The skin itself has two major tissue layers⎯the epidermis and the dermis. The epidermis is the outermost layer of skin, comprised of several sublayers. This layer of skin contains many cells, each called a keratinocyte, a keratin-producing cell found in the skin. Keratin is the structural protein that lends durability and water impermeability to skin, hair, and nails. Keratin also makes up the body's hair and nails. Cells that contain keratin are generated in the lower layers of the epidermis. These migrate upward, where they are progressively flattened. The topmost layer of the epidermis is composed of dead cells, filled with keratin.
The dermis is the thick, inner layer of skin just below the epidermis that makes up about 90 percent of the skin's thickness. It also contains blood vessels, which support these tissues. Finally, the dermis contains glands (including sweat glands) and hair follicles.Beneath the skin is a subcutaneous layer called the hypodermis. The hypodermis is the deepest layer of the skin, made up mostly of areolar connective tissue and adipose (fat) tissue.
Layers of the Epidermis
The epidermis is the outermost layer of the skin and is composed of several sublayers. There are four main types of cells throughout the epidermis. Keratinocytes produce keratin and form the skin's protective barrier. A melanocyte is a cell in the deepest layer of the epidermis that produces melanin. This is a pigment that protects the body from UV radiation. A Merkel cell is a type of receptor cell in the skin triggered by light touch. Finally, a Langerhans cell is an immune cell that engulfs foreign particles and pathogens when the skin is damaged. These cells in the epidermis are organized into either four or five layers, depending on which part of the epidermis is examined.
The bottom layer of the epidermis is called the stratum basale, which is a single-celled layer of basal cells. This layer is also called the stratum germinativum, or germinal layer, because it is where the new keratinocytes are produced from the stem cells. These new cells push the older cells toward the surface of the skin. In addition to these proliferating keratinocytes, this layer also contains melanocytes. There are also Merkel cells in this layer, particularly in skin covering touch-sensitive regions of the body, such as the fingertips.
The next layer of the epidermis is the stratum spinosum, or spinous layer, the layer of the epidermis that has a spiny appearance due to the keratinocytes that dominate it. It lends durability to skin, and also contains Langerhans cells, which are related to immune function.
The stratum granulosum, or granular layer, is the layer of the skin where keratinocytes produce keratin and then flatten and lose their nuclei.
The outermost layer in the epidermis is the stratum corneum, or horny layer, which serves as the primary barrier against microbes and keeps the deeper layers from dehydrating. Keratinocytes die when they are pushed into this layer, and they are then called corneocytes. These are keratin-filled cells that form the skin's protective barrier. They are embedded in a lipid matrix that helps repel water. The thickness of this layer varies throughout the body. Eventually these dead cells will shed off.In the epidermis that covers the thick skin on the palms of the hands and soles of the feet, there is the fourth layer present below the stratum corneum, called the stratum lucidum, or translucent layer. The stratum lucidum is a layer of the skin found only in certain areas of the body and provides extra padding and durability. Keratinocytes in this layer are mostly dead, and they appear translucent under a light microscope.
Layers of the Epidermis
Structure and Function of the Dermis
The dermis is the layer of the skin that sits directly beneath the epidermis. It is primarily composed of connective tissue that supports and strengthens the skin. It also contains sweat glands, sebaceous glands, nerves, blood vessels, hair follicles, and sensory receptors. The dermis is composed of two layers⎯the papillary layer and the reticular layer.
The papillary dermal layer lies directly under the stratum basale of the epidermis. The papillary layer contains a network of small blood and lymph vessels that provide nutrients to the epidermis. These blood vessels also play a key role in thermoregulation. They dilate to release heat and constrict to conserve it. The blood vessels make up networks of capillaries that are found in fingerlike projections called dermal papillae, which reach up into the epidermis. Within the papillae, capillaries are surrounded by loose networks of connective tissue. The papillae also contain tactile corpuscles, such as Meissner's corpuscles, which are nerve endings that detect light touch.The reticular layer is the deep layer of the dermis that contains hair follicles and sweat and oil glands. A sudoriferous gland, also known as a sweat gland, helps regulate body temperature through evaporative cooling. Some types of sweat glands also produce body odor. A sebaceous gland is a gland that is joined to a hair follicle and produces oil to keep the skin supple and prevent the growth of microorganisms. Nerve endings and hair follicles are also found in the reticular layer.
Structure and Function of the Hypodermis
The hypodermis, also called the subcutaneous layer, is the innermost part of the integumentary system. The hypodermis is not technically a part of the skin, or dermis. Instead, it separates the dermis from the muscles and is composed of adipose tissue, or fat, along with loose connective tissue called areolar tissue.
Adipose tissue is composed of adipocytes, or fat cells. The number and size of these adipocytes depend on the part of the body in which they are located, as well as the nutritional state of the organism. Adipose tissue serves as an energy reserve for the organism, as adipocytes store energy in the form of lipids. Adipose tissue provides important cushioning for the internal organs in the body. It insulates the body and therefore plays a role in thermoregulation. Finally, adipose tissue also plays a significant role in hormonal signaling and regulation of appetite.
Areolar tissue is a loose connective tissue primarily composed of interlaced collagenous fibers⎯bundles of coiled collagen molecules. It may also contain stretchable fibers made of elastin called elastic fibers. Finally, areolar tissue may contain reticular fibers⎯collagen fibers that join connective tissues together. In areolar tissue, these fibers are spaced widely apart. This provides space for interstitial fluid to pass while still providing structure and elasticity.
In addition to adipose tissue and areolar tissue, the hypodermis contains blood vessels and nerve fibers that pass through the hypodermis on their way to the dermis. It also contains the glandular portions of some sweat glands, including mammary glands, which produce breast milk. Hair follicle roots also sit in the subcutaneous layer.