Alexa Patzelt

Follicular transport route--research progress and future perspectives.

The important role of hair follicles as penetration pathways and reservoir structures for topically applied compounds has been validated in numerous animal models as well as in humans. Follicular penetration rates are modulated by regional variations in size and proportions and the functional status. Advances have especially been made in the targeting of hair follicle-associated cell populations including antigen-presenting cells and stem cells. Improved investigative methods based on differential stripping, spectrophotometry and confocal laser scanning microscopy have led to the determination of the penetration profiles and kinetics for a multiplicity of drugs and drug delivery systems. The observation that particulate delivery systems aggregate and remain in hair follicle openings and their penetration along the follicular duct occurs in a size-dependent manner, which has led to advanced concepts of targeted drug delivery of bioactive compounds in the field of solid particles, as well as semi-solid particles, such as liposomes. This review summarizes the recent progress in this field, and underlines the necessity for pilot studies in human volunteers to further the development of clinical applications for follicular targeting.

Selective follicular targeting by modification of the particle sizes

Hair follicles represent interesting target sites for topically applied substances such as topical vaccinations or agents used in the field of regenerative medicine. In recent years, it could be shown that particles penetrate very effectively into the hair follicles. In the present study, the influence of particle size on the follicular penetration depths was examined. The penetration depths of two different types of particles sized 122 to 1000 nm were determined in vitro on porcine skin. The results revealed that the particles of medium size (643 and 646 nm, respectively) penetrated deeper into the porcine hair follicles than smaller or larger particles. It was concluded that by varying the particle size, different sites within the porcine hair follicle can be targeted selectively. For the human terminal hair follicle, the situation can be expected to be similar due to a similar size ratio of the hair follicles.

Hair Follicles – An Efficient Storage and Penetration Pathway for Topically Applied Substances

In the past, it was assumed that the intercellular route was the only relevant penetration pathway for topically applied substances. Recent results on follicular penetration obtained at the Center for Experimental and Applied Cutaneous Physiology, Charité – Universitätsmedizin Berlin, Germany, emphasize that the hair follicles represent a highly relevant and efficient penetration pathway and reservoir for topically applied substances.

Cutaneous concentration of lycopene correlates significantly with the roughness of the skin

Antioxidant substances in the skin are expected to slow down photo ageing. We therefore developed the hypothesis that high levels of antioxidant substances may be correlated to lower levels of skin roughness. By utilizing modern optical non-invasive in vivo methods, the structures of the furrows and wrinkles as well as the concentration of lycopene were analyzed quantitatively on the forehead skin of 20 volunteers aged between 40 and 50 years. In a first step, the age of the volunteers was correlated to their skin roughness. Here, no significant correlation was found. In a second step, a significant correlation was obtained between the skin roughness and the lycopene concentration (R=0.843). These findings indicate that higher levels of antioxidants in the skin effectively lead to lower levels of skin roughness, and therefore support our hypothesis.

Differential stripping demonstrates a significant reduction of the hair follicle reservoir in vitro compared to in vivo

Penetration studies are commonly performed on in vitro models, presumably due to a lack of non-invasive in vivo methods. To date, it is not clear whether in vitro models are suitable to reflect the in vivo conditions for percutaneous penetration. Apart from inter and intraspecies skin differences, the excision of a skin sample may influence the penetration rate inter alia as a result of the contraction of the elastic fibres in the skin during excision. Therefore, the aim of the present study was to investigate the follicular reservoir of the hair follicles of human skin in vivo and in vitro utilizing the method of differential stripping. The results obtained revealed a significantly reduced follicular reservoir in vitro, which was only 9.5+/-10.6% of the in vivo reservoir. These results are important for the interpretation of earlier and future penetration investigations. It can thus be assumed that excised skin models are suitable for penetration studies only to a limited extent, as follicular penetration is greatly diminished due to the contraction of the elastic fibres of the skin.

