Skin is composed of the skin itself and supporting tissue
(hypodermis). There are two skin layers: the epidermis is surface layer,
one-tenth of a millimeter thick. It is the first barrier preventing
invasion of foreign microbes and substances. The epidermis is a
multilayered keratinized squamous epithelium. Keratinocytes produce
keratin, the proteins that form the most exterior skin layer (horny
layer). Keratinocytes gradually migrate from deeper layers to the skin
surface and are shed.
The dermis below is a complex tissue made of collagen fibers, sweat
glands, hair roots, nerve cells, nerve fibers, blood and lymph vessels.
Cone-shaped loops of collagen fibers rise as papillas into the
epidermis linking the two layers of skin. Beneath the dermis is
the sub cutis, a layer of loose, chamber-structured connective tissue
There are two types of sweat glands, eccrine and apocrine. There are
about 3 million eccrine sweat glands that have a tubular shape and are
found all over the body. About 3,000 sweat glands per square inch are
found in the palm of the hand. The secretory base of the gland lies
below the dermis in the subcutaneous tissue. A duct follows a spiral
course through the dermis to the skin surface. An adult secretes a
minimum of 100 ml sweat each day; heat exposure, and work increase sweat
production to a maximum of 10 liters per day. The most important
function of sweat production is body cooling. Water evaporation requires heat taken from the skin, cooling the skin
surface. Sweating under the arms is suppressed by antiperspirants
containing aluminum salts.
Hair is produced in hair follicles by cells at the base of each hair.
Follicular cells are added to the hair fiber and become compressed and
molded as the hair root moves up the follicle. The shape of the follicle
determines whether the hair will be straight or curly. As more cells are
added, the hair fiber is pushed up and out and keratinized. By the
time the hair emerges from the follicle all the cells are dead. The
cells layers are a cortex, or core, surrounded by the cuticle. There may
be a hollow center or medulla.
Hair follicles secrete keratin proteins that form the main structure
of hair. Exposed hair is a non-living product of cells just as nails are
non-living products. Hair follicles extend from the base of the dermis and
emerge through openings in the dermis. The papilla cells are a stem cell
pollution that create all the structures of the hair follicle. Hair growth is
periodic; the papillary cells at the base of the germinal matrix at the base of
the hair follicle take rests between periods of active hair growth. Germinal
matrix cells in the papilla grow into the lumen of the external root sheath.
The root of the hair becomes detached from its matrix and gradually moves up the
follicle, attaching for a time to the external root sheath. The deep part of the
external root sheath grows downward again to cover the old papilla, which
becomes rejuvenated, or a new one develops. Sebaceous glands sit adjacent
to the hair follicles and secrete sebum into the follicle supplying an oily
coating for the hair as it emerges from the follicle.
Skin color is developed by melanocytes, cells that produce the brown/black
pigment, melanin in the basal cell layer of the epidermis. Melanocytes form long
finger-like protuberances (dendrites) which grow into and through the
neighboring keratinocytes. UV light simulates biosynthesis of melanin that
occurs in melanosomes, components of the melanocytes that fill with melanin as
they move to the periphery of the cell and into the dendrites, filling up
originally colorless skin cells (keratinocytes) with pigment. Melanin
absorbs UV radiation and protects other skin structures from damage. Hence, sun
exposure increases melanin production. Humans who have lived in sunny locations
on the planet have dark skin, and humans who lived in cold dark zones lost most
of their skin pigmentation. Paradoxically, white skin people often want to
become tanned and dark skinned people often want to become lighter. White skin
is often interrupted by patches of pigmentation, described as freckles, moles
There are many variations in skin pigmentation and some skin lesions are
pigmented. A surface fungal infection, for example, will change skin color
paradoxically and is called tinea versicolor. Dark skin becomes
lighter and light skin becomes darker. Sometimes a single application of Nizoral
shampoo, painted on with cotton tipped applicator to cover the affected skin
will kill the fungus.
The skin is a meeting place for many characters in an ecological drama that
unfolds every day. Numerous little animals live on the skin, mostly peacefully.
Pimples erupt in most people regularly and reveal the presence of microorganisms
that are always ready to take advantage of the host and invade. The pimple is
also evidence of immune cells that patrol below the skin surface. An
inflammatory bump is full of immune cells and the chemicals they secrete. The
pustule is made of immune cells and micro-organisms that died in battle. The
animals that live on the skin are mostly bacteria, some fungi and some mites.
Viruses invade the skin and live inside skin cells, often disrupting cell
growth. A wart is an expression of abnormal growth in virus-infected skin cells.
Herpes viruses infect skin cells, but invade nerve cells that innervate the
skin. They travel to the nerve cell body where they remain resident for the
lifetime of the infected host.
Herpes virus travels down neuron axons from
time to time invading and killing skin cells. Herpes eruptions are local
interactions between immune cells and infected skin cells that display viral
antigens on their surface and are killed by killer lymphocytes and eaten by
Nutrients and problems arrive at the skin from inside through
the blood stream. Immune cells travel in the blood and enter the skin whenever
they are needed. When immune cells enter skin, they often take over and produce
an inflammatory response that causes the skin to swell, itch, burn, and turn
red. A hive is a prototype of immediate allergic or immune-mediated skin
inflammation. Usually a hive is a brief event that resolves spontaneously
Foreign molecules arrive from the blood and trigger mast cells that are
patrolling the deep layers of the skin. The mast cells release histamine and
other chemicals that cause the inflammation. The foreign molecules are ingested
as food or drugs