Cosmeceuticals : Function and the Skin Barrier

Introduction

In the past several years, our fundamental understanding of the macro- and micro-structure and function of the stratum corneum (SC) has evolved greatly. Advances in biomolecular and human measurement capabilities, in particular minimally- or non-invasive methodologies, have not only reinforced the fact that the SC is incredibly responsive and adaptive, but also continue to provide new insights and opportunities for cosmeceutical scientists. Despite inroads in our understanding of the physiology of the skin's barrier, one major challenge remains – a means to enhance penetration of actives across the skin barrier. Notwithstanding recent advancements in our understanding of how devices can be used to deliver active agents deep into skin, enhancing delivery of cosmeceuticals remains a significant challenge, particularly within the confines of cosmetic regulatory framework.

The function of the stratum corneum

It is well known that water is essential for life. Cells, in all the organs in our body, critically depend upon the availability of water. The functioning of skin, which encases all organs, also depends upon the availability of water. The living skin (about as thick as paper) is about 70–80% water. Left unprotected from the environment, the water inside the skin and organs would evaporate, ending life as we know it! However, nature devised a marvelous semi-permeable layer for the skin and thus also for the body: a membrane that is indispensable for life – the stratum corneum (SC). In fact, it is widely held that SC is the fundamental evolutionary adaptation that made terrestrial life possible.

SC, our body's first contact with the external environment, is a composite multifunctional membrane engineered by nature to maintain water, yield mechanical strength, provide selective transport of molecules and keep infections away. SC is the end product of the skin cell's life cycle and is formed from the basal cells of the skin that embark upon a well-orchestrated terminal differentiation pathway with regular frequency throughout the life-time, ending in the formation, removal and renewal of SC layers. The critical steps involved in maturation of the SC and their functional role are summarized in Table 1.1 .

Table 1.1

Key processes for the formation and functioning of the stratum corneum

Key process	Essential features	Significant function	Diagram
Maturation of corneocytes
Corneocytes are specialized cells located within the stratum corneum (SC). Corneocytes are composed mainly of proteins (keratin) and are surrounded by a cornified envelope. They also contain intracellular humectants called natural moisturizing factors (NMFs)	Keratinocytes are the predominant cells generated within the basal layer of the skin. Over time, these cells divide and migrate upwards to the surface of the skin in a process called differentiation. As the keratinocytes reach the outer epidermis they transform to form enucleated, flat, proteinaceous cells called corneocytes. Since corneocytes are no longer living they cease to divide and remain in the SC until they are actively removed in a process called desquamation	Corneocytes form the basic structure of the SC, the ‘bricks’ in the ‘bricks and mortar’ structural analogy. They serve as the skin's physical barrier against entry of pathogens (e.g. viruses, bacteria), chemicals, pollution, and loss of water from the skin. Protein – in the form of keratin – is able to hold water within corneocytes, thus maintaining skin's hydration. Hydrated corneocytes are more elastic and give the skin more resilience against the environment
Development of the SC structure
Corneocytes align and are cohesively linked together to form the structural basis of the SC – the ‘bricks and mortar’	During cornification, the corneocyte cell membrane is replaced by a layer of long chain ceramides which becomes covalently linked to the proteins (e.g. loricrin, involucrin) that form the cornified envelope. This produces a highly resistant and insoluble outer envelope. The SC structure is further strengthened by protein links between adjacent corneocytes called corneodesmosomes. Corneodesmosomes are composed of three major specialized proteins: corneodesmosin; desmoglein-1; and desmocollin-1. These anchor the corneocytes within the SC by allowing alignment and organization of the lamellar lipid structure between layers of corneocytes	As the ‘mortar’ in the ‘bricks and mortar’ skin analogy, corneodesmosomes serve to link the corneocytes tightly to each other to protect the deeper, living skin and body by maintaining moisturization and preventing damage from the environment, as described above
Formation of the SC lipid matrix
Specialized lipids are packed and organized in layers between the corneocytes	The spaces between corneocytes (intercellular spaces) are filled by specialized lipids. These lipids are produced in epidermal keratinocytes and carried in lipid- and protein-rich structures called lamellar bodies (LB). The SC lipids are subsequently released from LB at the stratum granulosum (SG): SC transition. Three major lipid classes comprise the intercellular lipids – fatty acids, ceramides and cholesterol. These lipids self-organize into multiple bilayers (lamellar structure). These lipid structures are often referred to as the SC barrier lipids. The conformational organization and packing of lamellar structures effects SC function	The SC lipid bilayers form the effective moisture barrier of the SC. The characteristics of the lamellar structures (e.g. relative lipid composition, intercellular compartmentalization and conformation) regulate SC water content. The tight packing of lamellar lipids physically prevents penetration and entry of many classes of chemicals and pathogens. However, due to the characteristics of the lamellar structure, during certain conditions, SC permeability can be enhanced resulting in penetration of chemicals of different classes into the SC. The majority of materials that enter the SC do so via the hydrophobic or hydrophilic regions of the lipid bilayers
Production of skin's natural humectants
Release of natural moisturizing factor (NMF) occurs within the protein matrix of the corneocyte	The skin remains hydrated by its ability to bind and retain water within the SC. A mix of low molecular weight, water soluble molecules, called NMFs, reside in the corneocytes and serve as the skin's own humectants. NMF is approximately 50% amino acids and 50% salts including lactate and urea. The amino acid component of NMF is primarily derived from degradation of a high molecular weight multimeric SC protein called filaggrin. Filaggrin is formed as a precursor in epidermal keratinocytes and subsequently processed into amino acid NMF (e.g. pyrrolidone carboxylic acid (PCA) and urocanic acid) within corneocytes	NMF is the skin's natural mechanism to keep moisturized. As water levels in the skin decrease, the skin responds by initiating a series of biological processes resulting in the breakdown of filaggrin into amino acids. NMF attract and bind water, keeping the SC hydrated
Removal of corneocytes by desquamation
Old, spent corneocytes are detached and shed from the surface of the skin in a regulated process called desquamation	Desquamation refers to the highly regulated, enzyme-mediated degradation of the protein links between corneocytes (corneodesmosomes). Activity of the hydrolytic enzymes that degrade corneodesmosomes is tightly regulated to ensure appropriate shedding of corneocytes at the skin's surface as new cells move upwards from the epidermis. Certain environmental conditions, such as low hydration, inhibit the ability of these enzymes to function properly	Orderly release of old corneocytes at the surface of the skin is required for skin to appear and feel smooth, soft and attractive. If the process of desquamation is inhibited (e.g. by skin dryness or in certain disease conditions) corneocytes accumulate at the surface of the skin giving rise to the visual signs of dry skin including the appearance of flakes

