Epidermis structure, density, and Significance in needle ...

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2. Understanding epidermis

Basic facts and background Etymologically, the term “epidermis” originates from the Ancient Greek words “epi,” meaning “over” or “upon,” and “derma,” meaning “skin.” Thus, 'epidermis' refers to the layer of cells atop the skin. Any aspect related to this outermost skin layer is described using the adjective “epidermal.”

From the biological perspective, the epidermis is the outermost layer of the skin, serving as a critical barrier between the body and its external environment. This layer is distinct from the two deeper layers of the skin - the dermis and the hypodermis. One of the primary functions of the epidermis is to act as a defense mechanism, protecting the body from environmental pathogens. Additionally, it plays a crucial role in regulating water loss from the body into the atmosphere, a process known as transepidermal water loss.

Structurally, the epidermis comprises several layers of flattened cells known as keratinocytes, which lie above a basal layer of columnar cells. These basal cells are oriented perpendicularly and are pivotal as they contain stem cells that generate new epidermal cells. As these cells mature, they move upwards, flattening out to form the skin's protective outer layers.

The human epidermis exemplifies an epithelial tissue classified as a stratified squamous epithelium. This classification is based on its structure comprising multiple cell layers, with the surface cells appearing flattened. Such a structural design significantly bolsters the epidermis's protective role. The epidermis undergoes a continual renewal process; new cells are generated through mitosis at the basal layer, which is situated at the epidermis's lower boundary, adjacent to the basement membrane. These new cells gradually migrate to the surface, replacing older cells. This entire cycle of epidermal renewal typically spans approximately 28 days, resulting in the replacement of the epidermis with a completely new layer.

Consistency of epidermis

The epidermis primarily consists of keratinocytes (proliferating basal and differentiated suprabasal), which comprise 90% of its cells but also contain melanocytes, Langerhans, Merkel, and inflammatory cells. Epidermal thickenings called Rete ridges (or rete pegs) extend downward between dermal papillae. Blood capillaries are found beneath the epidermis and are linked to an arteriole and a venule. The epidermis itself has no blood supply and is nourished almost exclusively by diffused oxygen from the surrounding air. Cellular mechanisms for regulating water and sodium levels are found in all epidermis layers. Basically, this means that we do not feel our epidermis, nor can the removal or piercing of the epidermis actually hurt us. It protects us from bacteria entering our body, but, as ancient Greeks realized, the epidermis is just like a cover, something that is on top of what our body is. Let us know and explore further how the epidermis protects us.

Why does the epidermis “feel no pain”?

The perception of pain in the skin is primarily mediated by nociceptors, which are sensory neurons responsive to potentially harmful stimuli. In the context of the epidermis and its lack of pain sensation, a few key factors are involved.

Location of Nociceptors. While the epidermis contains free nerve endings, most nociceptors are located in the dermis, the layer just beneath the epidermis. This deeper positioning of nociceptors is crucial for sensing pain, as these nerve endings are better protected and positioned to signal harm that could lead to tissue damage accurately.
Structure of the Epidermis. The epidermis primarily comprises keratinocytes, specialized cells that do not have the necessary components (like nociceptors) to sense pain. The topmost layer of the epidermis, the stratum corneum, is composed of dead and flattened keratinocytes, which further diminishes any pain sensitivity in this outermost layer.
Function of the Epidermis. The primary function of the epidermis is to act as a barrier, protecting the underlying layers of skin and the rest of the body from environmental factors such as pathogens, chemicals, and physical abrasion. Sensing pain is not a direct function of the epidermis; this role is reserved for the deeper layers of the skin where nociceptors are more prevalent.

While the epidermis contains some free nerve endings that can sense temperature and other stimuli, the bulk of sensory processing, especially for pain, occurs in the dermis. This anatomical arrangement allows the epidermis to fulfill its role as a protective barrier while relying on the dermis for more complex sensory functions, including pain perception. Therefore, all claims by some artists refer to the possibility of feeling the pain when piercing the epidermis as greatly exaggerated.

Junctions between the epidermal cells

Although the “flaking off” mechanics of the epidermis may make an average artist think that the epidermis is very fragile and weak, that is not necessarily true. Epidermal cells are tightly interconnected to serve as a tight barrier against the exterior environment.

