Your skin’s middle layer, Dermis, is made up of two layers of connective tissue. The thin papillary layer contacts epidermis, while the thick reticular layer contains bundles of collagen and is densely packed with fibroblasts.
It contains blood vessels that open to release heat as your body temperature rises or close to preserve heat in cool weather, nerve endings and hair follicles. It also houses sweat glands and oil glands.
The Papillary Layer
The papillary layer, the thinnest of the two dermal layers, is also named for the fingerlike projections (papillae) that extend into the epidermis from this layer. The papillae increase the strength of the connection between the skin’s epidermis and dermis, helping to protect the epidermis from water loss. They are more visible in thicker skin, such as on the palms of the hands and fingers, where they create the epidermal ridges commonly known as fingerprints (Figure 10.4.3).
The papillae are made of loose areolar connective tissue that contains fibroblasts and scattered mast cells, leukocytes, and lymphocytes. The papillae also contain terminal networks of blood vessels and tactile touch receptors called Meissner corpuscles.
In addition to providing a source of nutrients, the blood vessels in this layer regulate skin temperature and carry away cellular wastes that would otherwise kill the epidermis cells. The dilated blood vessels found in this layer cause the pink tint seen in light-skinned people. The same blood vessels are responsible for blushing, when they expand in response to emotions such as anger and fear.
The reticular layer is much deeper than the papillary layer and forms the bulk of the dermis. It is densely woven with a network of thick collagen bundles. The reticular layer also contains glands and hair follicles. These include the eccrine sweat glands, which help control body temperature by producing sweat and the sebaceous glands, which secrete oil.
The Reticular Layer
The reticular layer of the dermis, also called corium, is the thick area beneath your epidermis that cushions subcutaneous tissues of your body from stress and strain. It is made up of dense irregular connective tissue, including a network of coarse collagen fibers and elastin fibers that give the skin its strength and elasticity. It houses sweat glands, hair follicles, nerves, blood vessels, and fat cells. Its mechanoreceptors provide the sense of touch, while thermoreceptors detect temperature changes in the skin.
It is a tough, dense connective tissue with a gel-like consistency. It contains a protein complex known as proteoglycan, chondroitin sulfates, and glycoproteins. Dermal fibroblasts create a strong matrix of these compounds, which give the skin its firmness and elasticity.
Fibroblasts also produce collagen, which gives the dermis its tensile strength. These fibers are cross-linked to form strong parallel layers. The reticular layer is deeper than the papillary layer, but it merges with the inferior aspect of the papillary layer, making its border difficult to distinguish.
It contains bundles of collagen that extend from the base of the papillary layer to the hypodermis. It is less cellular than the papillary layer, but it contains a dense reticular matrix of irregular fibrous tissue and a few elastic fibers. It houses the sweat glands, hair follicles, and nerve endings, as well as adipose lobules.
The subcutis is a network of septa that contain fat lobules and connective tissue. Its function is to act as a gliding plane between the skin and fascia, protect deeper structures from acute and chronic trauma, store fat, and participate in temperature control. It also provides a limited network of blood vessels and serves as an additional, albeit relatively limited, pathway for drug absorption.
The subcutis of the dermis is divided into two regions: the papillary region which has finger-like projections that push into the epidermis and create a bumpy surface of the skin; and the reticular region which contains dense, irregularly organized connective tissue that gives the skin its strength and elasticity. Both regions of the subcutis contain hair roots, sebaceous glands, sweat glands, and nerves.
Fibroblasts and mast cells populate the subcutis, forming tough supportive tissue. It contains a network of collagen fibers and structural proteoglycans and glycoproteins that support the epidermis, hair follicles, arrector pili muscle, and glands. Its primary function is to provide support and protection of the skin and deeper structures, assist in thermoregulation, and aid sensation. In the absence of normal subcutis, shear stress can develop and lead to abnormal mechanical loading of the overlying epidermis and dermis. This can contribute to recurrent wound breakdown and unhealed scarring. When the subcutis is damaged by surgery, radiation injury, or extensive necrotizing soft tissue infection, the insulating effect of the skin is lost.
The Blood Vessels
The dermis, which contains blood vessels and nerves, provides support and adds pliability to the skin. It also plays a role in thermoregulation and sensation. Fibroblasts in this layer synthesize collagen and elastin, which make the skin strong and flexible. This layer is also home to hair follicles, sweat glands and fat lobules. It is the source of the oils that keep your skin healthy and waterproof. The nerves in this layer include sensory cells that detect touch, pressure and vibrations. They also send signals to the brain to perceive pain, cold and other sensations. The dermis contains a network of blood vessels that nourish the epidermis and carry away waste products. When the skin is exposed to hot temperatures, the blood vessels in this layer enlarge (dilate), allowing warm blood to bypass the epidermis and cool the body. In response to cold temperatures, these same blood vessels narrow (constrict), retaining the body’s heat.
This layer also contains a specialized group of skin cells called phagocytes, which consume harmful bacteria and other impurities that penetrate the epidermis. Finally, it contains fat that provides insulation and padding for the muscles, bones, and organs. This layer is also responsible for regulating your body temperature by keeping your muscles and organs warm when it’s cold, and cooler when it’s hot.