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Sperm nuclei entering parthenogenetically activated mouse oocytes before the first mitosis transform into pronuclei an ultrastructural study.

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THE ANATOMICAL RECORD 243:516-518 (1995)
Sperm Nuclei Entering Parthenogenetically Activated Mouse
Oocytes Before the First Mitosis Transform Into Pronuclei
AN ULTRASTRUCTURAL STUDY
MAREK MALESZEWSKI
Department of Embryology, Institute of Zoology, University of Warsaw, Warsaw, Poland
ABSTRACT
Background: This report is an extension of previous observations (Maleszewski 1992. Mol. Reprod. Dev., 33:215-221) on the behavior
of mouse sperm nuclei incorporated into parthenogenetically activated
mouse oocytes prior to the first cleavage division and undergoing transformation during mitosis.
Method: Artificially activated mouse oocytes were inseminated in vitro
and an ultrastructural analysis was performed of sperm-derived nuclei
present in two parthenogenetic two-cell embryos.
Results: Both chromatin and nuclear envelope of sperm derived-nuclei
are structurally identical with those of oocyte-derived nuclei and of the
nuclei of blastomeres of normal two-cell embryos.
Conclusions: Cytoplasm of the parthenogenote during the first mitotic
division has the ability to transform sperm nucleus into a male pronucleus
just like the cytoplasm of a metaphase I1 oocyte. Q 1995 Wiley-Liss, Inc.
Key words: Fertilization, Sperm Remodelling, Parthenogenetic Embryo,
Mitosis, Mouse
The ability of a mammalian oocyte to transform the
sperm nucleus into a pronucleus varies according to the
developmental stage of the oocyte (Usui and Yanagimachi, 1976; Balakier and Tarkowski, 1980; Borsuk
and Tarkowski, 1989; Szollosi et al., 1990, 1994).
Breakdown of the sperm nuclear envelope (NE) is one
of many steps required for remodeling of the sperm
nucleus during fertilization (Yanagimachi, 1994).
Breakdown of sperm NE takes place during M phase
(Szollosi et al., 1990; Maleszewski, 1992) and also during the transition period to the zygote interphase (Szollosi et al., 1994). In a previous paper Maleszewski
(1992) demonstrated that when mouse oocytes are activated by ethanol and inseminated during the second
half of the first embryonal cell cycle (between 9 and 13
hours after the onset of activation), sperm nuclei remain condensed until the first mitosis. During the first
mitotic M phase, sperm nuclei decondense, subsequently recondense, and are passively displaced into the
daughter blastomeres. In the two-cell embryos, sperm
nuclei form interphase nuclei that are able to replicate
DNA and to condense into discrete chromosomes during
the M phase of the following division. These observations suggest that the cytoplasm of the parthenogenote
during the first mitotic division has the ability to transform the sperm nucleus into a male pronucleus just like
the cytoplasm of metaphase I1 oocytes.
The present electron microscopic study gives data
confirming suggestions coming from a light microscopic study and reports some new information.
nol, and then freed of zonae pellucidae (Maleszewski,
1992). Between 9 and 13hours after ethanol activation,
zona-free oocytes were inseminated in vitro (Maleszewski, 1992). Three hours after the completion of the first
cleavage, two-cell embryos were examined under a n
inverted microscope equipped with Nomarski interference optics. Embryos containing sperm-derived nuclei
were fixed and processed for electron microscopic observations a s previously described (Szollosi et al.,
1994).
MATERIALS AND METHODS
Received June 5, 1995; accepted July 25, 1995.
Address reprint requests to Marek Maleszewski, Dept. of Embryology, Inst. of Zoology, University of Warsaw, 00-927 Warsaw 64 Poland.
Metaphase I1 oocytes from F1 (C57BlxCBA/H) females were collected, artificially activated with etha0 1995 WILEY-LISS, INC.
RESULTS AND DISCUSSION
Ultrastructural examination was performed on two
parthenogenetic two-cell embryos that were inseminated before the first cleavage division and were fixed
3 hours after completion of cleavage. In both embryos
one blastomere had only one nucleus, while the other
contained two nuclei (Fig. 1A). In one embryo we could
identify the sperm-derived nucleus because sperm tail
components associated with the implantation fossa
(structure that attaches sperm tail to the head; Fawcett, 1975) were in close proximity to its NE (Fig. 1B).
Furthermore, part of the sperm tail was found in proximity to the mid-body, inside the cytoplasmic bridge
that connected the blastomeres (not shown).
We found that both nuclei in one blastomere and the
SPERM NUCLEI IN CLEAVING PARTHENOGENETIC EGGS
Fig. 1. A: Two nuclei present in one of the blastomeres of a two-cell
parthenogenetic embryo. x 4,000. B Fragment of the NE of spermderived nucleus. Implantation fossa (IF) is still visible in proximity to
NE. BL, bleb. x 40,000. C: Portion of the NE of the sperm-derived
nucleus with nuclear lamina (NL) associated with the internal mem-
517
brane of NE. Arrowhead, nuclear pore. x 67,500. D Nucleolus precursor body (NPB) in the sperm-derived nucleus has chromatin (CHI
associated with its surface. In the fragment of NE are visible bleb (BL)
and nuclear pore (arrowhead). x 37,500. E: Interchromatin granules
(IG) present in the sperm-derived nucleus. x 36,000.
