At least one polar body, almost invariably the second, persists intact to the early blastocyst stage in nearly two-thirds of mouse conceptuses of the PO strain. The distribution in early blastocysts of these surviving polar bodies was highly non-random. Most not only lay in the mid-region of the embryonic-abembryonic axis but, on discovering that early blastocysts are bilaterally rather than radially symmetrical about this axis, were found to align with the bilateral axis. Cell marking experiments failed to detect movement of polar bodies relative to the surface of the conceptus during either cleavage or blastulation. That the distribution of degenerating polar bodies and their presumed debris was similar to intact ones also argued against their motility, as did the finding that at all stages second polar bodies were attached to conceptuses by a thin, extensible, weakly elastic ‘tether’. Although the transfer of small fluorochromes between them was rarely observed beyond second cleavage, the second polar body and conceptus could remain coupled ionically up to the blastocyst stage. It is concluded that the second polar body normally remains attached to the conceptus through persistence of the intercellular bridge formed during its abstriction, and therefore provides an enduring marker of the animal pole of the zygote. Hence, according to the distribution of polar bodies, the axis of bilateral symmetry of the early blastocysts is normally aligned with the animal-vegetal axis of the zygote and its embryonic-abembryonic axis is orthogonal to it. Such relationships suggest that, at least in undisturbed development, specification of the axes of the blastocyst depends on spatial patterning of the zygote.

REFERENCES

Bellairs
R.
,
Bancroft
M.
(
1975
)
Mid-bodies and beaded threads.
Am J. Anat
143
,
393
398
Calarco
P. G.
(
1995
)
Polarization of mitochondria in the unfertilized mouse oocyte.
Dev. Genet
16
,
36
43
Cameron
R. A.
,
Hough-Evans
B. R.
,
Britten
R. J.
,
Davidson
E. H.
(
1987
)
Lineage and fate of each blastomere of the eight-cell sea urchin embryo.
Genes Dev
1
,
75
84
Champlin
A. K.
,
Dorr
D. L.
,
Gates
A. H.
(
1973
)
Determining the stage of the estrous cycle in the mouse by the appearance of the vagina.
Biol. Reprod
8
,
491
494
Copp.
A. J.
(
1978
)
Interaction between inner cell mass and trophectoderm of the mouse blastocyst. I. A study of cellular proliferation.
J. Embryol. Exp. Morph
48
,
109
125
Gardner
D. K.
,
Sakkas
D.
(
1993
)
Mouse embryo cleavage, metabolism and viability: role of medium composition.
Hum. Reprod
8
,
288
295
Gardner.
R. L.
,
Meredith
M. R.
,
Altman
D. G.
(
1992
)
Is the anterior-posterior axis of the fetus specified before implantation in the mouse?.
J. Exp. Zool
264
,
437
443
Goodall
H.
(
1986
)
Manipulation of gap junctional communication during compaction of the mouse early embryo.
J. Embryol. Exp. Morph
91
,
283
296
Goodall
H.
,
Johnson
M. H.
(
1984
)
The nature of intercellular coupling within the preimplantation mouse embryo.
J. Embryol. Exp. Morph
79
,
53
76
Goodall
H.
,
Maro
B.
(
1986
)
Major loss of junctional coupling during mitosis in early mouse embryos.
J. Cell Biol
102
,
568
574
Gulyas
B.
(
1975
)
A reexamination of cleavage patterns in eutherian mammalian eggs: rotation of blastomere pairs during second cleavage in the rabbit.
J. Exp. Zool
193
,
235
248
Howlett
S. K.
,
Bolton
V. N.
(
1985
)
Sequence and regulation of morphological and molecular events during the first cycle of mouse embryogenesis.
J. Embryol. Exp. Morph
87
,
175
206
Jones-Seaton
A.
(
1950
)
Etude de l'organisation cytoplasmic de l'oeuf de Rongeurs principalement quant a la basophile ribonucleique.
Arch. Biol
61
,
291
444
Karnovsky
M. J.
(
1965
)
A formaldehyde-glutaraldehyde fixative of high osmolarity for use in electron microscopy.
J. Cell Biol
27
,
137
–.
McCain
E. R.
,
McClay
D. R.
(
1994
)
The establishment of bilateral symmetry in sea urchin embryos.
Development
120
,
395
404
Mullins
J. M.
,
Biesele
J. J.
(
1977
)
Terminal phase of cytokinesis in D-985 cells.
J. Cell Biol
73
,
672
684
Nichols
J.
,
Gardner
R. L.
(
1989
)
Effect of damage to the zona pellucida on development of preimplantation embryos in the mouse.
Hum. Reprod
4
,
180
187
Smith
L. J.
(
1980
)
Embryonic axis orientation in the mouse and its correlationwith blastocyst relationships to the uterus. I. Relationships between 82 hours and 4 1/4 days.
J. Embryol. Exp. Morph
55
,
257
277
Smith
L. J.
(
1985
)
Embryonic axis orientation in the mouse and its correlation with blastocyst relationships to the uterus. II. Relationships from 4 1/4 to 9 1/2 days.
J. Embryol. Exp. Morph
89
,
15
35
Tarkowski
A. K.
,
Wroblewska
J.
(
1967
)
Development of blastomeres of mouse eggs isolated at the 4-and 8-cell stage.
J. Embryol. Exp. Morph
18
,
155
180
Wilson
I. B.
,
Bolton
E.
,
Cuttler
R. H.
(
1972
)
Preimplantation differentiation in the mouse egg as revealed by microinjection of vital markers.
J. Embryol. Exp. Morph
27
,
467
479
Zamboni
L.
(
1970
)
Ultrastructure of mammalian oocytes and ova.
Biol. Reprod. Suppl.
2
,
44-63
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