Epidermis explained

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1. Background

Understanding the epidermis's role is essential in pigmentation procedures like lip blush, powder brows, microblading, and hairstrokes. This article explores the epidermis, the skin's outermost layer, often misunderstood in semi-permanent makeup. Based on 47 interviews with experienced pigmentation artists and supported by two dermatologists and a cellular biology expert, we aim to clarify the epidermis's role in these popular cosmetic procedures.

We aim to offer an in-depth understanding of the epidermal layer's dermatological impact. We also want to evaluate the validity of widespread beliefs regarding lip blush, powder brows, microblading, and hairstroke techniques. Through this investigation, we intend to provide a detailed comprehension that connects dermatological science with practical use in semi-permanent makeup.

2. Understanding the Epidermis

Basic facts and background

Etymologically, "epidermis" comes from the Ancient Greek "epi" (over or upon) and "derma" (skin), referring to the skin's top layer. The term "epidermal" describes anything related to this outermost layer.

From a biological viewpoint, the epidermis is the skin's outer layer, a crucial barrier between the body and external elements. Unlike the deeper dermis and hypodermis layers, the epidermis primarily serves as a defense mechanism, protecting against environmental pathogens and regulating water loss into the atmosphere, known as transepidermal water loss.

Structurally, the epidermis consists of flattened cells called keratinocytes above a basal layer of columnar cells containing stem cells that produce new epidermal cells. As these cells mature, they move upward, forming the skin's protective outer layer.

The epidermis is an example of stratified squamous epithelium, characterized by multiple cell layers with flattened surface cells, enhancing its protective function. It undergoes continuous renewal, with new cells generated at the basal layer and moving to the surface over approximately 28 days.

Consistency of the Epidermis

The epidermis mainly comprises keratinocytes, making up 90% of its cells, and includes melanocytes, Langerhans cells, Merkel cells, and inflammatory cells. Epidermal thickenings, known as Rete ridges, extend downward between dermal papillae. Blood capillaries are located beneath the epidermis, which has no blood supply and relies on oxygen from the air. The epidermis contains cellular mechanisms for regulating water and sodium levels. Essentially, the epidermis does not have a sensation, meaning its removal or piercing cannot cause pain. It acts as a barrier against bacterial entry, but as the ancient Greeks recognized, it's essentially a cover over the body.

Why the Epidermis Feels No Pain

Skin pain perception involves nociceptors, sensory neurons responsive to harmful stimuli. Nociceptors are primarily found in the dermis, beneath the epidermis, mainly composed of keratinocytes lacking nociceptors. The epidermis's top layer, the stratum corneum, contains dead keratinocytes, reducing pain sensitivity. The epidermis's role is protective, with pain-sensing delegated to deeper skin layers where nociceptors are prevalent. Hence, any claims of pain during epidermal piercing are exaggerated.

Junctions Between Epidermal Cells

Despite appearances, the epidermis is not fragile due to tightly interconnected cells forming a barrier. These connections are adherens junctions involving cadherins linked to actin filaments inside the cell, appearing as borders in immunofluorescence microscopy due to tight cell proximity.

Layers of the Epidermis

Depending on the skin region, the epidermis contains 4 or 5 layers: Stratum Corneum, Stratum Lucidum (in some areas), Stratum Granulosum, Stratum Spinosum, and Stratum Basale.

3. Stratum Corneum

Outermost Cornified layer

The stratum corneum, known as the "horny layer" for its tough characteristics, is the epidermis's topmost layer. It comprises 10 to 30 layers of corneocytes, the final form of keratinocytes. These cells are polyhedral and anucleated, indicating they lack a nucleus. The layer is notably thicker on the palms and soles to meet the higher mechanical demands of these areas.

Structure of Corneocytes

Corneocytes have a protein coating beneath their plasma membrane, filled with keratin proteins vital for water retention. They are connected by corneodesmosomes and enveloped by lipids, significantly enhancing the skin's barrier function.

Function of the Stratum Corneum

This layer is the primary barrier against environmental factors and plays a key role in hydration regulation. The cells, essentially dead, are continuously shed in the skin's natural renewal process.

