The Skin Structure
- [Stanford University]
- The Skin Structure
Microscopic examination reveals the layered structure of the skin, as well as the many smaller elements within these layers that help the skin perform its primary protective function.
The skin consists of two main layers: the superficial epidermis and the deep dermis:
- The epidermis consists of several layers, the uppermost layer consisting of dead cells that are periodically shed and gradually replaced by cells that form from the basal layer.
- The dermis connects the epidermis with the subcutaneous tissue and provides strength and elasticity due to the presence of collagen and elastin fibers. Located deep in the dermis of the skin, the hypodermis is the connective tissue that connects the dermis to the underlying structures; it also contains adipose tissue for fat storage and protection.
- The Epidermis
The outer surface of the skin is the epidermis, which itself consists of several layers - the basal cell layer, acanthocyte layer, granule cell layer, and stratum corneum. The cells in the epidermis are called keratinocytes.
The deepest layer of the epidermis is the basal cell layer. Cells here are constantly dividing, producing plump new skin cells (millions per day). These cells move towards the surface of the skin, driven by the dividing cells below.
Blood vessels in the dermis (located below the basal cell layer) provide nutrients to support the active growth of new skin cells. As basal cells move up and away from their blood supply, their cellular content and shape change as follows.
Cells above the basal cell layer become more irregular in shape and form spinous layers. Above this, cells move to the granular layer. With the blood supply away from the dermis, the cells begin to flatten and die, accumulating a substance called keratin. Keratin is a protein also found in hair and nails.
The stratum corneum ("stratum corneum") is the top layer of the epidermis -- it's the layer of skin we see from the outside. Here the cells are flat and scaly ("squamous"). These cells are dead, contain large amounts of keratin, and are arranged in overlapping layers that give the skin's surface its tough and waterproof properties.
Dead skin cells are constantly shed from the surface of the skin. This is balanced by dividing cells in the basal cell layer to generate a state of constant renewal. Also in the basal cell layer are cells called melanocytes, which produce melanin. Melanin is a pigment that is absorbed into dividing skin cells to help protect them from sun (UV) damage. The amount of melanin in your skin depends on your genetics and your exposure to sunlight. The more melanin you have, the darker your skin will be.
The epidermis also contains dendritic (Langerhans) cells, which are part of the immune system and help protect the body from foreign substances.
- The Dermis
Beneath the epidermis is a layer called the dermis. The top layer of the dermis (the layer just below the epidermis) has many ridges called papillae. At the fingertips, the surface of the skin follows this pattern of ridges to form our individual fingerprints. Therefore, the ridges are not located in the outermost layer of the skin as they appear.
The dermis contains varying amounts of fat, as well as collagen and elastin fibers that provide the skin with strength and flexibility. In older people, elastin fibers break down and most of the skin's elasticity is lost. This, along with a loss of subcutaneous fat, leads to wrinkles.
When the skin is exposed to sunlight, modified cholesterol in the dermis produces vitamin D, which helps the body absorb calcium and maintain bone health.
- Below are some of the other structures within the dermis that enhance the function of the skin.
- Blood vessels provide nutrients to dividing cells in the basal layer and remove any waste products. They also help maintain body temperature by expanding and carrying more blood when the body needs to lose heat from surfaces. They narrow and carry less blood when the body needs to limit heat loss from its surface.
- Specialized nerves in the dermis detect heat, cold, pain, pressure and touch and relay these messages to the brain. In this way, the body senses changes in the environment that could potentially cause harm to the body.
- Hair follicles are embedded in the dermis and spread all over the body except the soles of the feet, palms and lips. At the base of each follicle is a layer of cells that is constantly dividing, pushing the covering cells up into the follicle. These cells keratinize and die, just like the cells in the epidermis, but here form the visible hair shaft above the skin. Hair color is determined by the amount and type of melanin in the outer layer of the hair shaft.
- The sebaceous glands ("oil") go to each hair follicle and produce sebum, which acts as a lubricant for the hair and skin, helps waterproof and destroys chemicals and microbes ("germs").
- Attached to each hair follicle are small arrector pili muscle fibers. These muscle fibers contract in cold weather and sometimes when you are frightened – this pulls the hair up which in turn pulls on the skin, causing 'goosebumps'.
- All skin areas have sweat glands – each individual has more than 2 million sweat glands. These glands produce sweat (a mixture of water, salt, and some waste products such as urea) when the body needs to lose heat. Sweat moves to the surface of the skin through the sweat ducts, and the evaporation of this water from the skin has a cooling effect on the body.
Different parts of the body have different thicknesses of skin, as well as different numbers of hair follicles, sebaceous glands, and sweat glands. The thickest skin is on the soles of the feet and palms. There are a large number of hair follicles on the top of the head.
- [Human Skin - Wikipedia]
- Subcutaneous Fat
The innermost layer of the skin is the subcutaneous fat layer, which varies in thickness in different parts of the body. The fat stored in this layer is the body's source of energy and helps insulate the body from changes in external temperature.
- Skin Functions
As you can see, there are many different structures within the skin. Together, these structures give the skin many protective properties that help protect the body from damage from external influences. Thus, the skin:
- Protect the body from water loss and damage from impact, chemicals, sunlight or microorganisms (“germs”);
- Helps control body temperature;
- is a sensor that notifies the brain of changes in the surrounding environment; and
- Synthesize vitamin D.
[More to come ...]
