399Fig. 8.4 (continued) body (Bb), it fragments into small clusters composed of several mitochondria
surrounding the mitochondrial cement (4). At this point, Sm proteins leave the mitochondrial cement
and are recycled back into the nucleus (red arrow). At stage I, the mitochondrial cement, releases the
mitochondria and transforms into granulo-fibrillar material (GFM, 4), the precursor of the germinal
granules (GG). At this stage, the germ plasm is located at the vegetal tip of the Bb. Germ plasm RNAs
present in the GFM includes nanos1 (red dots), xpat (purple dots), xdazl (black dots), and deadsouth
(blue dots). Germinal granules (in gray) are mainly concentrated at the vegetal tip of the Bb (5)
Fig. 8.5 Germ plasm formation and distribution in Xenopus embryo. (a–d) Ultrastructural
analysis of germ plasm formation. (a) Interconnected oogonia at nest stage showing nuage
closely apposed to mitochondria. (b) Inter-mitochondrial cement or nuage, closely apposed to
nucleus, pre-stage 1 oocyte; (c) Germ plasm within Bb in Stage I oocyte; note the electron dense
germinal granules, mitochondria, smooth endoplasmic reticulum, ribosomes; (d) EM in situ
hybridization showing labeled nanos RNA within germinal granule, stage 1 oocyte. (e–j) Whole
mount in situ hybridizations showing Xpat expression in germ plasm. (e) Stage I oocytes show-
ing clear nucleus and closely apposed Bb; (f) Vegetal pole of stage VI fully grown oocyte; (g)
Xpat staining within germ plasm at tips of vegetal pole blastomeres of 32-cell embryo; (h)
Gastrulation showing PGCs within vegetal mass, closely apposed PGCs are likely mitotic sis-
ters; (i) Sagittal cut of early neurula stage showing PGCs close to the posterior floor of the
archenteron; (j) Tailbud with PGCs clustering at the dorsal aspect of endoderm. M mitochondria,
N nucleus, GG germinal granules, ER endoplasmic reticulum. (Malgorzata Kloc and Kenneth
Dunner, unpublished micrographic data)8 Mechanisms of Vertebrate Germ Cell Determination