Skin Structure Biology PDF

**Skin Structure** Introduction The skin, along with its accessory structures (eg, nails, hair, oil glands, sweat glands), make up the body\'s **integumentary system** (Figure 19.1). Skin consists of multiple layers, which are arranged to form two main structural elements (ie, main skin layers), t...

**Skin Structure** Introduction The skin, along with its accessory structures (eg, nails, hair, oil glands, sweat glands), make up the body\'s **integumentary system** (Figure 19.1). Skin consists of multiple layers, which are arranged to form two main structural elements (ie, main skin layers), the **epidermis** and **dermis**. The dermis rests on a deeper layer of connective tissue called the **hypodermis** (ie, subcutaneous layer), which is not technically part of the skin but contributes to the function of the integumentary system. This lesson outlines important integumentary system structures and their respective locations in the skin. **Figure 19.1** Integumentary system components. 19.1.01 Epidermis The uppermost (ie, most superficial) layer of the skin is the **epidermis**. The epidermis is a **stratified squamous epithelium** that consists of multiple types of [epithelial cells](javascript:void(0)), including keratinocytes, melanocytes, Langerhans cells, and Merkel cells. A diagram of a skin structure Description automatically generated Chapter 19: Skin System 601 The epidermis is divided into five structurally distinct layers (strata). From deepest to most superficial, these layers include the **stratum basale**, **stratum spinosum**, **stratum granulosum**, **stratum lucidum** (present only in skin found on the palms of the hands and soles of the feet), and **stratum corneum** (Figure 19.2). **Figure 19.2** Layers of the epidermis. The stratum basale consists of a single row of [stem cells](javascript:void(0)) that continually divide to give rise to new stem cells. Of the two daughter cells produced from each [mitotic division](javascript:void(0)) in the stratum basale, one cell remains in the basal layer (to continue proliferating) and the other begins differentiating into a mature keratinocyte. As constant cell division in the stratum basale pushes cells outward, these cells mature and are sequentially organized to form the other layers of the epidermis, as shown in Figure 19.3. During their maturation, keratinocytes fill with **keratin proteins**, flatten, lose their organelles, and ultimately die. The stratum corneum (ie, outermost layer of the epidermis) is composed of 20--30 layers of dead, keratin-filled cells, which are constantly shed from the surface of the skin, being replaced by deeper, younger keratinocytes. **Figure 19.3** Replacement of dead cells lost from the surface of the skin.  A diagram of a keratin Description automatically generated Chapter 19: Skin System 602 19.1.02 Dermis The layer that contributes most to the thickness of skin is the **dermis**, which is located between the epidermis and the hypodermis. The skin\'s [blood vessels](javascript:void(0)), [lymph vessels](javascript:void(0)), and [sensory nerve fibers](javascript:void(0)) are located within the dermis, as are sweat glands, sebaceous (oil) glands, and hair follicles. The dermis is composed of connective tissue, contains abundant fibroblast cells, and consists of two main layers: the **papillary layer** and the **reticular layer** (see Figure 19.4). The papillary layer lies just beneath the epidermis and consists of loose (areolar) connective tissue, which contains collagen fibers and elastic fibers produced by fibroblasts. The reticular layer, which makes up about 80% of the thickness of the dermis, is the deeper of the two dermal layers. The reticular layer is composed of dense irregular connective tissue, which contains thick bundles of collagen fibers along with elastic fibers. The **hypodermis**, which is composed primarily of **adipose tissue**, is located just below the dermis. **Figure 19.4** Structure of the dermis. 19.1.03 Pilosebaceous Unit Hair, hair follicles, and sebaceous (oil) glands are accessory structures associated with the skin. A **hair follicle** is a living organ that produces a hair, whereas the hair itself is composed primarily of dead, keratinized cells. Each hair follicle is connected to a **sebaceous gland**, which secretes an oily lipid  Chapter 19: Skin System 603 substance called **sebum** through a duct into the hair follicle and ultimately onto the surface of the skin, where it contributes to the barrier function of the skin, as described in Lesson 19.2. In addition, an **arrector pili muscle**, made up of a bundle of [smooth muscle cells](javascript:void(0)), is attached to each hair follicle. Contraction of arrector pili muscles in an area of skin causes the hairs in that area to stand upright (ie, piloerection), which can play a role in regulation of body temperature in animals with abundant hair. A hair follicle, along with its associated sebaceous gland and arrector pili muscle, is referred to as a **pilosebaceous unit** (Figure 19.5). **Figure 19.5** Components of a pilosebaceous unit. 19.1.04 Skin Capillaries Unlike the epidermis, which contains no [blood vessels](javascript:void(0)), the dermis of the skin is **vascularized** (ie, contains blood vessels). **Arteries** that supply blood to the skin are present in the hypodermis, and branches from these arteries carry blood through the reticular layer of the dermis. These branches (ie, smaller arteries) eventually give rise to **arterioles**, which supply blood to **capillary loops** present in the papillary layer of the dermis. Blood drains from these capillary loops via **venules**, which join to form **veins** that carry blood away from the skin through the hypodermis (see Figure 19.6). A diagram of the skin Description automatically generated **Skin Function** Introduction The complex structural organization of the skin allows it to carry out multiple important functions. These functions include providing protection against various potentially harmful environmental factors as well as participating in the maintenance of [homeostasis](javascript:void(0)) within the body. This lesson explores the various functions that are performed by the skin. 19.2.01 Skin Barrier A primary means by which the skin functions to protect the body is by serving as a barrier between the body\'s external and internal environments. This function is performed largely by the skin\'s [epidermis](javascript:void(0)), particularly by the outermost layer of the epidermis, the **stratum corneum**. The dead **keratinocytes** (ie, specialized cells) that make up the stratum corneum are filled with keratin proteins that serve to increase the skin\'s resistance to [abrasion](javascript:void(0)) and chemicals. The stratum corneum also functions to inhibit water loss from the body. Keratins and other proteins within the keratinocytes, lipids between these cells, and **sebum** secreted onto the surface of the epidermis make the stratum corneum highly water resistant. Damaged skin or skin that has lost sebum (eg, through frequent handwashing) will undergo excessive water loss through the stratum corneum (see Figure 19.7). **Figure 19.7** Role of stratum corneum in prevention of excessive water loss. The epidermis also serves as a barrier to pathogens and ultraviolet radiation. The tightly packed keratinocytes of the epidermis help to physically block pathogen entry into the body. Furthermore, **Langerhans cells**, which are especially abundant in the stratum spinosum, perform surveillance to detect invading microorganisms. These [phagocytic cells](javascript:void(0)) are active participants in the body\'s immune response, which is discussed in Lesson 20.2. In addition, **melanocytes** in the stratum basale produce the pigment **melanin**, which absorbs ultraviolet radiation. After being synthesized, melanin granules (melanosomes) are transferred to nearby keratinocytes (see Figure 19.8), where the granules are arranged to shield the keratinocytes\' nuclei from the DNA-damaging effects of exposure to ultraviolet radiation (discussed in Concept 1.3.02).  Chapter 19: Skin System 606 **Figure 19.8** Transfer of melanosomes from melanocytes to keratinocytes. In addition to the epidermis performing barrier functions, the [dermis and hypodermis](javascript:void(0)) also play roles in protecting the body. **Collagen fibers** present in the [dermis](javascript:void(0)) cause the skin to be strong and resist tearing, and **elastic fibers** contribute to the skin\'s resilience. The dermis contains phagocytic **dendritic cells**, which, like Langerhans cells in the epidermis, function in the body\'s immune response. In addition, adipose tissue in the hypodermis cushions the body and serves as an insulating layer to reduce heat loss. Various environmental factors can result in harm to the body. Table 19.1 summarizes components of the skin that function to protect the body from these potentially harmful factors. **Table 19.1** Skin components that protect the body from potentially harmful environmental factors. A diagram of a human body Description automatically generated  Chapter 19: Skin System 607 19.2.02 Skin\'s Role in Osmoregulation **Homeostasis** refers to the ability of an organism to maintain a relatively stable physiological environment in response to external or internal changes. One important aspect of homeostasis involves maintaining appropriate water and solute concentrations in the body via the process of **osmoregulation**. Although the kidneys are the body\'s main osmoregulatory organs (see Concept 16.2.01), skin also functions in an osmoregulatory capacity. **Sweat glands** (sudoriferous glands) in the dermis of the skin secrete **sweat**, which is a mixture of water, salts, trace nitrogenous waste, and antimicrobial proteins (Figure 19.9). The epidermis, which functions as a water barrier, prevents most direct water loss through the skin; however, sweat is deposited on the surface of the epidermis via ducts that travel through the dermis and epidermis. These ducts allow sweat to bypass the epidermal barrier and be released into the environment as necessary. **Figure 19.9** Secretion of sweat. 