The Epidermis

The epidermis is the innermost layer of the skin. It contains column-shaped basal cells that constantly divide and are pushed to the surface of the skin. This layer also contains Merkel cells that send pain, pressure, and temperature sensations to the brain for interpretation.


Keratinocytes are responsible for forming the epidermal water barrier and regulating calcium absorption by activating cholesterol precursors to form vitamin D. They also secrete a protective waxy coating called cuticle.

Stratum basale

The epidermis is a layer of flat, hard, tightly packed cells found everywhere on the body. The cells are composed of keratin, a protein rich in lipids. The keratin provides a waterproof barrier that protects the underlying layers of the skin. The keratin cells are shed periodically and replaced by cells moving up from lower levels of the epidermis.

The innermost layer of the epidermis is called the stratum basale. This layer contains stem cells that are responsible for producing all the other layers of the epidermis. Eventually, these stem cells mature into keratinocytes that are cuboidal or columnar in shape. These cells sit on the basement membrane and are bound to it by desmosomes and hemidesmosomes. They secrete a waxy substance called hyaluronate, which helps to maintain hydration in the skin.

This layer is followed by the granule cell layer (stratum granulosum) and then the stratum corneum. In thick skin, a fifth layer, the stratum lucidum, can sometimes be seen, but is difficult to recognise in routine histological sections. The keratinocytes in this layer are dead and flat, but remain translucent due to the presence of a hydrophobic lipid-rich secretory product called eleiden. The keratinocytes are also surrounded by a layer of keratinous acids, which prevents the penetration of water into the cells.

The granule cells in the stratum granulosum contain small organelles known as melanosomes, which are filled with dark brown pigment granules. The melanosomes absorb UV light that would otherwise damage the underlying layers of the skin. This is a very important function of the stratum basale, and it protects the skin from both UVA and UVB radiation.

Stratum granulosum

The cells in the bottom layer of the epidermis are constantly dividing and moving up toward the surface. This process is called keratinization, which creates the protective barrier that protects the body from harmful substances. Cells in the bottom layer (stratum basale or germinativum) fill with keratin, which gives the skin a rough texture. They also secrete defensins, which are part of the body’s first immune defenses.

The next layer is the stratum spinosum, which has a more granular appearance. As the keratinocytes in this layer are pushed upward, they start to change their shape and become flattened and diamond shaped. They also start to produce a protein called keratohyalin, which helps aggregate the keratin filaments and promotes keratinization. They also accumulate lamellar bodies, which are lipids that help the keratinocytes form a lipid barrier. The granular cells in this layer eventually transform into cornified cells (i.e., a horny layer of dead keratinocytes).

The next layer is the stratum granulosum, which is characterized by a spiky appearance. The spiky appearance comes from the prominent cell-to-cell junctions called desmosomes, which look like spiky membrane projections on histology slides. These proteins anchor the keratinocytes to their neighbors and give the cells significant structural integrity. The keratinocytes in this layer can also synthesise tonofilaments, which are thick tufts of intermediate filaments. These tonofilaments have a fibrous texture and help aggregate keratin filaments.

Stratum spinosum

Stratum spinosum is the thickest layer of the epidermis and also contains a small number of blood vessels. It is also home to touch receptors called Meissner corpuscles. This layer of skin is primarily made of loose connective tissue. The basal cells in this layer are cuboidal in shape and act as a precursor to the keratinocytes that make up the epidermis. These keratinocytes are the building blocks of hair, nails and skin, and they produce a protective protein called keratin. In addition, they are a source of Vitamin D when the body is exposed to sunlight. Molecules of the lipid compound 7-dehydrocholesterol are found in the epidermis and convert to the active form of Vitamin D, calcitriol.

The keratinocytes in the stratum spinosum are polyhedral with large pale-staining nuclei and are known as prickle cells. These cells are surrounded by an extracellular lipid membrane that is derived from the plasma membrane and is composed of cross-linking proteins. This lipid membrane is hydrophobic, and it prevents water from entering the cell.

The stratum spinosum layer lies between the stratum basale layer and the stratum granulosum. It is a spinous layer and consists of keratinocytes held together by sticky proteins called desmosomes. This layer helps to make your skin flexible and strong. It also contains melanocytes (mel-ann-o-site) stem cells that produce the protein keratin. This helps to form hair, nails and the outer layer of your skin, which protects you from harsh environments.

Stratum squamous

The epidermis of most skin (thin skin) is a keratinized stratified squamous epithelium that contains four or five layers, depending on the body region. It has no blood vessels within it and is avascular. The cells of this layer are arranged in a flattened pattern, called orthokeratosis, and they are lined with a tough, hydrophobic layer of lipids. This enables them to repel water, making the epidermis an effective barrier against invasion by microorganisms.

The outermost cell layer of the epidermis is the stratum corneum, also known as the squamous cell layer, or keratinized squamous epithelium. This layer is composed of dead, keratinized cells that are known as squames and are filled with densely packed keratin. The squames have an insoluble, hydrophobic membrane and contain a high concentration of keratin. These properties enable the squames to prevent microbe penetration, protect underlying cells from physical insults, and prevent water loss. The cells of this layer are shed periodically by a process called desquamation. They are replaced by squames that have been pushed up from the stratum granulosum or, in the case of the palms and soles, the stratum spinosum.

Besides providing mechanical protection, the epidermis is also important in wound healing. It is highly reactive to injury, with basal cells proliferating rapidly and migrating upward to replace damaged cells. The basal layer is also reinforced by cytoplasmic fibers called tonofilaments, and it is attached to the basement membrane by hemidesmosomes.