The skin is the heaviest single organ of the body, accounting for about 16% of total body weight.
It is composed of the EPIDERMIS, DERMIS, HYPODERMIS .
The junction of dermis and epidermis is irregular, and projections of the dermis called PAPILLAE interdigitate with evaginations of the epidermis known as EPIDERMAL RIDGES.
Epidermal derivatives include HAIRS, NAILS, AND SEBACEOUS AND SWEAT GLANDS.
Beneath the dermis lies the hypodermis (Gr. hypo, under, + derma, skin), or subcutaneous tissue, a loose connective tissue that may contain a pad of adipose cells. The hypodermis, which is not considered part of the skin, binds skin loosely to the subjacent tissues and corresponds to the superficial fascia of gross anatomy.
The junction of dermis and epidermis is irregular, and projections of the dermis called papillae interdigitate with evaginations of the epidermis known as epidermal ridges. In three dimensions, these interdigitations may be of the peg-and-socket variety (thin skin) or formed of ridges and grooves (thick skin). Epidermal derivatives include HAIRS, NAILS, AND SEBACEOUS AND SWEAT GLANDS. Beneath the dermis lies the hypodermis (Gr. hypo, under, + derma, skin), or subcutaneous tissue, a loose connective tissue that may contain a pad of adipose cells, the panniculus adiposus. The hypodermis, which is not considered part of the skin, binds skin loosely to the subjacent tissues and corresponds to the superficial fascia of gross anatomy.
STRATUM BASALE (STRATUM GERMINATIVUM)
The stratum basale consists of a single layer of basophilic columnar or cuboidal cells resting on the basement membrane at the dermale-pidermal junction .Desmosomes bind the cells of this layer together in their lateral and upper surfaces.
The stratum basale, containing stem cells, is characterized by intense mitotic activity and is responsible, in conjunction with the initial portion of the next layer, for constant renewal of epidermal cells.
All cells in the stratum basale contain intermediate keratin filaments about 10 nm in diameter. As the cells progress upward, the number of filaments increases until they represent half the total protein in the stratum corneum.
The stratum spinosum consists of cuboidal, or slightly flattened, cells with a central nucleus and a cytoplasm whose processes are filled with bundles of keratin filaments. These bundles converge spiny projections .The cells of this layer are firmly bound together by the filament-filled cytoplasmic spines and desmosomes that punctuate the cell surface, providing a spine-studded appearance. These keratin bundles, visible under the light microscope, are called tonofilaments; they end at and insert into the cytoplasmic densities of the desmosomes. The filaments play an important role in maintaining cohesion among cells and resisting the effects of abrasion. The epidermis of areas subjected to continuous friction and pressure (such as the soles of the feet) has a thicker stratum spinosum with more abundant tonofilaments and desmosomes.
Stratum spinosum of the skin from the sole of the foot (thick skin) showing the spiny projections that strongly bind the cells of this layer together to resist abrasion.
The stratum granulosum consists of three to five layers of flattened polygonal cells whose cytoplasm is filled with coarse basophilic granules ,called keratohyalin granules. The proteins of these granules contain a phosphorylated histidine-rich protein as well as proteins containing cystine. The numerous phosphate groups account for the intense basophilia of keratohyalin granules, which are not surrounded by a membrane.
Another characteristic structure in the cells of the granular layer of epidermis that can be seen with the electron microscope is the membrane-coated lamellar granule, a small ovoid or rodlike structure containing lamellar disks that are formed by lipid bilayers. These granules fuse with the cell membrane and discharge their contents into the intercellular spaces of the stratum granulosum, where they are deposited in the form of sheets containing lipid. The function of this extruded material is similar to that of intercellular cement in that it acts as a barrier to penetration by foreign materials and provides a very important sealing effect in the skin. Formation of this barrier, which appeared first in reptiles, was one of the important evolutionary events that permitted development of terrestrial life.
More apparent in thick skin, the stratum lucidum is a translucent, thin layer of extremely flattened eosinophilic epidermal cells .The organelles and nuclei are no longer evident, and the cytoplasm consists primarily of densely packed keratin filaments embedded in an electron-dense matrix. Desmosomes are still evident between adjacent cells.
The stratum corneum consists of 15-20 layers of flattened nonnucleated keratinized cells whose cytoplasm is filled with a birefringent filamentous scleroprotein, keratin.
After keratinization, the cells consist of only fibrillar and amorphous proteins and thickened plasma membranes; they are called HORNY CELLS. During keratinization, lysosomal hydrolytic enzymes play a role in the disappearance of the cytoplasmic organelles. These cells are continuously shed at the surface of the stratum corneum.
This description of the epidermis corresponds to its most complex structure in areas where it is very thick, as on the soles of the feet. In thin skin, the stratum granulosum and the stratum lucidum are often less well developed, and the stratum corneum may be quite thin .
In PSORIASIS, a common skin disease, there is an increase in the number of proliferating cells in the stratum basale and the stratum spinosum as well as a decrease in the cycle time of these cells. This results in greater epidermal thickness and more rapid renewal of epidermis.
In bullous pemphigoid and related disorders, the affected patient has an antibody which reacts against a specific antigen (BPAG) located in the hemidesmosomes and lamina lucida; an antigen-antibody reaction occurs triggering a sequence of changes damaging the basement membrane and leading to separation of the epidermis and blistering.
Section of the stratum spinosum showing the localized deposits of melanin covering the cell nuclei. Melanin protects the DNA from the UV radiation of the sun. This explains why people with light skin have a higher incidence of skin cancer than people with dark skin. The highest concentration of melanin occurs in the cells that are more deeply localized; these cells divide more actively. (The DNA of cell populations that multiply more actively is particularly sensitive to harmful agents.)