Your skin’s dermis supports and enables the epidermis to thrive. It also houses connective tissue, blood vessels, oil and sweat glands, nerves and hair follicles.
Your dermis has two layers — the papillary layer and the reticular layer — which merge without clear demarcation. The reticular layer contains dense connective tissue and bundles of elastic fibers while the papillary layer is thinner and composed of loose connective tissue.
The epidermis is the outermost of two major layers of skin and is a tough, waterproof barrier. It has no blood vessels within it and is the thinnest of the two skin layers, averaging just 0.10 mm in thickness. It covers almost all of the body surface and is continuous with — but structurally distinct from — the mucous membranes that line the mouth, anus, urethra, and vagina.
It is made of keratinized stratified squamous epithelium and has four or five layers of cells, depending on the location of the skin. The innermost layer, called the stratum basale, contains stem cells that divide to form all the keratinocytes of the epidermis. The next layer, the stratum spinosum, is thicker and contains immunologically active Langerhans cells and spiny keratinocytes.
The final layer, the stratum corneum, is dry and dead and gives the skin its characteristic scaly appearance. This layer protects the underlying tissues from microbes, chemicals, and UV radiation. It also serves as a mechanical barrier against abrasion. The dead cells in this layer can be so hardy that they form keratinous materials such as hooves, claws, horns, and feathers that resist impact (as in hooves), external attack, and aerodynamic forces (as in horns). This keratinization helps to explain the protective properties of these structures.
The layer beneath the dermis is called subcutaneous tissue (synonymous with hypodermis). It contains fat and connective tissue, and it has a thicker texture than the skin on other parts of your body. It also includes hair follicles and blood vessels.
It is responsible for protecting the skin and insulates the body from cold temperatures. In addition, it stores energy and helps regulate your internal temperature.
Fat cells, called adipocytes, make up the majority of this tissue, and its thickness varies throughout the body. It is thickest in the buttocks, palms of your hands and soles of your feet.
The subcutaneous tissue houses larger blood vessels and nerves than those in the dermis. It also functions as a buffer between deeper muscles and bones, protecting them from shock and helping to control body temperature. This tissue is the primary site for the absorption of many medications, which is why injectable treatments can be used to deliver certain drugs. The loss of this fat tissue as you age can reduce the ability of your body to retain heat, which could lead to hypothermia.
The subcutis is the third layer of skin, located beneath the dermis. Also known as superficial fascia, cutaneous fascia or panniculus adiposus, the subcutis is a dense network of fat cells and connective tissue. It functions as a heat-insulator and shock absorber, helping to protect the organs below. In addition, it stores adipose tissue for energy. The subcutis also contains blood vessels, hair follicles and lymphatic tissue.
It is thickest on the abdomen, arms, lower back and shoulders. This layer is yellowish in color due to the presence of a pigment called carotene.
The fibroblasts of the subcutis produce collagen that gives structure to the skin. They also create the proteoglycan and elastin that give the skin its toughness and elasticity.
The upper dermal layer, the papillary layer, contains a thin arrangement of collagen fibers. It supplies nutrients to select layers of your epidermis and regulates temperature. Its vascular system — which functions like other vascular systems in your body — can constrict and expand to control how much blood moves through your skin. This layer also contains touch and pain receptors that transmit sensations of pain, itch and pressure to your brain for interpretation and to trigger shivering to generate additional body heat.
The deeper reticular layer is dense, irregular connective tissue with thick bundles of collagen and elastic fibers that extend in all directions but are mainly parallel to your skin’s surface. Its extracellular matrix includes a gel-like substance made of proteins and a network of blood vessels that provides nourishment to cells in the papillary and reticular layers and removes wastes from those cells.
This layer supports hair follicles, sweat glands and sebaceous (oil) glands. It also carries lymphatic vessels, nerves and a network of blood vessels. Light microscopy of H&E-stained skin samples shows a pattern of alternating dermal papillae and rete ridges. These fingerlike projections increase the strength of the attachment between the epidermis and the dermis.
The reticular layer of the dermis is thicker than the papillary layer and consists of dense irregular connective tissue with mats of dense collagen and elastin fibers. This layer is responsible for much of the strength and elasticity of the skin. It also contains hair follicles, sweat glands, oil glands, nerves and blood vessels.
It is also known for the ridges that appear as our fingerprints and give the layer its name – dermal papillae. The papillae are full of capillaries that supply the epidermis with blood. Meissner’s corpuscles – which sense light touch – are found in this layer, while Pacinian corpuscles are located deeper in the reticular layer and sense vibration and pressure.
The reticular layer also contains fat cells which provide the skin with a cushion and a layer of insulation against heat loss through conduction, radiation and evaporation. In addition, this layer contains lipids (fatty acids) which act as an energy reserve and are the source of vitamin A. The hypodermis, a subcutaneous layer of fat that is highly vascularized and innervated, lies below the reticular layer.