Flexible, waterproof, and tough, the skin is the body’s largest organ and its multiple layers comprise about 15% of an adult’s total body weight. The dermis is the skin layer that lies between the epidermis — the tough, outer layer of the skin — and the subcutaneous layer, which consists mostly of fat.
Thicker than the epidermis, this connective tissue layer contains both elastic and fibrous tissue, which give the skin its strength and flexibility. Collagen is the primary structural component, but the dermis also contains hair follicles, sebaceous glands, blood vessels, nerves, specialized cells, and structures that serve a variety of essential functions.
The primary role of the dermis is to support and sustain the epidermis and to protect the structures beneath it. The vasoactive dermal vessels within the dermis help regulate body temperature. Receptors in the dermis allow the body to sense not only pleasant and painful stimuli but deeper sensations such as pressure and vibrations.
Two layersmake up the dermis.
The papillary dermis, the thinner top layer, accounts for about 20% of the dermis. It contains both blood vessels and loose connective tissue and provides nutrition to the epidermis. The papillary dermis also controls skin temperature.
A deeper, reticular layer forms the bulk of the dermis, along with the thick elastin fibers and bands of collagen that run parallel to the skin’s surface. The reticular layer contains the hair follicles, sweat glands, and sebaceous glands.
Dermal fibroblasts are the primary cells in the dermis. Scientists once believed that these cells served a passive role, but research shows that fibroblasts not only regulate inflammation but can secrete molecules that communicate with the surrounding cells — a role essential to the wound-healing process. Dermal mast cells also play a similar role in wound healing by promoting angiogenesis, re-epithelization, and scar development.
The dermis provides the skin’s blood supply through two networks of blood and lymphatic vessels. The first network is between the reticular and papillary layers. The second lies between the dermis and the skin layer beneath it, the subcutaneous tissue. The epidermis does not contain blood vessels, so it depends on nourishment provided through the diffusion of the blood capillaries in the upper layers of the dermis.
The dermis layer also contains several sensory mechanoreceptors. The papillary layer contains Meissner corpuscles, or tactile corpuscles, which are responsible for detecting touch, pain, and low-frequency vibration. Pacinian or lamellar corpuscles are much larger than the former. These touch receptors sense deep or transient pressure and high-frequency vibration.
The arrector pili muscles are small muscles that attach to hair follicles. Every arrector pili consists of a bundle of smooth muscle fibers that connect with a follicular unit and dermal tissue on opposite ends. When the body senses cold, the arrector pili muscles simultaneously contract, which causes the hairs on the body to stand up straight and generates heat.
Within the dermis are the dermal papillae, a group of bone marrow stem cells that exist just under the hair follicles. The dermal papillae play a key role in the size, shape, and color of hair and its regeneration frequency. The fingerlike projections increase the surface area and fortify the connection between the dermis and the epidermis, allowing for the exchange of blood, oxygen, nutrients, and waste products.
Several disorders and diseases can affect the dermis, some of which affect other organs as well:
UVA rays are a type of UV radiation that damages all levels of the skin, starting with the surface layer and working its way into the dermis. This damages the collagen and elastin fibers, reducing the skin’s strength and elasticity. It may even affect various blood vessels and capillaries within the skin.
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