Animals have been used in biomedical and bioscientific research for over one hundred years (AMP, 2020). To scientists, animals are good models for systems representing the human body and human disease, and therefore, this in vivo method of testing is, and has been for many decades, widely used across toxicological and other biomedical research. This type of research has played an essential part in the development of masses of veterinary, medical and other scientific discoveries and breakthroughs, including the production of a variety of medical treatments and vaccinations (Kaushik and Vaswani, 2018).
Now more than ever, the use of animals in this way is an extremely controversial matter in society. While many believe that laboratory animals are mistreated and kept in poor conditions, there are government legislations in place to ensure that this is not the case. The most prevalent legislation effective in the protection of laboratory animals currently is the Animals (Scientific Procedures) Act, 1986 – otherwise known as ASPA. This legislation was put in place to regulate all procedures that are carried out on protected animals. A protected animal is classed as any living vertebrae, other than humans, or any living cephalopod (Government Legislation UK, 2020). Vertebrates such as birds, mammals and reptiles are protected once they have reached two thirds of their gestation periods, while in any other case, the animal is protected once it has reached the stage where it becomes capable of feeding independently. Cephalopods are not protected if they are in their embryonic form. A procedure is classed as regulated if it may have any effect of pain, suffering, distress or lasting harm equivalent to, or higher than, that caused by the introduction of a needle in accordance with good veterinary practice, and these must only be carried out if they serve an educational or scientific purpose (Home Office, 2014). The class of regulated procedures also encompasses breeding programmes in which the animals bred are genetically modified. These procedures are regulated by the Home Office and the Department of Health, Social Security and Public Safety across the UK – they make sure that any establishment and persons carrying out regulated procedures on protected animals have the correct licensing to do so.
ASPA ensures that all animals that are bred, supplied and used for the purpose of scientific procedures and research are looked after in accordance with the best possible standards of animal husbandry, so that scientific researchers treat laboratory animals with the respect and compassion that they deserve and provide them with the appropriate housing and enrichment that they need, as well as seeing to their social needs. If animal husbandry is not to a good enough standard, it can have severe effects on both the physical health and welfare of the animal. The Animals (Scientific Procedures) Act protects animals used in biomedical and bioscientific research through the implementation of the principles of the 3R’s: replacement, reduction and refinement. The concept of the 3R’s was introduced by William Russel and Rex Burch in 1959, with the intention of enabling biomedical and bioscientific research to become more humane (Baumans, 2004). One of the main purposes of the 3R’s is to encourage the use of alternatives to protected animals where possible in scientific research (Rai and Kaushik, 2018).
The first of the 3R’s – replacement – involves replacing protected animals used in scientific research either with non-protected animals (partial replacement), or replacing them with a source other than an animal (absolute replacement) (Parker and Browne, 2014). Examples of partial replacement could include using non-protected animals such as fruit-flies or worms, while examples of absolute replacement could include the use of human tissue cell cultures or computer models (Schechtman, 2002). Reduction simply encourages that where possible, if protected animals are absolutely necessary for a science research project, then the minimum number of animals possible should be used, without it having a negative impact and compromising the objectives of the study (Clark, 2017). What this means is that the population sample of protected animals used must be as little as possible, whilst still being of an appropriate size for the results of the study to be valid and representative of a wider population. The final of the 3R’s is refinement. Refinement is about minimizing the pain, suffering, stress or lasting harm that the animal experiences, in order to be able to maximize their wellbeing (Faisal, 20005). Where animals must be used, techniques such as administering analgesics and sedatives could be used to minimise the pain experienced by the animal. Refinement also involves providing the animals with the correct enrichment and only using them a minimal number of times to obtain results for the study, in order to make their day-to-day lives as stress free and pain free as possible.
One constantly growing and developing example of an alternative method to using animals in research involves the use of human cells and tissue cultures. This is an in vitro method that requires scientists to grow cultured human tissue cells in a suitable growth medium outside of the body, to form tiny structures that function like human organs such as the heart, lungs, liver and kidneys (Amelian et al, 2017). This review will analyse the efficacy of human tissue culturing as an alternative method to using protected animals in biomedical and bioscientific research.
In vitro tissue models: Human tissue cultures in biomedical and bioscientific research
Clinical studies can take years to be completed and cost millions or even billions of pounds, all while animals are being subjected to harm and stress, and many lose their lives. Using human tissues can be a particularly effective method in the biomedical industry when it comes to developing and testing the effects of new treatments and drugs, and it does not require the use of any protected or non-protected animals. Not only does it eliminate the controversial issue of using laboratory animals in biomedical and bioscientific research, but it also provides more human-relevant results during medicine and vaccination production than animal experiments do (Polini et al, 2014).
When an animal is used in a science laboratory for research purposes, it has to have first been obtained from purpose breeders. Transportation of the animals to the required establishment can take anything from a couple of hours to several days, which in itself can severely stress the animals (Ashenafi, 2018). Once they arrive in the laboratories and are placed in their new environments, they have to be left to settle for seven days before any research can be carried out. This is so that the animals can adapt to their new surroundings without it having an effect on their behaviour, which could impact the experimental results. Common laboratory animals that are used for scientific research include mice and rats. Guinea pigs and rabbits are also sometimes used along with some species of primate. Most animals are only used to collect data over a period of several days or weeks. However, due to primates having a much longer life span, they are often used for long-term studies which can take several years (Knight, 2008). In most cases, after laboratory animals have fulfilled their purpose in the research, they are humanely killed. Using human tissue cell cultures saves this entire process that a protected animal would face in its lifetime.
