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Fig. 2. Germ cell propagation in vitro and in vivo. (A-C) The germ cell cycle in
the mouse. (A) Primordial germ cells (PGCs) appear at E7.25 as a small group
of cells (red) in the extra-embryonic mesoderm. (B) After E8.5, PGCs start to
migrate to the gonadal anlagen and contribute (C) during puberty to oocytes or
sperm. (D) Embryonic stem (ES) cells are in vitro derivatives of inner cell
mass (ICM) cells. (E,F) The formation of ES cells occurs either directly from
the ICM/primitive ectoderm (E) or according to our hypothesis, through in
vitro differentiation of ICM outgrowth into primitive ectoderm, then into
extra-embryonic mesoderm and finally into PGCs (F). (G) PGCs that form in vivo
(A-C) give rise to embryonic germ (EG) cells in vitro. Germ cells, PGCs, ES
and EG cells are all capable of forming teratomas and teratocarcinomas. ES and
EG cells can reintegrate into the normal embryo after injection into the
blastocyst (not shown). Circular green arrows denote unlimited
self-renewal.
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