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Primordial germ cells (PGCs) of Xenopus laevis have been isolated from early embryos and kept alive in vitro, in order to study the structural basis of their motility, using the transmission and scanning electron microscope. The culture conditions used mimicked as closely as possible the in vivo environment of migrating PGCs, in that isolated PGCs were seeded onto monolayers of amphibian mesentery cells. In these conditions we have demonstrated that: (a) No significant differences were found between the morphology of PGCs in vitro and in vivo. (b) Structural features involved in PGC movement in vitro include (i) the presence of a filamentous substructure, (ii) filipodial and blunt cell processes, (iii) cell surface specializations. These features are also characteristic of migratory PGCs studied in vivo. (c) PGCs in vitro have powers of invasion similar to those of migrating PGCs in vivo. They occasionally become completely surrounded by cells of the monolayer and, in this situation, bear striking resemblance to PGCs moving between mesentery cells to the site of the developing gonad in stage-44 tadpoles. We conclude that as far as it is possible to assess, the behaviour of isolated PGCs in these in vitro conditions mimics their activities in vivo. This allows us to study the ultrastructural basis of their migration.
Fig. 1. Phase-contrast light micrograph of PGC moving on a cellular substrate. Note
the single filopod extending from one pole, and refractile yolk platelets in the PGC
cytoplasm.
Fig. 2. Phase-contrast light micrograph of a PGC which has extended a long and
well defined single filopod over the cellular substrate.
Fig. 3. Scanning electron micrograph of the same PGC as photographed for Fig. 1.
Fig. 4. Scanning electron micrograph of a PGC during the rounded up phase of its
activity. This cell is attached to a broken piece of Pasteur pipette covered in cells.
Yolk platelets (Y) and lipid droplets (L) protrude upwards underneath the cell
membrane, and small cytoplasmic processes extend along the substrate (arrow).
Fig. 5. Electron micrograph of an elongated PGC, cut in longitudinal section, and
lying on several layers of adult mesentery cells. Note the high electron density of
PGC cytoplasm compared to that of the underlying cells. Y= yolk platelet, L = lipid
droplet.
Fig. (5a). High power view of region A of Fig. 5 to show the close association of
this pole of the cell with the underlying cell. The area of contact is stained with
ruthenium red (arrow). Notice the well-defined bundle of microfilaments (M) in
the cytoplasm of the substrate cell.
Fig. 6. Electron micrograph to show characteristic cytoplasmic structures of PGC
in vitro. Note the Golgi body (G), clusters of mitochondria (M), smooth endoplasmic
reticulum (S) and a lobe of nucleus (N).
Fig. 7. A microvillus protruding from the surface of a PGC. The microfilamentous
substructure is not well-defined here.
Fig. 8. The upper surface of a PGC to show the microfilamentous substructure (M)
beneath the cell membrane. This area is not penetrated by cell organelles such as
mitochondria, granular e.r., lipid droplets and yolk platelets.
Fig. 9. Electron micrograph of a filopod cut in longitudinal section. Mitochondria,
ribosomes and agranular endoplasmic reticulum are found in the cytoplasm. Note the
electron density of the cytoplasm compared to that of the underlying cell.
Fig. 10. High power view of filopod of PGC scanned in Fig. 3. The main filopodial
trunk has microspikes (M) and lamella-like areas (L) protruding from it. Note the
longitudinal ridges (arrows) in the upper surface of the filopod.
Fig. 11. High power electron micrograph to show a membrane specialization between
a PGC (below) and somatic cell (above). Note the numerous free ribosomes in the
PGC cytoplasm.
Fig. 12. A broad process of a PGC invaginating the surface of the underlying cell.
The process has a rather indistinct substructure of microfilaments (M) and contains
many free ribosomes. Note the cell surface specialization (arrow) between the PGC
and somatic cell.
Fig. (12a). A high power view of the same blunt processes, as seen in Fig. 12, from
a nearby section which cuts one process in the mid saggital plane. A microfilamentous
substructure can be distinguished in the protrusions (arrow).
Fig. (126). Scanning electron micrograph to show blunt processes protruding from
a rounded PGC.
Fig. 13. A process extending from the undersurface of a PGC and penetrating
between adjacent cells of the substrate. The cellular monolayer is attached to
tissue culture plastic (P). Note microfilamentous bundles (M) in the somatic cell
cytoplasm and cell surface specializations (arrows) between the PGC process and
the adjacent cells.
Fig. 14. PGC which has been in culture for 10 days and is completely surrounded by
somatic cells of the substrate. The cells are lying on tissue culture plastic (P). Note
the overlapping somatic cell processes (S) covering the upper surface of the PGC.
Fig. 15. PGC fixed in vivo during its migration along the mesentery of stage-44
tadpole. Note the electron density of PGCcytoplasm in vivo, and the blunt process
(Pr) extending from the leading end of this PGC.
Fig. 16. Cell surface specialization between a germ cell (above) and mesentery
cell (below) fixed in vivo in a stage-44 tadpole. Its structure bears considerable
resemblance to that seen in vitro in Fig. 11.
Fig. 17. Cell surface specialization between germ cell (below) and mesentery cell
(above) fixed //; vivo in a stage-44 tadpole. Compare with Fig. 11.
