Do you want to know more about your skin, hair, or nails? This is the place to look for the basic information to get you started. Learn more about how your skin works so you can better understand what happens when it doesn't work right. Don't forget to check out the glossary if you need to look up a medical term.

Skin Anatomy: 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.

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:

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.

| Illustration of maturation of epidermis cells |

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

Epidermis Anatomy: Keratinocyte Maturation: There are four major layers of keratinocytes (the structural cells) in the epidermis and one layer that is present only in certain parts of the body. 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 cells move into the higher layers, they flatten and eventually die. We will take a closer look at the characteristics of each of these layers.

Stratum Basale: The stratum basale is the bottom layer of keratinocytes in the epidermis and is responsible for constantly renewing epidermal cells. This layer contains just one row of undifferentiated columnar stem cells that divide very frequently. Half of the cells differentiate and move to the next layer to begin the maturation process. The other half stay in the basal layer and divide over and over again to replenish the basal layer.

Stratum Spinosum: Cells that move into the spinosum layer (also called prickle cell layer) change from being columnar to polygonal. In this layer the cells start to synthesize keratin.

Stratum Granulosum: The cells in the stratum granulosum, or granular layer, have lost their nuclei and are characterized by dark clumps of cytoplasmic material. There is a lot of activity in this layer as keratin proteins and water-proofing lipids are being produced and organized.

Stratum Lucidum: The stratum lucidum layer is only present in thick skin where it helps reduce friction and shear forces between the stratum corneum and stratum granulosum.

Stratum Corneum: The cells in the stratum corneum layer are known as corneocytes. The cells have flattened out and are composed mainly of keratin protein which provides strength to the layer but also allows the absorption of water.

Stratum Corneum - Close-Up: The structure of the stratum corneum layer looks simple, but this layer is responsible for maintaining the integrity and hydration of the skin - a very important function. There are actually complex processes that are at work in the stratum corneum and minimal disruptions of any of these processes can cause a variety of skin problems.

Stratum Corneum Anatomy - The Key to Healthy, Attractive Skin: The stratum corneum is the outermost of the 5 layers of the epidermis and is largely responsible for the vital barrier function of the skin. Before the mid-1970's the stratum corneum was thought to be biologically inert, like a thin plastic sheet protecting the more active lower layers of the skin. In the past 30 years, and especially the past 5 years, scientists have discovered that the biological and chemical activity of the stratum corneum is very intricate and complex. Understanding the structure and function of the stratum corneum is vital because it is the key to healthy skin and its associated attractive appearance. These illustrations will take you through the important components of the stratum corneum.

Stratum Corneum Anatomy - The Corneocyte: The structure of the stratum corneum has been described as a "brick and mortar" type structure. In this analogy, the corneocytes are the bricks. A corneocyte is a protein complex that is made of tiny threads of keratin in an organized matrix. The keratin can hold large amounts of water between the fibers/threads. The stratum corneum is contains about 12-16 layers of corneocytes and each corneocyte has a mean thickness of 1 micrometer, depending on the following factors:

| Age | Anatomical location | Exposure to UV radiation |

Stratum Corneum Anatomy - Lamellar Bodies: Lamellar bodies are formed in the keratinocytes of the stratum spinosum and stratum granulosum. When the keratinocyte matures to the stratum corneum, enzymes degrade the outer envelope of the lamellar bodies releasing types of lipids called free fatty acids and ceramides.

Stratum Corneum Anatomy - Intercellular Lipids: Free fatty acids and ceramides that are released from the lamellar bodies fuse together in the stratum corneum to form a continuous layer of lipids. Because there are two types of lipids, this layer is referred to as a lamellar lipid bilayer. This lipid bilayer plays a major role in maintaining the barrier properties of the skin and is analagous to the "mortar" in the brick and mortar model.

Stratum Corneum Anatomy - Cornified Envelope: Each corneocyte is surrounded by a protein shell called a cell envelope. The cell envelope is composed primarily of two proteins, loricirn and involucrin. These proteins contain extensive links between each other making the cell envelope the most insoluble structure of the corneocyte. The two sub-types of cell envelopes are described as "rigid" and "fragile" based on the interaction of lamellar bilayer with the cell envelope.

Stratum Corneum Anatomy - Cornified Envelope Lipids: Attached to the cell envelope is a layer of ceramide lipids that repel water. Because thelamellar lipid bilayer also repels water, water molecules are held between the cell envelope lipids and the lipid bilayer. This helps maintain the water balance in the stratum corneum by trapping water molecules instead of letting them absorb into the lower layers of the epidermis.

Stratum Corneum Anatomy - Corneodesmosomes: The "rivets" that hold the corneocytes together are specialized protein structures called corneodesmosomes. These structures are also a part of the "mortar" in the "brick and mortar" analogy. Corneodesmosomes are the major structure that must be degraded for the skin to shed in a process called desquamation.

Stratum Corneum Anatomy - Natural Moisturizing Factor (NMF): Natural moisturizing factor (NMF) is a collection of water-soluble compounds that are only found in the stratum corneum. These compounds compose approximately 20-30% of the dry weight of the corneocyte. NMF components absorb water from the atmosphere and combine it with their own water content allowing the outermost layers of the stratum corneum to stay hydrated despite exposure to the elements. Because NMF components are water soluble, they are easily leached from the cells with water contact - which is why repeated contact with water actually makes the skin drier. The lipid layer surrounding the corneocyte helps seal the corneocyte to prevent loss of NMF

Stratum Corneum Anatomy - Desquamation Process: The desquamation, or exfoliation, process of the stratum corneum is actually very complex and only parts of this process are fully understood. We do know that several enzymes degrade the corneodesmosomes in a specific pattern, but we don't know the exact nature of these enzymes or how they become activated to start the exfoliation process. We do know that water and pH play a significant role in the activity of these enzymes.