The skin is the heaviest organ of the human body accounting for 16% of total body weight. It is relatively impermeable to water which prevents excessive water loss. It is also an important receptor organ in continuous communication with the environment in order to protect us from injuries. To understand why different skin diseases occur, you first have to know the intricate structures of the skin and their roles in contributing to its functions.

The skin is composed of two sections of cells: the epidermis and the dermis. The dermis has these finger-like projections called papillae that interdigitate with the grooves of the epidermis. The epidermis is composed of a type of cell that produces keratin, a fibrous structural protein. These cells organized themselves into five different layers: stratum basale, stratum spinosum, stratus granulosum, stratum lucidum and stratum corneum. Starting from the layer closest to the dermis, stratum basale consists of a single layer of cells that line alone the basement membrane at the dermal-epidermal junction. These cells are bound together via adhesive proteins called desmosomes. They are rapidly dividing cells that are largely responsible for the constant renewal of epidermal cells which takes about 15 to 30 days to complete depending on age and the region of the body. All the cells in stratum basale contains intermediate keratin filaments that increase in number as the cells progress upward towards the outer surface of the skin.

Above stratum basale is stratum spinosum, which consists of slightly flattened cells with even more bundles of keratin filaments with the tips extending to the desmosomes that glue the cells together, thereby giving the cells that spine-studded appearance. These keratin bundles reinforces the cohesion among cells and resists the effects of abrasion, without which the skin would easily break down with minor forces.

On top of stratum spinosum is stratum granulosum, named because the cells are filled with granules such as keratohyalin granules and lamellar granules. Lamellar granules are important in that they extrude substances out of the cells to form sheets of lipids that give rise to the water repellent nature of skin and function as a barrier to heat, light, chemicals and bacteria.

The next layer of cells on top of stratum granulosum is stratum lucidum which is composed of extremely flattened cells with densely packed keratin filaments, and as they migrate to the top, they become part of stratum corneum, the outermost part of the skin where the cells are continuously shed. This is the toughest layer of cells meant to protect the inner layers. The spaces between the cells are filled with lipids to further fortify the skin against moisture loss and infection.

Besides these keratin producing cells, which are the building blocks of the epidermis, interspersed among them you can also find cells like melanocytes, Langerhan cells, and Merkel’s cells. Melanocytes are cells that produce melanin, a pigment not only contributes to the color of the skin but also protects the skin against UV rays from the sun. Langerhan cells are star-shaped cells that have an important role in immunological activity in the skin as they bind, process, and present foreign proteins to T lymphocytes, which are a type of white blood cells that participate in the immune response of the body. Merkel’s cells work like sensory receptors and allow the skin to sense light touch.

Below the epidermis lies the dermis, the connective tissue that supports the epidermis. It is much thicker than the epidermis and is composed of two layers: a papillary layer and reticular layer. The papillary layer is where you will find capillaries, lymphatic channels and sensory neurons all couched within loose connective tissue. Its function is to supply nutrients to the epidermis above. The reticular layer is comprised of thicker dense connective tissue along with elastic and collagen fiber. They give rise to the skin’s strength and flexibility.

In summary, there are two major layers of the skin: the epidermis and dermis. Each layer is also divided in the subsections, each with a role to play in determining the physical properties and functions of the skin. Many skin diseases arise from breakdown of these structures, and treatments are therefore targeting towards restoring their natural integrity.