/ . Embryo/, exp. Morph. Vol. 59, pp. 1-17, 1980
Printed in Great Britain @ Company of Biologists Limited 1980
The migration of presumptive
primordial germ cells through the endodermal
cell mass in Xenopus laevis: A light and
electron microscopic study
ByMICHIKO KAMIMURA 1 , MINORU KOTANI 2
AND KENZO YAMAGATA 3
From the Department of Anatomy, Osaka City University Medical School,
Japan
SUMMARY
Presumptive primordial germ cells (pPGCs) were examined during migration from their
deep endodermal position to the endodermal crest in Xenopus laevis, using light and electron
microscopy with Epon sections, and several morphological characteristics of pPGCs, associated with their migration, were revealed. pPGCs displayed polymorphism, with smooth
contours. The intercellular space around the pPGCs was large and variable in width and
cytoplasmic processes from pPGCs were occasionally observed in it. It was shown quantitatively that pPGCs at the migratory stage had a tendency to move with the leading end,
towards which the nucleus was localized, dragging the germinal plasm behind. These polarized pPGCs were frequently associated with large intercellular spaces, both at their leading
and trailing ends. Cytoplasmic processes of polarizing pPGCs found in the large intercellular space at the leading end were conspicuous. Ultrastructurally, the nuclei of pPGCs
were euchromatic, and the nucleolus was prominent. The germinal plasm at the light microscope level corresponded to the cytoplasmic area near the nucleus where a large number of
mitochondria with well-developed cristae and most of the other organelles were aggregated.
Centrioles and centriole-associated microtubules observed in the aggregate were thought to
be important structures responsible for the cell polarization mentioned above. It was demonstrated quantitatively that the size of mitochondria in pPGCs was larger on average than that
of mitochondria in neighbouring somatic endodermal cells. Numerous irregularly shaped
small yolk platelets characterized pPGCs. These ultrastructural features suggested that
pPGCs were in an activated metabolic state. It was concluded that the migration of pPGCs
was attributable to active movement with high cell metabolism, causing the formation of
cell processes and intracellular polarization.
1
Author's address: Department of Anatomy, Kinki University School of Medicine,
Sayama-cho, Osaka 589, Japan.
2
Author's address: Department of Natural Science, Osaka Women's University, Daisencho, Sakai, Osaka 590, Japan.
3
Author's address: Department of Anatomy, Osaka City University Medical School,
Abeno-ku, Osaka 545, Japan.
2
M. KAMIMURA, M. KOTANI AND K. YAMAGATA
INTRODUCTION
The extragonadal origin of germ cells has been demonstrated in the embryos
of many vertebrate groups (e.g. Swift, 1914; Bounoure, 1934; Everett, 1943;
Witschi, 1948). Presumptive primordial germ cells (pPGCs) migrate from their
site of origin to the future gonadal ridge.
In the anuran Xenopus laevis, it is known that during the cleavage stages a
microscopically identifiable cytoplasm known as 'germinal plasm' becomes
included in some cells, which are differentiated into primordial germ cells
(PGCs) in the genital ridge (see reviews by Beams & Kessel, 1974; Blackler,
1966; 1970; Eddy, 1975). The locations of pPGCs at successive developmental
stages are summarized as follows, based on previous descriptive studies (Bounoure, 1934; Blackler, 1958; Whitington & Dixon, 1975; Kamimura, Ikenishi,
Kotani & Matsuno, 1976). pPGCs lie near the vegetal pole in the earliest
cleavage stages and are usually found in the lower part of the presumptive
endoderm at the blastula and early gastrula stages. Throughout the stages of
neurula and tail bud they are located deeply within the endodermal cell mass
below the archenteron cavity. Later they are found in the lateral, and then dorsal
parts of the endoderm. They leave the endoderm at the endodermal crest and
come to lie in the dorsal mesentery, and are finally located in the paired genital
ridges at the feeding tadpole stage.
pPGCs have been considered to be translocated passively from their position
near the vegetal pole to their deep endodermal location by shuffling of cells
during the cleavage stages and by the morphogenetic movements of gastrulation
(Whitington & Dixon, 1975). But the translocation from the deep endodermal
position to the endodermal crest seems to be due to their active movements
since there are not any known morphogenetic events during the relevant stages
which could account for the translocation (Blackler, 1958; Whitington &
Dixon, 1975; Kamimura et al. 1976).
