Exploring Anatomy & Physiology In The Laboratory 3rd Edition – Figure 1. Layers of skin. The skin is composed of two main layers: the epidermis, composed of closely packed epithelial cells, and the dermis, composed of dense, irregular connective tissue that contains blood vessels, hair follicles, sweat glands, and other structures. Beneath the skin is the hypodermis, which is mainly composed of loose connective and fatty tissue.

Although we don’t usually think of the skin as an organ, it’s actually made up of tissues that work together as a single structure to perform unique and critical functions. The skin and its accessory structures form the integumentary system, which provides complete protection to the body. The skin is composed of several layers of cells and tissues, which are held to the underlying structures by connective tissue (Figure 1). The deep layer of the skin is well vascularized (has numerous blood vessels). It also contains numerous sensory, and autonomic and sympathetic nerve fibers that ensure communication to and from the brain.

Exploring Anatomy & Physiology In The Laboratory 3rd Edition

Exploring Anatomy & Physiology In The Laboratory 3rd Edition

Figure 2. Thin skin versus thick skin. These slides show cross-sections of the epidermis and dermis of (a) thin and (b) thick skin. Note the significant variation in the thickness of the epithelial layer of the thick skin. From top, LM × 40, LM × 40. (Micrograph provided by the Regents of the University of Michigan Medical School © 2012)

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The epidermis is composed of keratinized, stratified squamous epithelium. It is composed of four or five layers of epithelial cells depending on its location in the body. It does not contain any blood vessels (ie, it is avascular). Skin with four layers of cells is called “thin skin”. From deep to superficial, these layers are the stratum basale, stratum spinosum, stratum granulosum, and stratum corneum. Most skin can be classified as thin skin. “Thick skin” is found only on the palms of the hands and the soles of the feet. It has a fifth layer, called the stratum lucidum, which is located between the stratum corneum and the stratum granulosum (Figure 2).

Figure 3. Epidermis. The epidermis is an epithelium composed of several layers of cells. The basal layer consists of cuboidal cells, while the outer layer consists of squamous, keratinized cells, so the epithelium as a whole is described as keratinized stratified squamous epithelium. LM × 40. (Micrograph provided by the Regents of the University of Michigan Medical School © 2012)

Cells in all layers except the stratum basale are called keratinocytes. A keratinocyte is a cell that produces and stores the protein keratin. Keratin is an intracellular fibrous protein that gives hair, nails, and skin their hardness and water-resistant properties. Keratinocytes in the stratum corneum are dead and regularly shed, replaced by cells in the deeper layers (Figure 3).

The stratum basale (also called stratum germinativum) is the deepest epidermal layer and connects the epidermis to the basal lamina, which lies beneath the layers of dermis. Cells in the stratum basale are attached to the skin by a tangle of collagen fibers, called the basement membrane. Finger-like projections or folds known as dermis papillae (plural = dermal papillae) are found in the superficial part of the skin. Dermal papillae increase the strength of the connection between the epidermis and the dermis; The greater the folding, the stronger the connection (Figure 4).

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Figure 4. Layers of the epidermis. The epidermis of thick skin consists of five layers: stratum basale, stratum spinosum, stratum granulosum, stratum lucidum, and stratum corneum.

The stratum basale is a layer of cells composed mainly of basal cells. A basal cell is a dense-shaped stem cell that is the precursor of the keratinocytes of the epidermis. All keratinocytes are formed from this single layer of cells, which continuously undergo mitosis to produce new cells. As new cells are formed, existing cells are pushed away from the superficial stratum basale. The other two cell types are found scattered among the basal cells in the stratum basale. The first is the Merkel cell, which acts as a receptor and is responsible for stimulating the sensory nerves that the brain perceives as touch. These cells are especially abundant on the surface of the hands and feet. The other is the melanocyte, a cell that produces the pigment melanin. Melanin gives hair and skin its color and helps protect living cells of the epidermis from damage by ultraviolet (UV) radiation.