The junctions between the epidermal cells are of the adherens junction type, formed by transmembrane proteins called cadherins. Inside the cell, the cadherins are linked to actin filaments. In immunofluorescence microscopy, the actin filament network appears as a thick border surrounding the cells, although the actin filaments are actually located inside the cell and run parallel to the cell membrane. Because of the neighboring cells' proximity and tightness of the junctions, the actin immunofluorescence appears as a border between cells.

Layers of epidermis

The epidermis comprises 4 or 5 layers, depending on the skin region being considered. Those layers from outermost to innermost are Stratum Corneum, Stratum Lucidum, Stratum Granulosum, Stratum Spinosum, and Stratum Basale.

3. Stratum Corneum

Outermost Cornified layer, Latin for “horny layer” (10-30 cell layers)

Firs, let us explore the composition and characteristics of the stratum corneum, often referred to as the "horny layer" due to its tough and resilient characteristics, is the topmost layer of the epidermis. It consists of 10 to 30 corneocytes layers, the final, differentiated form of keratinocytes. These cells are polyhedral and anucleated, meaning they no longer contain a nucleus. This layer is thicker on the palms and soles, adapting to the higher mechanical demands of these areas.

Structure of Corneocytes

Corneocytes are enveloped in a protein coating beneath their plasma membrane and filled with keratin proteins, essential for retaining water. They are linked by corneodesmosomes and surrounded by multiple layers of lipids in the extracellular space. These structural elements contribute significantly to the skin's barrier function. Function of the Stratum Corneum This layer is the primary barrier against environmental factors and is crucial in hydration regulation. The cells in this layer are essentially dead and continually shed as part of the skin's natural renewal process.

Etymology of "Horny Layer"

The term "horny" in Latin (corneum) refers to the resemblance of these cells to an animal's horn, highlighting their toughness and durability. As the cells age and wear down, they are replaced by new, robust cells from the lower layers. Cell Turnover Rate The stratum corneum undergoes a continual renewal process. In young adults, complete cell turnover – the process by which new ones replace old cells – occurs approximately every 28 to 30 days. However, this process slows with age, taking about 45 to 50 days in older adults.

Research and Analysis

Due to its location at the skin's surface, the stratum corneum has been extensively studied, providing valuable insights into its composition, function, and the overall health of the skin.

Thus, the stratum corneum plays a vital role in protecting the body, regulating moisture, and maintaining skin integrity. Its unique composition and constant renewal are essential for healthy skin function.

Detailed Anatomy of the Stratum Corneum

The stratum corneum is primarily composed of flattened corneocytes, which are organized into two distinct layers: the stratum disjunctum and the stratum compactum.

Stratum Disjunctum. This is the uppermost layer of the stratum corneum. It is looser and contains the larger, more rigid, and more hydrophobic corneocytes. It also hosts the skin's protective acid mantle and lipid barrier.
Stratum Compactum. Located beneath the stratum disjunctum, this layer is more compact and cohesive. The corneocytes here are smaller and less hydrophobic compared to those in the stratum disjunctum.

Protective Functions

The stratum corneum serves several vital functions.

Barrier Protection. It acts as a barrier to protect underlying tissues from infection, dehydration, chemicals, and mechanical stress.
Physical and Biological Defense. The layer provides mechanical resistance (shear and impact) and regulates microbial proliferation and invasion. It also plays a role in initiating inflammation through cytokine activation and dendritic cell activity.
Selective Permeability. It selectively permits the exclusion of toxins, irritants, and allergens.
Cell Composition and Structure. Comprising often 15–20 layers of flattened, anucleated cells, the corneocytes in the stratum corneum contain filamentous keratin and are embedded in a lipid matrix of ceramides, cholesterol, and fatty acids.
Desquamation Process. Desquamation, the shedding of cells from the stratum corneum's surface, balances the proliferation of keratinocytes originating in the stratum basale. These cells migrate through the epidermis to the surface, taking about two weeks. This constant renewal process maintains the integrity and functionality of the stratum corneum.
Physiological Roles. In addition to its protective barrier function, the stratum corneum is instrumental in hydration regulation and water flux, thus playing a crucial role in the skin's overall health.