The present study was attempted using sectioned materials, to find morphological evidence relating to the migratory mechanism of pPGCs in the
endodermal cell mass. The results suggested that the migration was attributable
to an active movement of pPGCs manifested by the formation of cell
processes, intracellular polarization and ultrastructural features implying a
high level of cell metabolism.
MATERIALS AND METHODS
Freshly laid fertilized eggs were obtained from X. laevis by injecting 200 and
300 i.u. of gonadotropic hormone (Teikoku Zoki Co.), respectively into sexually
matured males and females. Embryos were staged after Nieuwkoop and Faber
(1967).
Light microscopy. The serially sectioned materials stained with toluidine blue
Migration of germ cells in Xenopus
3
in the previous study (Kamimura et al. 1976) were used again for the observations in this study to obtain more detailed information concerning migration.
Embryos at stages 18, 28, 31, 33/34 and 35/36 were examined.
Electron microscopy. Specimens at stage 35/36 (migratory stage of pPGCs)
were fixed with Karnovsky fixative (Karnovsky, 1965) for 30 min at room
temperature. Then the cranial and the caudal body parts of the larvae were cut
off in the fixative with a razor blade and the trunk only was fixed with fresh
fixative for a further 2 h. Postfixation was with 2-0 % osmium tetroxide in
0-1 M cacodylate buffer at pH 7-4 in an ice-bath for 2 h. After fixation all specimens were embedded in Epon (Luft, 1961). Sections were cut on a PorterBlum MT-2B ultramicrotome. Thin sections were stained with uranyl acetate
and Millonig's lead procedure (Millonig, 1961), and were examined in a Hitachi
HS-8 electron microscope. Identification of pPGCs was done through the
inclusion of numerous mitochondria near the nucleus and this was further
confirmed by the existence of the 'germinal granules' (Williams & Smith, 1971),
i.e. the electron dense bodies considered to be confined to the pPGCs (Czolowska, 1972; Ikenishi & Kotani, 1975).
RESULTS
Light microscopy
The individual shape and apparent size of pPGCs in cross sections varied
considerably (Figs. \b, c, 2). Some were oval or rounded, whereas others
appeared elongated. All pPGCs observed in four or five embryos from the same
batch were examined for size at three developmental stages respectively, i.e.
54 pPGCs at stage 18, 58 pPGCs at stage 31 and 81 pPGCs at stage 35/36 (Table
1). The average major and minor diameters of pPGCs, with standard deviations,
were 60-8 ±14-1 jtim and 33-1 ±12-0 fim at stage 18, 42-8 ±9-4 /tm and
28-0 ±7-3 /on at stage 31 and 32-4 + 8-9 /an and 22-0 ±4-4 /on at stage
35/36. There was a gradual decrease in size of pPGCs during this developmental period.
Examination of serial sections showed that in a few cases a pPGC was
encircled almost completely by a neighbouring endoderm cell (Fig. 1 c).
Polarization of internal structure
The nuclei of pPGCs during migration were located in the centre (Fig. 2 a) or
an eccentric part of the cell (Fig. 2 b-f). The germinal plasm was located on one
side of the nucleus (Fig. 2 a, b,f) or around the nucleus (Fig. 2 e). The polarization of intracellular structures in pPGCs was examined quantitatively.
Because of the difficulty of expressing the degree of polarization in pPGCs
accurately, those with an extremely eccentric nucleus, as shown in Fig. 2 b, were
regarded as cells in which polarization must have taken place, and their
M. KAMIMURA, M. KOTANI AND K. YAMAGATA
Migration of germ cells in Xenopus
5
frequency of occurrence was examined at five developmental stages (Table 1).
Table 1 clearly shows that the percentage of pPGCs with very eccentric nucleus
is high in the migratory stages (stages 31, 33/34 and 35/36), but low before
migration (stages 18 and 28). On the other hand, a considerable number of
somatic endodermal cells, which were thought not to be migrating, also possessed an extremely eccentric nucleus in the peripheral region of the endodermal
cell mass (Fig. 1 a, d), where many pPGCs at the migratory stages were localized
(Kamimura et ah 1976). Nevertheless in that region the ratio of pPGCs with
very eccentric nucleus was higher than that of the endodermal cells. Therefore
the extremely eccentric location of the nucleus in pPGCs was considered at
least to be related to their migration.
