Experimental methodology used in hair biology


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Hair biology experimental techniques

The regulation and structure of hair follicle are rather complex - they are composed of more than 20 cells and involve numerous signalling events. These complexities make the experimental hair research a challenging task. The scientists proceed with utmost care in choosing the experimental model for finding the answer to each question related to hair growth or diseases.

In situ analysis in humans

The task of monitoring hair growth and hair abnormalities apparently appears to be easy due to their localization on the skin surface. Then you can easily access the hair fibers and hair roots for non-invasive collection. Then, the hair cycling anomalies can also be easily evaluated in clinical diagnostics, using simple light microscopy. Phototrichograms, and the recent development of the “trichoscan™” technique, allow quantification of hair growth in vivo. Moreover, the use of global photography techniques has emerged as the most popular approach in treatment evaluation. It is true that these approaches prove to be effective in the diagnosis of disease and clinical treatment trials; but they provide only limited information about the hair follicle in the skin. More invasive techniques are needed to characterise hair follicles. So far the clinics use techniques such as tissue biopsy and histological analysis of the hair follicle structure. A number of laboratory research scientists are on their way to find out more advanced techniques that would help in better understanding of skin appendage formation and growth.

All experimental techniques have their respective advantages and limitations. For the evaluation and validation of expression patterns of RNA in situ hybridization, the conventional analysis of sections of cryo-preserved or paraffin-embedded tissue is still regarded as the most valuable tool. The same is true for the protein using techniques in immunohistochemistry. Plucked hair follicles can also be good source of material for gene expression profiling. As little as one single hair fiber will provide you sufficient quantity and quality of RNA to use for microarray hybridizations. These approaches using tissues allow evaluative studies to be performed, but they do not permit studies to determine the functional significance of a particular gene or its product in hair growth or disease. To make these studies functional, the researchers need to use living tissue in some fashion.

Animal models

Using animal models in the experimentations relating to hair follicle development and hair diseases have become more popular in recent decades. Developmental biology is increasingly making use of model organisms such as Drosophila, Xenopus and Zebrafish. These have provided valuable insights into the role of conserved signalling pathways including Hedgehog or Wingless in pattern formation and formation of skin appendages in general. Genetically engineered mice are being used as mammalian models, to study the regulatory events in hair follicle development and hair growth. These studies are directly relevant to hair follicle biology in humans. Rodent models are especially valued in many researches due to a number of reasons, such as 1. their ready availability, 2. rapid breeding, 3. known genetics in inbred strains, 4. the ability to modify their genetic profile, 5. the ability to control environmental input, and 6. the ability to conduct invasive procedures. Rodent models are categorized into two broad groups - A. those with spontaneous hair follicle abnormalities or disease, B. those where genetic manipulation has been conducted to produce transgenic (gene over-expression) or knockout (gene inactivation) mice for a specific gene of interest.

The rodent models that are used both for hair biology and hair disorder researches can be further classified on the basis of the nature of the trait features they come with and that include those with: 1) a failure in hair follicle formation and consequently abnormally low numbers of hair follicles, 2) disorders of hair morphogenesis where hair follicles form but fail to fully develop, 3) hair follicle cycling disorders where hair follicles form and develop but fail to cycle correctly, 4) hair follicle structure and/or sebaceous gland structure disorders often leading to hair shaft defects and alopecias, 5) disorders of hair fiber pigmentation, 6) immunological abnormalities resulting in alopecias, 7) neuroendocrine abnormalities, and 8) models of environmentally mediated diseases where exogenous factors are introduced to modify hair growth. Till date, perhaps a few hundred models with spontaneous murine hair disorder have been used in the researches. But since the introduction of transgenic technology, literally thousands of genetically modified mice have been used in the researches. They have been specially generated to throw light in the understanding of hair biology and disease.

Majority of the regulatory events in hair follicle growth have been carried on both rodents and humans. Some critical differences have been noticed at the molecular level and as such animal models of hair disorders failed to fully explain the morphological presentation of disease in humans. That is why, studies on animal models are not sufficient; you also need to consider human hair follicles especially when it comes to the pathogenesis of human diseases. However, there are numerous ethical limitations to experimental research directly on human volunteers. Often you will not find adequate number of volunteers willing to take part in research. In addition to that, the human volanteers’ genetics are often unknown, their environment cannot be regulated, and invasive approaches are limited. But better understanding of human hair disorders is never possible without human volunteers. Where direct human involvement is not possible, then animal tissues can be utilized.

Transplant models

Tissue transplant models open up new possibilities; this approach involves the transplantation of diseased hair-bearing human skin onto immunodeficient mice. This approach is helpful in the study and manipulation of human hair growth. This method has been applied in research on androgenetic alopecia where no satisfactory rodent model was available, alopecia areata, and for the study of several forms of genetic trichoses. This method was also proved to be helpful in understanding basic hair biology. The advantage of this approach particularly relates to the evaluation of drugs and their impact on hair growth. However, the approach has its limitations too. When a human tissue is transplanted on to a mouse host, the tissue gets disconnected from any systemic effects from elsewhere in the body; this is particularly relevant in inflammatory diseases. Then, biochemical signals in mice may not be cross reactive with human cell receptors. As such, a mouse host environment differs from that of a human hair follicle’s in situ environment.

Cell culture

Culture techniques provide another useful tool for functional research. Hair follicles are composed of populations of interacting cells that can be readily identified and are clustered in discrete sites. This allows the isolation of different cell types to observe pronounced interactive abilities when cultured in vitro. Primary cell cultures from mouse and human hair follicles, including hair follicle keratinocytes, dermal papilla cells and melanocytes, provide important information on the expression of mediators and the behaviour of single cell populations. However, they offer you only an incomplete picture because they tend to loose their organ-specific characteristics within a few passages. With the increasing complexities in the knowledge and regulation of hair growth the scientists need more sophisticated in vitro models to carry on with the investigation of the complex interactions during normal hair growth and in response to bioactive compounds. Thus the whole organ culture model as first developed and described by Michael Philpott provides a very effective research tool.

Today hair research is often conducted using animal models, “human models” and in vitro models. Computer modelling is perhaps the model of the future, but it is not here yet for hair research. Each approach has its limitations and no single model can provide the complete picture. A fuller understanding of hair biology and hair disease requires collating and assimilating of information from diverse sources and then piecing together of those information items - a very complex task indeed.

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