Showing posts with label stratum spinosum. Show all posts
Showing posts with label stratum spinosum. Show all posts
C. Langerhans' cells
C. Langerhans' cells: These star-shaped cells lack tonofilaments and occur mainly in the stratum spinosum (400-1000 cells/mm2 of skin surface). They stain selectively with gold chloride and contain numerous rodlike or racket-shaped cytoplasmic granules (Birbeck's granules). They are thought to be antigen-presenting cells that process and present to the lymphocytes any antigenic material that penetrates the skin's surface. Of mesodermal origin, they arise in bone marrow and may belong to the mononuclear phagocyte system. Langerhans' cells also occur in oral and vaginal epithelia as well as in the thymus.
B. Pigmentation System
B. Pigmentation System: Skin color is conferred mainly by the pigments melanin and carotene, the thickness of the epidermis, the number of dermal blood vessels, and the color of the blood in those vessels.
1. Melanins contribute to skin, eye, and hair color. Synthesized by melanocytes, they include the dark brown pigment eumelanin, found in the epidermis, iris, and brown and black hair; and the cysteine-rich pigment pheomelanin, found in red hair. 2. Melanocytes derive from the neural crest and migrate into the epidermis during em bryogenesis. Although they are scattered among the keratinocytes of the stratum basale, they are not attached to them by desmosomes. They have round cell bodies, central nuclei, and long cytoplasmic processes that pass between the cells of the strata basale and spinosum and terminate in small indentations on the keratinocyte surfaces. Melanocytes make up 10-25% of this layer's cells but do not participate in keratinization.
1. Melanins contribute to skin, eye, and hair color. Synthesized by melanocytes, they include the dark brown pigment eumelanin, found in the epidermis, iris, and brown and black hair; and the cysteine-rich pigment pheomelanin, found in red hair. 2. Melanocytes derive from the neural crest and migrate into the epidermis during em bryogenesis. Although they are scattered among the keratinocytes of the stratum basale, they are not attached to them by desmosomes. They have round cell bodies, central nuclei, and long cytoplasmic processes that pass between the cells of the strata basale and spinosum and terminate in small indentations on the keratinocyte surfaces. Melanocytes make up 10-25% of this layer's cells but do not participate in keratinization.
layers of the epidermis
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.
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.
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