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Epidermis explained

Feb 01, 2024, Update: Feb 01, 2024, author: Hairstrokes.com / Holistic PMU

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https://hairstrokes.com/blogs/resources/epidermis-explained

"The skin's outermost layer. The epidermis significantly impacts semi-permanent makeup techniques like hairstrokes, powder brows, and microblading. This article explores the epidermis's structure, examining its density and thickness. We analyze how these factors should influence the selection of pigmentation needles for the best results in semi-permanent makeup procedures."

1. Background

In the world of pigmentation procedures, such as powder brows, microblading, and hairstrokes, understanding the role of the epidermis is crucial. This article explores the epidermis, the outermost layer of the skin, often subject to misconceptions in the realm of semi-permanent makeup. Drawing insights from 47 interviews with seasoned pigmentation artists and corroborated by two dermatologists and a cellular biology expert, we aim to demystify the epidermis's role in these popular cosmetic procedures.

Our goal is to provide an in-depth understanding of the epidermal layer's dermatological impact and assess the accuracy of common claims in powder brows, microblading, and hairstroke techniques. Through this exploration, we seek a comprehensive understanding that bridges the gap between dermatological science and practical application in semi-permanent makeup.

2. Understanding the 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, known as transepidermal water loss.

Structurally, the epidermis comprises several 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 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, 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 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 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 in the dermis, the layer 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 majority of sensory processing, especially for pain, occurs in the dermis. This structural organization enables the epidermis to act as a protective barrier, with the dermis handling more complex sensory functions, such as pain perception. Hence, some practitioners often overstate the notion that piercing the epidermis can cause significant pain.

Junctions between the epidermal cells

Despite the epidermis's "flaking off" mechanics, which might lead one to believe it is fragile, epidermal cells are tightly interconnected, forming a robust barrier against external elements.

The connections between epidermal cells are known as adherens junctions, consisting of transmembrane proteins called cadherins. Within the cell, cadherins are linked to actin filaments. In immunofluorescence microscopy, this actin filament network is visible as a pronounced border around the cells, although these filaments are situated inside the cell, parallel to the cell membrane. The proximity and tightness of these junctions cause the actin immunofluorescence to appear as a distinct border between cells.

Layers of the epidermis

The epidermis comprises 4 or 5 layers, varying according to the specific area of skin. These layers, listed from the outermost to the innermost, include the Stratum Corneum, Stratum Lucidum (present only in certain parts of the body like the palms and soles), Stratum Granulosum, Stratum Spinosum, and Stratum Basale.

3. Stratum Corneum

The stratum corneum, often called the "horny layer" for its tough and resilient nature, is the outermost layer of the epidermis. It comprises 10 to 30 layers of corneocytes, which are keratinocytes' final, differentiated state. These cells are polyhedral and lack a nucleus, making them anucleated. This layer is notably thicker on the palms and soles to accommodate the higher mechanical demands of these areas.

Structure of Corneocytes

Corneocytes are wrapped in a protein coating beneath their plasma membrane and filled with keratin proteins, crucial for water retention. They are connected by corneodesmosomes and enveloped by multiple layers of lipids in the extracellular space, significantly contributing to the skin's barrier function.

Function of the Stratum Corneum

This layer is the primary barrier against environmental factors and is key in regulating hydration. The cells are dead and continuously shed as part of the skin's natural renewal process.

Etymology of "Horny Layer"

The term "horny," derived from Latin (corneum), reflects the cells' resemblance to animal horns, emphasizing their durability. As cells age and wear down, they are replaced by newer, robust cells from the lower layers.

Cell Turnover Rate

The stratum corneum undergoes continuous renewal. In young adults, complete cell turnover occurs every 28 to 30 days. This rate slows with age, extending to 45 to 50 days in older adults.

Research and Analysis

Given its surface location, the stratum corneum has been the subject of extensive research, yielding valuable insights into its composition, function, and overall skin health.

Detailed Anatomy of the Stratum Corneum

The stratum corneum consists mainly of flattened corneocytes organized into two layers: the stratum disjunctum and the stratum compactum.