Penetration and storage of particles in human skin: Perspectives and safety aspects

The application of particles in dermatology and cosmetology represents an emerging field and is closely connected with the question of risk assessment as the potential for, and consequences of, penetration of such particles into the living tissue has not been determined conclusively. In the medical sector, extensive research activities are in progress to develop particles, which can be used as efficient carriers for drug delivery through the skin barrier. In contrast, in cosmetic products, particles are mostly required to remain on the skin surface to fulfill their beneficial effect. Whereas the intercellular penetration of particles seems to be unlikely, the hair follicle has been shown to be a relevant penetration pathway for particles as well as an important long-term reservoir. It has been demonstrated that the penetration depth of the particles can be influenced by their size resulting in the possibility of a differentiated targeting of specific follicular structures. In the present review, the follicular penetration mechanisms and storage properties of particles are discussed.

Distribution of Bacteria in the Epidermal Layers and Hair Follicles of the Human Skin

Previous studies over recent years have revealed the presence of a resident bacterial population in the human skin throughout the entire body. However, the localization and composition of the bacteria within the epidermis and the skin appendages have not been fully investigated. Using differential tape stripping, cyanoacrylate skin surface biopsies and mapping of hair follicles, bacteria on the forearms of study participants were isolated, mapped, cultured and identified with respect to their origin within the epidermis and the hair follicles. Our studies showed that 85% of the bacteria were found within the first 6 corneocyte layers and roughly 25% of the cutaneous bacterial population were localized within the hair follicles. The microbial flora of the skin between individuals is subject to considerable fluctuations. Micrococcaceae represent the biggest fraction of hair-follicle-associated bacteria. The techniques developed for this study allowed us to selectively investigate the bacterial population within the hair follicles. Our results point out the role of skin appendages as potential microbial reservoirs and the need to develop new antiseptic formulations that sufficiently penetrate into the hair follicles.

In vivo distribution of carotenoids in different anatomical locations of human skin: comparative assessment with two different Raman spectroscopy methods

Background:  The cutaneous antioxidants form an efficient protection system against the destructive potential of free radicals, produced by environmental factors, such as UV‐sun irradiation, hazardous substances and lifestyle habits. Most of the antioxidants cannot be produced by the human organism. Thus, they have to be incorporated by food and beverages.

Permeation of topically applied caffeine through human skin – a comparison of in vivo and in vitro data

AIMS Due to ethical reasons, in vivo penetration studies are not applicable at all stages of development of new substances. Therefore, the development of appropriate in vitro methods is essential, as well as the comparison of the obtained in vivo and in vitro data, in order to identify their transferability. The aim of the present study was to investigate the follicular penetration of caffeine in vitro and to compare the data with the in vivo results determined previously under similar conditions. METHODS The Follicular Closing Technique (FCT) represents a method to investigate the follicular penetration selectively. In the present study, FCT was combined with the Franz diffusion cell in order to differentiate between follicular and intercellular penetration of caffeine into the receptor medium in vitro. Subsequently, the results were compared with the data obtained in an earlier study investigating follicular and intercellular penetration of caffeine in vivo. RESULTS The comparison of the data revealed that the in vitro experiments were valuable for the investigation of the follicular penetration pathway, which contributed in vivo as well as in vitro to approximately 50% of the total penetration, whereas the kinetics of caffeine penetration were shown to be significantly different. CONCLUSIONS The combination of FCT with the Franz diffusion cell represents a valuable method to investigate follicular penetration in vitro. Nevertheless, in vivo experiments should not be abandoned as in vitro, structural changes of skin occur and blood flow and metabolism are absent, probably accounting for reduced penetration rates in vitro.

Which skin model is the most appropriate for the investigation of topically applied substances into the hair follicles

It has recently been demonstrated by the utilization of different techniques, such as differential stripping and laser scanning microscopy, that the hair follicles represent an efficient long-term reservoir for topically applied substances. In the present paper, the different in vivo techniques are compared to those of in vitro analysis. It was found that in vitro measurements on pig ear skin are highly superior for the analysis of follicular penetration, as compared to in vitro investigations on excised human skin, mainly due to the fact that the human skin contracts after removal. Restretching of the skin to its original size mainly stretches the interfollicular fibres, whereas the fibres around the hair follicles remain contracted. In contrast to excised human skin, pig ear tissue does not contract when the cartilage is not removed. Moreover, it has an intact barrier on both sides of the ear. Regardless of the fact that the hair follicles on pig ear skin are larger than those of humans, the porcine ear skin represents a more suitable in vitro model for the analysis of the penetration and storage of topically applied substances in the hair follicles than excised human skin.