Next, the direction towards which the nucleus was localized in the cell body
was examined in the pPGCs with extremely eccentric nucleus, to clarify the
relationship between the direction of polarization of pPGCs and their dorsalwards migration (Table 1). Cells with nucleus closest to the dorsal, lateral or
ventral surface were so classified. Cells with nucleus close to two or more sides
of the cell were labelled 'undecidable' (e.g. Fig. 2 / ) .
Table 1 demonstrated that at stages 33/34 and 35/36 during migration the
frequency of occurrence of pPGCs with 'dorsal' nuclei was significantly higher
than that of pPGCs with 'lateral' or 'ventral' nuclei. On the other hand, such
a tendency of unidirectional orientation of the internal structure was not noticed
in those somatic endoderm cells with extremely eccentric nuclei.
FIGURE 1
Keys to abbreviations in all figures: A, archenteron; C, centriole; EC, somatic endodermal cell; GC, presumptive primordial germ cell; GP, germinal plasm; L, Jipid
droplet; M, mitochondrion; N, nucleus; NO, nucleolus; P, pigment granule; S,
somite; Y, yolk platelet.
Light micrographs of 0-5-0-7 /tm Epon cross-sections from a tadpole at
migratory stage of pPGCs, stained with toluidine blue, (a) A pPGC (large arrow) is
located in the lateral region of the endoderm. pPGC can be distinguished from the
somatic endodermal cells by the possession of granular cytoplasm, i.e. 'germinal
plasm'. pPGC appears to be isolated from the surrounding endoderm cells owing
to the round shape and the existence of large intercellular space around itself.
Regarding the intracellular location of the nuclei of somatic endodermal cells, note
that cells with extremely eccentric nuclei are seldom observed in the deep endodermal region, but are occasionally found in the peripheral region of the endoderm
(small arrows), (b) Higher magnification of the pPGC shown in (a). The germinal
plasm is clearly seen near the nucleus, (c) A pPGC and a somatic endodermal cell
adjacent to the pPGC. Note that the somatic cell is modified and encircles the
pPGC almost completely with thin cytoplasmic flaps, (d) Higher magnification of
one of the somatic endodermal cells with extremely eccentric nucleus indicated by
small arrows in (a).
M. KAMIMURA, M. KOTANI AND K. YAMAGATA
Migration of germ cells in Xenopus
7
Large intercellular space and cytoplasmic processes
We reported previously that one of the morphological characteristics of
pPGCs during migration was that the intercellular space between pPGCs and
neighbouring endoderm cells was larger and of more irregular width than that
between somatic endodermal cells (Kamimura et al. 1976). In this study we
noticed that the wide spaces were frequently seen both at the cell side where the
eccentric nucleus was localized (Figs. 2 b, 3 a) and at the opposite side (Fig.
3 b, d). Among 58 pPGCs with extremely eccentric nucleus at stage 35/36, 44
pPGCs showed undoubtedly wide spaces on the cell surface near the nucleus
and/or at the opposite side, i.e. 20 pPGCs had a wide space at the cell side near
the nucleus, 4 pPGCs had it at the opposite side to the nucleus, and remaining
20 pPGCs had it at both the cell sides.
Cytoplasmic processes were rarely seen in pPGCs except the processes with
various lengths occasionally found in the large intercellular space near the
extremely eccentric nucleus (Fig. 3 a). Serial sectioning demonstrated that the
processes ordinarily gathered in a small region of the cell surface and did not
protrude into the narrow intercellular space between endoderm cells surrounding a pPGC, but usually into the large intercellular space between a pPGC and
a surrounding cell (Fig. 3a).
The wide space on the cell surface near the nucleus was also seen in the case of
somatic endodermal cells with extremely eccentric nucleus and the shape of the
space gave us the impression that it was formed by artificial retraction caused
by the fixation procedure used (Fig. la, d). However, comparing the spaces
related to somatic endodermal cells with those related to pPGCs, the latter were
usually larger than the former and the cytoplasmic processes seen in the latter
were seldom seen in the former. Therefore it was considered that the wide space
Fig. 2. Light micrographs of pPGCs during migration, showing various features of
pPGCs regarding the intracellular location of the nucleus and germinal plasm. The
dorsal side of the cell is directed upward in photographs, (a) The nucleus is located
in a central part of the cell. The germinal plasm is seen on one side of the nucleus.
