40.1: Introduction to the Skin and Its Function
By the end of this section, you should be able to:
- 40.1.1 Discuss the skin and its two major layers.
- 40.1.2 Identify the five layers of the epidermis and their purposes.
- 40.1.3 Describe the two layers of the dermis and their purposes.
- 40.1.4 Explain the role of the hypodermis as it relates to the skin and dermatologic disorders.
The epidermis , the dermis , and the hypodermis are the layers that comprise the skin. The foundation of the skin is known as the hypodermis (also subcutaneous tissue or superficial fascia ). The hypodermis is composed of fatty tissues and membranes that hold the dermis to the underlying muscle (see Figure 40.2).
Above the hypodermis lies the dermis. The dermis is rich in vascularity, nerves, and sensory receptors embedded in a more elastic, collagenous system of fibers. The bottom dermal sublayer—the reticular layer—is much thicker and more vascular than the other skin layers and contains more nerve and sensory functions. The uppermost sublayer—the papillary layer—contains papillae , which extend upward into the epidermis to hold the dermis and epidermis together. The papillary layer also contains phagocytes, which act to destroy bacteria that may penetrate the epidermis. New dermal growth pushes skin cells through the dermis and upward into the epidermis.
The epidermis is the outermost layer of skin comprising the external skin surface of humans. Unlike the dermis, the epidermis does not contain blood vessels and nerves, but is composed mainly of cellular sublayers that protect the dermis. The basal layer ( stratum basale ) is the foundation of the epidermis in areas where the stratum lucidum is not present. (The stratum lucidum is an additional layer that provides a thicker layer of epidermis in areas that need more protection, such as the palms of the hands and soles of the feet.) The basal layer is the only epidermal sublayer that is not made from keratin cells. This layer bonds to the dermis to connect it to epidermal tissues. The remaining sublayers of the epidermis are composed of keratinocytes, cells that are protein based, giving rise to hair, nail, and skin cells that are strong and water-resistant. Just above the basal layer is the spinous layer ( stratum spinosum ), which is composed of a depth of 8–10 cells that have spiny processes that help to hold the cell layers together. Encompassed in this layer are Langerhans cells , which eliminate foreign bacteria and wastes that enter the spinous layer, providing protection from invading organisms.
Human skin has a high mitotic rate and has more rapid cell development and cell death than many other body tissues. In addition to rapid growth, skin cells travel from the dermis through the epidermis until they die and are shed from the outermost skin surface. The third sublayer of the epidermis is the granular layer ( stratum granulosum ), which is composed of a depth of 3–5 keratinous cells that are flatter and more protein based than the other sublayers. These cells essentially die, leaving behind a fibrous protein layer that gives rise to the fourth layer, the cornified layer ( stratum corneum ). The cornified layer is the outermost layer that forms the visible, external skin and is 20–30 cells thick (Yousef et al., 2022).
When conditions threaten skin integrity, prompt diagnosis and treatment enable the primary defense of the body systems to remain strong. As skin integrity becomes compromised, bacteria, other organisms, and contaminating substances may enter the body through breaks in the skin. Dermatologic medications assist in healing breaks in the skin, preventing infection, and maintaining healthy skin.
Trending Today
Dermatologic Diagnosis and Treatment Disparities in People of Color
One issue that has been identified as a barrier to treatment of skin conditions it the correct diagnosis and treatment for people of color. Skin conditions in darker-skinned individuals often present differently than in lighter-skinned individuals. This article from American Family Physician discusses the need for better educating dermatologists in the proper recognition and treatment of skin issues in clients with darker skin (Frazier et al., 2023).