394
During the first 24 h of development, the PGCs follow a route that can be divided
into six migration steps where they seem to have a certain association with specific
embryonic structures. In the end, two cell clusters form in the region where the
future gonad will be (Fig. 8.3). PGCs appear to leave certain regions and migrate in
the direction of several intermediate target sites before reaching their final destina-
tion (Raz 2003 ). The use of tissue-specific mutants facilitated understanding the
nature of the signals that guide the migrating cells. For example, the absence of
some mesodermal tissues does not interfere with the ability of PGCs to leave the
medial positions and reach their intermediate target. But a change in the position of
their target leads to alterations in PGC migration, strongly suggesting that PGCs
actively migrate towards their targets (Weidinger et al. 1999 , 2002 ).
Interestingly, isolated PGCs undergo apoptosis after prolonged incubation peri-
ods suggesting that the extracellular matrix and cell surface signals may play impor-
tant roles in PGC development (Di Carlo and De Felici 2000 ). In addition, Dnd
activity is required for PGCs to begin their migration (Raz and Reichman-Fried
2006 ). Specific knockdown of dnd blocks the polarization and migration of PGCs.
Fig. 8.3 Migration of zebrafish primordial germ cells: Schematic representation of embryos from
early gastrula stage (shield) to 24 hpf showing the positions and movements of the PGC clusters.
During gastrulation, four clusters of PGCs are found close to the blastoderm margin. (1)
Concomitantly with gastrulation, lateral and ventral clusters move towards the dorsal region, ven-
tral clusters migrate more slowly. (2) Clusters located very close to the dorsal side migrate away
from the midline. (3) Ventrally located clusters align at the lateral border of the mesoderm. (4) In
the early somite stages, most PGCs have arrived in two lines at the level of the first somite. These
anterior located PGCs migrate towards the lateral region, forming two lateral PGC clusters. Cells
that were initially located ventrally migrate towards the anterior along the anlage of the proneph-
ros. (5) At the 8-somite stage, all anterior PGCs are found lateral to the paraxial mesoderm in a
cluster extending from first to third somites. These clusters start to move towards the posterior area,
while the ventral cells tightly align on the lateral border of the pronephros and continue to migrate
anteriorly. (6) At the 19-somite stage, the main clusters have shifted to more posterior positions.
(7) At 24 hpf, the PGC clusters are located at the genital ridges in the anterior end of the yolk exten-
sion, which corresponds to the eighth to tenth somite level
T. Aguero et al.