(6) The nucleus is located in an eccentric part of the cell. As the nucleus is closely
adjacent to a cytomembrane of the cell, the nucleus appears to form a part of the
outline of the cell under the light microscope. The germinal plasm is located below
the nucleus, (c) The nucleus is located in an eccentric part of the cell. The nucleus is
closely adjacent to a cytomembrane of the cell over a small region (arrow). The
germinal plasm is not clear in this section, {d) The nucleus is located in an eccentric
part of the cell. Though the nucleus is adjacent to the cytomembrane, granular cytoplasm, one part of the germinal plasm, exists between the nucleus and the adjacent
cytomembrane. (e) The nucleus is located somewhat eccentrically. Yolk granules,
lipid droplets and granular cytoplasm exist between the nucleus and the adjacent
cytomembrane. Germinal plasm is seen around the nucleus. (/) The nucleus is
closely adjacent to the cytomembrane on two sides of the cell (arrows), i.e. both on
the dorsal and ventral sides of the cell.
(early migratory stage)
33/34
(migratory stage)
35/36
(migratory stage)
31
(just before migration)
28
(stationary stage)
18
Stage
58(71-6%)
81
4
13
10
31
1
2
35
10
3
2
Lateral
* All animals examined were obtained from the same batch except stage 28.
71 (64-0%)
18(31-0%)
111
58
11 (16-9%)
5
5
65
3
10(18-5%)
54
4
5
Dorsal
No. of pPGCs with
extremely eccentric
nucleus
No. of
pPGCs
examined
No. of
animals
A
13
14
7
4
3
Ventral
4
9
0
2
2
Undecidable
No. of pPGCs showing various localizations of eccentric
nuclei
Table 1. Relationship between the polarization ofpPGCs and their dorsalwards migration''
>
>
>
>
c
>
00
Migration of germ cells in Xenopus
Fig. 3. Light micrographs of pPGCs during migration, showing cytoplasmic processes of pPGCs and the large intercellular space around them. The dorsal side of the
cell is directed upward in photographs, (a) Cytoplasmic processes (arrows) are seen
in the large intercellular space near the very eccentric nucleus. Note that the processes protrude towards the large intercellular space between a pPGC and a surrounding cell, not towards the narrow intercellular space between endoderm cells
surrounding a pPGC. (b) Section through another part of the same pPGC in (a).
Large intercellular space is seen at the ventral side of the cell, (c) Large intercellular
spaces are seen on the lateral and ventral sides of the cell (arrows). The space may
have been formed by contraction of one part of the cell, (d) Large intercellular space
is seen on the ventral side of the cell. It appears that the space was formed by retraction of the cytoplasm having several contact points with the surrounding cells.
facing the cell surface near the nucleus might partly reflect an intrinsic condition of pPGCs, different from that of the somatic cells.
A general feature of the large intercellular space at the opposite side of
the cell body to the nucleus is shown in Fig. 3 b. We occasionally observed
10
M. KAMIMURA, M. KOTANI AND K. YAMAGATA
appearances which gave us the impression that this space was formed by contraction of one part of the cell (Fig. 3 c) or by retraction of the cytoplasm leaving
several contact points between the adjacent cells (Fig. 3d).
Electron microscopy
The nuclei of pPGCs displayed a somewhat irregular shape and ordinarily had
a low affinity for the electron-microscopic stains compared with somatic endodermal cell nuclei. The pPGC chromatin was finely granular and uniformly
dispersed, and heterochromatin was never observed (Figs. 4a, 5 a). Nucleoli
were prominent (Fig. 5 a).
The germinal plasm included a cluster of mitochondria with well-developed
cristae (Fig. 4 a, c). 1032 mitochondria observed in 9 sections of 7 different
pPGCs and 782 mitochondria from nearly 37 neighbouring somatic endodermal
cells in the same sections were measured for size. Since the mitochondria were
not spherical, diameters across the long and the short axes of each mitochondrion were examined. Charted results (Fig. 6) indicated that both long and short
diameters of mitochondria of pPGCs were larger respectively, on average, than
those of somatic endodermal cells.
Most other cytoplasmic organelles were concentrated within the area of the
mitochondrial aggregation. Centrioles (Fig. 4 b) and microtubules radiating
from the centriole region (Fig. 5b) were sometimes seen. Golgi apparatuses with
several stacks of parallel, flattened cisternae and small associated vesicles were
frequently observed (Fig. 5 c). pPGCs had markedly less granular endoplasmic
reticulum in comparison with the neighbouring somatic endodermal cells
(Fig. 4 c), which provided one aid for the identification of pPGCs at the electron
microscope level. Germinal granules (Williams & Smith, 1971) (Fig. 5d) and
pigment granules (Fig. 4a) were also observed in the area.
