1. Stratum basale (stratum germiuativum). This single layer of columnar basophilic keratinocytes rests on the basal lamina that separates epidermis from dermis. These cells divide continuously and give rise to the keratinocytes in all other layers. They attach to their neighbors by desmosomes and to the basal lamina by hemidesmosomes. Cytokeratin intermediate filaments (tonofilaments) are important components of both junctions. The cytokeratin content increases as these cells approach the stratum corneum, where it constitutes about 50% of their total protein. The basophilia of the basal layer is caused by ribosomes.
2. Stratum spinosum. This comprises several layers of large keratinocytes overlying the stratum basale. The cells are cuboidal or polygonal in the deeper layers and slightly flattened in the upper layer. Tonofibrils (tonofilament bundles) fill the cytoplasm, extend into the numerous cell processes that give these cells their spiny appearance, and insert into the desmosomes that attach the tips of these processes to those of adjacent cells. The mitotic rate here is lower than in the stratum basale. Mitosis occurs only in the malphighian layer, which includes the stratum basale and stratum spinosum.
3. Stratum granulosum. This lies above the stratum spinosum and, in thick skin, consists of 3-5 layers of flattened polygonal (often diamond-shaped) cells that contain numerous mem braneless keratohyalin granules. The intense basophilia of these granules is caused by their content of a phosphorylated histidine-rich precursor of the protein filaggrin. Cells in this layer also contain small ovoid or rodlike lamellar granules. These fuse with the plasma membrane and release their contents (glycosaminoglycans and phospholipids) into the inter cellular spaces. This material may be important in sealing the deeper layers of the skin from the external environment and in protection from dehydration.
4. Stratum lucidum, This layer overlies the stratum granulosum and is apparent only in thick skin. It is a narrow, acidophilic, translucent band of flattened keratinocytes whose nuclei, organelles, and intercellular borders are not visible. The cytoplasm contains dense cytokeratin aggregates embedded in an amorphous electron-dense matrix derived from the keratohyalin granules. This intracellular mixture of intermediate filaments and matrix con stitutes the immature keratin, sometimes called eleidin,
5. Stratum corneum, The outermost layer, this consists of many layers of dead, platelike enucleate keratinocytes with thickened plasma membranes. These cells represent the final stage of keratinization and are filled with mature keratin, a birefringent scleroprotein consisting of at least 6 polypeptides. The molecular weights of the polypeptides of mature keratin in the stratum corneum is higher than those of immature keratin in deeper, less differentiated cells. Keratin's substructure includes tonofilament subunits formed by 3 coiled and intertwined polypeptide chains. Nine of these subunits coil together to form each IO-nm thick intermediate filament. As they aggregate end to end, the tonofilament increases in length. Tonofilaments are embedded in and bound together by the amorphous matrix first found in keratohyalin granules. Dead cells are continuously sloughed (exfoliated) from the surface and replaced, through successive waves of mitosis and differentiation, by cells from the deeper waves.
EPIDERMIS
The epidermis contains 2 major and 2 minor cell populations specialized for specific functions. Major populations include the keratinocytes and melanocytes. Minor populations include Langerhans' and Merkel's cells.
A. Keratinizing System: The keratinocytes make up most of the epidermis. They participate in the continuous turnover (renewal) of the skin surface by passing through 4 overlapping processes: cell renewal, or mitosis; differentiation, or keratinization; cell death; and exfoliation (the sloughing of dead cells from the skin surface). The entire process takes 15-30 days and occurs in waves. A cell layer produced by a mitotic wave in the basal layer undergoes keratinization in synchrony. Each wave pushes the cell layers produced in earlier waves toward the surface. The layers from several waves, each at a different depth and step in the process, give a stratified appearance to vertical sections of the epidermis. The 5 layers of the epidermis are distinguished by the shape, staining properties, contents, and orientation of the keratinocytes they contain.
A. Keratinizing System: The keratinocytes make up most of the epidermis. They participate in the continuous turnover (renewal) of the skin surface by passing through 4 overlapping processes: cell renewal, or mitosis; differentiation, or keratinization; cell death; and exfoliation (the sloughing of dead cells from the skin surface). The entire process takes 15-30 days and occurs in waves. A cell layer produced by a mitotic wave in the basal layer undergoes keratinization in synchrony. Each wave pushes the cell layers produced in earlier waves toward the surface. The layers from several waves, each at a different depth and step in the process, give a stratified appearance to vertical sections of the epidermis. The 5 layers of the epidermis are distinguished by the shape, staining properties, contents, and orientation of the keratinocytes they contain.
