Structure, Function and Care of the Human Skin

by Dr Danny T. Siegenthaler

 

The Functions of the Skin

Before we look at the structure of the human skin, let's review the roles our skin plays and what its functions are in the maintenance and health of our bodies.

Skin serves several functions, which are introduced here.

1. Regulation of body temperature:
   • High environmental temperatures, fever or strenuous exercise leads to an expansion of blood vessels in the skin and an increase of perspiration. This aids in the cooling of the blood and thus provides a regulatory function on body temperature and blood pressure.
   • Cold air temperature leads to a reduction in perspiration, thereby reducing evaporation and thus conservation of body heat.
   • Temperature regulation is important, as various organs, including our brain can not operate outside a certain temperature range.
     
     
2. Protection:
   • The skin covers the body and provides a physical barrier that protects underlying tissues from physical abrasion, bacterial invasion, dehydration, and ultraviolet (UV) radiation. Hair and nails also have protective functions.
     


3. Sensation:
   • The skin contains abundant nerve endings and receptors that detect stimuli related to temperature, touch, pressure, and pain.
     


4. Excretion:
   • Besides removing heat and some water from the body, sweat also is the vehicle for excretion of a small amount of salts and several organic compounds.
     


5. Immunity:
   • Certain cells of the epidermis are important components of the immune system, which fends off foreign invaders.
     


6. Blood reservoir:
   • The dermis of the skin houses extensive networks of blood vessels that carry 8 to 10% of the total blood flow in a resting adult. In moderate exercise, skin blood flow may increase, which helps dissipate heat from the body. During hard exercise, however, skin blood vessels constrict (narrow) somewhat, and more blood is able to circulate to contracting muscles.
     


7. Synthesis of Vitamin D:
   • Vitamin D is a group of closely related compounds.  Synthesis of vitamin D begins with activation of a precursor molecule in the skin by ultraviolet (UV) rays in sunlight. Enzymes in the liver and kidneys then modify the molecule, finally producing calcitriol, the most active form of vitamin D. Calcitriol contributes to the homeostasis (equilibrium) of body fluids by aiding absorption of calcium in foods. Therefore, vitamin D is a hormone, since it is produced in one location in the body, transported by the blood, and then exerts its effect in another locations. In this respect, the skin may be considered an endocrine organ (hormone producing organ or gland).

 

The Structures of the Skin

The human skin consists primarily of two major structures:

• Epidermis
• Dermis
  

The Epidermis

The Epidermis is further subdivided into 5 Layers (from deepest to most superficial layer):

• Stratum basale
• Stratum spinosum
• Stratum granulosum
• Stratum lucidum
• Stratum corneum

The Stratum basale (also called Stratum germinativum): This is the deepest layer of the epidermis and it is here that new cells are generated for the renewal of the epidermal layers of the skin. A process of cell division referred to as mitotic division is responsible for the generation of the new epidermal skin cells. After the mitotic division (cell division leading to the formation of a new cell) a newly formed cell will undergo a progressive maturation called keratinisation as it migrates to the surface of the skin (1).

The Stratum spinosum: The cells that divide in the stratum germinativum soon begin to accumulate many desmosomes (structures that join adjacent cells together) on their outer surface (1).

The Stratum granulosum: As keratinocyes (these are the basic cell of which the epidermis is composed) progressively mature they accumulate a protein called keratin (this process is called keratinisation). In addition, the cells of the stratum granulosum accumulate dense basophilic keratohyalin granules (Granules found in living cells of keratinizing epithelia) (1).

The Stratum lucidum: This is the second layer of the epidermis and varies in thickness throughout the body depending mainly on frictional forces and is thickest on the palms of the hands and soles of the feet. The stratum lucidum is normally only well seen in thick epidermis and represents a transition from the stratum granulosum to the stratum corneum (1).