Therefore, with its unique structure and composition, the stratum corneum is essential for the skin's protective role, balancing various physiological functions while continually renewing itself to maintain skin health.

4. Stratum Lucidum

The "Transparent Layer” (3-5 cell layers)

The stratum lucidum, Latin for "Clear layer," is a thin, transparent epidermis layer. It's typically found only in areas of thick skin, such as the palms of the hands and the soles of the feet, and is absent in thinner skin areas, including the facial skin above the eyes in the brow area.

Microscopic Appearance and cell composition

Under a microscope, this layer appears translucent. It sits between the stratum granulosum and stratum corneum in the epidermal structure. The stratum lucidum consists of three to five layers of dead, flattened keratinocytes. These cells have indistinct boundaries, contributing to the layer's translucent nature.

Keratinocyte Composition

The keratinocytes in this layer are filled with eleidin, an intermediate form of keratin. This transformation occurs as keratinocytes migrate from the lower stratum spinosum and stratum granulosum layers.

Oily Surroundings and thickness regulation

These cells are encased in an oily substance resulting from the exocytosis of lamellar bodies. This process starts as the cells move through the lower layers of the epidermis. The thickness of the stratum lucidum is influenced by the rate of cell division, or mitosis, in the epidermis. This rate determines how quickly cells are replenished and subsequently transition into this layer.

Role of Melanosomes

The darkness of the stratum lucidum is affected by melanosomes present in the stratum basale. These melanosomes influence the overall pigmentation of this layer, although it generally remains relatively clear.

Thus, the stratum lucidum is a specialized, thin epidermis layer predominantly present in thicker skin regions. Its unique cellular composition and position within the epidermal layers contribute to its clear appearance and functional role in skin protection.

5. Stratum Granulosum

The Granular Layer of the Epidermis (1-5 cell layers)

Position and Composition The stratum granulosum, or the granular layer, is in the middle of the epidermis. It lies above the stratum spinosum and beneath the stratum corneum. In areas with thick skin, like the palms and soles, it's under the stratum lucidum. This layer is not a prominent feature in the brow skin.

Cellular Transition and Keratohyalin Granules

Keratinocytes ascending from the stratum spinosum transform into granular cells in this layer. The presence of keratohyalin granules characterizes these granular cells. These granules are rich in proteins containing histidine and cysteine. Their primary role is to bind intermediate keratin filaments together, aiding in the cell structure's robustness.

Formation of the Skin Barrier As cells transition from the stratum granulosum to the stratum corneum, they release lamellar bodies containing lipids and proteins into the extracellular space. This secretion creates a hydrophobic lipid envelope, a crucial component of the skin’s barrier properties.

During this transition, cells undergo significant changes – they lose their nuclei and organelles, signaling the end of their cellular life cycle. Lipid Transformation The polar lipids within these cells are converted to non-polar lipids and are reoriented parallel to the cell surface. For instance, glycosphingolipids transform into ceramides, and phospholipids convert into free fatty acids. These lipid changes further enhance the protective barrier function of the skin.

Therefore, the stratum granulosum is pivotal in skin health and protection. Its unique cellular composition and the transformation that occurs within this layer contribute to the skin's barrier function, preventing moisture loss and protecting against external contaminants.

6. Stratum Spinosum

The Prickle Cell Layer (8-10 cell layers)

The stratum spinosum, also known as the spinous or prickle cell layer, is a key contributor to the epidermis's thickness. It resides between the stratum granulosum and the basal layer (stratum basale).

Cellular Composition

This layer comprises several layers of polyhedral keratinocytes. These keratinocytes are interconnected by desmosomes, which are specialized cell structures that provide strength and cohesion between cells.

Spiny Appearance

This layer's characteristic “spiny” appearance, from which it derives its name spinosum, results from the contraction of microfilaments between the desmosomes. This appearance becomes evident when the layer is stained using hematoxylin and eosin (H&E). Keratinization

Process and Formation of Desmosomes

The keratinization process, the protein keratin development, starts in the stratum spinosum. Keratinocytes in this layer are active in synthesizing fibrillar proteins called cytokeratin.These proteins accumulate within the cells, forming tonofibrils. Tonofibrils play a crucial role in creating desmosomes, thereby facilitating robust connections between adjacent keratinocytes. This structural feature is essential for maintaining the integrity and resilience of the skin. Presence of Langerhans Cells Within the stratum spinosum, Langerhans cells are present. These cells function similarly to macrophages, engulfing bacteria, foreign particles, and damaged cells found in this layer. Their role is vital for the skin's immune defense mechanism.