Stratum Disjunctum: This top layer is looser and contains larger, more rigid, and more hydrophobic corneocytes. It also houses the skin's protective acid mantle and lipid barrier.
Stratum Compactum: Beneath the stratum disjunctum, this layer is denser and more cohesive, with smaller, less hydrophobic corneocytes.

Protective Functions

The stratum corneum serves several crucial functions:

Barrier Protection: It shields underlying tissues from infection, dehydration, chemicals, and mechanical stress.
Physical and Biological Defense: The layer offers mechanical resistance and regulates microbial growth and invasion. It also initiates inflammation through cytokine activation and dendritic cell activity.
Selective Permeability: It selectively blocks toxins, irritants, and allergens.
Cell Composition and Structure: Typically comprising 15–20 layers of flattened, nucleus-free cells, the corneocytes contain filamentous keratin and are set in a lipid matrix of ceramides, cholesterol, and fatty acids.
Desquamation Process: Desquamation, or cell shedding from the surface, balances the proliferation of keratinocytes from the stratum basale. These cells migrate to the surface in about two weeks, ensuring the constant renewal of the stratum corneum.
Physiological Roles: Beyond its protective function, the stratum corneum is crucial in regulating hydration and water flux, playing a vital role in skin health.

Thus, the stratum corneum's unique structure and composition are indispensable for the skin's protection, balancing various physiological functions while continuously renewing itself to maintain skin health.

4. Stratum Lucidum

The stratum lucidum, known as the "Clear layer" in Latin, is a thin, transparent epidermis layer. It is primarily located in areas of thick skin, such as the palms and soles, and is notably absent in thinner skin regions, for example, the facial skin above the eyes in the brow area.

Microscopic Appearance and Cell Composition

Under microscopic examination, the stratum lucidum is translucent, positioned between the stratum granulosum and stratum corneum within the epidermal structure. It comprises three to five layers of dead, flattened keratinocytes, with indistinct boundaries contributing to its transparent appearance.

Keratinocyte Composition

The keratinocytes within this layer contain eleidin, an intermediate keratin acquired as they migrate from the lower stratum spinosum and stratum granulosum layers.

Oily Surroundings and Thickness Regulation

These cells are enveloped in an oily substance resulting from lamellar bodies' exocytosis, which begins as the cells advance through the lower epidermal layers. The stratum lucidum's thickness is dictated by the rate of cell division (mitosis) in the epidermis, determining the speed at which cells are replenished and transition into this layer.

Role of Melanosomes

Melanosomes from the stratum basale influence the pigmentation or darkness of the stratum lucidum, although this layer generally maintains its clarity.

Consequently, the stratum lucidum represents a specialized, thin layer of the epidermis, predominantly found in thicker skin areas. Its distinct cellular composition and placement within the epidermal layers underscore its transparent characteristic and protective function in the skin.

5. Stratum Granulosum

The granular layer of the epidermis, known as the stratum granulosum, occupies a middle position within the epidermal structure. It is situated above the stratum spinosum and below the stratum corneum, and in areas of thick skin, such as the palms and soles, it is positioned beneath the stratum lucidum. This layer is less pronounced in the brow skin area.

Cellular Transition and Keratohyalin Granules

Keratinocytes progressing from the stratum spinosum transform into granular cells within this layer, marked by keratohyalin granules. These granules, rich in histidine- and cysteine-containing proteins, play a crucial role in binding intermediate keratin filaments, enhancing cell structure robustness.

Formation of the Skin Barrier

The transition of cells from the stratum granulosum to the stratum corneum involves the release of lamellar bodies filled with lipids and proteins into the extracellular space, forming a hydrophobic lipid envelope essential for the skin's barrier function. This stage marks significant cellular changes, including the loss of nuclei and organelles, signaling the end of the cellular lifecycle.

Lipid Transformation

Within this transition, polar lipids within the cells are converted into non-polar lipids and reoriented parallel to the cell surface, enhancing the skin's protective barrier. For example, glycosphingolipids are transformed into ceramides, and phospholipids into free fatty acids.

The stratum granulosum is crucial for skin health and protection, with its unique cellular composition and transformation processes contributing significantly to the skin's barrier function, aiding in moisture retention and protection against external contaminants.

