Clothing the Nude: A New Model for Trichogenesis

Abstract

Considering how hard evolution has worked to denude humans of our fur covering it seems paradoxical how much time and effort we spend on enhancing and preserving the little that remains. This effort is mirrored in the huge (and continually growing) scientific literature on hair. What all this study has revealed is that the hair follicle, and related structures, is an excellent, accessible and fruitful model for many fields such as developmental biology, epithelial– mesenchymal interactions, regeneration and stem cell biology. Indeed a great deal of extremely interesting and useful data has been obtained from studies of hair development and cycling, particularly with the advent of transgenic tools. These studies are, however, currently limited by two important factors. One is the relative inaccessibility of mammalian embryos to manipulation during development. Thus researchers using the chick system to study skin and feather development have benefited from the ability to manipulate and follow the developmental embryos in ovo. In the chick system it has been possible to observe and intervene at all stages of skin development, even before dermis development. Moreover, by using transfection, via electroporation or viral infection, experiments, the critical importance of different pathways in initiation and morphogenesis of epidermal appendages has been highlighted (Yu et al, 2004). In mammalian systems, on the other hand, until the advent of targeted transgenic methods, our understanding of hair induction and development was restricted to the results obtained via dermal–epidermal recombination experiments. The other major impediment is the lack of a dermal specific promoter for targeted transgenics. Since the introduction of epidermal specific promoters, the cre-lox system and other mechanisms of controlling transgene expression, numerous, very elegant studies have been carried out where specific genes can be expressed or knocked out in an epidermal specific manner. The lack of a dermal specific promoter, however, means that genes of the mesenchymal component can, for the most part, only be studied indirectly, for example by expression of a diffusible inhibitor in the epidermis (Andl et al, 2002). This is except in the rare cases when a general knockout affecting hair formation can be shown not to have an effect in the epidermis (Botchkarev et al, 2002). The lack of an in vitro or easily manipulated in vivo system is thus a great impediment to experimental study of hair development and induction. There is hence a great need for a simple and repeatable in vitro or in vivo assay for hair reconstitution and growth and methods to test for the inductiveness of dermal cells. Lacking these, using cells manipulated ex vivo in order to study hair induction and development has been difficult. Previous models for trichogenesis in vivo have been developed, including the implantation of dermal papillae (Cohen, 1961), and subsequently cultured dermal papilla cells (Jahoda et al, 1984) in rat ear or grafts under the kidney capsule of nude mice (Inamatsu et al, 1998; Takeda et al, 1998). More recently models that allows grafting of defined mesenchymal and epidermal cell populations, for example, using silicone chambers and nude mice (Lichti et al, 1993; Weinberg et al, 1993) have been developed and have proved extremely useful. These assays are, however, quite time and materials intensive, particularly in the large number of (often precious) cells used and the time required for a positive result. It is for this reason that the method presented in this issue by the group of Kurt Stenn (Zheng et al, 2005) and similar methods recently developed (Morris et al, 2004; Pouliot et al, 2005) is of interest. In their paper they present a detailed and elegant system that they call the ‘‘hair patch’’ assay, for studying trichogenesis, using dissociated cells transplanted in the dermis of nude mice. The chief advances and advantages of this system over earlier methods are its simplicity, relative rapidity in terms of seeing results, and the small numbers of cells required. The method involves injecting a mixture of isolated neonatal dermal cells with epidermal aggregates into the dermis of nude mice. These are then able to interact and undergo relatively normal hair morphogenesis to give rise to cycling hair follicles within 8–12 d. Because of the simplicity of the method (a simple injection) and the fact that the ‘‘grafts’’ are small and spatially restricted, a number, six or more, can be performed on the same mouse, which represents large potential savings in terms of animals and their upkeep. In addition relatively small numbers of cells, i.e., 10 dermal and 5 10 epidermal cells, are used as compared with other methods, for example the chamber graft assay requires 10-fold more cells. A number of interesting experiments can be envisaged with this system as is in terms of testing the interaction of mesenchymal and epithelial cells from different sources or genetic backgrounds. It will, moreover, be extremely inter-