Epidermis

The epidermis is composed of stratified squamous epithelium and contains four principal types of cells. About 90% of the epidermal cells are keratinocytes (ker-aTIN-b-sitq kerato = horny). They produce the protein keratin that helps waterproof and protect the skin and underlying tissues. Anchoring junctions called desmosomes weld keratinocytes to one another.

 

Melanocytes (MEL-a-no-sits), which produce the pigment melanin, comprise about 8% of the epidermal cells. Their long, slender projections extend between and transfer granules of melanin to keratinocytes. Melanin (melan = black) is a brown-black pigment that contributes to skin colour and absorbs ultraviolet (UV) light. Once inside keratinocytes, the melanin granules cluster to form a protective veil over the nucleus, on the side toward the skin surface. In this way they shield the genetic material from damaging UV light.

The third type of cell in the epidermis is known as a Langerhans (LANG-er-hans) cell. These cells arise from bone marrow and migrate to the epidermis. They interact with white blood cells called helper T cells in immune responses and are easily damaged by UV radiation.

A fourth type of cell found in the epidermis is called a Merkel cell. These 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.

Four or five distinct layers of cells form the epidermis. In most regions of the body the epidermis is about O. 1 mm thick and has four layers. Where exposure to friction is greatest, such as in the palms and soles, the epidermis is thicker (l to 2 mm) and has five layers. Constant exposure of thin or thick skin to friction or pressure stimulates formation of a callus, an abnormal thickening of the epidermis. (See Diagram)

The names of the five layers (strata), from the deepest to the most superficial, are:

1. Stratum basale. This single layer of cuboidal to columnar cells contains stem cells, which are capable of continued cell division, and melanocytes. The stem cells multiply, producing  keratinocytes,  which  push  up toward the surface and become part of the more superficial layers. The nuclei of the keratinocytes degenerate, and the cells die. Eventually, the cell remnants are shed from the top layer of the epidermis. Other stem cells in the stratum basale migrate into the dermis and give rise to sweat and oil glands and hair follicles. The stratum basale is sometimes referred to as the stratum germinativum (jer'-mi-na-Ti-vum) to indicate its role in germinating new cells.  The stratum basale also contains tactile (Merkel) discs that are sensitive to touch.

2. Stratum spinosum. This layer of the epidermis contains 8 to 10 rows (sheets) of polyhedral (many sided) cells that fit closely together. The cells here appear to be covered with prickly spines (spinosum prickly) because the cells shrink apart when the tissue is prepared for microscopic examination. At each spinelike projection, filaments of the cytoskeleton insert into desmosomes, which tightly join the cells to one another. Long projections of the melanocytes extend among the keratinocytes, which take in melanin by phagocytosis of these melanocyte processes.

3. Stratum granulosum. The third layer of the epidermis consists of three to five rows of flattened cells that develop darkly staining granules of a substance called keratohyalin (ker'-a-tb-HI -a-lin). This compound is the precursor of keratin, a protein found in the outer layer of the epidermis. Keratin forms a barrier that protects deeper layers from injury and microbial invasion and makes the skin waterproof. The nuclei of the cells in the stratum granulosum are in various stages of degeneration. As their nuclei break down, the cells can no longer carry on vital metabolic reactions, and they die. Stratum lucidum. Normally, only the thick skin of the palms and soles has this layer.

4. Stratum lucidum consists of three to five rows of clear, flat, dead cells that contain droplets of an intermediate substance that is formed from keratohyalin and is eventually transformed to keratin.

5. Stratum corneum. This layer consists of 25 to 30 rows of flat, dead cells completely filled with keratin. These cells are continuously shed and replaced by cells from deeper strata. The stratum corneum serves as an effective barrier against light and heat waves, bacteria, and many chemicals.

In the process of keratinization, cells newly formed in the basal layers undergo a developmental process as they are pushed to the surface. As the cells relocate, they accumulate keratin. At the same time the cytoplasm, nucleus, and other organelles disappear, and the cells die. Eventually, the keratinised cells slough off and are replaced by underlying cells that, in tum, become keratinised. The whole process by which a cell forms in the basal layer, rises to the surface, becomes keratinised, and sloughs off takes two to four weeks.

Epidermal growth factor (EGF) is a protein hormone that stimulates growth of epithelial and epidermal cells during tissue development, repair, and renewal. Certain oncogenes, genes that can turn normal cells into cancerous ones, cause tumours by permanently turning on EGF stimulation of cells, which then proliferate without control.

 

Dermis

The second principal part of the skin, the dermis, is composed of connective tissue containing

collagen and elastic fibres. The few cells in the dermis include fibroblasts, macrophages, and adipocytes. The dermis is very thick in the palms and soles and very thin in the eyelids, penis, and scrotum. It also tends to be thicker on the dorsal than the ventral aspects of the body and thicker on the lateral than the medial aspects of the extremities. Blood vessels, nerves, glands, and hair follicles are embedded in the dermis.

 
The outer portion of the dermis, about one-fifth of the thickness of the total layer, is named the papillary region (layer). It consists of areolar connective tissue containing fine elastic fibers. Its surface area is greatly increased by small, finger like projections called dermal papillae (pa-PIL-e; papilla = nipple). These nipple-shaped structures indent the epidermis, and many contain loops of capillaries. Some dermal papillae also contain tactile receptors called corpuscles of touch (Meissner's corpuscles), nerve endings that are sensitive to touch. Dermal papillae cause ridges in the overlying epidermis. It is these ridges that leave fingerprints on objects that are handled.

   The deeper portion of the dermis is called the reticular (rete = net) region (layer). It consists of dense, irregular connective tissue containing interlacing bundles of collagen and coarse elastic fibres. Within the reticular region, bundles of collagen fibres interlace in a netlike manner. Spaces between the fibres are occupied by a small quantity of adipose tissue, hair follicles, nerves, oil glands, and the ducts of sweat glands. Varying thicknesses of the reticular region contribute to differences in the thickness of skin.

The combination of collagen and elastic fibres 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 (STRI-e) or stretch marks.

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.