Dermis

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General histology of the skin.

The dermis is the layer of skin found deep to the epidermis and superficial to the hypodermis. Thickness of the dermis varies and can range from 0.6 mm (eyelid) to 3 mm (palmar and plantar skin). 

The dermis contains a mixture of vessels, nerves and epidermal derivatives (hair follicles, arrector pili muscle, glands) embedded in a tough fibroelastic tissue consisting of collagen (mainly types I and III) and elastic fibers, surrounded by an amorphous ground substance (glycosaminoglycans, glycoproteins and bound water). The fibroelastic tissue of the dermis is composed of many different cell types which include fibroblasts, macrophages, adipocytes, mast cells, Schwann cells, and stem cells. 

The dermis is divided into two main layers: 

• Papillary layer
• Reticular layer 

The papillary layer forms the superficial layer of the dermis and is located just deep to the epidermis. It is composed of loose connective tissue which forms numerous papillae that extend and interdigitate with the epidermal ridges. The papillary layer is mainly composed of a combination of type III collagen and elastic fibers as well as a capillary loop which supplies blood to the epidermis. This capillary network also aids in regulating the temperature of the epidermis and superficial dermis. 

The reticular layer of the dermis merges with the inferior aspect of the papillary layer and therefore its border may be difficult to decipher. It forms the deepest layer of the dermis and is much thicker than it superficial counterpart. The reticular layer contains dense irregular connective tissue, composed of mainly type I collagen with a lesser amount of elastic fibers. Inferiorly, the reticular layer is continuous with the hypodermis. At this junction is a deep plexus of large blood vessels which supply numerous branches of capillaries to the papillary layer. Free nerve endings extend through the reticular and papillary layers to reach the epidermis. 

Identify the structures of the skin with this interactive quiz.

The main function of the dermis is to support and add strength and pliability to the skin. Due to the presence of blood vessels and nerves, it also plays an active role in thermoregulation and sensation. Cells of the dermis (fibroblasts, macrophages, mast cells, dermal adipocytes) also contribute to collagen synthesis, the inflammatory response as a result of injury/inflammation to the skin and energy storage.