Skin cells are the basic building blocks of skin; A large, complex organ forms a protective barrier between our inner being and the outer environment. The most common skin cell type is the keratinocyte, whose main function is to form a tough, waterproof layer against UV radiation, harmful chemicals and infectious agents.

However, the skin also contains highly specialized cells with important immunological, photoprotective and sensory functions. The term “skin cell”, therefore, can refer to one of the four main types of cells found in the epidermis (or outer layer) of the skin.

Skin cells of the epidermis are highly specialized
The epidermis contains highly specialized skin cells

Functions of the skin

The skin is the largest organ in the human body and has a number of vital functions that aid survival. The main function of the skin is to form a physical barrier between the internal environment of an organism and the outside world. This protects internal organs and structures from injury and infection.

The skin also helps maintain homeostasis by preventing water loss and regulating body temperature. It protects organisms from the harmful effects of UV light and helps produce vitamin D when exposed to the sun. After all, the skin acts as a sensory organ and enables us to perceive touch, temperature changes and pain.

The skin can perform all of these functions thanks to the highly specialized cells that make up the skin epidermis (the outermost layer of the skin).

The skin protects organisms from UV radiation, pathogens and harmful toxins
The skin forms a physical barrier between the inside of an organism and its external environment

Structure of the skin

The skin is made up of three main layers; the Epidermis, the Dermis, and the Hypodermis (AKA the lower layer of skin).

The epidermis is the outermost layer of the skin. This waterproof barrier protects the underlying skin layers and other internal structures from injury, UV damage, harmful chemicals, and infection from pathogens such as bacteria, viruses, and fungi. The thickness of the epidermis varies between different parts of the body. In the thin, delicate skin of the eyelids, the epidermis is only about 0.5 mm thick, while the more elastic skin of the palms and feet is about 1.5 mm thick.

The Dermis is located just below the epidermis and is the thickest of the three layers of skin. This layer contains a complex network of specialized structures, including blood vessels, lymph vessels, sweat glands, hair follicles, sebum glands, and nerve endings. It also contains collagen and elastin, structural proteins that make the skin strong and flexible. The main functions of the dermis are to provide oxygen and nutrients to the epidermis and to regulate body temperature.

The Hypodermis (or lower layer of skin) is the oily, innermost layer of the skin. It consists mainly of fat cells and acts as an insulating layer that helps regulate the internal body temperature. The hypodermis also acts as a shock absorber that protects the internal organs from injury.

The skin is made up of three layers;  the epidermis, the dermis and the hypodermis
The skin is made up of three main layers

What is a skin cell?

The term “skin cell” can refer to one of the four main types of cells found in the epidermis. These are keratinocytes, melanocytes, Langerhans cells and Merkel cells. Each skin cell type has a unique role that contributes to the overall structure and function of the skin.

Skin cells of the epidermis

Keratinocytes

Keratinocytes are the most abundant skin cell types in the epidermis and make up about 90-95% of epidermal cells.

They produce and store a protein called Keratin, A structural protein that makes skin, hair and nails tough and waterproof. The main function of the keratinocytes is to form a strong barrier against pathogens, UV radiation and harmful chemicals while minimizing the loss of water and heat from the body.

Keratinocytes come from stem cells in the deepest layer of the epidermis (the basal Layer) and are pushed up through the layers of the epidermis as new cells are produced. As they move upwards, keratinocytes differentiate and undergo structural and functional changes.

The epidermis consists of several layers
Keratinocytes differentiate as they move through the layers of the epidermis

The Layer basal (or ground layer) is where keratinocytes are produced by mitosis. Cells in this layer of the epidermis can also be referred to as Basal cells. As new cells are constantly being produced, older cells are pushed into the next layer of the epidermis. the stratum spinosum.

In the layer spinosum (or Squamous epithelial layer) keratinocytes take on a prickly appearance and are known as Thorn cells or tingling cells. The main function of this epidermal layer is to maintain the skin’s firmness and flexibility.

Next, the keratinocytes migrate into the Stratum granulosum. Cells in this layer are heavily keratinized and have a grainy appearance. As they get closer to the surface of the skin, the keratinocytes begin to smooth and dry out.

At that point, keratinocytes enter the stratum lucidum (AKA the clear layer) they have flattened and died thanks to their increasing distance from the nutrient-rich blood supply to the stratum basal. The Stratum corneum (the outermost layer of the epidermis) consists of 10-30 layers of dead keratinocytes that are constantly being shed from the skin. Keratinocytes of the stratum corneum can also be referred to as Corneocytes.

The morphology of the keratinocytes changes as they differentiate
The structure of the keratinocytes is different in each layer of the epidermis

Melanocytes

Melanocytes are another major type of skin cell, making up 5-10% of the skin cells in the base layer of the epidermis.

The main function of melanocytes is production Melanin, Which is the pigment that gives skin and hair its color. Melanin protects the skin cells from harmful UV radiation and is produced as a reaction to solar radiation. With constant exposure to the sun, melanin accumulates in the skin and causes it to become darker, ie it creates a suntan.

Melanocytes produce the pigment melanin
Melanocytes produce melanin in response to exposure to sunlight

Langerhans cells

Langerhans cells are immune cells of the epidermis and play an essential role in protecting the skin against pathogens. They occur throughout the epidermis, but are most concentrated in the stratum spinosum.

Langerhans cells are antigen-presenting cells which, when encountered with a foreign pathogen, devour it and digest it into protein fragments. Some of these fragments appear as part of theirs on the surface of the Langerhans cell MHCI complex and are presented to naive T cells in the lymph nodes. The T cells are activated to trigger an adaptive immune response and effector T cells are used to find and destroy the invading pathogen.

Langerhans cells are usually located in the thorn layer of the epidermis
Langerhans cells are the immune cells of the epidermis

Merkel Cells

Merkel cells are located in the base layer of the epidermis and are particularly concentrated on the palms of the hands, finger pads, feet, and the undersides of the toes. They are located very close to sensory nerve endings and are supposed to act as touch-sensitive cells. Merkel cells enable us to perceive sensory information (such as touch, pressure and texture) from our external environment.

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