The cytoplasm of both pPGCs and somatic endodermal cells was stippled by
numerous electron dense particles, presumably free ribosomes and glycogen,
uniformly distributed throughout the ground substance (Fig. 4c). The cytoplasm
Fig. 4. Electron micrographs of pPGCs in cross section from a tadpole at stage
35/36. (a) A large number of mitochondria exist in aggregate near the euchromatic
nucleus and there are centrioles, Golgi apparatus and pigment granules. Many yolk
platelets and lipid droplets are seen in the peripheral region of the cell. Irregularly
shaped small yolk platelets characterise pPGC. (b) High power view of the area marked
in (a). Pair of centrioles are seen and many electron dense structures are seen around
them, (c) Micrograph showing cytoplasm of pPGC (left) and adjacent somatic endoderm cell (right). Note that the cristae of mitochondria of pPGCs are well developed
and the size of mitochondria in pPGCs is apparently larger than that of mitochondria
in somatic cells. Granular endoplasmic reticulum (arrow) is seen in the cytoplasm
of somatic cell, but not in that of pPGC. The cytoplasm of both cells is stippled by
numerous electron dense particles uniformly distributed. More particles are seen in
the cytoplasm of somatic cells than in that of pPGC.
Migration of germ cells in Xenopus
11
12
M. KAMIMURA, M. KOTANI AND K. YAMAGATA
Fig. 5. Electron micrographs showing characteristic structures of pPGCs at stage
35/36. (a) Irregularly shaped euchromatic nucleus. Note the conspicuous nucleolus.
(b) Microtubules (arrows) radiating from centriole region, (c) A well-developed
Golgi apparatus, id) Germinal granules (arrows), (e) Irregularly shaped yolk
granule internally subdivided. (/) Irregularly shaped yolk granule with internal cavities, (g) Round yolk granule with internal cavities.
Migration of germ ceils in Xenopus
13
A. Long diameter
40
I 30
g
£ 20
10
pPGC*
(01)
0-2
04
0-6 0-8
10
1-2
1-4
1-6
1-8
20
2-2 2-4
pPGC
(01)
2-6
2-
2-6
2-8
(Aim)
B. Short diameter
70
60
50
40
30
20
10
1
0-2
0-4
0-6 0-8
pPGC
(01)
i
1-0
1-2
1-4
1-6
1-8
2-0 2-2
2-4
Qmi)
Fig. 6. Frequency distribution of long and short diameters of mitochondria of
pPGCs and somatic endodermal cells. 1032 and 782 mitochondria of pPGCs and
somatic endodermal cells were examined respectively. Abscissa: diameter of mitochondria. Ordinate: relative frequency of mitochondria. * figures in parenthesis
mean % of mitochondria of pPGCs. D, somatic endodermal cells. • , pPGCs.
of pPGCs was usually sparser than that of neighbouring somatic endodermal
cells, possibly due to the high concentration of the particles present in the latter
(Fig. 4c).
pPGCs had a large quantity of yolk platelets and lipid droplets in their cytoplasm, as did somatic endoderm cells. In addition to the large yolk platelets with
elliptic shape, irregularly shaped small yolk platelets, as shown in Fig. 5e-g,
were present in pPGCs cytoplasm. Those small yolk platelets were more conspicuous in pPGCs cytoplasm than in that of somatic endodermal cells (Figs.
4 a, la-c), which seemed to be in the course of utilization.
2
EMB 59
14
M. KAMIMURA, M. KOTANI AND K. YAMAGATA
Migration of germ cells in Xenopus
15
Pseudopod-like protrusions (Fig. la-d) or filopodial processes (Fig. 7e, f)
were found in the large intercellular space between pPGCs and adjacent somatic
endodermal cells. The examination of serial sections of a pPGC with a pseudopod-like protrusion disclosed that the large intercellular space was formed
around the protrusion (Fig. la-d). The filopodial processes were variable in
both length and diameter (Fig. le,f). The range of the diameter of the processes
was 0-06-0-55 /*m.