GENERAL FEATURES OF THE SKIN
A. General Functions: The skin is the largest and heaviest organ. It protects against microorganisms, toxic substances, dehydration, ultraviolet radiation, impact, and friction. It also acts as a sensory receptor and has a role in excretion, vitamin D metabolism, and regulation of blood pressure and body temperature.
B. General Organization: Human skin (the integument) is of 2 types. Thick skin, restricted to the palms of the hands and soles of the feet, lacks hairs and has abundant sweat glands. Thin skin, which has hairs, covers the rest of the body. Thick or thin, the skin consists of 2 distinct but tightly attached layers, the epidermis and dermis, which are underlain by the hypodermis.
1. Epidermis. This outer (superficial) layer of skin, composed of keratinized stratified squamous epithelium, derives from embryonic surface ectoderm. It is avascular, receiving nourishment from vessels in the underlying dermis. Its only innervation is by unencapsulated (free) nerve endings. The epidermal layer is further divided into 5 stratea; these layers, in order from superficial to deep, are the stratum corneum, stratum lucidum, stratum granulosum, stratum spinosum, and stratum basale. The thickness of these layers differs in thick and thin skin.
2. Dermis, This inner (deeper) layer is a vascular connective tissue of mesodermal origin. It can be further divided into a superficial papillary layer and a deeper reticular layer. The papillary layer contains extensive capillary networks, which nourish the epidermis. The reticular layer contains many arteriovenous anastomoses that help regulate blood pressure and body temperature. It is richly supplied with free nerve endings, a variety of encapsulated sensory receptors, and autonomic fibers that control the vascular smooth muscle. Even in thick skin, the dermis is much thicker than the overlying epidermis.
3. Hypodermis, Although not a part of the skin, this layer of mesoderm-derived loose connective and adipose tissue underlying the dermis flexibly binds the skin to deeper structures. Its thickness varies, depending on nutritional status, level of activity, body region, and gender. The hypodermis is also called the subcutaneous fascia and, where thick enough, the panniculus adiposus
Structures Associated With the Skin: Glands (sebaceous and sweat), hairs, and nails arise from epidermal downgrowths into the dermis during embryonic development. These structures, which are mainly of epithelial origin, require epitheliomesenchymal interactions between the epidermis and dermis for their formation and maintenance
B. General Organization: Human skin (the integument) is of 2 types. Thick skin, restricted to the palms of the hands and soles of the feet, lacks hairs and has abundant sweat glands. Thin skin, which has hairs, covers the rest of the body. Thick or thin, the skin consists of 2 distinct but tightly attached layers, the epidermis and dermis, which are underlain by the hypodermis.
1. Epidermis. This outer (superficial) layer of skin, composed of keratinized stratified squamous epithelium, derives from embryonic surface ectoderm. It is avascular, receiving nourishment from vessels in the underlying dermis. Its only innervation is by unencapsulated (free) nerve endings. The epidermal layer is further divided into 5 stratea; these layers, in order from superficial to deep, are the stratum corneum, stratum lucidum, stratum granulosum, stratum spinosum, and stratum basale. The thickness of these layers differs in thick and thin skin.
2. Dermis, This inner (deeper) layer is a vascular connective tissue of mesodermal origin. It can be further divided into a superficial papillary layer and a deeper reticular layer. The papillary layer contains extensive capillary networks, which nourish the epidermis. The reticular layer contains many arteriovenous anastomoses that help regulate blood pressure and body temperature. It is richly supplied with free nerve endings, a variety of encapsulated sensory receptors, and autonomic fibers that control the vascular smooth muscle. Even in thick skin, the dermis is much thicker than the overlying epidermis.
3. Hypodermis, Although not a part of the skin, this layer of mesoderm-derived loose connective and adipose tissue underlying the dermis flexibly binds the skin to deeper structures. Its thickness varies, depending on nutritional status, level of activity, body region, and gender. The hypodermis is also called the subcutaneous fascia and, where thick enough, the panniculus adiposus
Structures Associated With the Skin: Glands (sebaceous and sweat), hairs, and nails arise from epidermal downgrowths into the dermis during embryonic development. These structures, which are mainly of epithelial origin, require epitheliomesenchymal interactions between the epidermis and dermis for their formation and maintenance
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