As well as this method of research being beneficial in the sense that no animals have to be put through any pain, suffering or distress, human tissue culturing techniques can be much less time consuming and cheaper to run (Doke and Dhawale, 2015). Almost all drugs, chemicals and cosmetics are now tested for their efficacy and toxicity using human tissue cell cultures as opposed to protected animals. For example, in the year 2000, Ke-Ping Xu et al undertook a study which looked at eye irritancy. Previous to this study, chemical irritancy was analysed by a method called the Draize Test, which was carried out on animals – particularly on rabbits – and caused them a great level of pain and discomfort (Yun et al, 2016). However, Ke-Ping Xu and his team of researchers instead used organ tissue cultures, which saw a cornea cultured for three weeks before it was then analysed to look for the effects of the chemical irritancy. This way no animals were harmed in the process of the research.
One of the current top ten emerging human tissue culturing tools developed to aid in drug discovery programmes is a method called “Organs-on-a-chip” (Wu et al, 2020). While many clinical trials on animals often fail to predict accurate human responses to the treatments, due to the lack of ability of the animal models to accurately mimic human pathophysiology, organs-on-a-chip studies are an alternative way to accelerate the progression of drug development for humans (Selimovic et al, 2013). The cultures are grown on scaffolds embedded on plastic chips, and the growth of the organs enable modelling of human physiology and disease. These biomimetic organ systems have the ability to regulate key parameters found in human organs, such as maintaining concentration gradients, cell patterning, interactions between tissues and organs, and so on (Zhang et al, 2018). This form of microengineering enables definite control over the cellular microenvironment, therefore resulting in more representative microsystems for the true in vitro environment (Wikswo et al, 2013).
Furthermore, the organs-on-a-chip research method can replace the use of whole live animals in early research stages, when only certain systems need to be tested on. Each organ chip consists of a small, clear, flexible polymer, which contains hollow microfluidic channels that are lined by living human organ-specific cells. These cells are connected with human endothelial cell-lined artificial vasculature, and when mechanical forces are applied, this can mimic the physical microenvironment of true living organs in humans (Ronaldson-Bouchard and Vunjak-Novakovic, 2018). This can include mimicking functions such as peristalsis-like deformations in the intestine, as well as respiratory motions in the lungs. Their ability to be able to host and combine different cell and tissue types to form human organs, means that organ chips are ideal microenvironments for studying molecular-scale activities in organ functions, as well as mimicking human-specific disease states and enabling researchers to identify new therapeutic targets in vitro. Additionally, the fact that they can recreate therapeutically relevant connections, such as the blood-brain-barrier, means that it is possible for scientists to be able to more easily investigate drug delivery and effects (Takebe et al, 2017).
Organ-on-a-chip technology gives scientists the ability to be able to reproduce physiological functions of in vivobody tissue, with much more accuracy than if they did so with the standard cell-based model systems (Sung, 2018). The fact that the organ-on-a-chip technology consists of different micro channels and allows for the formation of sub-compartments, means that it provides the advantages of scientists being able to manipulate the communication between different tissue types, as well as allowing them control the physical conditions so precisely (Ingber, 2018). Considering the fact that drug trials using animals often fail, due to the animal models not being particularly predictive of the human condition, and the fact that they are much more expensive and time-consuming, organ-on-a-chip cultures will be able to provide a better alternative platform for drug and toxicology testing, whilst only requiring micro-sale quantities of cell and drug samples (Sosa-Hernandez et al, 2018).
Although organ-on-a-chip systems represent many aspects of real body tissues, many tissue functions are not represented, which can cause certain limitations. This concept is specifically predominant in the organ-on-a-chip recreation of the hepatic system. The liver is responsible for an abundancy of toxicology mechanisms, including interactions with circulating immune cells to pathogenesis of liver fibrosis. The sinusoidal tissue unit responsible for this is not fully established in organ-on-a-chip cell cultures, which could mean that not all toxicological effects of drug treatments trialled on the liver in this way are effectively displayed (Reif, 2014). As well as this, alveoli (the structures responsible for regulating gas exchange in the lungs) can be extremely challenging to produce in vitro. Although this is the case, a team of scientists led by Dongeun Huh, in 2012, created a lung-on-chip model which used soft lithography to separate the chip into several regions, which were partitioned by membranes with an extracellular matrix. The upper regions contained alveolar epithelial cells, while the lower regions had human microvascular endothelial cells, which enabled the organ-a-chip model to mimic the alveolar-capillary barrier. This highlights that although organ-on-a-chip technologies are an extremely enhanced development as an alternative to using animals in the biomedical and bioscientific research field, further developments still need to be made for them to act as a completely accurate representation of an organ’s entire function.
In addition, the process of actually obtaining human tissue cell samples for the purpose of research has to follow several government legislations. The Human Tissue Act 2004 regulates the use of tissue donated by either deceased or living organ donors. A deceased donor must have given full consent to the removal of their organs for the sole purpose of scientific research, prior to their death (Government Legislation UK, 2020). Living donors must also give their full consent and a government ethics committee must review it before any research can be carried out.
Biomedical and bioscientific research involving animals will always remain a crucial part of scientific developments, particularly in the medical health field. Although human cell cultures are an extremely effective in vitro method that avoid the use of protected animals, and the use of this method in the development of different drugs and vaccinations will continue to increase over the coming years, it will never completely eliminate the use of laboratory animals in research, even in combination with other alternatives. An overall faster turnaround of analysis via the means of organ-on-a-chip technology could lead to a reduction in the cost of drug trials, and the use of human cells produce greater precision. However, even in times like the current, with all of the rapid progression of advances in technology, using animals in biomedical research will remain a vital component of scientific discoveries, with beneficial results for both animals and humans, for the foreseeable future.
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