What is the Epidermis?

The epidermis is the outer layer of the skin and provides protection. It contains the cells keratinocytes that make the protein keratin. It also contains melanocytes that produce the pigment that gives your skin its color.

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The epidermis is composed of a stratified squamous epithelium. The bottom layer of the epidermis is called the stratum basale. This layer is rich in tonofibrils that connect adjacent keratinocytes. It is also home to dendritic cells of the immune system.

Stratum basale

The epidermis is a multilayered, stratified, squamous epithelium made of keratinocytes. It protects the underlying dermis against infection and dehydration. It also protects the body from physical damage. It has four tightly-adherent layers, referred to as strata: the stratum basale, stratum spinosum, stratum granulosum, and the stratum corneum. The layer closest to the skin is the stratum basale, which consists of a single row of tall, simple columnar epithelial cells. These cells rest on the basement membrane zone and undergo rapid cell division (mitosis) to make replacement keratinocytes for the layers above them. The resulting keratinocytes then migrate upward to form the outer layers of the epidermis.

The next layer is the stratum spinosum, which consists of a few cellular layers. This layer contains granules that are a type of lipid secretion and help reduce friction between the layers above it. The granules also contain dark clumps of cytoplasmic material, which are sometimes visible under a microscope. The cells in this layer have many desmosomes, which anchor them to each other. These connections give the cells a prickly appearance, hence the name prickle cells. They are often stained black and can be easily recognized during a histological section of the skin.

The granule cell layer, known as the stratum granulosum, consists of 3-5 cellular layers. These cellular layers have diamond-shaped keratinocytes with granules filled with a lipid rich secretion called keratohyalin. The granules also contain lamellar granules, which are crosslinked to form bundles of keratin. These granules are responsible for the epidermal water barrier, which prevents the penetration of bacteria and viruses and regulates calcium absorption.

Stratum spinosum

The stratum spinosum is a layer of 8-10 layers of cells that develops as the result of cell division in the basal cell layer. It contains keratinocytes (kur-atin-oh-site) stem cells that produce the protein keratin, which helps form hair and skin. It also contains melanocytes, which are responsible for producing the pigment melanin that gives skin its color.

This layer is found in thick skin of the palms, soles, and digits. It consists of flattened keratinocytes that are packed with a lipid-rich secretory product called keratohyalin, which helps to provide a barrier against water. It is also rich in a clear substance called eleiden, which appears translucent. The layer is thought to be important in regulating the amount of hydration that a person receives from the skin.

It is also a sensory component of the epidermis. It contains oval-shaped modified epidermal cells that are bound to adjoining keratinocytes by desmosomes and serve as mechanoreceptors that respond to light touch and are most abundant in the fingertips, but also occur on the lips, nose, and oral and genital mucosa.

The keratinocytes in this layer are also known as horny cells. They are dead, and they have become flattened and densely packed with a lipid-rich secretory fluid called keratohyalin. Occasionally, the cells in this layer will become distorted and produce thick finger-like downgrowths that are sometimes referred to as pseudoepitheliomatous hyperplasia.

Stratum granulosum

The top layer of the epidermis is called stratum granulosum. It is a clear layer that is thicker in the palms and soles. This layer is rich in keratin cells and contains several glycolipids that act as a glue. These lipids form a hydrophobic lipid barrier to protect the skin from water loss and microbial infection. It also plays a role in providing a sense of touch and warmth.

Cells in this layer are known as keratinocytes and are responsible for producing a protein called keratin, which forms the outermost layer of the skin, nails, and hair. It also contains cells that produce melanin, which gives the skin its color. The keratinocytes in this layer are also responsible for the movement of new cells to the surface of the skin.

This layer is found in between the stratum spinosum and the stratum corneum (or stratum lucidum, when present). Its name, which means “granular” layer, refers to the fact that its cells are distinctively granular in appearance. These keratinocytes also contain a molecule called keratohyalin, which helps to bond keratin filaments together.

The cells in this layer are able to withstand physical damage, such as minor scrapes and scratches. Sharp objects and rough surfaces have a hard time penetrating the tough outer layer of this layer, which is made up of dead keratin cells. In addition, this layer provides protection from pathogens and the sun’s UV rays.

Stratum lucidum

The skin is the body’s primary protective barrier. It keeps pathogens away, regulates temperature, and is a shield against UV rays. This layer also contains fats that prevent water from easily entering or leaving the body. The epidermis consists of five layers. The outermost layer is the stratum lucidum, which is a clear, translucent layer of keratinized, squamous epithelium. The epidermis has a vascular system that supplies nutrients to select layers of cells and controls the flow of blood in response to stimuli such as heat or cold.

The second layer of the epidermis, the stratum granulosum, has a grainy appearance. This layer consists of three to five layers of keratinocytes that become flattened and lose their nuclei and cytoplasmic organelles. In this layer, the keratinocytes produce large amounts of keratin and keratohyalin, which are proteins that accumulate as lamellar granules.

In thick skin, a fifth layer (stratum lucidum) is sometimes observed between the stratum granulosum and the stratum corneum layer. This is a thin transparent layer that can be difficult to distinguish in routine histological sections.

The avascular epidermis provides protection against physical damage, including scratches and scrapes. Sharp objects, rough surfaces, and UV rays have difficulty penetrating the tough, dead keratin-filled cells in this layer. The layer also protects the underlying dermis from abrasions, heat, and pathogens.