DISCUSSION
Certain relationships between intracellular polarization and the direction of
migration have also been recognized in mouse PGCs (Clark & Eddy, 1975),
migrating mesenchymal cells of the chick embryo (Trelstad, Hay & Revel,
1967), and human neutrophils under chemotactic conditions (Maleck, Root &
Gallin, 1977). Maleck et al. (1977) commented as follows on the mechanism
of intracellular polarization in the human neutrophil: ' Microtubules (radiating
from centriole) modulate pseudopod formation by providing a cytoskeleton'
and' the radial array of microtubules may prevent the random drift of the nuclear
lobes by locking them into position on the side'. Though this explanation for
neutrophil polarization cannot be adopted directly for Xenopus pPGCs because
of the difference of the nuclear location in relation to the direction of migration,
the centriole found in the area of germinal plasm and its associated array of
microtubules may be involved in establishing the internal polarization of
Xenopus pPGCs.
The euchromatic nucleus, conspicuous nucleolus, numerous large sized
mitochondria with well-developed cristae and irregularly shaped small yolk
platelets of pPGCs seem to indicate high metabolic activity of the cell. The
mitochondria especially should be noted, because it is widely held that there are
positive correlations between the metabolic activity of a tissue and the number,
size and ultrastructure of mitochondria (e.g. Ghadially, 1975). It seems likely
that the large size of mitochondria of pPGCs compared with that of surrounding
somatic endodermal cells is concerned with the energy requirements for their
migration through the endodermal cell mass and may also correlate with yolk
utilization.
Fig. 7. Electron micrographs showing cytoplasmic processes of pPGCs. (a), (b), (c),
(d) Four transverse sections through the anterior and posterior part of the same
pPGC. The pPGC has no prominent processes and no conspicuous large intercellular space in (a). However, pseudopod-like process and large intercellular space
around it are seen in {b) and several thick processes (arrows) branching from the
process are seen in (c). In (d), a cross section of the thick processes in (c) is shown.
(e) Filopodial process from pPGC. (f) Cross section of processes from pPGC
(arrows). The diameter of the processes nearly corresponds to that of microvilli.
16
M. KAMIMURA, M. KOTANI AND K. YAMAGATA
We found in a few cases microfilaments, which have been considered to have
an important role in the movement of various cells, in filopodial processes of
pPGCs and in peripheral cytoplasm of somatic endodermal cells adjacent to
pPGC. Our observations in this study have not yet been sufficient to demonstrate the role of the microfilaments in migration of pPGCs. Further study is
required to accumulate data on microfilaments.
The problem of how pPGCs move through the endodermal cell mass is not
yet resolved. Furthermore very little is known about the intercellular relationship
between pPGCs and somatic endodermal cells except the observation that the
shape of the latter is frequently highly modified. Recently, the in vitro locomotion of pPGCs in X. laevis was examined by time-lapse cinematography (Heasman, Mohum & Wylie, 1977) and by electron microscope (Heasman & Wylie,
1978). They have reported that PGCs 'move by elongation, coupled with the
extrusion of filopodia (at their leading end), followed by waves of contraction,
and ultimately by retraction of the trailing end of the cell'. Although the stage
and environmental condition of germ cells examined are not identical in their
study and in the present one, the behaviour of PGCs observed by these authors
is in accord with the morphological events of pPGC migration presented here.
Based upon the above-mentioned discussions, the present results suggest: (1)
The polymorphism of pPGCs reflects the morphological transformation of
pPGCs under cell movement. (2) The existence of pPGCs showing various
features concerned with the intracellular location of the nucleus and germinal
plasm reflects the phase of cell movement accompanied by a change of internal
structure in the pPGCs. (3) The high ratio of pPGCs showing polarization
of internal structure and that of pPGCs whose nucleus is localized at the
dorsal part of the cell suggest that the polarization is a morphological feature
of their directed movement and that the leading end of pPGCs is the cell
part where the nucleus is localized. (4) Cytoplasmic processes at the leading
end may be responsible for forming the wide intercellular space around the
processes. (5) A large intercellular space at the opposite side to the nucleus
reflects the retraction of the trailing end. From these suppositions, pPGCs
are considered to progress by both the formation of cell processes at the
leading end and retraction of the trailing end, accompanying a change of internal
structure.
We are grateful to Dr Kenjiro Wake, Tokyo Medical and Dental University, for his useful
advice in the course of this study and for technical instruction in electron microscopy. We are
also thankful to Mr Yasuyoshi Nishio, Osaka City University Medical School, for his help
with the photography.
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(Received 13 November 1979, revised 31 March 1980)