“Your epidermis is showing” said 10 year old me to a boy at recess. Oh, young me thought I was so witty…
The epidermis is the skin that covers the entire exterior surface of the human body and is the primary site of interface with the surrounding world. It functions as a protective barrier that prevents internal tissues from exposure to trauma, toxins, temperature extremes, ultraviolet (UV) radiation, and bacteria. The skin is made up of three layers; Epidermis, the Dermis, and the Subcutaneous Layer.
We will be discussing each layer of the skin in this months skin series. Today we are starting from the top, literally..
Your epidermis is a fairly thin, tough, outer layer of the skin. The majority of the cells in the epidermis are keratinocytes. Keratinocytes originate from cells in the deepest layer of the epidermis, known as the basal layer. New keratinocytes slowly migrate up toward the surface of the epidermis. Once the keratinocytes reach the skin surface, they are gradually shed and replaced by newer cells pushed up from below.
There are five cell layers of the epidermis:
Stratum Corneum: The outermost portion of the epidermis, known as the stratum corneum, is moderately waterproof, and when unharmed, prevents most bacteria, viruses, and other foreign substances from entering the body. This layer consists of fully mature keratinocytes, which contain fibrous proteins (keratins) that continually shed.
Stratum Lucidum: Can only be found in those regions of the body where the skin is particularly thick, such as the palms or the soles.
Stratum Granulosum: Keratinocytes from the squamous layer are pushed up through the layers of the stratum lucidum and stratum granulosum. As these cells move further towards the surface of the skin, they get bigger and flatter and adhere together, eventually becoming dehydrated and die.
Stratum Spinosoum: Otherwise known as the “spiny layer” due to the fact that the cells are held together with spiny projections. Within this layer there are basal cells that have been pushed upward, however these maturing cells are now called squamous cells, or keratinocytes. Keratinocytes produce keratin, a tough, protective protein that makes up the majority of the structure of the skin, hair, and nails.
Stratum Basale: The basal layer is the deepest layer of the epidermis, and contains basal cells. This layer of skin cells is 1-2 layers thick. The basal cells continually divide, forming new keratinocytes that replace the cells that shed from the stratum corneum.
Stay tuned, we are going deeper next week: Skin Anatomy: Dermis