6. Stratum Spinosum

The prickle cell layer, or stratum spinosum, contributes significantly to the epidermis's overall thickness. It is located between the stratum granulosum and the basal layer (stratum basale).

Cellular Composition

This layer comprises several polyhedral keratinocytes, interconnected by desmosomes, specialized cell structures that provide strength and cohesion.

Spiny Appearance

The "spiny" appearance of this layer, from which its name derives, is due to the contraction of microfilaments between desmosomes, becoming evident when stained with hematoxylin and eosin (H&E).

Keratinization Process and Formation of Desmosomes

The keratinization process begins within the stratum spinosum, with keratinocytes actively synthesizing fibrillar proteins known as cytokeratin. These accumulate within the cells as tonofibrils, crucial for desmosome formation and ensuring robust connections between adjacent keratinocytes, vital for skin integrity and resilience.

Presence of Langerhans Cells

Langerhans cells, which function similarly to macrophages by engulfing bacteria, foreign particles, and damaged cells, are also present within this layer and play an essential role in the skin's immune defense.

The stratum spinosum is integral to maintaining skin structure and strength, thanks to its dense network of interconnected keratinocytes. It plays a pivotal role in the skin's immune response, underscored by the presence of Langerhans cells. Its structural and functional attributes are key to the skin's overall 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 pivotal role in generating new skin cells and typically comprises a single row of cells.

Overall Thickness

Contrary to cellular, the basal membrane is a thin, fibrous structure at the interface between the epidermis and the dermis. It serves as a specialized type of extracellular matrix. The basement membrane is categorized into two main layers.

Lamina Lucida. Lamina Lucida is the layer nearest to the epidermis. It is visible under an electron microscope as a clear layer containing proteins such as laminin and integrins.
Lamina Densa. Located beneath the lamina lucida, this denser layer consists of a network of collagen fibers, predominantly type IV collagen, and other elements like heparan sulfate proteoglycans. It measures approximately 40-120 nm in thickness.

Cellular Composition and Melanocyte Function

The basal layer primarily comprises small, round cells that continually divide, pushing new cells upwards toward the surface for further maturation and eventual shedding.

Within this layer, melanocytes are specialized cells responsible for melanin production, the pigment giving skin its tan or brown color, crucial for protection against ultraviolet rays from the sun. Sunlight exposure triggers increased melanin production, leading to tanning, with variations in melanin distribution causing freckles, birthmarks, and age spots. Melanoma, a form of skin cancer, originates from the malignant transformation of melanocytes. Merkel cells, tactile in function and neuroectodermal in origin, are also situated in the basal layer.

Constant Cell Renewal

The term "stratum germinativum" emphasizes the basal layer's perpetual cell generation, consistently replenishing cells lost from the skin's surface.

Therefore, the stratum basale is the epidermis's foundational layer, essential for new skin cell generation. It houses melanocytes and Merkel cells, instrumental in skin pigmentation and tactile sensation. The basal layer's ongoing cell division is vital in skin renewal and repair.

Holistic View of the Epidermis

To understand the epidermis comprehensively, it is crucial to perceive it as an integrated and dynamic system. The basal layer, or stratum basale, operates harmoniously with the outermost stratum corneum, creating a continuous skin renewal cycle. The basal layer serves as the origin of new skin cells, which embark on a journey upwards through the intermediary layers, undergoing various transformations along the way. Eventually, these cells arrive at the stratum corneum, where they exist as flattened, lifeless cells that the body can naturally shed.

The Life Cycle of Skin Cells

This skin cell turnover and renewal process can be likened to a 'circle of life' within our body's biological systems. This turnover occurs relatively swiftly during our youth, but as we age, the pace gradually diminishes. This phenomenon mirrors the broader aging processes that affect the human body. Ultimately, like many bodily functions, the efficiency and speed of this cycle wane over time, continuing until the end of life.

In summary, the epidermis functions as an intricately connected system, with the stratum basale initiating the generation of new skin cells, setting in motion a cycle that progresses through the epidermal layers, culminating in the shedding of aged cells from the stratum corneum. This process is fundamental to our body's natural renewal and maintenance mechanisms.

