Click Image to Enlarge. The skin is the body's largest organ. It covers the entire body. It serves as a protective shield against heat, light, injury, and infection. The skin also:. Your skin takes on different thickness, color, and texture all over your body. For example, your head contains more hair follicles than anywhere else. But the soles of your feet have none. The cells in all of the layers except the stratum basale are called keratinocytes. A keratinocyte is a cell that manufactures and stores the protein keratin.
Keratin is an intracellular fibrous protein that gives hair, nails, and skin their hardness and water-resistant properties. The keratinocytes in the stratum corneum are dead and regularly slough away, being replaced by cells from the deeper layers Figure 3.
Figure 3. The epidermis is epithelium composed of multiple layers of cells. The basal layer consists of cuboidal cells, whereas the outer layers are squamous, keratinized cells, so the whole epithelium is often described as being keratinized stratified squamous epithelium.
The stratum basale also called the stratum germinativum is the deepest epidermal layer and attaches the epidermis to the basal lamina, below which lie the layers of the dermis.
The cells in the stratum basale bond to the dermis via intertwining collagen fibers, referred to as the basement membrane. Dermal papillae increase the strength of the connection between the epidermis and dermis; the greater the folding, the stronger the connections made Figure 4.
Figure 4. Layers of the Epidermis. The epidermis of thick skin has five layers: stratum basale, stratum spinosum, stratum granulosum, stratum lucidum, and stratum corneum. The stratum basale is a single layer of cells primarily made of basal cells. A basal cell is a cuboidal-shaped stem cell that is a precursor of the keratinocytes of the epidermis. All of the keratinocytes are produced from this single layer of cells, which are constantly going through mitosis to produce new cells.
As new cells are formed, the existing cells are pushed superficially away from the stratum basale. Two other cell types are found dispersed among the basal cells in the stratum basale. The first is a Merkel cell , which functions as a receptor and is responsible for stimulating sensory nerves that the brain perceives as touch.
These cells are especially abundant on the surfaces of the hands and feet. The second is a melanocyte , a cell that produces the pigment melanin. Melanin gives hair and skin its color, and also helps protect the living cells of the epidermis from ultraviolet UV radiation damage.
In a growing fetus, fingerprints form where the cells of the stratum basale meet the papillae of the underlying dermal layer papillary layer , resulting in the formation of the ridges on your fingers that you recognize as fingerprints. Fingerprints are unique to each individual and are used for forensic analyses because the patterns do not change with the growth and aging processes. As the name suggests, the stratum spinosum is spiny in appearance due to the protruding cell processes that join the cells via a structure called a desmosome.
The desmosomes interlock with each other and strengthen the bond between the cells. Unstained epidermis samples do not exhibit this characteristic appearance.
The stratum spinosum is composed of eight to 10 layers of keratinocytes, formed as a result of cell division in the stratum basale Figure 5. Interspersed among the keratinocytes of this layer is a type of dendritic cell called the Langerhans cell , which functions as a macrophage by engulfing bacteria, foreign particles, and damaged cells that occur in this layer.
Figure 5. Cells of the Epidermis. The cells in the different layers of the epidermis originate from basal cells located in the stratum basale, yet the cells of each layer are distinctively different.
The keratinocytes in the stratum spinosum begin the synthesis of keratin and release a water-repelling glycolipid that helps prevent water loss from the body, making the skin relatively waterproof. As new keratinocytes are produced atop the stratum basale, the keratinocytes of the stratum spinosum are pushed into the stratum granulosum. The stratum granulosum has a grainy appearance due to further changes to the keratinocytes as they are pushed from the stratum spinosum.
The cells three to five layers deep become flatter, their cell membranes thicken, and they generate large amounts of the proteins keratin, which is fibrous, and keratohyalin , which accumulates as lamellar granules within the cells see Figure 4. These two proteins make up the bulk of the keratinocyte mass in the stratum granulosum and give the layer its grainy appearance.
The nuclei and other cell organelles disintegrate as the cells die, leaving behind the keratin, keratohyalin, and cell membranes that will form the stratum lucidum, the stratum corneum, and the accessory structures of hair and nails.
The stratum lucidum is a smooth, seemingly translucent layer of the epidermis located just above the stratum granulosum and below the stratum corneum. This thin layer of cells is found only in the thick skin of the palms, soles, and digits.
The keratinocytes that compose the stratum lucidum are dead and flattened see Figure 4. These cells are densely packed with eleiden , a clear protein rich in lipids, derived from keratohyalin, which gives these cells their transparent i. The stratum corneum is the most superficial layer of the epidermis and is the layer exposed to the outside environment see Figure 4.
The increased keratinization also called cornification of the cells in this layer gives it its name. There are usually 15 to 30 layers of cells in the stratum corneum.
This dry, dead layer helps prevent the penetration of microbes and the dehydration of underlying tissues, and provides a mechanical protection against abrasion for the more delicate, underlying layers.
Cells in this layer are shed periodically and are replaced by cells pushed up from the stratum granulosum or stratum lucidum in the case of the palms and soles of feet. The entire layer is replaced during a period of about 4 weeks. Figure 6. Layers of the Dermis. This stained slide shows the two components of the dermis—the papillary layer and the reticular layer. Both are made of connective tissue with fibers of collagen extending from one to the other, making the border between the two somewhat indistinct.
The dermal papillae extending into the epidermis belong to the papillary layer, whereas the dense collagen fiber bundles below belong to the reticular layer. It contains blood and lymph vessels, nerves, and other structures, such as hair follicles and sweat glands. Second, in areas of thick skin like this, they provide a large surface area, to nourish the epidermal layer. Don't forget the epidermis is a stratified squamous epithelium, so it does not have its own blood supply.
It relies solely on the blood supply from the dermis. The dermis is a connective tissue layer, that contains collagen and elastin fibres, and fibroblasts, macrophages and adipocytes, as well as nerves, glands and hair follicles.
The dermis is tough, and is the layer used to make leather. This layer contains loose connective tissue, and it has many capillaries. It extends up into the epidermis in small projections called dermal papillae. This region also contains Meissners corpuscles, which are touch receptors, as well as free nerve endings non-myelinated that are sensitive to temperature. The collagen bundles are woven into a coarse network.
This layer contains fibroblasts, macrophages and fat cells. The arteries supplying the skin are deep in the hypdermis. Branches from the arteries pass upwards to form a deep and a superficial plexus. It supplies the fatty tissue of the hypodermis, and the deeper parts of the dermis, including the capillaries for hair follicles, deep sebaceous glands and sweat glands.
The superficial subpapillary plexus lies just beneath the dermal papillae, and supplies the capillaries in the dermal papillae. The pink colour of skin is mainly due to the blood seen in venules of this plexus.
There are many arteriovenous anastomoses in the dermis, which can prevent blood from entering the superficial cutaneous plexus. This strategy is used as a response to cold as a way of conserving heat. The danger is that if the epidermis loses its blood supply for too long, it will die frostbite! Alternatively, when it is hot, more blood is allowed into the superficial plexus, and the skin flushes.
The blood in the superficial capillaries is cooled by the evaporation of sweat of the surface of skin.
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