19.2.03 Skin\'s Role in Thermoregulation **Thermoregulation**, the maintenance of body temperature within the normal physiological range, is a major homeostatic function of skin. Thermoregulatory centers in the hypothalamus monitor body temperature by processing information from internal and surface (skin) thermoreceptors. When a significant change in body temperature is detected, the hypothalamic center coordinates the physiological responses necessary to reset the temperature back within the normal range. When body temperature is above normal, **vasodilation** (widening) of skin [arterioles](javascript:void(0)) increases blood flow to skin capillaries and maximizes heat loss through the skin (Figure 19.10). Increased heat loss occurs because blood, which is warmed in the body core, transfers heat to the cooler skin surface as it passes A diagram of a skin structure Description automatically generated Chapter 19: Skin System 608 through capillaries near the surface. Vasodilation occurs when the [smooth muscle](javascript:void(0)) surrounding blood vessels relaxes. The process of sweating (ie, **perspiration**) occurs when sweat is secreted onto the skin\'s surface by sweat (sudoriferous) glands. Heat loss and subsequent cooling occur due to **evaporation** of the water in the sweat, an endothermic process that absorbs heat from the body. **Figure 19.10** Thermoregulatory mechanisms carried out by the skin.  Chapter 19: Skin System When body temperature is below normal, **vasoconstriction** (narrowing) of skin arterioles minimizes heat loss by diverting warm blood away from capillaries near the skin\'s surface and toward blood vessels in the interior of the body (Figure 19.10). Vasoconstriction occurs when the smooth muscle surrounding blood vessels contracts. Adipose tissue in the hypodermis functions as an insulating layer between the body\'s interior and the outside environment and aids in the retention of core body heat. In cold environments, sympathetic signaling causes contraction of arrector pili muscles (attached to hair follicles in the skin), which causes **piloerection** (ie, hairs standing upright). Piloerection, which impedes heat loss by trapping warm air near the skin\'s surface, is an inefficient means of heat retention in humans due to insufficient body hair. **Concept Check 19.1** Fill in the blanks in the sentences below with the words \"evaporation,\" \"facilitate,\" \"hinder,\" \"insulation,\" \"piloerection,\" \"sweat,\" \"vasoconstriction,\" and \"vasodilation.\" When body temperature is too low, \_\_\_\_\_\_\_\_\_\_ occurs and arterioles within the skin undergo \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ to \_\_\_\_\_ the loss of heat from the body. In addition, adipose tissue beneath the skin provides \_\_\_\_\_\_\_\_\_ to help retain body heat. Conversely, when body temperature is too high, \_\_\_\_\_\_\_\_\_\_ of skin arterioles and \_\_\_\_\_\_\_\_\_\_\_ of \_\_\_\_\_ from the skin \_\_\_\_\_\_ transfer of heat from the body to the environment. [**Solution**](javascript:void(0)) 19.2.04 Regulation of Skin Function The ability of the skin to participate in thermoregulation and osmoregulation depends on controlling vasodilation, vasoconstriction, and sweat gland function within the skin. These functions are regulated by the nervous system and hormonal inputs, as summarized in Table 19.2. When body temperature increases above a set point established by the hypothalamus, vasodilation and activation of sweat glands occur as a result of stimulation of arterioles and sweat glands via the [sympathetic nervous system](javascript:void(0)). Although [postganglionic](javascript:void(0)) sympathetic nerve terminals typically release the neurotransmitter norepinephrine, these skin responses to elevated temperature are triggered instead by the release of acetylcholine. In addition, emotionally stressful situations can cause sweat secretion as a result of the production of the fight-or-flight hormone [epinephrine](javascript:void(0)) by the adrenal medulla. Vasoconstriction of arterioles in the skin, which can occur in response to cold environmental temperatures and emotionally stressful situations, is regulated by the sympathetic nervous system via the release of norepinephrine.