Epidermis in the Brow Area

When examining the epidermis in the brow area, referring to studies on epidermal thickness in facial regions is informative. Generally, the epidermis in the eyebrow area displays less variability in thickness. Accounting for individual differences in skin properties, the epidermis in this region typically measures between 50-100 micrometers (0.05-0.1 mm).

This range aligns with established dermatological knowledge, indicating that the epidermis is notably thinner than the dermis and exhibits less thickness variation across different body areas. Compared to body parts like the palms or soles, the epidermis on the face, including the brow area, tends to be thinner.

Correlation Between Dermal and Epidermal Thickness
Evidence suggests a correlation between dermal and epidermal thickness, although this relationship is not strictly linear and can be influenced by various factors. Some possible factors contributing to this correlation include the following.
Collagen and Elastin Density: A thicker dermis, rich in collagen and elastin, may support a healthier and thicker epidermis due to improved skin integrity and nutrient supply.
Blood Supply: A well-vascularized thicker dermis could enhance epidermal health and thickness.
Genetic and Hormonal Factors: These factors may simultaneously influence dermal and epidermal thickness.

However, while a general trend may exist, individual variations are substantial. A thicker dermis does not universally guarantee a thicker epidermis. Skin physiology is intricate; numerous independent and overlapping factors influence these interconnected layers.

Epidermis Thickness and Needle Selection

When considering different needles for pigmentation procedures in the brows, such as powder brows, combo brows, or hair strokes, the thickness of the epidermis should be analyzed about needle selection. Realistic analysis involves examining the thickness of various needles and their impact on the epidermis.

For the sake of simplicity, let's compare three possible needle sizes: 1RL 0.25, 1RL 0.3, and 1RL 0.35. These needles have different tapers, the length from the tip to where the needle achieves its full diameter. To simplify the analysis, let's assume equal tapers for these needles.

When using a 1 RL 0.3 needle with a medium stroke, the resulting puncture diameter can range from 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 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

It's worth noting that standard images or diagrams depicting skin layers often inaccurately represent the relative thickness of the epidermis, especially in the brow area. These images may wrongly convey that the epidermis is much thicker in proportion to the dermis. In reality, the epidermis is significantly thinner than typically depicted and thinner than the needles commonly used in pigmentation procedures. This understanding is crucial for pigmentation artists to accurately assess the impact of their tools on the skin layers, particularly when considering techniques and outcomes in semi-permanent makeup applications.

Resistance of Cellular Structure

Nevertheless, the resistance of the epidermis still plays a role in needle selection. The uppermost layer, the stratum corneum, consists of densely packed, dead, and keratin-rich cells, offering significant mechanical resistance due to its density and keratinization. This layer presents notable resistance to needle penetration.

Comparatively, when a needle pierces through the papillary dermis, it encounters less resistance due to the looser connective tissue matrix, fewer keratinized cells, and more elastic fibers and blood vessels. However, it's essential to recognize that factors like skin hydration, age, and specific anatomical locations can influence the resistance experienced during needle penetration.

In summary, a needle generally encounters more resistance under certain conditions when piercing through the epidermal layers, particularly the stratum corneum, compared to the papillary dermis. This variation in resistance should be considered when selecting needles for pigmentation procedures, as it can impact the technique and outcome.

Stratum Corneum vs. Needle: David vs. Goliath

In skin anatomy, a significant battle unfolds when a needle encounters the stratum corneum, the outermost layer of the epidermis. This layer is a formidable barrier to mechanical penetration, akin to the tale of David vs. Goliath. The primary reason behind this resistance lies in the stratum corneum's high concentration of keratin within its keratinocytes. Keratin is a robust protein that bestows resilience and strength, rendering the stratum corneum exceptionally resistant to physical intrusions.

In stark contrast, just beneath the epidermis lies the papillary dermis, characterized by a more relaxed connective tissue matrix. This matrix includes components such as collagen fibers, elastin fibers, and a gel-like ground substance. Due to its less dense and pliable nature, the papillary dermis offers notably less resistance to needle penetration compared to the densely packed and keratin-rich stratum corneum.