The stratum corneum structure

The thickness of SC varies from site to site on the body. The palm and foot, which need to constantly bear mechanical challenges, have the thickest SC (≈150 microns), whereas eyelids have the thinnest SC (≈10 microns). The bricks and mortar structure of SC ( Fig. 1.1 ) is well known and comprises a milieu of specialized cells (called corneocytes), embedded in a complex mortar (lipids and proteins). However, it tells only a part of the story of the organization of SC. The SC not only needs to manage the exfoliation of cells on a regular basis (through an orchestrated process called desquamation) but also needs to respond to environmental changes that may cause insults such as drying and oxidative stresses. Therefore, the detailed structure of brick and mortar is complex at both the single-cell and multicellular scales.

The macro-structure of SC

The macro-structure of the SC, since it needs to withstand the adverse challenges of the environment, reflects additional, higher length scale features – called skin canyons ( Fig. 1.2 ). Unlike glyph lines, which can be readily visualized on skin surface, skin canyons are anatomical micro-folds that can be visualized in vivo only under special imaging conditions ( ). The key macro-structure features of the SC and their relative sizes are listed in Table 1.2 . In the SC, the corneocytes (bricks) and intervening mortar are organized into clusters that can range from 100 microns to 250 microns in width across the surface. These cell clusters are separated by inter-cluster spaces – which are referred to as canyons. The canyons range in width between 10 microns and 30 microns in the SC. These micro-folds in the SC often extend down to deeper layers of viable skin cells. Of note, canyons are deep invaginations of the SC reaching the depths of viable cells – which are unprotected by SC. It is probable that nature engineered the extended organization of SC to provide ‘space’ for skin cells to respond to environmental humidity fluctuations or other insults. Because of this, these unique micro-folds may represent possible routes to deliver active and beneficial agents into the skin.

Table 1.2

Dimensions of macro-structural components of the stratum corneum

Skin structure	Dimensions
Glyph lines	50 microns and above
Canyons	10–30 microns
Clusters	100–200 microns
Corneocytes	20–30 microns
Lamellar lipids	Multi-layers of 4.7 nm bilayers