The Epidermis

The deepest cells of the epidermis, which are next to the dermis, form a layer called the stratum germinativum. These cells are nourished by dermal blood vessels and are capable of reproducing. As they divide, older epidermal cells are slowly pushed away from the dermis toward the surface of the skin. 

The further the cells travel, the poorer their nutrient supply becomes, and in time they die. Meanwhile, the membranes of the cells become thickened and develop numerous intercellular junctions (desmosomes) that fasten them to adjacent cells. 

At the same time the cells undergo a hardening process called keratinization, during which the cytoplasm develops strands of tough, fibrous, waterproof protein called keratin. As a result, many layers of tightly packed dead cells accumulate in the outer portion of the epidermis.

Epidermis

This outermost layer is the stratum corneum, and the dead cells that compose it are easily rubbed away. Other layers of epidermal cells occur between the stratum germinativum and the stratum corneum. They include the stratum lucidum, the stratum granulosum, and the stratum malpighii. The cells of these layers are characterized by changes they undergo as they are pushed toward the surface of the skin.

In healthy skin, the production of epidermal cells is closely balanced with the loss of stratum corneum, so that skin seldom wears away completely.

Skin Layer	Location	Characteristics
Stratum corneum (horny layer)	Outermost layer	Many layers of keratinized, dead epithelial cells that are flattened and Nonnucleated
Stratum lucidum	Beneath the stratum corneum	A thin, translucent layer found only in the skin of the palms and soles.
Stratum granulosum	Beneath the stratum lucidum	One or more layers of granular cells that contain shrunken fibers of keratin and shriveled nuclei
Stratum malpighn	Beneath the stratum granulosum	Many layers of cells with centrally located, large oval nuclei and developing fibers of keratin
Stratum germmativum (basal cell layer)	Deepest layer	A single row of columnar cells that undergo mitosis; this layer also includes pigment-producing melanocytes.

In fact, the rate of cellular reproduction tends to increase in regions where the skin is being rubbed or pressed regularly. This response causes the growth of calluses on the palms and soles as well as the development of corns on the toes when poorly fitting shoes excessively rub the skin. 

The epidermis has important protective functions. It shields the moist underlying tissues against water loss, mechanical injury, and the effects of harmful chemicals. When it is unbroken, the epidermis prevents the entrance of many disease-causing microorganisms. 

Melanin, a pigment that occurs in deeper layers of the epidermis and is produced by specialized cells known as melanocytes, absorbs light energy. In this way, it helps to protect deeper cells from the damaging effects of ultraviolet rays of sunlight. 

Although melanocytes are usually located in the deepest portion of the epidermis and are the only cells that can produce melanin, the pigment also occurs in nearby epidermal cells. 

This happens because melanocytes have long cellular extensions that pass upward between neighboring epidermal cells, and they can transfer granules of melanin into these other cells by a process sometimes called cytocrine secretion. 

Consequently, nearby epidermal cells may contain more melanin than the melanocvtes. The protective stratum corneum of an infant's skin is relatively thin. Consequently, infant skin is more easily injured and more easily penetrated by chemicals than that of an adult