The stratum spinosum is vital for maintaining skin structure and strength due to its dense network of interconnected keratinocytes. It also plays a significant role in the skin's immune response, thanks to the presence of Langerhans cells. The layer's structural and functional properties contribute significantly to overall skin health and resilience.

7. Basal membrane

Stratum Basale: The Foundation of the Epidermis

The stratum basale, also known as the basal layer or stratum germinativum, is the innermost layer of the epidermis. It plays a crucial role in generating new skin cells. Typically, this layer consists of a single row of cells.

Overall thickness

The basal membrane is not composed of cells but is a thin, fibrous structure that lies at the interface between the epidermis and the dermis in the skin. It's a specialized type of extracellular matrix. In terms of its structure, the basement membrane is divided into two primary layers:

Lamina Lucida. This is the layer closest to the epidermis. A clear (lucid) layer is visible under the electron microscope, containing proteins like laminin and integrins.
Lamina Densa. This layer lies beneath the lamina lucida and is denser. It comprises a network of collagen fibers, mainly type IV collagen and other components like heparan sulfate proteoglycans. The dense basal lamina is closest to the basal aspect of the epithelium. It is about 40-120 nm thick.

Cellular Composition and Melanocyte Function

The basal layer is primarily composed of basal cells, which are small and round. These cells constantly divide, propelling the newer cells upwards toward the skin's surface, where they undergo further maturation and are eventually shed.

Melanocytes are present Within the basal layer, specialized cells that produce the pigment melanin. Melanin is responsible for giving the skin its tan or brown color and is pivotal in protecting the skin from the sun's harmful ultraviolet rays. Exposure to sunlight stimulates melanocytes to produce more melanin, leading to tanning. Variations in melanin distribution cause freckles, birthmarks, and age spots. Melanoma, a type of skin cancer, arises from the malignant transformation of melanocytes. Merkel cells are also located in the basal layer. These cells are tactile, which is associated with the sense of touch, and are of neuroectodermal origin.

Constant cell renewal

The term “stratum germinativum” underscores the basal layer's continuous cell generation. This ongoing process ensures a steady supply of new cells to replace those shed from the skin's surface.

Thus, the Stratum basale is the foundation layer of the epidermis, where new skin cells are generated. It hosts melanocytes and Merkel cells, contributing to skin pigmentation and tactile functions. The basal layer's continuous cell division is key to the skin's ability to renew and repair itself.

9. Epidermis in the Brow area

When we analyze the epidermis in the brow area, we can utilize the studies of the epidermis thickness in the facial area. It can be said that in the eyebrow area, the epidermis shows less variation in thickness. Accounting for different skin properties, the epidermis in this region typically measures between 50-100 micrometers (0.05-0.1 mm).

It is safe to say that this range aligns with established dermatological knowledge. The epidermis is considerably thinner than the dermis and varies less in thickness across different body areas. The epidermis on the face, including the brow area, is typically thinner than other body parts like the palms or soles.

Correlation Between Dermal and Epidermal Thickness

Here is some evidence suggesting a correlation between dermal and epidermal thickness. However, this relationship is not strictly linear and can be influenced by many factors.

Possible Causes for that

Collagen and Elastin Density. A thicker dermis, with more collagen and elastin, might support a healthier and possibly thicker epidermis due to better overall skin integrity and nutrient supply.
Blood Supply. A richer blood supply in a thicker dermis could potentially enhance epidermal health and thickness.
Genetic and Hormonal Factors. These might simultaneously influence both dermal and epidermal thickness.

However, it's important to note that while there might be a general trend, individual variations are substantial, and a thicker dermis does not universally guarantee a thicker epidermis. Skin physiology is remarkably complex, and a myriad of independent and overlapping factors influence these interconnected layers.

