LANDMARK 3-D MODEL

Formation of self-assembling retina from embryonic stem cells is a ‘game changer’

[caption id='attachment_262' align='alignright' width='300' caption='The self-assembly of a murine retina from 3-dimensional cultures of embryonic stem cells is among the most complex tissue engineering achievements yet. The success, reported from the Organogenesis and Neurogenesis Group at the RIKEN Center for Developmental Biology, is likely to have a fundamental impact on both research and drug development in the field of retinal medicine'][/caption]

Remarkable research has shown for the first time the formation of a three-dimensional retina in a laboratory dish beginning with no more than a culture of floating embryonic stem cells.

While sounding more like science fiction than fact, the breakthrough is set to fundamentally impact on both academic studies of retinal biology and on the pharmacological testing of new compounds and treatment strategies for a broad range of retinal disorders, including retinitis pigmentosa and age-related macular degeneration (AMD).

The self-assembly of a murine retina from 3-dimensional cultures of embryonic stem cells is among the most complex tissue engineering achievements yet. While of immediate interest to the field of ophthalmology, the advance has already made an impact on stem cell biology and regenerative medicine, disciplines keen to explore the potential application of the research for brain and other tissues.

Research data showed that the use of embryonic stem cells in assembly of the retina emerged spontaneously in vitro suggesting the capability of self-assembly may be applied to many other organs and, as such, may radically alter not only basic research but also the cost and development timelines for introducing new drugs for retinal degenerations.

The researchers behind the new development, Dr Mototsugu Eiraku and Prof Yoshiki Sasai of the RIKEN Center for Developmental Biology, in Kobe, Japan, published their findings in the journal Nature [2011;472:51-56]. In a series of time-lapse images of laboratory culture – available on the Nature website (www.nature.com) – the self-assembling retina begins as a floating 'cloud' of embryonic stem cells (ESCs) which, in the presence of matrigel, begins to express genes known to be markers of the retinal fate.

Within approximately six days, the cloud becomes a hollow sphere containing polarised epithelial cells. Specific cells self-associate into islands that form vesicles that eventually involute to the classic C shape of the developing retina. A rudimentary retinal pigment epithelium slowly becomes discernible, and between days 20 to 24, the full neuro-sensory retina is complete with photoreceptors, all in their correct anatomical location.

While the lab-based self-assembly appeared to follow the natural temporal sequence of retinal formation and the differentiated cells appeared to be organised into the correct cellular layers, whether or not the organ functions in capturing and processing light inputs remains to be tested. Regardless, the supplementary videos accompanying the paper are in themselves worth viewing if only to see first-hand the incredible assembly of one of the body's most complex organs.

A number of research groups around the world had previously shown retinal differentiation from ESCs and the formation of lens-like structures and retinal rosettes from a variety of studies all of which were homing in on defining the illusive elixir of protocols and culture conditions aimed at optimising the development of more organised tissues.

The RIKEN research team themselves have demonstrated the self-formation of cerebral cortical tissues in culture from floating aggregates of ESCs and of retinal differentiation from similar ESCs however, formation of retinal epithelial structures was not happening. A key step in the process that led to the successful self-assembly was the addition of basement-membrane matrix components – matrigel – to support the formation of a stable epithelial structure.

According to Prof Sasai, without the matrigel, 'cells tend to fall apart,' presumably due to the absence of the mechanical support which would be available under normal organogenesis in vivo.

Given the significant cost and timelines involved in developing new therapeutic strategies, the availability of 3-D models could provide a considerable resource in accelerating the testing of new compounds.

Furthermore, one of the significant challenges in tackling disorders such as retinitis pigmentosa is that as time progresses the patient loses first rod and then cone photoreceptors which are not replaced. Any viable treatment needs to apply a potential therapy to a dwindling number of photoreceptors. If the RIKEN research successes can be replicated in the development of self-assembling 3-D human retinas then clinicians and patients may benefit from a renewable source of photoreceptors to be transplanted at an optimal development stage.

Dr Eriaku and Prof Sasai commented that 'self-formation of fully stratified 3-D neural retina tissues heralds the next-generation of generative medicine in retinal degeneration therapeutics, and opens up new avenues for the transplantation of artificial retinal tissue sheets, rather than simple cell grafting.' Projecting such technology onto the broad landscape of many other degenerative disorders, while many years away, may open numerous new approaches for treatment.

While the medical and research applications of the studies are destined to attract much commentary, the evolutionary context of the findings is notable. The eye has long represented a major battlefield in historical and contemporary debates on evolution by natural selection. If, as the argument goes, such a slow gradual piecemeal process underlies the formation of organs of such exquisite perfection as the human eye, then what use was there for the endless varieties that preceded the finished product in humans?

While modern scientific thought universally accepts that half an eye or even a fraction of an eye is fundamentally better than none at all, we witness in the RIKEN research, unfolding frame by frame, the ontogeny of a mammalian eye in a laboratory dish. We see several million years of evolution condensed into 24 days – for this alone the studies are destined to represent a major scientific landmark.