The Stratum corneum: This layer consists of primarily dead skin cells. As a cell accumulates keratinohyalin granules, it is thought that rupture of lysosomal membranes (membrane covering lysosomal enzymes) release lysosomal enzymes (Lysosomal enzymes are those enzymes which are responsible for breaking down complex chemicals within a cell which have expended their useful life) that eventually cause cell death (5). The dead and dying cells filled with mature keratin form the stratum corneum (SC). The deeper cells of the stratum corneum retain their desmosomal junctions, but as they are pushed to the surface by newly forming cells of the stratum germinativum (SG), the dead cells gradually break apart and are lost, a process called desquamation (1).

Skin Renewal Process In the Epidermis

The epidermis is composed of stratified squamous epithelium (cells) and contains four principal types of cells. About 90% of the epidermal cells are keratinocytes (i.e.: cells with finger-like or 'horny' projections). They produce the protein keratin. Keratin helps waterproof and protect the skin and underlying tissues (2).

Keratinocytes in the stratum basale of the epidermis can undergo mitosis (cell division). The formation of new cells in this basal layer gradually pushes previously formed cells upward through the stratum spinosum.  As keratinocytes approach the surface of the epidermis, they accumulate intracellular keratin and secrete a waxy material into the intercellular space; these changes are visible in the stratum granulosum, a distinctive layer which is diagnostic for a keratinized epithelium.  As maturing keratinocytes seal off the intercellular spaces through which they receive nutrients, they eventually die and form the stratum corneum, a tough and relatively impermeable layer of hardened, dead cells.  Eventually, as cells reach the surface, they are sloughed off.  The entire epidermis above the basal layer is replenished (replaced by new cells) within about two weeks (3).

Epidermal cells

There are several cells that make up the epidermis. Although the keratinocytes are by far the most common, they are just one of the cells found in the epidermis.

Others include:

Melanocytes: The main function of melanocytes is to produce melanin, which is responsible for the colour of our skin (4).

 Langerhans Cells: The function of Langerhans cells is largely unknown, however they form part of a system of cells which fix and process exogenous antigen and are found mainly in the stratum spinosum of the epidermis (4). They arise from bone marrow and migrate to the epidermis. Langerhans cells interact with white blood cells called ‘helper T cells’ in immune responses and are easily damaged by UV radiation (2).

Merkel Cells: Merkel cells are located in the deepest layer (stratum basale) of the epidermis of hairless skin, where they are attached to keratinocytes by desmosomes. Merkel cells make contact with the flattened portion of the ending of a sensory neuron (nerve cell), called a tactile (Merkel) disc, and are thought to function in the sensation of touch (2).

 

Dermo-Epithelial Junction

The Epidermis and Dermis are separated by the Dermo-Epithelial Junction. This junction holds the epidermis and dermis together and this is achieved by various fibers including collagen and desmosomes. This prevents the two layers becoming separated in areas of high shearing stress such as fingertips, palms of the hands and soles of the feet.

The Dermis

The Dermis consists of two sub-layers:

• The Papillary dermis and
• The Reticular dermis

The Papillary dermis (sub-epithelial layer) includes areolar connective tissue, dermal papillae (finger like projections that increase the surface area) and ridges that extend into the epidermis.

These nipple-shaped structures protrude into the epidermis, and many contain loops of capillaries (very small blood vessels). Dermal papillae cause ridges in the overlying epidermis. It is these ridges that leave fingerprints on objects that are handled (2).

The Reticular dermis consists of dense, irregular connective tissue containing interlacing bundles of collagen and coarse elastic fibers. Within the reticular region, bundles of collagen fibers interlace in a netlike manner. A small quantity of adipose tissue, hair follicles, nerves, oil glands, and the ducts of sweat glands occupy spaces between the fibers. Varying thickness of the reticular region contribute to differences in the thickness of skin. The combination of collagen and elastic fibers in the reticular region provides the skin with strength, extensibility, and elasticity. (Extensibility is the ability to stretch; elasticity is the ability to return to original shape after stretching.) The ability of the skin to stretch can readily be seen in pregnancy, obesity, and oedema. Small tears that occur in the dermis during extreme stretching are initially red and remain visible afterward as silvery white streaks called striae or stretch marks (2).

