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.
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.
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.
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.
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.
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).
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.
Thick Versus Thin Skin
The thickness of thick skin is a function of the four upper layers of the epidermis: the stratum spinosum, stratum granulosum, stratum lucidum, and stratum corneum. The stratum corneum, consisting of keratin-packed dead cells, is substantially thicker in thick skin than in thin skin (more than 300 layers versus 15 layers of cells). The palms of your hands, soles of your feet, and your lips have thick skin. Thick skin is adapted to specialized activities, including gripping, walking and suckling, and the wear and tear that goes with those activities. Thick skin does not have hair, and has few glands.
Most of the rest of your skin is thin. The stratum lucidum isn’t even present in thin skin. The packed keratin provides most of the protective properties associated with the epidermis. Whereas the stratum corneum of thin skin may be completely shed and replaced in about a week, this replacement may take about a month in thick skin.
When you wear shoes that do not fit well and are a constant source of abrasion on your toes, you tend to form calluses at the point of contact. This occurs because the basal stem cells in the stratum basale are triggered to divide more often. This increases the thickness of the skin at the point of abrasion so as to provide greater protection to the underlying tissue. Calluses can also form on your fingers if they are subject to constant mechanical stress like long periods of writing or playing stringed instruments or video games. The formation of calluses is an example of how tissues can adapt to a minor or local stress. Corns are a specialized form of calluses. They are formed due to the abrasion on the skin as a result of an elliptical-type motion.
Organ Level — Dermis
The skin’s dermis is made up of two distinct layers of connective tissue. The papillary layer is made up of areolar connective tissue and the underlying reticular layer is composed of dense irregular connective tissue. This dermal part of the skin (organ) is vasculated (has blood vessels) and is innervated (has nerves). As described earlier, the dermis is sparsely populated with fibroblasts that produce collagen and elastin fibers in the extracellular matrix. This leads to a strong and elastic tissue structure. The matrix can also contain mast cells involved in allergic reactions.
The fibroblasts are dispersed within the collagen and elastin fibers of the areolar tissue (loose connective tissue) of the papillary layer. This forms a loose mat, which contains an abundance of small blood vessels. The dermal papillae with blood capillaries interdigitate (become interlocked) with the epidermal ridges of the stratum basale. In addition, the papillary layer contains phagocytes — defensive cells that help fight bacteria or other infections that have breached the skin. This layer is also interspersed with lymph vessels and sensory receptors.
The reticular layer appears “reticulated” (net-like) because it is composed of a mesh of collagen fibers and elastin fibers. Fibrocytes form the bundles of collagen that extend into the papillary layer and the hypodermis, making these layers hard to distinguish. The flexible collagen provides structure and strength, while elastin lends limited elasticity to the skin. Collagen also binds with water, keeping the skin hydrated. Water is necessary to maintain the normal elasticity and resiliency (called “turgor”) of the skin. Dehydration causes a loss of turgor; if the skin of a dehydrated person is pinched it remains domed and does not immediately flatten out.
Collagen injections and Retin-A creams help restore skin turgor by introducing collagen externally in the former case or by stimulating blood flow and repair of the dermis in the latter case.
By NIST database (Fingerprint_Whorl). Public Domain
The hypodermis (also called the subcutis or subcutaneous layer) functions to connect the integument (epidermis and dermis) to the underlying muscles and organs. The hypodermis is not considered part of the skin, but has several important functions. Like the dermis, the hypodermis is made up of areolar tissue, collagen, and elastic fibers, providing it with some elasticity. Additionally, it contains adipose tissue, which functions as a mode of fat storage. The hypodermis is vascular and contains arteries, veins, and blood capillaries.
Adipose tissue present in the hypodermis accumulates fat, which serves as an energy reserve, insulates the body, and prevents heat loss. The fat distribution changes as our bodies mature and age. It is also hormone-dependent. Men tend to accumulate fat in different areas (neck, arms, lower back, and abdomen) than do women (breasts, hips, thighs, and buttocks). Physical inactivity due to a lack of exercise and sedentary jobs, combined with the consumption of high-calorie foods, has resulted in the highest rates of obesity ever seen in our country. While accumulation of fat provided an evolutionary advantage to our ancestors, who experienced unpredictable bouts of famine, it is now considered a major health threat. Improved diet and increased exercise are the best ways to control body fat accumulation, especially when it gets to levels that increase the risk of heart disease.
Accessory structures of the skin include hair, nails, sweat glands, and sebaceous glands. Although these structures appear to be part of the dermis, they are actually derived from the epidermis. The hair shaft is made of dead, keratinized cells and gets its color from melanin pigments. Nails are also keratinized and protect the extremities of our fingers and toes from mechanical damage. Sweat glands and sebaceous glands produce sweat and sebum, respectively. Each of these fluids has a role to play in maintaining homeostasis. Sweat helps the body remove excess fluids and electrolyte wastes and also cools down the body surface when it gets overheated. Sebum acts as a natural moisturizer of the dead, flaky outer keratin layer of skin and hair. Sebum is also known for its microbicidal and microbiostatic properties.