Anatomy of the Epidermis

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Epidermis Anatomy - Keratinocyte Maturation

The skin is made up of three layers:

  • the epidermis
  • the dermis
  • the subcutaneous tissue

The outermost layer of the skin is the epidermis. Within the epidermis, there is four major layers of keratinocytes (the cells that provide structure to the epidermis) and one layer that is present only in certain parts of the body. Keratinocytes within the epidermis begin dividing in the bottom layer of the epidermis, pushing already formed cells into higher layers. As cells move into the higher layers, they flatten and eventually die.

Let's take a closer look at the characteristics of each of these layers within the epidermis.

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Stratum Basale

The bottom layer of the epidermis is called the stratum basale. This layer contains one row of keratinocytes that are shaped liked columns called basal cells. These basal cells are constantly dividing and pushing already formed cells into higher layers towards the skin's surface. As cells move into the higher layers, they flatten, eventually die, and are shed.

Melanocytes, which are cells that produce melanin (a pigment produced in your skin that gives it its color) are also found in this layer.

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Stratum Spinosum

The spinosum layer lies over the stratum basale and is about five to ten cells thick. Cells that move into the spinosum layer of the epidermis (also called prickle cell layer or squamous cell layer) change from being columnar to polygonal. This means that the shape of the cells morphs from a column-like shape into a polygon-like shape.

In this layer, the cells start to make keratin—a protein that is found in your skin, hair, and nails and serves a protective role. 

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Stratum Granulosum

The cells in the stratum granulosum, or granular layer, have lost their nuclei and now appear as flattened cells containing dark clumps of cytoplasmic material. There is a lot of activity in this layer as keratin proteins and water-proofing lipids are being produced and organized.

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Stratum Lucidum

The stratum lucidum layer is only present in thick skin (the palms of the hands and soles of the feet) where it helps reduce friction and shear forces between the stratum corneum and stratum granulosum.

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Stratum Corneum

The cells in the stratum corneum layer are known as corneocytes or horny cells. These cells have flattened out, are considered dead, and are composed mainly of keratin protein, which provides strength to the layer but also allows the absorption of water. 

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A Close-Up of the Stratum Corneum

The structure of the stratum corneum layer looks simple, but this layer is responsible for maintaining the integrity and hydration of the skin—a very important function. There are actually complex processes that are at work in the stratum corneum and minimal disruptions of any of these processes can cause a variety of skin problems.

Source:

Kolarsick P, Kolarsick MA, Goodwin C. Anatomy and Physiology of the Skin. Journal of the Dermatology Nurses' Association. 2011. 3(4):203-13.

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