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Growing human cells in culture

Our body is made up of over two hundred different cell types. Even before we are born, as we grow in the womb, our cells have learnt the jobs they are destined for. Red blood cells carry gases to tissues, neurons relay electrical signals to control body functions and pancreatic endocrine cells secrete insulin to maintain our glucose blood level. By and large, these cells, called differentiated cells have these highly specialised tasks and do not do anything else. To become differentiated, cells adopt unique shapes and structures that enable them to do their specific tasks, making most types of cells distinguishable when observed using a microscope. Once they have specialised, most cells stop dividing although some specific cell types continue to divide or resume dividing after injury. However, as we grow older, the ability of our cells to divide is reduced and they divide fewer times.

Scientists have been researching how to grow cells in a culture dish for over a hundred years. In 1951 the first permanent human cell line was established from a malignant tumour. At this time, it was believed that any of our body cells could be grown in a tissue culture dish. However, in 1961 Leonard Hayflick meticulously showed that normal body cells had limited lifespan in culture. As animals age, the lifespan of a cell shortens. Thus, if our body’s differentiated cells are extracted and transferred into a culture dish they will only divide a set number of times and then stop. Being able to grow and expand human body cells in a laboratory would enable many different kinds of research. For example, human cells could be used as factories for vaccine production, human antibody production and drug and toxicology testing. Further, this would allow the generation of cell therapies for different diseases or injuries. Many diseases, such as Parkinson’s disease and juvenile-onset diabetes mellitus result from the dysfunction of a specific cell type. The replacement of such cells could provide treatments for such diseases. Further, a greater understanding of cells and in particular, stem cells and how they work in our body will give us a better understanding of how our organs and tissues regenerate when damaged. This information could provide a way to promote regeneration in certain conditions or diseases.

Stem cells

Stem cells have been identified in various organs and tissues of our body although they exist in very few numbers in comparison to differentiated cells. Stem cells differ from other cells in our body as they continue to divide making more identical stem cells or they may differentiate, adopting particular destined traits. In our body, stem cells commonly produce only cells indicative of the organ or tissue in which they reside. A different type of stem cell the embryonic stem cell is isolated from early embryos. Although stem cells come from a number of sources they are broadly categorised into two groups that reflect their origins; adult and embryonic stem cells.

Adult stem cells

Many different stem cells are classified as adult stem cells, or cells derived from organs in our body as well as cells from a baby’s placenta, amniotic sac and umbilical cord. Adult stem cells develop into specific cells mainly reflective of the organs in which they reside. For example, in adults, liver stem cells produce hepatocytes as well as cells around the liver biliary duct; bone marrow stem cells produce immune cells or blood cells. Although bone marrow stem cells produce blood which travels within vasculature throughout the body. Mostly, bone marrow stem cell therapies treat patients who have blood or immune disorders. However, some researchers believe that once removed from the body, stem cells can be coaxed to become other cell types. Presently, most of this work is being undertaken in laboratories, although some transplantation of bone marrow stem cells for disorders such as heart disease, or muscle injuries are underway. Many researchers are working to better identify and isolate different types of adult stem cells from various organs and tissues. Conditions to be able to expand adult stem cell numbers are actively being pursued as so far, they have limited growth potential in a laboratory. 

Human embryonic stem cells

Human embryonic stem cells are cells that have been isolated from early human embryos that have been in a culture dish for five days. Most human embryos available for research are generated in assisted reproduction units following procedures for fertility treatments. At this stage, these embryos are called blastocysts. One human embryonic stem cell line comes from one blastocyst, which is destroyed in the process of its making. Blastocysts are made up of two progenitor cell types, the inner cell mass and trophectoderm. The inner cell mass is a cluster of cells which develop into all of our body’s cell types. Trophectoderm cells develop into the placenta. Thus, inner cell mass cells are called pluripotent, capable of forming all cell types, including all differentiated cells of the body including germ cells (egg and sperm). When the inner cell mass cellular clumps are extracted from the blastocyst and transferred into particular conditions they are encouraged to grow and become human embryonic stem cells. If not used for such research, blastocysts stop growing and die in culture. The human embryonic stem cells differ from adult cells in that they continuously divide in culture and can be expanded. Further, they can veer from this pathway and start to differentiate. It is precisely due to these unique abilities, that researchers want to make human embryonic stem cell lines. Human embryonic stem cells offer a potential to produce plentiful numbers of normal human cells such as cardiac cells and neurons. That human embryonic stem cells can divide endlessly provides limitless human pluripotent cells to be studied and distributed. Scientists saw the potential of these human pluripotent cells at once, and rushed to experiment with them or make more cell lines. By 2006, over four hundred and fifty human embryonic stem cell lines had been catalogued. However, most researchers use two of the cell lines derived by Thomson in 1998 and many lines are speculative, having not been fully characterised.

Induced pluripotent stem cells or iPS cells- another type of stem cells

Induced pluripotent stem cells are cells that, like human embryonic stem cells, are artificially made in laboratories. They were coined ‘iPS cells’ by their inventor Shinya Yamanaka. In 2006, he and his colleagues devised a method to make specialised body cells from a mouse behave like early embryonic cells, able to divide and be expanded as well as generate all cell types of the body.  As human iPS cells can be made from patients who require cellular therapy based treatments they are genetically identical and thus, could overcome them being rejected. This problem, of course, is no different than that experienced by recipients of donor organs, who take drugs to dampen their body’s immune system. Human iPS cell lines can be made from individuals with specific genetic disorders or diseases. This would enable particular diseased cells to be studied in laboratories, providing cellular targets for drug therapies.

The original method to make these cells, devised by Yamanaka and colleagues, is based on a routine experimental technique used to introduce specialised pieces of DNA called genes, into cells using a retrovirus like a Trojan horse. Once a particular set of four genes were inserted into the specialised cell, the cell took on a different behaviour and appearance, becoming similar to embryonic-like cells. This process is often called re-programming.

 

Since 2006, the research using iPS cells has escalated, mostly resulting in improved methods for their generation. Many researchers are now producing iPS cells without using retroviruses, making them applicable for clinical therapies.

Written by Susan Hawes PhD,  
Monash Immunology and Stem Cell Laboratories,
Monash University

Non-human Animal Stem Cells- Agricultural & Companion Animals

Livestock stem cells can be isolated from either embryonic or adult tissue from animals such as cows, pigs or sheep. They can have a broad range of agricultural applications and have the potential to be used for both veterinary research and medical models for human research.  One of the main goals of livestock stem cell research is to enhance the efficiency of animal production through superior trait selection (for example, finer grade wool or leaner meat). A routinely exploited method to disseminate the elite genetics of an animal is via germ cells, such as sperm cells. By using current reproductive technologies, progeny can be produced to display a desired trait. Trait selection can be performed in conventional breeding programs. However, in vitro stem cell screening can expedite trait selection. 

The use of animal stem cells is not limited to livestock.  A lot of effort is being dedicated to studies in athletic (horses) and companion animals (dogs and cats). One example is using stem cells for tendon, ligament or joint repair in horses, cats and dogs. The stem cells can be obtained by collecting the animal’s blood or bone marrow - this is an example of the use of adult stem cells.  Pharmacological medicine can be given to alleviate the pain associated with the injury, however its really only stem cells that have the ability to accelerate repair of the damaged site. This prime example of regenerative veterinary medicine can have major benefits to the horse racing industry.  

Written by Billie Murray,  
PhD student Livestock Reproductive Technologies,
CSIRO Livestock Industries