Skin

The skin is an ever-changing organ that contains many specialized cells and structures. The skin functions as a protective barrier that interfaces with a sometimes-hostile environment. It is also very involved in maintaining the proper temperature for the body to function well. It gathers sensory information from the environment, and plays an active role in the immune system protecting us from disease. Understanding how the skin can function in these many ways starts with understanding the structure of the 3 layers of skin - the epidermis, dermis, and subcutaneous tissue.

Diagram of Skin Layers

Epidermis
The epidermis is the outer layer of skin. The thickness of the epidermis varies in different types of skin. It is the thinnest on the eyelids at .05 mm and the thickest on the palms and soles at 1.5 mm.

The epidermis contains 5 layers. From bottom to top the layers are named:

• stratum basale
• stratum spinosum
• stratum granulosum
• stratum licidum
• stratum corneum

The bottom layer, the stratum basale, has cells that are shaped like columns. In this layer the cells divide and push already formed cells into higher layers. As the cells move into the higher layers, they flatten and eventually die.

The top layer of the epidermis, the stratum corneum, is made of dead, flat skin cells that shed about every 2 weeks.

Specialized Epidermal Cells
There are three types of specialized cells in the epidermis.

• The melanocyte produces pigment (melanin)
• The Langerhans' cell is the frontline defense of the immune system in the skin
• The Merkel's cell's function is not clearly known

Dermis
The dermis also varies in thickness depending on the location of the skin. It is .3 mm on the eyelid and 3.0 mm on the back. The dermis is composed of three types of tissue that are present throughout - not in layers. The types of tissue are:

• collagen
• elastic tissue
• reticular fibers

Layers of the Dermis
The two layers of the dermis are the papillary and reticular layers.

• The upper, papillary layer, contains a thin arrangement of collagen fibers.
• The lower, reticular layer, is thicker and made of thick collagen fibers that are arranged parallel to the surface of the skin.

Specialized Dermal Cells
The dermis contains many specialized cells and structures.

• The hair follicles are situated here with the erector pili muscle that attaches to each follicle.
• Sebaceous (oil) glands and apocrine (scent) glands are associated with the follicle.
• This layer also contains eccrine (sweat) glands, but they are not associated with hair follicles.
• Blood vessels and nerves course through this layer. The nerves transmit sensations of pain, itch, and temperature.
• There are also specialized nerve cells called Meissner's and Vater-Pacini corpuscles that transmit the sensations of touch and pressure.

Subcutaneous Tissue
The subcutaneous tissue is a layer of fat and connective tissue that houses larger blood vessels and nerves. This layer is important is the regulation of temperature of the skin itself and the body. The size of this layer varies throughout the body and from person to person.

The skin is a complicated structure with many functions. If any of the structures in the skin are not working properly, a rash or abnormal sensation is the result. The whole specialty of dermatology is devoted to understanding the skin, what can go wrong, and what to do if something does go wrong.