Skin and Its Tissues
The integumentary system consists of the skin, hair,
nails, and cutaneous glands.
The skin, or integument, is the largest of the body
organs. It includes two distinct tissue layers—the
superficial layer, called the epidermis, and a deeper
layer, called the dermis. A third layer, the hypodermis
(sometimes called subcutaneous tissue), is not part of
the skin, but serves to anchor the skin to underlying
bone and muscle tissue.The skin provides a barrier
against microorganisms, water, solar radiation, and
others. Upon exposure to UV radiation, the skin
synthesizes an inactive form of vitamin D. The skin is
the most extensive sense organ and is important in
thermoregulation.
The epidermis consists of five distinct layers, none of which contain any blood vessels. The deepest layer of the epidermis, the stratum basale,
contains cells undergoing mitosis. The outermost layer, the stratum corneum, is composed of many layers of dead, flattened epidermal cells.
Most of the cells in the epidermis are keratinocytes, which produce a protein mixture called keratin. Keratinocytes are responsible for the
structural strength and permeability characteristics of the epidermis. Other cells of the epidermis include melanocytes, Merkel cells, and
Langerhans cells. Melanocytes produce melanin, a pigment that results in dark skin color and protects the skin from ultraviolet radiation. Merkel
cells are touch receptors. Langerhans cells are macrophage. The epidermis protects underlying tissue against water loss, mechanical injury, and
the effects of harmful chemicals.
The dermis binds the epidermis to underlying tissues. The dermis consists of areolar tissue and dense irregular connective tissue. Dermal blood
vessels supply nutrients to all skin cells and help regulate body temperature. Sensory nerve fibers, hair follicles, sebaceous glands, sweat glands,
and nail roots are all found within the dermis.
The hypodermis (subcutaneous layer) is deep to the dermis and consists of areolar tissue and adipose tissue. Approximately half of the body's
stored fat is found in the hypodermis, although the amount and location vary with age, sex, and diet. The fat in the hypodermis functions as
padding and insulation and is responsible for some of the difference in body shape between men and women.