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Fig. 3.1 Early human embryonic development and germ cell differentiation from human embry-
onic stem cells. Shown is a series of microscopic images taken of: ( d0, day 0 ) the primary oocyte
with an intact germinal vesicle ( arrow ); ( d1 ) one-cell embryo, commonly referred to as a zygote,
on day 1 as evidenced by the appearance of two pronuclei containing the male and female set of
chromosomes ( arrows ); ( d2 ) two-cell embryo on day 2; ( d3.1 ) four-cell embryo on day 3; ( d3.2 )
eight-cell embryo at the end of day 3; ( d4 ) morula on day 4; ( d5 ) the blastocyst showing the inner
cell mass ( white arrow ) on day 5; ( d6 ) hatching of the blastocyst from the zona pellucida on day 6;
( hESCs ) a human ES cell colony cultured on mouse embryonic fibroblast feeder cells for 4 days;
( EBs ) human ES cells spontaneously differentiating in suspension as embryoid bodies for
2 days; ( DhESC ) differentiating human ES cells in adherent culture on Matrigel for 5 days; ( GCs )
a germ cell colony after isolation of germ cells from EBs and subsequent culture in defined
medium on mouse embryonic fibroblast cells for 14 days ( d0 , d1 , d3.1 , and d3.2 adapted from
Dobson et al. 2004 ; d2 , d4 , d5 , and d6 from R. Reijo Pera; hESCs , EBs , and DhESC from S.
Schuh-Huerta; GCs image from K. Kee)
germ cells and further culturing with various defined factors or somatic cell types;
and adherent differentiation of ES cells cultured on Matrigel, and subsequent isola-
tion and culturing of putative germ cells (Fig. 3.1 ). In most cases germ cells are
identified and isolated by using either integrated genetic constructs fused with fluo-
rescent proteins that serve as germ cell reporters and/or germ cell-specific cell
surface markers and cell sorting techniques. This relatively new field of germline
stem cell biology has the potential to uncover the as yet to be identified unique
genetic requirements, signaling pathways, and developmental programs of the human
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