In the growing fetus, fingerprints form where the cells of the stratum basale meet the papillae of the underlying dermal layer (papillary layer), resulting in the ridges on your fingers that you know as fingerprints. Fingerprints are unique to each individual and are used for forensic analysis because the patterns do not change with growth and aging processes.

Exploring Anatomy & Physiology In The Laboratory 3rd Edition

Figure 5. Cells of the epidermis. Cells in the different layers of the epidermis arise from the basal cells located in the stratum basale, although the cells in each layer are distinctly different. EM × 2700. (Micrograph provided by the Regents of the University of Michigan Medical School © 2012)

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As the name suggests, the stratum spinosum is spiky in appearance due to the protruding processes of cells that join cells through structures called desmosomes. Desmosomes connect to each other and strengthen the bonds between cells. It is interesting to note that the “spiky” appearance of this layer is an artifact of the staining process. Specimens of unstained epidermis do not exhibit this characteristic appearance. The stratum spinosum is composed of eight to 10 layers of keratinocytes, which form as a result of cell division in the stratum basale (Figure 5). The keratinocytes of this layer contain a type of dendritic cell called a Langerhans cell, which acts as a macrophage by engulfing bacteria, foreign particles, and damaged cells originating in this layer.

Keratinocytes in the stratum spinosum initiate the synthesis of keratin and release water-resistant glycolipids that help prevent water loss from the body, making the skin relatively waterproof. As new keratinocytes form above the stratum basale, the keratinocytes of the stratum spinosum are pushed into the stratum granulosum.

Further changes in keratinocytes give the stratum granulosum a granular appearance as they are pushed through the stratum spinosum. Cells (three to five layers deep) become flattened, their cell membranes thicken, and they produce large amounts of the protein keratin, which is fibrous, and keratohyalin, which accumulates within the cells as lamellar granules (see Figure 4). These two proteins make up the bulk of the keratinocyte mass in the stratum granulosum and give the stratum its granular appearance. As cells die, the nucleus and other cell organelles disintegrate, leaving behind keratin, keratohyalin, and cell membranes that form the stratum lucidum, stratum corneum, and accessory structures of hair and nails.

The stratum lucidum is a smooth, superficial translucent layer of the epidermis that lies 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 make up the stratum lucidum are dead and flattened (see Figure 4). These cells are filled with a clear protein rich in lipids, elidin, derived from keratohyalin, which gives these cells their transparent (ie clear) appearance and water barrier.

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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. Normally the stratum corneum consists of 15 to 30 layers of cells. This dry, dead layer helps prevent microbial penetration and dehydration of the underlying tissue and provides mechanical protection from abrasion to the more fragile, underlying layers. Cells in this layer are occasionally shed and replaced by cells pushed up from the stratum granulosum (or stratum lucidum in the case of the palms and soles). The entire layer is replaced over a period of about 4 weeks. Cosmetic procedures, such as microdermabrasion, help remove some of the dry, top layer and aim to keep the skin “fresh” and healthy.

Figure 6. Layers of the dermis. This stained slide shows the two components of the skin – the papillary layer and the reticular layer. Both are composed of connective tissue with collagen fibers extending from one to the other, making the boundary between the two somewhat blurred. Diffuse dermal papillae in the epidermis belong to the papillary layer, while the dense collagen fiber bundles below belong to the reticular layer. LM × 10. (Credit: “Kilbad”/work modified via Wikimedia Commons)

The dermis can be considered the “core” of the integumentary system (derma- = “skin”), which is distinct from the epidermis (epi- = “above” or “over”) and the hypodermis (hypo- = “below”). It contains blood and lymph vessels, nerves, and other structures such as hair follicles and sweat glands. Skin is made up of two layers of connective tissue that form an interconnected mesh of elastin.

Exploring Anatomy & Physiology In The Laboratory 3rd Edition

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