Layers of the Skin

‘Skin’, accounting to about 20 square feet area and weighing about 5 kilograms, is the largest body organ. It is continually changing and comprises of complex structures and specialized cells. It acts as a protective barrier between the vital organs of the body, tissues, muscles, skeletal system and external environment. Thus, it offers a shield to the body from invading microbes, volatile temperatures, chemical exposures and other hazardous environmental elements. Skin also functions to gather sensory signals, regulate body temperature, prevent infections, provide hydration and produce vitamin D for stronger bone health.

As skin is most vital organ of the body, it functions in many useful ways.

Functions

• The key function of the skin is to protect the body from environmental impacts, climatic changes, microbes, hazardous chemicals and radiations.
• The skin controls various aspects of physiology, such as, regulating body temperature through sweat glands and hair, changes in blood circulation and maintenance of skin hydration. It also helps to synthesis Vitamin D, which promotes good bone health.
• The external layer of the skin (Stratum Corneum/Horny layer) plays an important role in preventing water loss from the epidermis. It is comprised of NMFs (Natural Moisturizing Factors) that are sourced from the sebaceous oils. The elasticity and firmness of the skin is maintained due to binding of NMFs and water.
• The epidermis is the primary layer of self-resistance. The hypo dermis acts as a shock-absorbent, because the subcutaneous fats in the layer provide with the necessary padding. Thus, enabling to protect the muscles and fibrous tissues (fascia) underneath.

The skin also responds adequately to the external stimulus-es. For example, due to repeated rubbing or frictions, the skin tends to form calluses caused by thickening of the horny layer (stratum corneum).

• The skin tends to protect the body from the hazardous alkaline chemicals/elements. The acid mantle or the hydro-lipid film found in the skin help to prevent foreign particles from entering into the body.

The hydro-lipid film is a homogeneous combination of aqueous and lipid stage. The aqueous stage which majorly comprises of sweat, is mainly responsible for keeping the acidity level of the skin in check by using the buffering capacity of amino acids. While the lipid stage, comprising of sebum and cell lipids, ensures that the skin is hydrated by preventing water loss. Thus, by attaining this balance, it acts as a preventive barrier against external agents, bacteria and viruses.

• Apart from being a barrier to environmental factors, the skin has an immune system that safeguards the body from toxic substances, cancer and other infective agents. The immune system of the skin is at times called as SALT (Skin-Associated Lymphoid Tissue), which is inclusive of the peripheral lymphoid organs. The immune system dwells in the epidermis and dermis.

The primary immune cells in the epidermis are Langerhans cells (Epidermal dendritic cells) and Skin cells (Keratinocytes). The dermis is comprised of lymph vessels, blood vessels and several immune cells (such as Dermal dendritic cells, T cells, B cells, Natural Killer cells and Mast cells). Also, the skin microbiome (the skin friendly micro-organisms that dwell on the skin) help in the homeostasis of the skin immune system.

• The skin helps in sensation, as the nerve endings in the skin enable to sense and detect temperature, touch, external pressure, injury and vibration.
• Skin also play an important role in regeneration, as it is equipped with the required cell apparatus that facilitate the healing process.
• The skin is an energy reservoir, because the fat cells in the subcutis are the storage systems for nutrients. As the body requires them, the nutrients are passed into the surrounding blood stream, that are carried to the places where required.

Structuring of the Skin:

The skin being one of the dynamic body organs, has a multiplexed structure. The skin is made up of 3 main layers and several sub-layers which are elaborated as below:

The 3 main layers of the skin

Epidermis:

It is the topmost or outer layer of the skin visible to the naked eyes, that prevents the body from harmful microbes, toxic substances and de-hydration. The thickness of this layer depends on its location in the body. It tends to be thinner on the eye-lids (about half a mm) and thickest on the palms and soles (about 1.5 millimeter in size). About 95 percent of keratinocytes make up the epidermis and are sub-divided into 5 layers. Keratinocytes metamorphose as they pass through the external layers of epidermis to the skin surface.

During this process, they form systematic structures and release lipids and keratins (proteins), that become a part of the ECM (Extra-Cellular Matrix) and form a solid connective barrier within the skin. This phenomenon is termed as ‘Keratinisation’ or ‘Cornification’ which makes each of the sub-layers in the skin different.

The epidermis is further stratified into 5 layers:

Anatomy of the Epidermis

• Stratum basale (Basal layer):  The Stratum basale is a singular row on the basement membrane made up of cuboidal or columnar stem cells and keratinocytes, that segregate the epidermis from the dermis. Keratinocytes are continually producing new cells and pushing off the older dead cells from the skin surface, by replacing with new epidermal cells.

Although, Keratinocytes are the commonly occurring cells found in the epidermis, the other cells include Melanocytes, Langerhans cells and Merkel cells. Melanocytes are neuroectodermally branched cells that are comprised of the intracellular system to produce ‘Melanin’, the pigment that gives skin its colour. Melanin protects the skin from harmful ultra-violet radiations of the sun. Langerhans cells are the dendritic cells found in the epidermis as well as other body areas. These cells send alerting signals to the immune system and prevent any pathogens or antigens from entering the body, thus offering prevention against any potential infections.

Merkel cells (Tactile epithelial cells) are neuroendocrine cells located in the stratum basale just above the basement membrane, connected to keratinocytes by desmosomes. These cells are in close contact with the terminal filaments of the sensory neurons (tactile disc), that play a role in the sensation of touch. Found on the boundary between the Epidermis and Dermis is the ‘Dermo-Epidermal junction’. The collagen and desmosomes hold the junction together, preventing the two layers from falling apart in areas of increased pressures such as soles, palms and finger tips.

