Stem cells and organoids: a revolution in biomedicine

The field of biomedical and pharmacological research has evolved significantly from the traditional use of potentially immortal cell lines (such as HeLa or HEK293 cells) which, despite their usefulness, often presented ethical or functional challenges due to their cancerous or fetal origin, and their genetic variation over time.

The search for a personalized medicine that avoids the problems of immune rejection and the bioethical arguments related to the use of embryonic stem cells prompted the development of more ethical alternatives. These include the application of adult stem cells and cell reprogramming technology, which enables differentiated cells to revert to a pluripotent state (induced pluripotent stem cells or iPSCs).

The most recent innovation is organoids, simple tissue-engineered in vitro organ models that assemble different types of cells (mainly iPSCs or adult stem cells) into 3D multicellular systems that are able to recapitulate many aspects of the complex structure and function of a living organ. These organoids represent a crucial breakthrough in regenerative medicine, diagnostics, modeling of cardiovascular, kidney, and lung diseases, and novel drug testing, although their growing application in germline and nerve tissue regeneration raises important safety issues and bioethical concerns.

Human cell lines or cultures have been a very useful tool in biomedical and pharmacological research. They are basically human cell lines grown in vitro in Petri dishes and maintained for generations in the laboratory. As a result, they are potentially immortal. Their sources have varied, but those derived from cancerous tumors have been the most widely used in research. These include HeLa cells derived from cervical cancer cells and kept in culture since 1951, and HEK293 cells from human embryonic kidney cells, maintained since 1973. Today, there are hundreds of lines that have enabled significant progress to be made in various types of research, especially in pharmacology, where they are used to study the effects of new drugs.

However, the current cells of many of these lines may have undergone genetic variations over time, which, together with their origin—from a cancerous tumor or from fetal tissues from abortions—present a range of challenges in their applications, either because they do not accurately represent the behavior of normal human cells in vivo or for ethical reasons.

Towards personalized medicine

In the late 1990s, embryonic stem cells burst onto the applied research field as a method for repairing damaged tissues and organs. This prompted a major bioethical debate due to their origin from the inner cell mass of human embryos produced by in vitro fertilization, which implies the instrumentalization of human life in its initial stage of development. Genetic differences in the histocompatibility system between the embryonic cells and the recipient meant that many of the attempts to repair damaged tissues were unsuccessful. More ethical and personalized alternatives had to be sought.

The failed attempts to use embryonic stem cells soon led to the development of new, more ethical, and better alternatives than embryonic stem cell lines, to advance both regenerative medicine, and biomedical and pharmacological research. As a result, the use of adult stem cells would soon emerge. At the beginning of the 21st century, many scientific journals began to publish studies demonstrating that at the base of many post-embryonic, fetal, and post-natal somatic tissues are pluripotent or multipotent stem cells that can be reprogrammed into the same or a different cell type. These are adult or post-embryonic stem cells that have been found in amniotic fluid, the umbilical cord, fetal tissues, and in most adult tissues. They are subcellular groups responsible for repairing the natural degradation of many tissues. Furthermore, these cells can be derived from the patient themselves, so that the resulting cell clones would have the same genetic profile as the patient, the same genetic identity, thereby addressing the problem of immune rejection after potential transplantation.

As a result, personalized medicine developed as a new approach that takes into account individual genetic variability. The practice of precision regenerative medicine requires disease models that can replicate the disease development processes in individual patients at the molecular, cellular, and organ levels.

In 2006, a third technological alternative appeared: “cellular reprogramming”, or “genetic reprogramming”. Japanese researchers at Kyoto University’s Center for iPS Cell Research and Application, led by Shinya Yamanaka, winner of the 2012 Nobel Prize in Medicine, announced the possibility of genetically modifying differentiated cells from adult tissues and returning them to an embryo-like state. To do so, they tested the introduction of four genes—Oct3/4, Sox2, c-Myc, and Klf4—into mouse skin fibroblasts using inactivated retroviruses, and then activated them. After culturing the modified cells, they observed the generation of cells that had lost their specialized characteristics and behaved like embryo-like pluripotent cells, so they named them induced pluripotent stem cells or iPSCs for short^[1]. When properly cultured in suitable media, these cells revealed the ability to differentiate into various tissue types from the three germ layers (ectoderm, mesoderm, and endoderm), offering a new approach to regenerating damaged tissues without using embryos. Injection of the iPSCs into mouse blastocysts showed that they integrated into the cells of the inner cell mass and behaved like the embryo’s own cells, demonstrating their pluripotency despite originating from adult somatic cells.

Since then, iPSC technology has grown steadily, leading to the development of a number of alternatives to the use of cell lines and embryonic cells for the treatment of degenerative diseases or other pharmacological applications^[2].

The advent of organoids

There are diseases in which the patient’s cells cannot be maintained in cell culture for a long time without losing their metabolic activities; however, this is not the case for the derived iPSCs. Nevertheless, a cell culture in the laboratory is not the same as the actual situation in the patients’ tissues and organs. For this reason, the development of more complex 3D multicellular systems was soon pursued, which, since they are composed of several cell types, would be better suited to the requirements of tissue repair in the case of regenerative medicine. Thus, tissue “organoids” emerged, derived from either adult stem cells or patient-specific pluripotent stem cells. In some cases, another tool from genetic engineering, “gene editing”, would have to be incorporated. As a result, organoids can be designed according to the genetic and epigenetic requirements of the patient’s target tissue or organ.

Organoids are simple models of organs produced in vitro, obtained through tissue engineering by assembling different types of cells, especially adult stem cells or iPSCs, which recapitulate many aspects of the complex structure and function of a living organ. They can be used for the regeneration and repair of damaged human tissues or for basic studies on organ development. They can also be used in diagnostics, disease models, novel drug testing, or for transplantation, helping to overcome the shortage of donor organs. So far, numerous organoid engineering strategies have been described that enable their culture, growth, proliferation, differentiation, and maturation.

Organoids exceed the applied therapeutic capabilities of cell lines, embryonic stem cells, adult stem cells, and iPSCs. The scientific literature is already very extensive. Below, we present several examples.

In August 2013, researchers at the University of Pittsburgh published a study describing the reconstruction of mouse hearts using a protein scaffold derived from previously decellularized hearts. After repopulating this scaffold with human iPSC-derived multipotential cardiovascular progenitor cells and perfusing it with blood, the structure exhibited spontaneous contractions, becoming a synthetic heart capable of beating^[3]. The construction of human cardiac organoids is already a reality in their use as a model for investigating cardiovascular diseases, including myocardial infarction, heart failure, genetic heart disease, and arrhythmia^[4].

Similarly, liver, kidney, embryonic and other organoids were soon created to facilitate research or indeed the treatment of various diseases. In this regard, a study led by Spanish research professor and geneticist Juan Carlos Izpisua and his group at the Salk Institute for Biological Studies in La Jolla, California, consisting of the construction of kidney organoids or “mini-kidneys”, from iPSCs^[5] aroused great interest. The pluripotent cells produced were able to differentiate into cells resembling the mesoderm progenitor cells of the kidney (ureteric bud progenitor-like cells). The growth factors enabled the cells to be directed to specialize into kidney cells in just 4 days of culture. In addition to opening up new research strategies due to their potential clinical application, these 3D structures would also make it possible to evaluate new drug treatments in vitro.

Also worth highlighting is the work carried out at the Institute for Bioengineering of Catalonia (IBEC) and at ICREA in Barcelona, where Dr. Nuria Montserrat and her team developed kidney organoids from iPSCs, with a transcriptomic profile identical to that of second-trimester human fetal kidneys^[6] (this was the subject of a presentation at the “Congress on Genetic Research: Possibilities and Risks. An approach from Bioethics”, organized by the UCV Bioethics Observatory in July 2024).

Equally noteworthy is the generation of 3D structures or organoids of the human airways from iPSCs. In a study conducted at the Hospital for Sick Children in Toronto (Canada), human airway organoids were obtained for the first time from iPSCs with a mutated CFTR gene; this gene encodes a transmembrane conductance regulatory protein, whose dysfunction causes cystic fibrosis^[7].

Similarly, in 2013, a team of scientists from the Hubrecht Institute in the Netherlands managed to grow stem cells capable of generating the two different cell types that make up a healthy pancreas. This was a breakthrough in the repair of insulin-generating beta cells, which are important in the treatment of diabetes. The research group led by Dr. Hans Clevers, Professor of Molecular Genetics at Utrecht University, developed a 3D culture system as a proof of concept for the research and treatment of this disease. Dr. Clevers is regarded as one of the leading experts in the creation of organoids from adult stem cells and iPSCs for subsequent application in personalized medicine. The researcher and his team collaborated with the Utrecht Children’s Hospital in the search for a suitable pharmacological treatment for cystic fibrosis using colon organoids, developed from cells obtained by rectal biopsy from the patients themselves. The group also developed a way to test the different drugs available on patient cell samples stored in the biobank of the aforementioned institution, which is a great advance in the personalized treatment of a disease that can be caused by a number of genetic alterations^[8].

In another experiment, scientists from Stanford University (USA) transplanted brain organoids from human neurons—obtained from aggregates a few millimeters in diameter containing millions of iPSCs—onto the cortex of the brain of newborn rats genetically modified to avoid immune rejection. After examining the rats’ brains after 6 months, the researchers found that human cells from these organoids had successfully integrated into the animal tissue, growing to occupy one-third of the cerebral hemisphere of the transplanted rats. The cells proliferated, matured, and were nourished through the blood vessels of the recipient brain, eventually developing axons. The functional interconnection was also successful, as evidenced by its influence on the animals’ behavior^[9].

Nowadays, it is possible to produce organoids from any type of organ, including those whose tissues lack adult stem cells, such as cardiac and nervous tissues. iPSCs combined with CRISPR-Cas9 gene editing—which enables their modification to achieve greater effectiveness—offer the potential to obtain organoids that can be tailored to treat multiple degenerative diseases and to meet the demand for personalized organs for transplants.

Finally, it is interesting to note the creation of “organoid biobanks”, not only for modeling various diseases such as those mentioned, but also using tumor cells of many different types of cancer. These organoids serve as a test bed for different alternative treatments, and to advance our understanding of new therapies that may emerge in future research.

Bioethical aspects

The Austrian philosopher of science Karl Popper (1902-1994) said that research involves ethics, and that all research must consider the risks to man and nature. It should therefore be borne in mind that the danger lies not in basic research, but rather the applications and the aims or objectives pursued, which is of particular importance when it comes to organoids.

As we pointed out at the beginning, experiments using somatic cell cultures from adult tissues, or iPSCs used for organoid production and applications, do not pose ethical problems and are preferable to the cell lines that have been maintained for generations. The same cannot be said of cells derived from embryos, or cells derived from human tissues obtained from induced abortions.

In principle, organoid research that is confined to the laboratory does not raise bioethical concerns, as long as its goals are directed towards personalized medicine, such as selecting the most suitable pharmacological treatments for individual patients.

Likewise, applications designed to regenerate damaged somatic tissues or organs are ethically acceptable, provided that they are preceded by the appropriate safety measures for the transplant technology to be employed. Gene editing performed in organoid-generating cells (in adult stem cells or iPSCs derived from somatic tissues) does not pose ethical problems either, as it is usually carried out ex vivo or in vitro.

A separate issue is posed by organoids that seek to simulate or reconstruct so-called “embryoids” or embryonic models, which, at the same time, are very useful for research into the genetic causes of infertility or the failures that occur during early human embryo development. The synthesis of viable human embryoids, or embryoids created using stem cells derived from human embryos, would not be ethically acceptable.

The transplantation of organoids aimed at regenerating germline organs or tissues that affect gamete stem cells would also be ethically unacceptable, for both ethical and safety reasons. Any errors in the processing or in the potential genetic modifications made to the cells from which the organoids are derived could be passed down to future generations. There is still no absolute certainty of the accuracy and fidelity of gene editing techniques; consequently, there is a general consensus against conducting this type of research on embryos or human germ tissue.

Finally, the application of organoids on human nervous tissue is equally debatable, both because of the negative or unknown effects of these technologies on the tissue that coordinates the functioning of all the other body organs, and because of the potential psychological or behavioral effects on the recipients. Obviously, for the same reasons, this technology must be prevented from becoming a test bed for those who promote transhumanism for enhancement or any other non-health-related purpose.