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the anterior primitive streak, exposure of PSCs to high levels of activin A, a Nodal
analogue that acts through the same receptors, results in induction of DE during
in vitro differentiation assays (D’Amour et al. 2005 ; Gadue et al. 2006 ; Kubo et al.
2004 ). Indeed, the recapitulation of key features of this developmental program has
been demonstrated by studies featuring mouse ESCs targeted with reporter genes to
read-out endodermal gene expression. These studies demonstrate that differentiat-
ing ESCs pass through a primitive streak-like intermediate phase (defined by co-
expression of Brachyury and Foxa2 together with the cell surface markers C-KIT
and CXCR4) before further differentiating into DE (defined by loss of Brachyury
expression in cells that continue to express Foxa2 and are C-KIT+/CXCR4+) over
approximately 6 days in culture (Christodoulou et al. 2011 ; Gadue et al. 2006 ;
Gouon-Evans et al. 2006 ). Protocols established in mouse PSCs have subsequently
been adapted for human PSC differentiation with some differences in differentiation
kinetics but continued reliance on the expression of cell surface markers including
C-KIT, CXCR4, and EPCAM to identify DE (D'Amour et al. 2005 ; Green et al.
2011 ). It is from this pool of multipotent cells that lung progenitors are ultimately
specified.
Fig. 13.1 Mouse embryonic gastrulation – the mouse epiblast is depicted at the late primitive
streak stage. Epiblast cells migrate through the primitive streak and are exposed to variable con-
centration of key morphogens. Cells that exit the anterior aspect of the streak form the definitive
endoderm (DE)
A. Wilson and L. Ikonomou