518
M. MALESZEWSKI
solitary nucleus in the second had uniformly distributed nuclear pores throughout the entire NE (Fig.
lC,D). The internal membrane of the NE formed blebs
(Fig. lB,D) similar to control embryos. Filamentous
material (probably nuclear lamina) was associated
with the internal membrane of the NE (Fig. 1C). Several nucleolus-precursor bodies (NPBs) (Kopecny et al.,
1989; Biggiogera et al., 1994) in the form of typical
densely packed masses of fibrillar material were detected in all nuclei (Fig. lA,D). Some partially condensed chromatin was also associated with the surface
of these inactive NPBs (Fig. 1D). Granular structures
(interchromatin granules, perichromatin granules, and
large granules typical of pronuclei and early blastomere nuclei) (Fakan and Odartchenko, 1980) were
present in every nucleus (Fig. 1E).
The presence of nuclear lamina and nuclear pores
throughout the entire surface of the NE distinguishes
the NE of the male pronucleus from sperm head NE,
which is almost entirely devoid of nuclear pores and
lamins (Eddy and O’Brien, 1994). Blebing of the inner
leaflet of the NE was described in pronuclei and in the
nuclei of two-cell embryos at the beginning of the second cell cycle (Szollosi and Szollosi, 1988).The presence
of blebs in our experimental sperm-derived nuclei demonstrates that they can resume this activity following
their remodeling in the parthenogenote during the first
mitotic M phase.
The above structural observations of sperm-derived
nuclei strongly suggest that sperm nucleus transformation in the cytoplasm of parthenogenetic mitotic embryos corresponds to the steps of the remodeling of
sperm nucleus during normal fertilization. These steps
are: breakdown of the original NE, sperm chromatin
decondensation followed by a period of recondensation,
formation of the new NE, chromatin decondensation,
and formation of nucleolus precursor bodies (Adenot et
al., 1991; Biggiogera et al., 1994; Yanagimachi, 1994).
Sperm chromatin can decondense under certain conditions in its original NE (Borsuk and Tarkowski, 1989;
Borsuk, 1991, Szollosi et al., 1994). In such cases recondensation of the chromatin does not take place and
the nuclei develop abnormally (Szollosi et al., 1994). It
has already been observed by Maleszewski (1992) by
light microscopy that chromatin of the sperm nuclei
entering parthenogenote at the end of the first embryonal cell cycle decondenses and recondenses, suggesting the removal of its original NE. Thus, the cytoplasm
of the mouse parthenogenote during its first mitotic
division has all the abilities to remodel freshly entered
sperm nucleus into active pronucleus [formation of a
new NE and its blebing (present paper); DNA synthesis
and ability of sperm-derived chromatin to condense
into discrete chromosomes during the M phase of the
following division (Maleszewski 1992)], as it does in the
cytoplasm of mature oocytes during normal fertilization.
ACKNOWLEDGMENTS
I a m grateful to Dr. Maria S. Szollosi for her invaluable help during the experimental work and the manuscript preparation. I wish to thank Professor A.K.
Tarkowski for his interest and helpful advice and Professor Ryuzo Yanagimachi for his valuable comments
on the manuscript. This work was partly financed by a
grant from the State Committee for Scientific Research
(no. 6.6401.91.02) and was carried out within the
framework of French-Polish Scientific Exchange Programme (ATP 12). A WHO Small Institutional Grant
to the Department of Embryology is gratefully acknowledged.
LITERATURE CITED
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Dynamics of paternal chromatin changes in live one-cell mouse
embryo after natural fertilization. Mol. Reprod. Dev., 28:23-34.
Balakier, H., and A.K. Tarkowski 1980 The role of germinal vesicle
karyoplasm in the development of male pronucleus in the mouse.
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Biggiogera, M., T.E. Martin, J. Gordon, F. Amalric, and S. Fakan
1994 Physiologically inactive nucleoli contain nucleoplasmic ribonucleoproteins: Immunoelectron microscopy of mouse spermatids and early embryos. Exp. Cell. Res., 213:55-63.
Borsuk, E. 1991 Anucleate fragments of parthenogenetic eggs and of
maturing oocytes contain complementary factors required for development of male pronucleus. Mol. Reprod. Dev., 29:150-156.
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Eddy, E.M., and D.A. OBrien 1994 The spermatozoon. In: The Physiology of Reproduction. E. Knobil and J.D. Neill, ed. Raven Press,
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Fakan, S., and N. Odartchenko 1980 Ultrastructural organization of
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Fawcett, D.W. 1975 The mammalian spermatozoa. Dev. Biol., 44:
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Kopecny, V., J.E. Flechon, S. Camous, and J . Fulka Jr. 1989 Nucleologenesis and the onset of transcription in the eight-cell bovine
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Maleszewski, M. 1992 Behavior of sperm nuclei incorporated into parthenogenetic mouse eggs prior to the first cleavage division. Mol.
Reprod. Dev., 33:215-221.
Szollosi, M.S., and D. Szollosi 1988 ‘Blebing’of the nuclear envelope of
mouse zygotes, early embryos and hybrid cells. J . Cell. Sci., 91:
257-267.
Szollosi, D., M.S. Szollosi, R. Czolowska, and A.K. Tarkowski 1990
Sperm penetration into immature mouse oocytes and nuclear
changes during maturation. An EM study. Biol. Cell, 69.53-64.
Szollosi, M.S., E. Borsuk, and D. Szollosi 1994 Relationship between
sperm nucleus remodelling and cell cycle progression of fragments of mouse parthenogenotes. Mol. Reprod. Dev., 37:146-156.
Usui, N., and R. Yanagimachi 1976 Behaviour of hamster sperm nuclei incorporated into eggs at various stages of maturation, fertilization and early development. J . Ultrastruct. Res., 57:276288.
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pronucleus, nuclei, mitosis, oocytes, mouse, sperm, entering, parthenogenetically, ultrastructure, first, transform, stud, activated
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