Primordial Germ Cells

Development begins with fertilization, the process
by which the male gamete, the sperm, and the
female gamete, the oocyte, unite to give rise to a zygote.
Gametes are derived from primordial germ cells (PGCs)
that are formed in the epiblast during the second week
and that move to the wall of the yolk sac (Fig. 1.1). During
the fourth week these cells begin to migrate from the yolk
sac toward the developing gonads, where they arrive by the
end of the fifth week. Mitotic divisions increase their number
during their migration and also when they arrive in the gonad.
In preparation for fertilization, germ cells undergo gametogenesis,
which includes meiosis, to reduce the number of chromosomes and
cytodifferentiation to complete their maturation.
C L I N I C A L C O R R E L A T E
Primordial Germ Cells (PGCs) and Teratomas
Teratomas are tumors of disputed origin that often contain a variety
of tissues, such as bone, hair, muscle, gut epithelia, and others. It is
thought that these tumors arise from a pluripotent stem cell that can
differentiate into any of the three germ layers or their derivatives.


Figure 1.1 An embryo at the end of the third week, showing the position of primordial
germ cells in the wall of the yolk sac, close to the attachment of the future umbilical
cord. From this location, these cells migrate to the developing gonad.
Some evidence suggests that PGCs that have strayed from their normal migratory
paths could be responsible for some of these tumors. Another source
is epiblast cells migrating through the primitive streak during gastrulation