The humen skin diseases and skin function

The human skin diseases and skin function

Skin is the largest single organ of the body. It is 16% of total body weight and, it presenting 1.2-2.3 m2 of surface. The junction of dermis and eider-mis is  irregular, and projections of the dermis called papillae interdigitate with evaginations of the epidermis known as epidermal ridges. In three dimensions, these prestidigitation  may be off the peg-and-socket variety ( thin skin ) or formed of ridges and grooves ( thick skin ). Provides mechanical protection  to underlying tissue. Prevents loss of water and salt. Pigment present in epidermis protect tissues against harmful effect of light ( ultraviolet ray ). Plays an important role in
regulating body temperature and vitamin-D synthesis. It is very important sensory organ.

Stratum basale :The stratum basale consists of a single layer of ba sophilic columnar or cuboidal cells resting on the basal lamina at the dermal-epidermal junction. Desmosomes bind the cells of this layer together in their lateral and upper surfaces.Hemidesmo-somes, found in the basal plasmalemma, help bind these cells to the basal lamina. The stratum basale 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.The human epidermis is renewed about every 15-30 dayes, depending on age, the region of the body and other factors.
Stratum spinosum : The stratum spinosum consists of cuboidal or slightly flattened, cells with a central nucleus or slightly flattened cells with a central nucleus and a cytoplasm whose processes are filled  with bundles of keratin filaments. These bundles converge into many small cellular extensions, terminating with dismosomes that punctuate the cell surface, giving a spine-studded appearance. These tonofilament bundles, visible under the light micriscope, are called tonofibrils ; they end at and insert into the cytoplasmic densities of the desmosomes.The filaments play and important role in the maintaining cohesion among cells and the effects of
abrasion. The epidermis of areas subjected to continuous friction and pressure  ( such as soles of the feel ) has a thicker stratum spinosum with more abundant tonofibrils and desmosomes. All mitoses are confined to what is termed the malpighian layer, which consists of both the stratum basale and the stratum spinosum.
Stratum granulosum : The stratum granulosum consists of three to five layers of flattened polygonal cells whose cytoplasm is filled with coarse basophilic granules called keratohyalin granules. These granules contain a phosphorylated histidine-rich protein as well as proteins containing cystine. The numerous phosphate groups account for the intense basophillia of keratohyalin granules, which are not surrounded by a membrane.
Stratum lucidum : More apparent in thick skin, the stratum lncidum is a translucent, thin layer of extremely flattened eosinophilic cells. The organelles and nuclei are not 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.
Stratum corneum : The stratum corneum consists of 15-20 layers of flattened nonnucleated keratinized cells whose cytoplasm is filled with a birefringent filamentous scleroprotein, keratin. After keratinzation, 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    thick                                    
skin, the stratum granulosum and the stratum lucidum are often less well developed,and the stratum corneum may be quite thin.Inpsoriasis, a common skin disease, there is an increase in the number of proliferrating 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 -7 day's instead of 15-30 day's

Mediacl Applications

In humans,lack of cortisol from the adrenal cortex causes overproduction of adrenocorticotropic hormone, which increases the pigmentation of the skin.An example of this is addison disease,which is caused by dysfunction of the adrenal glands.Albinism, a hereditary inability if the rnelanocytes to systhesize melanin,is caused by the absence of tyrosinase activity or the inability of cells to take up ty'rosine.As a result, the skin is not protected from solar radiation by melanin,and there is a greater incidence of basal and squamous cell carcinomas.The degeneration and disappearance of entire melanoey'tes results in a depigmentation disorder
called vitiligo. Langerhans cells:Langerhans cells, star-shaped cells found mainly in the stratum spinosum of the epidermis,represent 2-8% of the epidermis cells.They are bone marrow-derived macrophages that are capable of binding, processing and presenting antigens to T lymphocytes,thus participating in the stimulation of these cells.Consequently,they have a significant role in immunologic skin  reactions. Merkel's cells:Merkel's cells,generally present in the thick skin of palms arid soles,somewhat resemble the epidermal cells but
have small dense granules in their cytoplasm. These cells may serve as sensory mechanoreceptors, although other evidence suggests that they have functions related to the diffuse neuroendocrine system. Immunological activity in the skin:Because of its large size has an impressive number of lymphocytes and antigen-presenting cells (Langerhans cells),and because of its location it is in close contact with many antigenic molecules.For these reasons,the epidermis has an important role in some types of immune response.Most lymphocytes found in the skin are ''hormed'' in the epidermis.