Etymology of "Horny Layer”

The term "horny" (Latin: corneum) signifies the cells' horn-like toughness and durability. As cells age, they are replaced by newer ones from the lower layers.

Cell Turnover Rate

The stratum corneum undergoes constant renewal. Cell turnover occurs every 28 to 30 days in young adults, and in older adults, it slows to about 45 to 50 days.

Research and Analysis

Extensive studies on the stratum corneum have provided insights into its composition, function, and the skin's overall health, underscoring its critical protective, moisture-regulating, and skin integrity roles.

Detailed Anatomy of the Stratum Corneum

The stratum corneum consists of flattened corneocytes in two layers: the stratum disjunctum and the stratum compactum.

Stratum Disjunctum is the looser upper layer with larger, more hydrophobic corneocytes. It hosts the acid mantle and lipid barrier.
Stratum Compactum: The more cohesive lower layer with smaller, less hydrophobic corneocytes.

Protective Functions

The stratum corneum provides barrier protection against infection, dehydration, chemicals, and mechanical stress. It offers physical and biological defense, regulates microbial growth, and plays a role in inflammation initiation. Its selective permeability excludes toxins, irritants, and allergens.

Cell Composition and Structure

The stratum corneum's corneocytes, consisting of 15–20 layers of flattened, anucleated cells, contain keratin embedded in a lipid matrix, crucial for the desquamation process.

Desquamation Process

Desquamation balances keratinocyte proliferation from the stratum basale, with cells migrating to the surface in about two weeks, maintaining the stratum corneum's integrity and functionality.

Physiological Roles

Beyond its protective function, the stratum corneum is key in regulating hydration and water flux, essential for skin health.

With its distinct structure and composition, the stratum corneum plays a vital role in skin protection, balancing physiological functions while continuously renewing to maintain skin health.

4. Stratum Lucidum

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

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

Microscopic Appearance and Cell Composition

Under a microscope, the stratum lucidum appears translucent. It is located between the stratum granulosum and the stratum corneum. This layer consists of three to five dead, flattened keratinocytes with indistinct boundaries, contributing to its transparency.

Keratinocyte Composition

The keratinocytes in this layer contain eleidin, an intermediate form of keratin, which forms as keratinocytes move up from the stratum spinosum and stratum granulosum.

Oily Surroundings and Thickness Regulation

These cells are surrounded by an oily substance from the exocytosis of lamellar bodies, beginning as the cells progress through the lower epidermal layers. The thickness of the stratum lucidum varies with the rate of cell division in the epidermis, affecting how quickly cells replenish and transition into this layer.

Role of Melanosomes

The darkness of the stratum lucidum is influenced by melanosomes from the stratum basale, affecting this layer's pigmentation, though it remains relatively clear.

Therefore, the stratum lucidum is a specialized, thin layer within the epidermis, predominantly found in thicker skin regions. Its distinct cellular composition and placement within the epidermal structure contribute to its transparent appearance and role in skin protection.

5. Stratum Granulosum

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

The stratum granulosum, or granular layer, occupies a central position in the epidermis. It is situated above the stratum spinosum, below the stratum corneum, and under the stratum lucidum in areas of thick skin such as the palms and soles. This layer is less significant in brow skin.

Cellular Transition and Keratohyalin Granules

In this layer, keratinocytes from the stratum spinosum transform into granular cells characterized by keratohyalin granules. These granules, rich in histidine- and cysteine-containing proteins, help bind intermediate keratin filaments, enhancing cell structure strength.

Formation of the Skin Barrier

As cells move from the stratum granulosum to the stratum corneum, they release lamellar bodies filled with lipids and proteins into the extracellular space, forming a hydrophobic lipid envelope essential for the skin's barrier properties.

During this transition, cells lose their nuclei and organelles, marking the end of their life cycle.

Lipid Transformation

Polar lipids within these cells change into non-polar lipids, aligning parallel to the cell surface. For example, glycosphingolipids become ceramides, and phospholipids become free fatty acids, further boosting the skin's protective barrier.

Hence, the stratum granulosum is crucial in skin health and protection. This layer's unique cellular composition and transformations are key to the skin's barrier function, preventing moisture loss and shielding against external contaminants.