Epidermis thickness and needle selection

When we consider different needles that can be used for the pigmentation of brows, whether we are talking about powder brows, combo brows, or hair strokes.

When considering the needle selection to the thickness of the epidermis, we should first analyze if the properties of the epidermis of a concrete client should influence our needle choice.

Realistic analyzes

Let us analyze the thickness of three possible needles that can be used for pigmentation procedures: 1RL 0,25, 1 RL 0,3, and 1 RL 0.35. When we analyze the needle piercing through the epidermis and the Base membrane, we have to consider the needle's taper - the length from the tip to the part where the needle achieves its full diameter.

However, for the sake of simplicity of proving the insignificance of the epidermis, we can equalize the tapers on the three sizes of needles. When it comes to defining the exact ranges of short, medium, and long strokes, different produces have somewhat different classifications, and in most cases, relative lengths matter. For the sake of comparison, it can be said that if the medium taper punches the hole inside the skin, the diameter of the implantation hole is around half to the full diameter of the needle.

Analyzing Needle Penetration in Epidermal Layers

When considering a 1 RL 0.3 needle with a medium stroke, we can approximate the diameter of the resultant puncture to be between 150,000 to 300,000 nanometers (0.15 - 0.4 mm). In contrast, the thickness of the epidermis typically ranges from 50,000 to 100,000 nanometers (0.05 - 0.1 mm). This disparity suggests that the width of the ‘crater’ created by the needle’s penetration through the epidermis could be up to three to six times greater than the depth of the epidermis itself.

Misrepresentation in Standard Graphics

Often, stock images or default diagrams used to depict skin layers inaccurately represent the relative thickness of the epidermis, especially in the brow area. These images may misleadingly suggest that the epidermis is much thicker in proportion to the dermis. When a needle is added to these diagrams, it may appear disproportionately small compared to the depicted thickness of the epidermis, which is not accurate in real-world scenarios.

In reality, the situation is quite the opposite. The epidermis is considerably thinner than it is often portrayed and significantly thinner than the typical needle used in pigmentation procedures, regardless of the needle’s taper or diameter. This understanding is crucial for pigmentation artists to accurately conceptualize the impact of their tools on the skin layers, especially when considering techniques and outcomes in semi-permanent makeup applications.

Resistance of cellular structure

However, that does not yet indicate that the resistance of the epidermis would not have an influence on the needle choice. As we know, the uppermost layer, the stratum corneum, is made of dead, flattened, and keratin-rich cells that are tightly packed. This layer provides substantial mechanical resistance due to its density and keratinization. This part of the epidermis offers considerable resistance to the needle piercing the skin.

Comparative Analysis

When penetrating the epidermis, a needle encounters more resistance due to the density and keratinization of the cells, especially in the stratum corneum. This resistance is more pronounced in areas where the epidermis is thicker. In contrast, when piercing through the papillary dermis, the needle encounters comparatively less resistance due to the looser connective tissue matrix, fewer keratinized cells, and the presence of more elastic fibers and blood vessels.

It is important to note that individual variability, such as skin hydration, age, and specific anatomical location, can influence the actual resistance experienced during needle penetration.

Under the ceteris paribus condition, a needle would generally encounter more resistance piercing through the epidermal layers, especially the stratum corneum, compared to the papillary dermis, which is characterized by its more flexible and less dense connective tissue structure. It can be claimed that the actual resistance to the needle penetration does

10. Stratum Corneum vs the needle

David vs. Goliath - The Stratum Corneum

The stratum corneum, the outermost layer of the epidermis, presents a significant barrier to mechanical penetration, such as needle piercing. This resistance is primarily due to the high concentration of keratin in the keratinocytes within this layer. Keratin is a tough protein that imparts resilience and strength, making the stratum corneum particularly resistant to physical intrusions.

In contrast, the underlying papillary dermis, situated just beneath the epidermis, comprises a looser connective tissue matrix. This matrix includes elements like collagen fibers, elastin fibers, and a gel-like ground substance. Given its less dense and more pliable nature, the papillary dermis offers less resistance to needle penetration compared to the densely packed and keratin-rich stratum corneum.

Therefore, it's reasonable to assert that the primary resistance to needle penetration during procedures like semi-permanent makeup is encountered at the level of the stratum corneum. The difference in tissue composition between the stratum corneum and the papillary dermis significantly influences the ease with which a needle can penetrate these layers.