Hence, it is reasonable to conclude that the primary hurdle to needle penetration in procedures like semi-permanent makeup lies at the level of the stratum corneum. The variance in tissue composition between the stratum corneum and the papillary dermis plays a pivotal role in determining the ease with which a needle can traverse these layers.

Actual Factors Influencing Resistance

While there is no straightforward linear correlation between the thickness of the dermis and the resistance of the stratum corneum in an individual's epidermis, several factors can contribute to the thickness and density of the stratum corneum, particularly in the facial and brow areas. These factors include.

Age and Hormonal Changes: As individuals age or undergo hormonal fluctuations, the skin's turnover rate and texture can change, impacting the properties of the stratum corneum.
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 response to UV radiation. Conversely, exposure to harsh weather conditions can result in skin damage and potential alterations in stratum corneum thickness.
Hydration Levels: Skin hydration influences its elasticity and pliability. Dehydrated skin can lead to a denser and more brittle stratum corneum.
Skin Care Practices: Using moisturizers, exfoliants, and other skin care products can affect 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 alter the characteristics of the stratum corneum.
Occupational Factors: Repeated mechanical friction or pressure, as observed in manual laborers, can thicken the stratum corneum.

Additionally, various skin conditions and factors can influence stratum corneum thickness and density, affecting needle penetration:

Calluses and Corns: These form in response to pressure or friction, resulting in thickened skin that impedes needle entry.
Psoriasis: A chronic skin condition leading to thick, scaly plaques that may hinder needle penetration.
Eczema: Certain types of eczema, like hyperkeratotic eczema, can cause stratum corneum thickening.
Ichthyosis: A group of genetic skin disorders causing 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 thicken the outer skin layers as a protective measure.
Diabetes: Diabetes can lead to changes in skin texture and thickness.

In summary, the resistance posed by the stratum corneum during needle penetration is a multifaceted interplay of numerous biological, environmental, and lifestyle factors. It is not solely reliant on dermal thickness but encompasses a complex web of influences that impact the characteristics of this outermost skin layer. Understanding these factors is essential for those involved in semi-permanent makeup procedures, as they significantly affect the technique and outcome.

8. Conclusions

The epidermis, our body's outermost skin layer, unveils its intricacies and variations when analyzed in the context of semi-permanent makeup procedures. The epidermis maintains a delicate balance in the brow region, measuring approximately 0.05 to 0.1 mm in thickness. This seemingly unassuming layer reveals its complexity through its division into five distinct strata: the Stratum Corneum, Granulosum, Stratum Spinosum, Stratum Basale, and the notably absent Stratum Lucidum.

However, a fascinating juxtaposition emerges when we consider the dimensions of a pigmentation needle. Typically ranging from 150,000 to 300,000 nanometers (0.15 - 0.3 mm) in diameter, the needle dwarfs the depth of the epidermis in the brow area, which spans a mere 50,000 to 100,000 nanometers (0.05 - 0.1 mm). This stark contrast underscores a common misrepresentation found in semi-permanent makeup training materials. Graphics often depict the epidermis as thicker than reality dictates.

Yet, beneath this seemingly fragile surface lies the Stratum Corneum, aptly referred to as the "Horny layer." Like an animal's horn, its robust structure poses a formidable resistance to needle penetration. Despite its thinness, this layer is a staunch defender, influencing the outcome of semi-permanent makeup procedures.

Exploring further, we uncover an intricate web of factors that shape the characteristics of the Stratum Corneum. Genetics, environmental exposure, age, hormonal fluctuations, skincare routines, and overall health and nutrition all leave their indelible mark on the density and resistance of this crucial layer. Surprisingly, these multifaceted variables, rather than dermal thickness, dictate the Stratum Corneum's properties.

For semi-permanent makeup artists, these revelations carry profound implications. Selecting the appropriate needle for procedures necessitates a thorough understanding of these intricacies. Despite the needle's apparent dominance in size, the Stratum Corneum's resistance remains a pivotal factor in achieving optimal results. As the artist wields their tools, they navigate the intricate terrain of the epidermis, crafting beauty through the delicate interplay of science and artistry.