The evolution of research in ocular care—ranging from cosmetics like make-up removers and eye creams to medical devices such as artificial tears—is currently facing a pivotal challenge: moving beyond traditional macroscopic testing. While it was once sufficient to confirm the absence of acute inflammation or corneal opacity, the frontier has now shifted toward subclinical irritation. This phenomenon, while not manifesting in obvious clinical signs, generates chronic discomfort that undermines patient compliance and reflects a silent but progressive state of cellular distress.
The Paradigm Shift: From “Red Eye” to Cellular Vitality
Standard safety guidelines (such as OECD/GHS) rely on parameters that are reversible within 21 days. However, for products intended for chronic use or those containing potentially stressful ingredients like acids and sunscreens, these criteria are inadequate. The current objective is to identify subclinical reactions through ultra-sensitive cellular analysis. This is where the integrity of the ocular surface—the critical interface between the eye and the environment—becomes paramount.
Microvilli and the Glycocalyx: Sensors of Vitality
The surface of conjunctival and corneal epithelial cells is not smooth; it is characterized by microvilli. These dynamic protrusions of the plasma membrane are not mere anatomical details, but vital indicators of metabolic health. Their function is essential: they increase the cell’s absorptive surface area by up to 30 times, facilitating the exchanges necessary to maintain homeostasis.
Coating these microvilli is the glycocalyx, also known as the “Second Mucosal System” (SMS). This complex network of glycoproteins and transmembrane mucins (such as MUC1 and MUC4) is responsible for tear film stability and lubrication. When the glycocalyx is compromised or microvilli are reduced—a condition well-documented in pathologies like Dry Eye Syndrome—the result is a collapse in corneal wettability. Scientific evidence confirms that the loss of microvilli correlates almost perfectly ($rho = 0.796$) with subjective sensations of dryness and irritation.
The CitoSEM Methodology: Merging IC, SEM, and TEM
To map this “invisible world,” the CitoSEM protocol was developed. This non-invasive method combines Impression Cytology (IC) with high-resolution electron microscopy.
- Ex Vivo Phase: Using IC, superficial epithelial cell samples are collected painlessly. This allows for the observation of the actual state of the tissue without the need for invasive biopsies.
- In Vitro Phase: Substances are tested on Reconstructed Human Corneal-like Epithelium (RhCE) models to observe their preventive or toxic impact within a controlled timeframe.
The Role of SEM and TEM
The analysis utilizes two complementary tools:
- SEM (Scanning Electron Microscopy): Provides a three-dimensional view of the surface. It is the gold standard for quantifying the density and morphology of microvilli, detecting incipient damage long before the nucleus or cytoplasm show alterations under a conventional light microscope.
- TEM (Transmission Electron Microscopy): Provides an ultra-high-resolution two-dimensional cross-section. This allows for an analysis of “internal mechanics,” such as glycocalyx thickness, ingredient penetrability, and, crucially, vesicular transport. An increase in vesicular transport is a direct sign of metabolic recovery and cellular health.
The Del Prete Scale: Standardizing Damage
To objectify these findings, the Del Prete Classification Scale is employed—a five-point system that categorizes microstructural alterations:
| Grade | Microvilli Position | Surface condition | Microvillar distribution | Morphology |
| Grade 0 (Healthy) | Microvilli on site | Normal surface | High distribution of Microvilli | Tree structure |
| Grade 1 (low damage) | Microvilli on site | Normal surface | Low microvilliar distribution | Not totally tree structure |
| Grade 2 (moderate damage) | Microvilli on site | Little alteration of surface | Spotted Distribution | Pseudomicrovilli |
| Grade 3 (High damage) | Microvilli on site | Strong alteration of surface | Strong reduction with nude areas | Pseudomicrovilli |
| Grade 4 (critical damage) | Smooth surface – not microvilli | Strong alteration of surface | Absence of microvilli | Smooth surface (moon-surface) |
Toward the Ocular Exposome
The most innovative aspect of this approach is the integration of these models to address the concept of the Ocular Exposome, a framework pioneered by Christopher Paul Wild in 2005. The exposome represents the totality of exposures (environmental, chemical, pharmacological) that influence our biology over time.
By combining in vitro data (observed over a 24-hour delta T) with ex vivo clinical data (patients treated for 30 days with advanced tear substitutes containing Cross-linked Hyaluronic Acid, Cationic Liposomes, and Trehalose), researchers have validated that certain formulations act at a much deeper level than simple lubricants. They do not merely “wet” the eye; they promote the functional reactivation of cells, stimulating microvillar regeneration and glycocalyx synthesis.
This advanced staging system no longer evaluates only if a product causes irritation, but how it intervenes in vesicular transport and mucosal barrier protection. It marks the birth of a biofunctional model that transforms ultrastructural diagnosis into a predictive tool for long-term ocular health.
Contact
Dr. Salvatore Del Prete – AD Service Biotech
saldelp@servicebiotech.com
https://www.servicebiotech.com/