Dermis

The dermis is the connective tissue that supports the epidermis and brinds it to the subcutaneous tissue (hypodermis).It contains blood vessels,lymphatics and nerves of the skin.It also contain epidermal derivatives as the hair follicles,sweat and sebaceous glands. Thin papillary layer:Composed of loose connective tissue;fibroblasts and other connective tissue cells,such as must cells and macrophages,are present.Extravasated leukocytes are also seen.The papillary layer is so called because it constitutes the major
part of the dermal papillae. From this layer, special collagen fibrils insert into the basal lamina and extend into the dermis. They  bind the dermis to the epidermis and are called anchoring fibrils.
Reticular layer : Thicker,composed of irregular dense connective tis sue (mainly type l collagen),and therefore has more fibers and fewer cells than does the papillary layer. The principal glycosaminoglycan is dermatan sulfate. The dermis contains a network of fibers of the elastic system, with the thicker fibers characteristically found in the reticular layer. This elastic network is responsible for the elasticity of the skin.

Medical applications:Abnormalities of the dermal-epidermal junction can lead to one type of blastering disorder (bulbus pemphigoid).
Another type of blistering disorder (pemphigus) is caused by the loss of intercellular junctions between keratinocytes. In several disorders,such as cutis laxa and ehlersdanlos syndrome,there is a considerable increase in skin and ligament extensibility'caused by defective collagen-fibril processing.The dermis has a rich network of blood and lymph vessels. In certain areas of the skin,blood can pass directly from arteries to veins through arteriovenous anastomoses,or shunts play a very importent role in temperature regulation.There is a rich supply of nerves in the dermis,and the effector nerves to the skin are postganglionic fibers of sympathetic ganglia of the paravertebral chain.No parasympathetic innervation is present.The afferent nerve endings from a superficial dermal network with free nerve endings, a hair follicle network, and the innervations of encapsulated sensory organs ( Meissner's and pacinian corpuscles ).
Subcutaneous tissue : The subcutaneous tissue layer consists of loose connective tissue that binds the skin loosely to the subjacent organs, making it possible for the skin to slide over them. The hypodermis often contains fat cells that very in number according to nutritional state. This layer ia also referred  to as the superficial fascia and, where thick enough, the panniculus adiposus.
Hair   
Hairs are elongated keratinized structures derived from invaginations of epidermal epithelium. It is a flexible horney thread composed of cornified cells. Their color, size, and region of the body. Hairs are found everywhere on the body except on the palms, soles, lips glans penis, clitoris, and labia minora. Hair growth in such regions of the body as the scalp, sace and pubis is strongly influenced not only by sex hormones-especially androgens-but also by adrenal and thyroid hormones. Each hair arises from an epidermal
invagination, the hair follicle that during its growth period has a terminal dilatation called a hair bulb. At the basa of the hair bulb, a dermal papilla can be observed. The dermal papilla can be observed. The dermal papilla contains a capillary network that is vital in sustaining the hair follicle. The loss of blood flow or the vitality of the dermal papilla with result in death of the follical. The epidermal cells covering this dermal papilla from the hair root that produces and is continuous with the hair shaft, which protrudes beyond the
skin. In certain types of thick hairs, the cells of the central region of the root at the apex of the dermal papilla produce large, vacuolated, and moderately keratinized cells that from the medulla of the hair. Root cells multiply and defferentiate into heavily keratinized  
compactly grouped fusiform cells that form the hair cortex.
Nail                           
A similar process of keratinization produces the nails, which are hard, flexible plates of keratin on the dorsal surface of each distal phalanx. Nails are plates of keratinized epithelial cells on the dorsal surface of each distal phalanx.It is a modified epidermis groove is the nail root. The epithelium of the fold of skin covering the nail root consists of the usual layers of cells . The stratum corneum of this epithelium of cells. The stratum corneum of this epithelium forms the eponychium,or cuticle.