Actual factors causing the resistance

There is no linear correlation between the dermis's " thickness " and the Stratum Corneum's resistance in a person's epidermis. However, some factors can contribute to the thickness and density of Stratum Corneum in the facial area and in brows specifically. Some of those are.

Age and hormonal changes. As individuals age or undergo hormonal fluctuations, the skin's turnover rate and texture can change, impacting the stratum corneum's properties.

Genetics. Genetic predispositions play a role in determining skin characteristics, including the thickness of the stratum corneum.
Environmental Exposure. Prolonged exposure to the sun can lead to a thickening of the stratum corneum as a protective mechanism against UV radiation. Conversely, exposure to harsh weather conditions can lead to skin damage and potential alterations in stratum corneum thickness.
Hydration Levels. Hydration affects the elasticity and pliability of the skin. Dehydrated skin can lead to a denser and more brittle stratum corneum.
Skin Care Practices. Using moisturizers, exfoliants, and other skin care products can influence the skin cells' turnover rate and the stratum corneum's thickness.
Health and Nutrition. Overall health and nutrition, including hydration and vitamin intake, impact skin health and can affect the characteristics of the stratum corneum.
Occupational Factors. Repeated mechanical friction or pressure (as seen in manual laborers) can lead to thickening of the stratum corneum.

An important factor is also related to skin conditions. Here are some examples.

Calluses and Corns. These are areas of thickened skin that form as a response to pressure or friction, making needle penetration more difficult.
Psoriasis- This chronic skin condition can lead to the formation of thick, scaly plaques that may impede needle entry.
Eczema. In certain types, like hyperkeratotic eczema, the stratum corneum can thicken.
Ichthyosis. A group of genetic skin disorders that cause dry, thickened, scaly skin.
Actinic Keratosis. Rough, scaly patches on sun-exposed areas can be thicker and harder.
Lichen Planus. This condition can cause thick, scaly patches on the skin.
Chronic Sun Exposure. Long-term sun exposure can lead to the thickening of the outer skin layers as a protective measure.
Diabetes: This can lead to changes in skin texture and thickness.

Each of these factors and conditions can contribute to the variability in stratum corneum thickness and density, thereby influencing the ease with which a pigmentation needle can penetrate the skin. Thus, the resistance of the stratum corneum is a complex interplay of various biological, environmental, and lifestyle factors rather than being solely dependent on the thickness of the dermis.

11. Conclusions

The epidermis, as the outermost layer of the skin, varies in thickness across different body areas. In the brow region, its thickness is approximately 0.05 to 0.1 mm. The epidermis comprises five distinct layers: Stratum Corneum, Stratum Granulosum, Stratum Spinosum, and Stratum Basale. Notably, the Stratum Lucidum is absent in the brow area.

Needle Diameter vs. Epidermal Depth. Typically, the diameter of a pigmentation needle ranges from 150,000 to 300,000 nanometers (0.15 - 0.3 mm). This is significantly larger than the depth of the epidermis in the brow area, which spans only about 50,000 to 100,000 nanometers (0.05 - 0.1 mm). Therefore, commonly used graphics in semi-permanent makeup training often misrepresent the relative thickness of the epidermis, portraying it as thicker than it is in reality.

Epidermis Resistance to Needle Penetration. Despite its thinness, the epidermis, particularly the Stratum Corneum, plays a significant role in resisting needle penetration. This uppermost layer, also known as the "Horny layer," is likened to an animal's horn in terms of its tough structure.

Factors Influencing Stratum Corneum Characteristics. A range of factors, including genetics, environmental exposure, age, hormonal changes, skincare practices, and overall health and nutrition, influence the density and resistance of the Stratum Corneum. These factors, rather than the dermis's thickness, determine this layer's characteristics.

Implications for Semi-Permanent Makeup Artists. It's crucial for artists to consider these variables when selecting a needle for semi-permanent makeup procedures. Despite its relative thinness compared to the needle, the Stratum Corneum's resistance is an important consideration for achieving optimal results.

Epidermis structure, density, and Significance in needle selection

1. Background

2. Understanding epidermis