• Stratum Spinosum (Prickle layer): Just above the Stratum Basale is Stratum Spinosum, comprising of 8-10 layers of keratinocytes. It has a spiny appearance and the cells in this layer are connected to each other by desmosomes. These desmosomes intersect with each other anchoring the cells together. During this process of fixation, the cells tend to shrink slightly and the desmosomes in the adjacent cells remain closely inter-locked with each other. These interlocks tend to look like spines or spindles; hence is termed as the ‘Prickle layer’.
• Stratum Granulosum (Granular layer): Stratum granulosum comprising of 3-5 layers of keratinocytes are pushed upwards from the Stratum Spinosum, which further undergoes change. These cells tend to flatten and the cellular membranes get thickened, producing keratins (fibrous proteins) and keratohyalin (lamellar granules). The keratinocyte mass comprised of these two proteins (Keratin and Keratohyalin) give the layer a ‘granular’ or ‘grainy’ appearance, hence the name. As the cells shed off, the nuclei and other cell apparatus tend to degenerate, leaving behind the proteins and the cell membranes that make up the other layers.
• Stratum Lucidium (Clear layer): The transparent, smooth layer above the Stratum granulosum is Stratum lucidium. This thin layer is comprised of dead and flat keratinocytes found on the skin of the palms, soles, tip of the fingers and toes. The keratinocytes in this layer contain large amounts of eledin, a protein rich in translucent lipid derived from Keratohyalin. Thus, giving this layer a transparent/lucid appearance, justifying to its name.
• Stratum Corneum (Horny Layer): Stratum Corneum is the most outward layer of the skin comprising of 15 -30 layers of dead keratinocytes. This layer protects the more delicate internal layers from getting dehydrated and penetration of harmful microbes. This layer undergoes a process of ‘desquamation’ every 4 weeks and replaces the dead cells pushed upwards by the Stratum granulosum and Stratum lucidium in case of palms, soles or tip of the fingers/toes. As this layer undergoes a process of continual cornification, it is termed as the ‘Stratum Corneum’ (Horny layer).

Dermis:

The structure beneath the epidermis is called as the dermis, which is divided by the basal membrane. The primary structural elements of the dermis consist of elastin and collagen, connective tissues, that offer strength and elasticity to the skin. The fibroblasts produce collagen fibres, proteoglycan and elastin. Proteoglycan makes the dermis hydrated and viscid, while elastin gives flexibility. The other skin elements in this layer are hair follicles, sweat glands, blood and lymphatic vessels. The thickness of the dermis on the eyelids is 0.6mm and 3mm on the palms, soles and the back. The dermis consists of two layers as illustrated below:

 Layers of the dermis

 Papillary layer: The external layer of the dermis is made up of undetached, areolar connective tissues forming a netted structure of collagen and elastin fibres. This layer protrudes into the Stratum basale of the epidermis to form finger-shaped papillae. The layer is made up of fibroblasts, adipocytes (fat cells) and several small blood vessels. The layer is comprised of phagocytes, the cells that enable to defend against any bacterial infections from entering into the skin. Lymphatic capillaries, nerve fibres and Meissner corpuscles (touch receptors) also make up the layer. The papillary layer provides the epidermis with necessary nutrients.

Reticular layer: Beneath the papillary layer, is the much denser and thicker Reticular layer composed of irregular connective tissues. As the name indicates, the layer has a reticulated appearance due to a close web-work of elastin fibres. Elastin provides the skin with elasticity and movement flexibility. Also, the collagen fibres in the layer provide structure and robustness, with collagen threads stretching into the hypodermis and papillary layer. Hair follicles, sebaceous glands and sweat glands (apocrine and eccrine) too constitute the reticular layer.

Apocrine sweat glands are found in the armpits and groin area, while, eccrine glands are found all over the body. Apocrine glands contribute to the body scent and the eccrine glands regulate or monitor body temperature through the phenomenon of evaporation. The sebaceous glands that are a part of the dermis produce sebum that promote skin lubrication, preventing it from de-hydration.

Subcutaneous Tissue/Hypodermis: Hypodermis, also known as the subcutaneous tissue, is the innermost/deepest layer of the skin. It is majorly composed of adipose tissues (fat tissues), blood vessels and special collagen fibres (termed as tissue septa that bind the adipocytes). The adipose distribution varies throughout the body and as per the gender. Thus, the size of adipocytes depends upon the anatomic location and dietary habits of an individual. An individual with good dietary habits and work-out regimen have adipocytes of smaller size and tend to be less over-weight. This layer is composed of connective tissues and fat.

Hypodermis

The hypodermis acts as an energy resource and tends to swell/shrink depending upon the usage or storage of fat. If the body has consumed energy acquired from the carbohydrates, the adipose tissue acts as the fuel source, which can result into weight reduction. ‘Leptin’ is a hormone secreted by the fat cells/adipose cells that enables to monitor the appetite and signals when full. It is the protective layer that acts as a natural cushioning/padding, safeguarding the bones, muscles, organs and other delicate tissues in the body. Hypodermis also acts as a body insulator and regulates the temperature of the body, by ensuring the internal temperature of the body is not too high or low.

As the subcutaneous tissue is comprised of blood vessels, it gives a convenient medium for absorption of the medicines. Also, its high fat deposition allows for slow and gradual absorption of medications, thus making it a preferred route for drug delivery. Therefore, many types of subcutaneous injections are administered through hypodermis.

Thus, skin is a large organ which has significant role in the body, and has to be taken care of!