Keratinocytes are a type of epithelial cell and represent the vast majority of cells found in the epidermis, the outermost layer of the skin. This classification is based on their structure and location, aligning perfectly with the biological definition of epithelial tissue. As the primary building blocks of the skin’s surface, these cells generate the tough, protective layer that interfaces with the external environment. They are constantly undergoing a life cycle that results in the continuous renewal of the body’s largest organ.
Understanding Epithelial Tissue
Epithelial tissue is one of the four fundamental tissue types in the body, characterized by cells that are tightly packed together with minimal extracellular space. These cells form continuous sheets that cover body surfaces, line internal cavities, and constitute glandular tissue. A defining structural feature is that all epithelial tissue rests upon a basement membrane, a specialized layer that anchors the cells to the underlying connective tissue.
Epithelial tissues are unique because they are avascular, meaning they lack direct blood vessels and must receive nutrients through diffusion from the underlying tissue. Their functions are broad, encompassing protection from physical abrasion, selective absorption of substances, and secretion of materials. Based on the number of cell layers and the shape of the cells, epithelial tissue is classified into various types, such as simple or stratified, and squamous, cuboidal, or columnar.
Keratinocytes: Identity and Location
Keratinocytes are the most abundant cell type in the epidermis, making up approximately 90% of the cells in this layer of the skin. They are named for the specialized protein they produce in large quantities, called keratin, which is a tough, fibrous structural protein. The epidermis itself is classified as stratified squamous epithelium, a multi-layered sheet of flattened cells.
These cells originate from stem cells located in the deepest layer of the epidermis, known as the stratum basale. Here, they are columnar in shape and are firmly attached to the basement membrane, fulfilling the epithelial requirement of being anchored to this underlying support structure. As they divide and mature, they migrate upward through the different epidermal layers, constantly pushing older cells toward the skin’s surface.
How Keratinocytes Build the Skin Barrier
The primary function of keratinocytes is to form the body’s physical and waterproof barrier, a process achieved through terminal differentiation known as keratinization or cornification. This life cycle begins with cells in the basal layer. As the cells move into the upper layers, they begin to express different types of structural proteins, including keratin filaments, which provide mechanical strength.
Upon reaching the granular layer, the keratinocytes synthesize specialized structures called keratohyalin granules, which contain proteins such as profilaggrin and loricrin. These proteins are later processed to help aggregate the keratin filaments and create a tough protein shell just beneath the cell membrane. Simultaneously, the cells release lipids from lamellar bodies into the extracellular space, which is an event that forms a continuous lipid matrix.
This process culminates in the cell losing its nucleus and all internal organelles, transforming the keratinocyte into a flattened, dead cell called a corneocyte. The resulting structure, the stratum corneum, is often described as a “bricks and mortar” arrangement, where the corneocytes are the protein-rich “bricks” surrounded by the lipid-rich “mortar”. This final tough, water-resistant layer is responsible for preventing excessive water loss from the body and blocking the entry of microbes and environmental toxins.