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Components of Cells
The Macromolecules
Skin Morphology

What the skin does

The average human adult is covered by a remarkable organ; the skin.  This highly specialized organ serves many functions that range from protection (against attacks of all kinds), a sensitive interface between the outside world and the world inside the body (there a LOT of sensory nerves and organs located in the skin layer), temperature regulation (sweating), location of hair, storage of lipids and insulation (loss of heat from a body that is often well above the outside temperature).

Although it is only about 2 mm thick (about 0.07 inches) it covers about 20 square feet of surface and weighs about 3 killograms (just over 6 pounds).

Depending on how you count them, the human skin has three layers.

Epidermis

The outermost layer, the one you see, is the epidermis which is hard, tough, protective (against attack and the damage caused by the sun) and holds the specialized cells called melanocytes which produce the pigment melanin and give the skin its characteristic color.

When covering sensitive parts of the body, such as the eyelids, the epidermis is only 0.05 mm thick, but on heavily used parts of the body, like the palms of the hands or the soles of the feet, this layer can be at least 1.5 mm thick.

Thick or thin, the epidermis has five distinct layers or regions.  In the one furthest from the surface (the stratum basale) the cells are shaped like columns and divide constantly.  They push themselves upwards, moving into layers closer and closer to the surface where they eventually flatten and die.  The outermost surface of the human skin is dead, and is completely replaced about every two weeks!

Dermis

The next layer is the dermis, which also can vary in thickness depending on what part of the body it is covering.  On the back, it can be 3.0 mm thick and on the eyelids only 0.3 mm thick.

There are no distinct layers in this part of the skin, but more or less two regions that differ in the types of fibers that spread throughout them.  The outer part has thin, delicate collagen fibers, while the inner part has rougher and tougher collagen fibers that are arranged like a mesh (reticulum) parallel to the surface.

It is in the dermis that specialized structures such as hair follicles, erector pili muscles (that attach to the hair follicles and pull the hairs upright in the cold), oil glands, scent glands, and sweat glands (eccrine glands), blood vessels and nerves are located.  This is where very sensitive nerve cells (Meissner's and Vater-Pacini corpuscles) give us the sensation of touch and pressure.
Hypodermis
(subcutaneous tissue)

Furthest from the outside is the hypodermis, panniculus adiposus or subcutaneous tissue.  This is a layer of adipocyte cells and connective tissue that stores large amounts of lipids and thus carries out the two important functions of energy storage and thermal insulation.

This layer of the skin varies in depth enormously depending on where it is located and the individual person.

BIOdotEDU
© 2005, Professor John Blamire