The integumentary system comprises the skin and its appendages (hair follicles, nails, sebaceous glands, and sweat glands). The integument or skin, which is the largest organ system of the body, constitutes 15% to 20% of the body weight.1
Anatomically, the skin consists of three distinct layers of tissue: the epidermis, the dermis, and the hypodermis, a subcutaneous fat cell layer that is located directly under the dermis and above the muscle fascial layers.1
The epidermis is the most superficial layer and is made up of epithelial cells and no blood vessels. Its main function is protection, absorption of nutrients, and homeostasis. In structure, it consists of a keratinized stratified squamous epithelium comprising four types of cells:
- keratinocytes: produce keratin cells that toughen and waterproof the skin. The only portion of skin that is non-keratinized is the lining of skin on the inside of the mouth.
- melanocytes: provide a protective barrier to the ultraviolet radiation in the sunlight; produce melanin and are responsible for skin color.
- Merkel cells
- Langerhans' cells
Key functions of the skin include
- Cushioning and protecting against injury or invasion
- Lubrication through secretion of oils
- Maintenance of homeostasis: fluid balance, regulation of body temperature
- Excretion of excess water, urea, and salt via sweat
- Maintenance of body shape—provides cosmetic appearance and identity
- Vitamin D synthesis
- Storage of nutrients
- Provision of sensory information via receptors in the dermis
Keratin is a fibrous protein that aids in the protection and water-proofing of the skin.
The dermis, the middle layer of the skin, is considered the "true" skin because it contains blood vessels, lymphatics, nerves, collagen, sebaceous and sweat glands, and elastic fibers.1 The dermis is composed primarily of collagen and elastin fibrous connective tissue.
The amount of elastin decreases with age. The dermis can be subdivided into two layers, a superficial papillary layer and the deeper reticular layer1:
The interface between the epidermis and the dermis is termed the rete peg region. Rete pegs are the epithelial extensions that project into the underlying connective tissue and which act to overcome the frictional forces that skin is exposed to during daily activity.1
Hair is formed by epidermal cells that invaginate into the dermal layers. The distribution, function, intensity, and texture of the hair vary according to the region of the body. Certain regions, such as the palms, soles, lips, and nipples are hairless. The primary function of hair is protection, even though its effectiveness is limited. Humans have a number of distinct kinds of hair: