Over the past decades, the medical research community has relied on in vitro two-dimensional
(2D) cell lines and in vivo animal models to study cellular and molecular mechanisms
of pathogenesis. Indeed, the use of these approaches has enormously contributed to
a better understanding of the biology and pathophysiology of human diseases. Such
studies have been successful in uncovering many aspects of development and differentiation
and cellular behavior under various conditions, as well as molecular interactions
and networking. The use of animal models further facilitated a higher and complex
dimension of understanding and hence tremendous breakthroughs and discoveries have
been made using these model systems allowing bench-to-bedside translation and leading
to therapeutic interventions in many human diseases. Nevertheless, the vast majority
of drugs that were developed in animal models have had limited success in clinical
trials. In a large part, this can be attributed to the lack of models that can faithfully
phenocopy the unique nature of human biology. Furthermore, animal models used in preclinical
research are mostly based on inbred strains harboring genetic homogeneity and lacking
the diversity of heterogenous genetics seen in humans.
This has led to the emergence of alternatives that allow precise modeling and drug
screening. One such system is the use of human three-dimensional (3D) cell culture
approaches [1, 2]. These in vitro 3D cultures, often referred to as organoids, are
generated from embryonic or adult stem cells as well as from induced-pluripotent stem
cells (iPSCs) (Fig. 1A). Using particular supplements and small molecules in culture
media, organoid cultures were established to model specific organs containing multiple
cell types and displaying physiological and cellular traits of the primary modeled
organ [3, 4]. Organoids can be maintained and followed for extended time periods due
to their self-renewal and differentiation capabilities. Recent years have seen great
advancements in establishing multiple different organoid culture protocols that model
the various organ systems in the human body (Fig. 1B). This platform has facilitated
unforeseen modeling and studies recapitulating the development of human tissues and
diseases. A major advantage is the ability to model patient-specific disorders in
a personalized manner and the use of organoids to screen for drugs and possible treatments.
The ability of long-term culture of organoids has also enabled the use of high-throughput
technologies to address key questions at various levels. Researchers have applied
transcriptomics (bulk and single cell), proteomics, and metabolomics to organoid cultures
driving novel discoveries and complementing studies in animal models and in 2D cultures.
The use of organoids has been applied to the study of human brain disorders, response
to infections including the novel severe acute respiratory syndrome coronavirus (SARS-CoV-2),
malignancies, and more [3, 5, 6]. As such, the use of organoids holds great promise
for recapitulating complex pathogenesis thereby expediting personalized medicine applications
and drug screening.
Fig. 1
Organoid types and possible applications.
A Schematic illustration of the main organoids generation approaches. Stem cells are
either isolated from embryonic (ESCs) or adult (AdSCs) tissues or derived via reprogramming
of somatic cells (induced pluripotent stem cells—iPSCs). With or without genetic engineering,
and under specific conditions, these cells have the capability to self-assemble into
3D structures. Under certain differentiation conditions, these generate tissue-specific
organoids. B Outline of the different systems modeled by organoids that are discussed
in this current issue. These systems can be applied to study different genetic or
pharmacological interventions in a setting more representative of the in-vivo physiology
than 2D cultures. These organoids can be easily analyzed using advanced molecular
tools such as sequencing, mass spectrometry, and imaging. PBMCs, peripheral-blood
mononuclear cells. ESCs embryonic stem cells.
Nevertheless, organoid cultures also have limitations and should be used with great
caution [7]. Major challenges include their robustness, heterogeneity, and reproducibility.
The human tissue organs are usually composed of different cell types, often arising
from different germ layers, that need to be considered when modeling a given tissue.
For example, the lack of functional vasculature, nervous system, or immune system
is a drawback and sets organoid cultures far from in vivo models. There are also many
ethical issues related to use of organoids in research that should be considered and
regulated.
Given the great interest and importance of this new and emerging field, this special
issue of CDD comprises articles from a group of pioneer scientists who have reviewed
and discussed advances and challenges related to the use of organoids in modeling
different tissues. The review article by Sidhaye and Knoblich discusses the foremost
advances in organoids use to study human brain development and related neurological
diseases [8, 9]. The emergence of several methods and protocols modeling the different
parts of the brain and disease-associated phenotypes and mechanisms are extensively
reviewed. Brain organoid protocols of patterned and unpatterned organization, region-specific,
fusion organoids and co-culture with other cell types and tissues are compared in
the context of the biological question and the disease of interest to be studied.
The authors further reviewed the different neurological diseases that were studied
using the different protocols of brain organoids and conclude with a description of
the immense potential of brain organoid technologies to reveal disease mechanisms
and as a platform for drug screening, therapy and diagnosis.
Jay Gopalakrishnan leads a discussion of the use of brain organoids to model and study
glioma, an aggressive form of brain cancer. The organoids used to study cancer were
either established from a patient tumor biopsy, retained cancer stem cells, or via
genetic manipulation, for example using CRISPR/CAS9, of a healthy organoid by modeling
a specific mutation [10]. These so called tumoroids or glioblastoma organoids closely
mirror the nature of the human disease and is discussed by Mariappan and colleagues
from various aspects, including the challenges involved and their significance in
closing the gap between studies of 2D cultures and animal models of gliomas.
Another interesting field in organoid development is found in the success of modeling
the inner ear and its disorders. Such disorders are common and attempts to intervene
and restore sensory functions have been very limited due to lack of accurate models.
The Kohler group present in this special issue a perspective on the current stand
of inner ear models and the promising future directions to advance their use in translational
research. Indeed, there have been several breakthroughs over the last few years regarding
modeling the developmental stages of the inner ear using organoids and the development
of protocols to mimic key signaling modes. Consistent with the continuous expansion
of this field, the authors discuss the necessary next steps of development so that
inner ear organoids truthfully recapitulate neurosensory functions and dysfunctions.
Two review articles by Takanori Takebe and colleagues and the Maurice group provide
an overview of recent progress of tissue-derived and PSC-derived epithelial (intestinal,
gastric, liver, and pancreatic) organoids. As for many tissues, access to the digestive
tract tissues and biopsies is limited and hence organoid-derived systems provide an
excellent alternate system to model pathologies and study human-specific aspects of
disease kinetics and physiology. In addition, this digestive tract platform allows
screening and investigating new therapeutic approaches for many diseases related to
disorders in the digestive tracts. Funata et al. give examples and describe attempts
to study genetic diseases (such as cystic fibrosis, monogenic liver diseases and Hirschsprung’s
disease), infectious diseases (H. pylori and HBV), inflammatory diseases (steatohepatitis,
Wolman disease and inflammatory bowel diseases (IBD)) and malignant diseases (colorectal
cancer, hepatocellular carcinoma and pancreatic adenocarcinoma), all related to the
digestive system. Spangers and colleagues elegantly discuss how two key signaling
pathways, YAP and Wnt/β-catenin, regulate the dynamic intestinal repair response upon
inflammation and injury and the promise of intestinal organoids to model intestinal
regeneration. The challenges accompanying the use of digestive tract organoids including
optimizing clinical relevance, modeling the complexity of organ interaction, improving
reproducibility and quality as well as the standardization of culture to enable high-throughput
screening are also discussed.
Another exciting review article by Margherita Turco and co-authors discusses recent
developments in 3D organoid technology that model the different regions of the female
reproductive tract (FRT) including organoids of the endometrium, fallopian tubes,
ovaries, cervix and placental development. In-depth understanding of FRT was lacking
due to the limited available tools and systems. Furthermore, the complex nature of
FRT and its non-cell autonomous dependence on signals and hormones from other organs
(pituitary, ovary or placenta in pregnancy) prompted the use of the 3D organoid system
in studying these complicated and highly dynamic organs. Such organoid systems accurately
model the morphology and functions of the human organs making them an excellent model
to study physiology and related pathologies, including malignancies and fertility.
There is definitely much more to uncover and to improve in FRT-related organoids to
better improve women’s wellbeing and reproductive health.
All in all, the organoid field is exponentially growing with promising potential to
act as an excellent platform to understand human biology and treat diseases. Many
developments and new protocols are emerging at a continuous pace that will certainly
aid the scientific community in answering basic research questions and act as useful
tools to model, understand and treat human diseases. As with any new model, bottlenecks
and pitfalls accompany this development and hence validation in other model systems
is always required to ensure reliable conclusions.