The Hypodermis

The reticular region is attached to underlying organs, such as bone and muscle, by the subcutaneous layer, also called the hypodermis or superficial fascia. The subcutaneous layer also contains nerve endings called lamellated or Pacinian (pa-SIN-e-an) corpuscles that are sensitive to pressure. Nerve endings sensitive to cold are found in and just below the dermis, while those sensitive to heat are located in the middle and outer dermis (2).

Skin Care

Skin Care of the Epidermis

Exfoliation

Exfoliation of the skin affects the epidermis. The primary function of exfoliation is to:

• Remove dead skin cells
• Promote new skin cell growth
• Promote blood circulation

Exfoliating the skin's surface is an important step in the maintenance of healthy, vibrant looking skin. Products such as the Skin Renewal Gel, from Wildcrafted Herbal Products, utilises natural ingredients that gently remove the dead skin cells and nourish underlying layers. Keeping dead skin cells to a minimum, allows the skin to be able to breath better, absorb nutrients from moisturisers more easily and reduces the risk of infections such as Ring Worm and other pathogens.

In addition, removal of dead skin cells will reduce the potential for sweat glands to become blocked thus reducing white heads, blackheads and acne.

Cleansing

Following exfoliation, cleansing will remove more deep seated dirt and help to free pores of possible obstruction from the stale, natural skin oils and environmental particles that become lodged in the skin's folds, wrinkles and pores.

Wildcrafted's range of Natural Skin Cleansers are formulated using only the purest, natural and organic ingredients to suite your specific skin type whether you have dry, mature and sensitive skin types. Used before toning and moisturising as part of a daily skin care regime.

Our natural facial skin cleansers are 100% natural and do not contain anyartificial or synthetic chemicals. Wherever possible we use certified organic ingredients and never use potentially harmful substances.

 

Toning

Once the dead skin cells have been removed and the skin cleaned it is important to prevent pores from remaining open. Toning, utilises skin care products that contain astringent ingredients which will close opened pores and prevent particles from entering the pores while they are wide open.

Natural skin care products should be used at all times, as there is increasing evidence suggesting that some non-natural skin care products contain ingredients that may be harmful to your health, as they are absorbed by your skin into the blood stream.

Wildcrafted's range of natural skin toners is 100% natural and none of our products contain any artificial or synthetic chemicals. The range of natural facial skin toners includes toners for oily, normal, dry, sensitive and combination skin types. In addition we offer an all-over body toning lotion for those areas where uneven skin texture can be unsightly such as the hips, thighs, stomach, buttocks and upper arms.

Skin Care of the Dermo-Epithelium, Dermis and Hypodermis

Natural skin care products such as moisturisers and masks target the Dermo-epithelium, Dermis and Hypodermis.

Moisturisers

Moisturisers penetrate the epidermis as they are absorbed into the deeper layers of the skin and the nutrients from the herbal extracts and essential oils in these moisturisers have the ability to promote cell growth and collagen production.

Moisturisers are an important final step in your daily skin care regime. They moisturise and help protect your skin, they hydrate your skin and nourish the cells and other structures outlined above, thus helping in maintaining the health of your skin.

Clay masks

Once or twice a week, a deep cleansing mask should be used on your facial skin and neck. These masks not only help to deeply cleanse your skin, but provide important nutrients to the tissues of your skin and help to remove dead skin cells from your skin’s surface.

Using Wildcrafted's Natural Facial Clay Masks to remove toxins, dirt and other impurities as well as dead skin cells from your skin, is an ideal way to refresh, rejuvenate and revitalise your facial skin.

 

 

References:

• http://www.meddean.luc.edu/lumen/MedEd/medicine/dermatology/skinlsn/stspin.htm
• Tortora, G.J. & Grabowski, S.R. (1993) Principles of Anatomy and Physiology (7th Edition). HarperCollins College Publisher, New York. [ISBN0-06-046702-9]
• http://www.siumed.edu/~dking2/intro/skin.htm
• http://medic.med.uth.tmc.edu/Lecture/Main/integ1.htm
• http://www.ggc.org/Diagnostics/Biochemical/lysosomal_enyzmes.htm