43. https://doi.org/10.3390/jdb11040043

Ohno, Y., Nakamura, T., Iwasaki, T., Katsuyama, A., Ichikawa, S., & Kihara, A. (2023). Determining the structure of protein-bound ceramides, essential lipids for skin barrier function. iScience, 26(11), 108248. https://doi.org/10.1016/j.isci.2023.108248

Pham, Q. D., Biatry, B., Grégoire, S., Topgaard, D., & Sparr, E. (2023). Solubility of Foreign Molecules in Stratum Corneum Brick and Mortar Structure. Langmuir : the ACS journal of surfaces and colloids, 39(6), 2347–2357. https://doi.org/10.1021/acs.langmuir.2c03092

Raja, E., Clarin, M. T. R. D. C., & Yanagisawa, H. (2023). Matricellular Proteins in the Homeostasis, Regeneration, and Aging of Skin. International journal of molecular sciences, 24(18), 14274. https://doi.org/10.3390/ijms241814274

Rajkumar, J., Chandan, N., Lio, P., & Shi, V. (2023). The Skin Barrier and Moisturization: Function, Disruption, and Mechanisms of Repair. Skin pharmacology and physiology, 36(4), 174–185. https://doi.org/10.1159/000534136

Zhou, L., Jiang, A., Veenstra, J., Ozog, D. M., & Mi, Q. S. (2022). The Roles of Skin Langerhans Cells in Immune Tolerance and Cancer Immunity. Vaccines, 10(9), 1380. https://doi.org/10.3390/vaccines10091380