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## Integumentary System ### Stratum Granulosum The stratum granulosum (gran-u-lo'sum), or granular layer, consists of three to five layers of keratinocytes superficial to the stratum spinosum. Within this stratum begins a process called keratinization (ker'a-tin-i-za'shun), where the keratinocytes...

## Integumentary System ### Stratum Granulosum The stratum granulosum (gran-u-lo'sum), or granular layer, consists of three to five layers of keratinocytes superficial to the stratum spinosum. Within this stratum begins a process called keratinization (ker'a-tin-i-za'shun), where the keratinocytes synthesize significant amounts of the protein keratin. The granules seen in this layer contain proteins that will help aggregate the keratin filaments in the stratum corneum. This accumulation of keratin causes both the nucleus and the organelles of these cells to disintegrate, which ultimately results in the death of these cells. Keratinization is not complete until the keratinocytes reach the more superficial epidermal layers. A fully keratinized cell is dead because it has neither a nucleus nor organelles, but it is structurally strong because of the keratin it contains. ### Stratum Lucidum The stratum lucidum (lū'si-dum), or clear layer, is a thin, translucent region of about two to three keratinocyte layers that is superficial to the stratum granulosum. This stratum is found only in the thick skin within the palms of the hands and the soles of the feet. Keratinocytes occupying this layer are flattened, pale cells with indistinct boundaries. They are filled with the translucent protein called eleidin (ē-lē'i-din), which is an intermediate product in the process of keratin formation. ### Stratum Corneum The stratum corneum (kōr'nē-ŭm; corneus = horny), or hornlike layer, is the most superficial layer of the epidermis. It is the stratum you see when you look at your skin. The stratum corneum consists of about 20 to 30 layers of dead, scaly, interlocking, keratinized cells. The dead keratinocytes are anucleate (lacking a nucleus) and are tightly packed together. The keratinized, or cornified, cells of the stratum corneum layer contain large amounts of keratin. After keratinocytes are formed from stem cells within the stratum basale, they change in structure and in their relationship to their neighbors as they progress through the different strata until they eventually reach the stratum corneum and are sloughed off its external surface. Major changes during keratinocyte migration include synthesis of keratin and loss of the nucleus and organelles as described. What remains of these keratinocytes in the stratum corneum is essentially keratin protein enclosed in a thickened plasma membrane. The thickening of the plasma membrane is due to a coating of lipids, which helps form the water barrier in the epidermis. Migration of the keratinocyte to the stratum corneum occurs during the first 2 weeks of the keratinocyte's life. The dead, keratinized cells usually remain for an additional 2 weeks in the exposed stratum corneum layer, providing a barrier before they are shed. Overall, individual keratinocytes are present in the integument for about 1 month following their formation. The stratum corneum presents a thickened surface unsuitable for the growth of many microorganisms. Additionally, some exocrine gland secretions (e.g., sweat, which contains demicidin, an antimicrobial peptide) help prevent the growth of microorganisms on the epidermis, thus supporting its barrier function (see section 22.3a). ### Variations in the Epidermis The epidermis exhibits variations between different body regions within one individual as well as differences between individuals. The epidermis varies in its thickness, coloration, and skin markings. #### Thick Skin Versus Thin Skin Although the skin ranges in thickness from 1.5 mm to 4 mm (see section 6.1), its thickness over most of the body is between 1 mm and 2 mm. However, skin is classified as either thick or thin based on the number of layers and thickness of the stratified squamous epithelium only, rather than the thickness of the entire integument (figure 6.3). Thick skin is found on the palms of the hands and the soles of the feet. All five epidermal strata occur in the thick skin. The epidermis of thick skin is between 0.4 mm and 0.6 mm thick. It houses sweat glands but has no hair follicles or sebaceous (oil) glands. Thin skin covers most of the body. It lacks a stratum lucidum, so it has only four specific layers in the epidermis. Thin skin contains the following structures: hair follicles, sebaceous glands, and sweat glands. The epidermis of thin skin is between 0.075 mm to 0.150 mm thick. ### Learning Strategy 6.1 Integrate lab and lecture material: Follow these steps to help you identify the epidermal strata using light microscopy: 1. Determine if the layer is closer to the free surface or is deeper. Remember the stratum corneum forms the free surface, whereas the stratum basale forms the deepest epidermal layer. 2. Examine the shape of the keratinocytes. The stratum basale contains cuboidal to low columnar keratinocytes, the stratum spinosum contains polygonal keratinocytes (which appear "spiny"), and the stratum lucidum and corneum contain squamous keratinocytes. 3. See if the keratinocytes have a nucleus or are anucleate. When they are still alive (as in the strata basale, spinosum, and granulosum), you are able to see nuclei. The stratum lucidum and corneum layers contain dead, anucleate keratinocytes. 4. Count the layers of keratinocytes in the stratum. The stratum basale has only one layer of keratinocytes, and the stratum corneum contains 20 to 30 layers of keratinocytes. The other layers contain about two to five layers of keratinocytes. 5. Determine if the cytoplasm of each keratinocyte contains visible granules. If the keratinocytes contain visible granules, you likely are looking at the stratum granulosum. *** This text is describing the structure of the epidermis. It starts by discussing the different layers of the epidermis and how they are organized from the bottom layer (stratum basale) to the top layer (stratum corneum). Then it goes on to discuss the keratinization process, which is responsible for the formation of the tough, protective outer layer of skin. The document then focuses on the differences between "thick" and "thin" skin. Lastly, it provides a learning strategy to identify the different layers of the epidermis under a microscope. An image of the epidermis is included, with the different layers highlighted. It is not included in this markdown.