The epidermis.

Two major zones of epidermis can be distinguished: an inner region of viable, moist cells termed the malpighian layer and the outer layer of anucleate, flattened, nonviable, desccicated cells known as the stratum corneum, or horhy layer. Thee substrata of living cells are recognized: the basal, spinous, and granular layers. Theses 3 layers represent progressive stages of differentiation and keratinization of the living as they move toward the skin’s surface to become stratum corneum. Thus, the columnar basal cells continually divide and give birth daughter cells that are displaced toward the skin surface. Here in the spinous stratum they are polyhedral, but they flatten as they move into the granular layer. The spinous cells adhere to one another mechanically by numerous attachment devices called desmosomes, which represent complex modification of the cells membrane. Large bundles of submicroscopic keratin filament (formerly called tonofilaments) synthesized in the spinous cells attach or “loop through” the desmosomes. To early observers, these desmosomal junctions imparted a spinous profile to the cells in the mid portion of the stratum germinativum – hence the designation “stratum spinosum”.

As the cells move from basal to the spinous layer and become more flattened, several additional structural changes occur. Keratin filaments form a network within the cytoplasm, and refractile keratohyaline granules and submicroscopic lamillar granules appear. Bundles of keratin filaments fill the cytoplasm and form intracellular networks that extend across cells and loop about the perinuclear space of the cells in the stratum spinosum.

Keratin filaments of the epidermis – hair and wool – are structurally homologous fibrous proteins composed of “building blocks” of chains of fibrous proteins assembled around one another in an a- helix coil.

Keratohyaline granules are synthesized in the upper germinative layer account for the distinctive granules in the granular layer. Keratohyaline granules are probably composed of both sulfur-rich proteins (cysteine residues) and phosphorylated histidine-rich protein.

Also in the upper layers of the spinous layer are lamellar granules. These structures contain lipid lamellae that appear to be extruded from the cells into the intercellular spaces. The function of the lamellar granules is unknown but may have something to do with both accumulating lipid materials between the stratum corneum cells that add to the barrier properties of the corneum as well as adding to the thickening of the corneum cell membranes.

The final stages of keratinization – the abrupt change from the granular layer to the stratum corneum – is attended by a variety of dramatic morphologic and biochemical degenerative alterations, including the degradation of cellular organelles and nuclei (carried out by lysosomal hydrolytic enzymes) as well as by the appearance of thickened cellular envelope of stratum corneum cells that provides them with the useful property of extreme resistance to degradation by various chemicals. The cornified layer consists of up to 25 layers of anucleate dead cells held tightly together by means of desmosomes. Horny cells at the surface of the skin are continuously and imperceptibly shed. Normally, 0,5-1 g of horny cells are lost daily.

The basal layer of the epidermis has a permanent population of germinal cells known as stew cells whose progeny undergo the specific pattern of differentiation (keratinization) just described in a period of 28-42 days. New epidermal cells continually form to compensate for those that exfoliate. Cells in the germinativum are reguire 12-14 days to move from the basal layer to the stratum corneum and another 14 days for the corneum cells to be shed.

The control of this continuouse mitotic activity in the basal layer may be mediated by intracellular concentration of cAMF/cGMF, prostaglandins, and other compounds derived from arachidonic acid, polyamines, calcium, glucocorticoids, and variety of additional regulatory proteins such as chalones and epidermal growth factor.

Precise control of proliferation of basal cells and their subsequent orderly differentiation into keratinized stratum corneum cells result in a smooth, pliable skin surface. Alterations in the homeostatic state of basal cell division, defects in epidermal differentiation, or alterations in exfoliation of the stratum corneum cells all may lead to irregularities in the skin surface, characterized clinically as roughening, scaling, and fissuring of the skin. Thus, a variety of skin diseases present with alterations in the skin surface due to defects in these processes, ie,

Increased cell division is and turnover of basal cells (psoriases), altered keratinization (Darier’s disease), and defects in exfoliation (ichthyoids vulgaris).

Organisms must be protected from their environment to survive. In humans, a large measure of this protection is afforded by the epidermis. The major barrier function resides in the stratum corneum. The low permeability of this horny layer not only effectively retards water loss from the inner milieu but also shields against damage from the environment, preventing entrance of toxic substances, allergens, and infectious agents. The barrier properties of the horny layer are of practical importance in the physician’s care of patients from several viewpoints. First, excessive drying or inflammatory reactins of the skin lead to roughness and scaling as the normally uniformly packed layers of stratum corneum are disrupted. If extensive areas of the horny layer are disrupted there is impressive transepidermal water loss such that total fluid loss can be appreciable, contributing to life-threatening fluid and electrolyte imbalance. Second, with breaks in the horny layer, external substances gain entrance to the underlying epidermis. Haptens and antigens may induce delayed hypersensitivity and contact dermatitis. Unwanted absorption of toxic chemicals may damage underlying living cells directly. Third, the disruption of the barrier layer increases the chance of colonization by pathogenic bacteria.

The percutaneous absorbtion of various topical medications used in treating inflammatory skin conditions is regulated by the stratum corneum. Most medications are absorbed by passive diffusion.

Дата добавления: 2015-08-03