Human Anatomy

Skin

The skin is an ever-changing organ. 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 persues sensory information from the environment, and plays an active role in the immune system protecting us from various attacks of microorganisms. There are three layers - the epidermis, dermis, and subcutaneous tissue.

Epidermis
The epidermis is the outer most 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 having shape 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 body. 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 also known as oil glands and apocrine glands are associated with the follicle.
• This layer also contains 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.

Subcutaneous Tissue

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