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Keratinocyte

Keratinocytes are the predominant cell type of epidermis. They originate in the basal layer, and they are constantly pushed to the upper layers of skin, where they are eventually turned into corneocytes and are shed as dead skin.

Keratinocytes produce various compounds that enhance the functionality of the skin. They generate keratin and lamellar bodies that form epidermal water barriers. They also hold melanin given from melanocytes that protect the skin against harmful UV radiation.

Keratinocytes vary from layer to layer of the epidermis, and each of them undergoes changes while ‘traveling’ from the bottom to the uppermost layer of the epidermis.

This is a set of five images showing the transformation of keratinocyte cells throughout the different layers of the epidermis. The first image contains the transfiguration of the keratinocyte from a brick-shaped cell into a oval-shaped cell, both with a visible nucleus. The second image shows two oval-shaped keratinocytes tight together via desmosomes in the stratum spinosum. Keratin appears. The third image, when the cells are in the stratum granulosum, shows how the cells adopt a larger and more elongated shape while lamellar bodies and keratohyalin are appearing. The fourth image, in the stratum lucidum, shows how the cells have become smaller and now only contain desmosomes, keratin, and eleidin as an external structure. The last image shows how, in the stratum corneum, the cells turn into thinner cells containing only larger quantities of keratin.

Figure 1. Set of images showing transformation of keratinocyte cell throughout the layers of the epidermis.

Keratinocyte journey

Keratinocytes are created from basal cells in the stratum basale, where they receive plenty of nutrients to start assembling fibers of keratin. Upon reaching the stratum spinosum layer, keratinocytes start connecting with each other through desmosomes and produce more bundles of keratin while still getting nutrients to stay alive. In the stratum granulosum, the cells are still tightly connected through desmosomes with visible lamellar bodies, keratohyalin, and keratin molecules embedded in the cytoplasm. Lamellar bodies are responsible for releasing a lipid-rich secretion that serves as a water repellent sealant for this and upper layers of the epidermis. Keratohyalin assembles keratin intermediate filaments into keratin. The organs of the keratinocytes pushed into the stratum lucidum are deteriorated, and the cells are no longer alive. Keratyhoalin is secreted out of the cells and transformed into a transparent protein called eleidin. Eleidin tightens the connection between the cells, facilitating the protective and water-repellent function of the skin. In the most upper layer of epidermis stratum corneum, dead keratinocytes become corneocytes. These cells are filled with keratin and eventually shed off the skin. They are no longer connected to each other.