Properties of the Dermis
The dermis provides tensile strength and elasticity to the skin through an extracellular matrix composed of collagen fibrils, reticular, and elastic fibers, embedded in proteoglycans. These layers serve to give elasticity to the integument, allowing stretching and conferring flexibility, while also resisting distortions, wrinkling, and sagging.
The dermis contains many mechanoreceptors (nerve endings) that provide the sense of touch and heat. It also contains the hair follicles, sweat glands, sebaceous glands, apocrine glands, lymphatic vessels, and blood vessels. These blood vessels provide nourishment to, and remove metabolic waste products from, the epidermis.
The dermis is tightly connected to the epidermis through a basement membrane and is structurally divided into two areas: a superficial area adjacent to the epidermis called the papillary region, and a deep thicker area known as the reticular region (Figure 2).
The papillary region is composed of loose areolar connective tissue. It is named for its finger-like projections called papillae that extend toward the epidermis. These papillae provide the dermis with a "bumpy" surface that interdigitates with the epidermis, strengthening the connection between the two layers of skin.
The reticular region lies deep in the papillary region and it is usually much more dense and continuous with the hypodermis. It is composed of dense irregular connective tissue and receives its name from the dense concentration of collagenous, elastic and reticular fibers that weave throughout it. These protein fibers give the dermis its properties of strength, extensibility and elasticity (Figure 1).
Also located within the reticular region are hair follicles, sebaceous glands, sweat glands, receptors, nails, nerves, blood vessels and lymphatic tissue (Figure 2).
The hypodermis is not part of the skin and lies below the dermis. Its purpose is to attach the skin to underlying bone and muscle as well as to supply it with blood vessels and nerves. It consists of loose connective tissue and elastin. The main cell types found in this area are fibroblasts, macrophages and adipocytes (the hypodermis contains 95% of the body's fat). Fat serves as padding and insulation for the body.