Normal Skin Anatomy
The human skin, taken collectively, is
the largest organ in the human body. On average, it weighs between 4 and 5 kg.
It is vitally important to life. The skin is made up of three distinct layers:
the epidermis, the dermis, and the subcutaneous tissue; some anatomists do not
include the subcutaneous tissue as part of the skin and classify it separately
as the hypodermis. Each of these layers plays a pivotal role in the execution
of day-to-day functions of the skin. The skin’s main function is to protect the
interior of the body from the exterior environment. It performs this role in
many fashions: It acts as a semipermeable barrier to both hydrophilic and
hydrophobic substances; it is the first line of immunological defense against
invading microbes; it contains many components of the adaptive and innate
immune system; and it has many physiological roles, including metabolism of
vitamin D.
The majority of the epidermis is made up of keratinocytes. It also
contains melanocytes, Langerhans cells, and Merkel cells. The epidermis is
avascular and receives its nutrition from the superficial vascular plexus of
the papillary dermis.
Melanocytes are derived from neural crest and are responsible for
producing the melanin family of pigments, which are packaged in melanosomes.
Melanocytes are found in equal density in all humans, but darker-skinned
individuals have a higher density of melanosomes than those with lighter skin.
This is the reason for color variation among humans. Eumelanin, the predominant
type of melanin protein, is responsible for brown and black pigmentation.
Pheomelanin is a unique variant of melanin that is found in humans with red
hair.
The skin is found in continuity with the epithelial lining of the
digestive tract, including the oral mucosa and the anal mucosa. Distinct
transition zones are seen at these interfaces. The skin also abuts the conjunctival
mucosa of the globe and the mucosa of the nasal passages. The skin and its
neighboring epithelial components supply the human body with a continuous
barrier to protect it from the external world.
Many appendageal structures are present throughout the skin. The major
ones are the hair follicles, their associated sebaceous glands, and the eccrine
glands. Most of the skin is hair bearing. Fine vellus hairs make up the
preponderance of the skin’s hair production. Terminal hairs are much thicker and
are found on the scalp, eyebrows, and eyelashes; in the axilla and groin areas;
and in the beard region in men. Glabrous skin, which is devoid of hair
follicles, includes the vermilion border of the lips, the palms, the soles, the
glans penis, and the labia minora.
Human skin varies in thickness. It is thickest on the back, and the
thinnest areas are found on the eyelids and the scrotum. Regardless of
thickness, all skin possesses the same immunological function and barrier
activity.
Various appendageal structures are found in higher densities in certain
regions of the skin. Sebaceous glands are located predominantly on the face,
upper chest, and back. These glands play an instrumental role in the
pathomechanism of acne vulgaris. Because sebaceous glands are attached to hair
follicles, they are found only on hair-bearing skin. Eccrine sweat glands, on
the other hand, are found ubiquitously. The highest densities of eccrine glands
are on the palms and soles.
The other main sweat glands of the skin, the apocrine glands, are found
almost exclusively in the axillae and the groin. The apocrine glands, like
sebaceous glands, are found only in conjunction with hair follicles.
Nails are composed of specialized keratin proteins. These keratins make a
hard nail plate that is believed to be important for protection, grasp, and
defense. Fingernails and toenails are made of the same keratin structure and in
the same manner. The only difference is that
the fingernails grow slightly faster than the toe-nails. The average thumbnail
takes 6 months to replace itself, whereas the average great toenail takes 8 to
12 months.
Skin is also an important means of communication with other humans. The
sense of touch is mediated through specialized receptors within the skin. One
cannot underestimate the importance of this function in the formation of human relationships.
