Organ Level — Skin (Epidermis and Dermis)
The skin, the integumentary system's organ, is composed of the epidermis (epithelial tissue) and dermis (connective tissues), with an underlying hypodermis that is technically not part of the skin organ. Several layers of keratinocytes at the surface form the epidermis. The topmost layer is dead and sheds continuously. It is progressively replaced by stem cells that divide in the basal layer (stratum basale). The dermis connects the epidermis to the hypodermis and provides strength and elasticity due to the presence of collagen and elastin fibers. The hypodermis is the name for the layer of connective tissue that connects the dermis to the underlying organs. It also harbors adipose tissue for fat storage. Let's look at the structure and function of these parts of the skin organ in detail.
The epidermis (or epithelial layer) is made up of four or five distinct layers (strata), depending on the region of the body. From deep to superficial, they are named the stratum basale, stratum spinosum, stratum granulosum, stratum lucidum, and stratum corneum. The stratum lucidum is unique to areas like the palms of the hand (palmar surfaces) and soles of the feet (plantar surfaces), where the skin is thicker than it is in the rest of the body. The stratum basale is made up of the many cell types already discussed, including basal cells, melanocytes, Langerhans cells and Merkel cells. As you look at the more superficial layers, you see that they become mostly (or completely) composed of keratinocytes, which protect and waterproof the body. As the cells are pushed superficially (toward the surface) they make keratin. As the cells begin to fill with keratin, they become increasingly impervious to water, and it becomes more difficult for osmosis and diffusion to occur inside the cell. In addition, as cells enter each superficial layer (further away from the dermis, which contains the blood supply), the distance across which oxygen and other nutrients must diffuse increases, making it harder for the cells to get the nutrients they need. The keratinocytes in the stratum corneum (the most superficial layer) are usually inert, or dead, and periodically slough away, being replaced by cells from the deeper layers.
Stratum Basale The stratum basale (also called stratum germinativum) is the deepest epidermal layer and attaches the epidermis to the basal lamina, below which lie the layers of the dermis. The stratum basale is primarily made up of a single layer of basal cells. These cells are considered to be stem cells. The function of this layer is to divide to replicate the cells that are lost from the surface. The daughter cells then differentiate into keratinocytes. Merkel cells and melanocytes are also dispersed among the basal cells in the stratum basale. Fingerprints form in the growing fetus where the basal cells of the stratum basale meet the connective tissue of the underlying dermal layer (papillary layer). The basal cells form strong cell-to-cell junctions called desmosomes not only with adjacent cells, but also with the basal lamina between themselves and the underlying connective tissue. During development, some areas of basal cells divide at a different rate, forming epidermal ridges that extend down in the dermis, and the dermal tissue proliferates to form dermal papillae. This results in the formation of deep ridges that get transmitted through the other layers of the skin to form fingerprints on the surface. Fingerprints are unique to every individual and are used for forensic analysis because the patterns do not change with the growth and aging processes. Even identical twins with the same genes will have different fingerprints because of this random process.
Stratum Spinosum As the name suggests, the stratum spinosum is spiny in appearance due to the polyhedral shape of the cells and desmosomes visible when tissue is prepared for microscope slides. As basal cells divide at different rates, keratinocytes get pushed up but maintain these strong cell-to-cell connections, changing cell shapes and forming a protective barrier. This stratum is composed of eight to 10 layers of keratinocytes, formed as a result of cell division in the stratum basale. Interspersed among the keratinocytes of this layer are the Langerhans cells, which help with immunity.
Stratum Granulosum
The stratum granulosum has a grainy appearance due to further changes to the keratinocytes as they move up from the stratum spinosum. The cells (three to five layers deep) become flatter, and their cell membranes thicken. At this point, the keratinocytes generate large amounts of the proteins keratin and keratohyalin in the cytoplasm and, with other lipids and enzymes, form vesicles called lamellar granules, which may be secreted by exocytosis. The cellular secretions act to retard water loss and entry of foreign materials. These two proteins eventually make up the entire mass of the keratinocytes in the stratum granulosum (the nuclei and other cell organelles disintegrate) and mark the transition between the metabolically active strata and the dead cells of the superficial strata.
Stratum Lucidum
The stratum lucidum appears lucid, or clear, and is not present throughout the body, but only on parts with thick skin, such as the surface of the palms and the soles of the feet. The stratum lucidum is a smooth, clear, thin layer, just superficial to the stratum granulosum. The keratinocytes in this layer are derived from the stratum granulosum, and mainly consist of keratin fibers. They are flat and densely packed.
Stratum Corneum
The stratum corneum is the most superficial layer of the epidermis, and is the layer that is exposed to the environment. The increased keratinization (also called "cornification") of the cells in this layer gives it its name. There are usually 15 to 30 layers of dead cells in the stratum corneum. This dry, dead layer prevents the growth of microbes and keeps the rest of the underlying layers healthy. It is also resistant to penetration by water and protects the inner layers from environmental damage. Dead cells in this layer are shed periodically (approximately every two weeks) and are replaced by cells from the stratum granulosum (or stratum lucidum in the case of the palms and soles).
EXAMPLE
Exfoliation is the removal of the outermost layer of dead cells. Cosmetic procedures like microdermabrasion or chemical peels help remove some of the dry upper layer of the skin and aim to keep the skin looking “fresh” and healthy. The ancient Egyptians are credited with first discovering the beauty effects of exfoliation. Too much exfoliation can cause damage to underlying, living tissue.