Induced Pluripotent Stem Cells

Embryonic Stem Cells (ESC’s)

 

Fig.1: Isolation and Maintenance of Stem Cells, (BioRad)

 

Human derived ESC’s have been extremely controversial and under the Bush administration in August of 2001 the generation of new human derived ES cell line was banned. Despite the ban being lifted by the Obama administration in 2009, the usage of hESC’s has been limited. (Wolinsky) Thus, most ESCs used in laboratories are derived from mouse embryos, which present a useful model, but lack the interpretive power that come from using human ESCs.

 

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Induced Pluripotent Stem Cells 

Fig.2:Directed Differentiation of iPS Cells (R&D Systems)

Since 2006 the Stem cell field has been pushing the bounds of pluripotency with iPS cells. Yamanaka et al. found that by culturing somatic cells with a cocktail of Oct3/4, Sox2, Klf4, c-Myc factors they were able to de-differentiate and develop pluripotent characteristics. This discovery created the opportunity to take patient fibroblasts, induce them into a pluripotent stem cell state (iPSC), then alter the media conditions further to induce differentiation into many cell types. Though the efficiency of the de-differentiation is quite low (0.01%) once a health stock has been created, they can be kept healthy and proliferate with a daily passaging protocol.

 

A powerful application of these iPSC’s is the growing field of organoid culture. In brief, this technique allows for the differentiation of iPSC’s into a particular cell type, which are then allowed to aggregate to form an organoid. These aggregate tissues proliferate and grow in culture and have been shown to acquire cell specific markers associated with the tissues that the in vivo tissue would normally form. For instance, neural rosettes express neural tube markers, or eye organoids undergoing optic cup invagination and express retina specific markers.

 

Fig.3: Localization of neural stage-specific cells between the hESC derived neural rosette (left panel) and the embryonic cortex (middle panel.) (Grabiec et al.)

 

 

Fig.4: Self-organisation of optic cup in vitro (Sasai et al.)

 

These organoids provide the opportunity to study tissue level effects, without the ethical dilemmas associated with acquiring those tissues from actual humans. There are limitations however, these cultures require daily attention, and take months to grow, as well as exhibiting epigenetic memory of the tissue they were derived from. Further, due to the lack of vasculature they often become hypoxic when allowed to grow into large organoids. However, there are a number of efforts in the field to counteract the hypoxic effect by either inducing vascularization, or growing the tissues around a tubule matrix that could act as an artificial vasculature.

 

Fig.5: An overview of vascularization techniques (Grebenyuk et al.)