6. Stratum Spinosum

The Prickle Cell Layer (8-10 cell layers)

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

Cellular Composition

This layer contains several layers of polyhedral keratinocytes connected by desmosomes. These specialized cell structures ensure cell strength and cohesion.

Spiny Appearance

The "spiny" appearance, giving the stratum spinosum its name, results from the contraction of microfilaments between desmosomes. This is particularly noticeable when stained with hematoxylin and eosin (H&E).

Keratinization Process and Formation of Desmosomes

Keratinization, the development of the protein keratin, begins in the stratum spinosum. Keratinocytes synthesize cytokeratin, a fibrillar protein that accumulates within cells to form tonofibrils. These tonofibrils are crucial for desmosome formation, ensuring strong connections between keratinocytes and maintaining skin integrity and resilience.

Presence of Langerhans Cells

Langerhans cells, functioning similarly to macrophages by engulfing bacteria, foreign particles, and damaged cells, are found within the stratum spinosum. They are essential for the skin's immune defense.

Due to its dense network of interconnected keratinocytes, the stratum spinosum is crucial for skin structure and strength. It also plays a significant role in the immune response, highlighted by the presence of Langerhans cells. This layer's structural and functional properties are key to 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 and typically consists of a single row of cells.

Overall thickness

The basal membrane is not composed of cells but is a thin, fibrous structure 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 and contains proteins like laminin and integrins.
Lamina Densa: This layer is denser and lies beneath the lamina lucida. 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 small, round basal cells. These cells constantly divide, propelling the newer cells toward the skin's surface, where they undergo further maturation and are eventually shed.

Within the basal layer, melanocytes are 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, tactile and associated with the sense of touch, are also located in the basal layer. These cells are of neuroectodermal origin.

Constant cell renewal

The term “stratum germinativum” underscores the basal layer's continuous cell generation. This 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.

8. Holistic view of the epidermis

To fully comprehend the epidermis

It is essential to view it as an integrated and dynamic system. The basal layer, or stratum basale, functions in concert with the outermost stratum corneum, creating a continuous skin renewal cycle. The basal layer generates new skin cells, which then journey upwards through the intermediate layers, transforming. Eventually, these cells reach the stratum corneum, where they exist as flattened, dead 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. As we age, the rate of this cycle gradually decelerates. This turnover happens relatively quickly in our youth, but the process becomes slower with age, reflecting the broader aging processes in the human body. Ultimately, like many bodily functions, the efficiency and speed of this cycle diminish over time, continuing until the end of life.

9. Epidermis in Lips

The epidermis, serving as the outermost layer of the skin, exhibits distinct variations in its structure and thickness across different areas of the face, notably between the lips and the brow region. These differences are pivotal in understanding the specialized functions and responses of the skin in these areas.

One of the primary distinctions between the epidermis of the lips and other facial areas, such as the brows, lies in its thickness and the composition of its layers. The lips are characterized by a thinner epidermis, which is more delicate due to the absence of the stratum corneum layer prevalent in other facial regions. This layer, known as the "horny layer," is responsible for the skin's barrier function, offering protection against physical and chemical aggressions. Its absence in the lips makes them more vulnerable to environmental factors, leading to a higher tendency for dryness and chapping.

Furthermore, the lip epidermis contains more blood vessels and fewer melanocytes than the brow area epidermis. This increased vascularity contributes to the lips' red or pink hue as the blood vessels are closer to the surface. The reduced presence of melanocytes, which produce melanin, the pigment responsible for skin color, also means that the lips have less natural protection against UV radiation, making them more susceptible to sunburn.

Another notable difference is in the sensory perception capabilities of these areas. The lips are rich in tactile receptors, making them highly sensitive to touch, temperature, and pain. This heightened sensitivity is due to the lips' role as a sensory organ, which is essential for functions such as eating and speaking. In contrast, the epidermis of the brows and other facial regions is less sensitive, with a structure more geared toward protection and less toward sensory detection.

10. Conclusions

The epidermis, as the outermost layer of the skin

The epidermis varies in thickness across different body areas. In the brow region, it is approximately 0.05 to 0.1 mm thick. The epidermis comprises five distinct layers: Stratum Corneum, 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

Artists must 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.