Developmental potential of parthenogenetic cells: role of genotype-specific modifiers

DNA methylation and imprinting: why bother?

DNA methylation is crucial for mammalian development because embryos that cannot maintain normal methylation levels die after gastrulation. I propose that DNA methylation is only important for the somatic lineages, but has no role in embryonic lineages including the germ line. Among vertebrates, genomic imprinting is found only in mammals, and numerous hypotheses have ascribed an essential function to imprinting because of the uniquely mammalian developmental and physiological requirements. However, our understanding of molecular details of the imprinting process, as well as evolutionary considerations, is rather consistent with imprinting having no intrinsic role in mammalian development.

Primate brain evolution: genetic and functional considerations

Functionally distinct regions of the brain to which maternal and paternal genomes contribute differentially (through genomic imprinting) have developed differentially over phylogenetic time. While certain regions of the primate forebrain (neocortex, striatum) have expanded relative to the rest of the brain, other forebrain regions have contracted in size (hypothalamus, septum). Areas of relative expansion are those to which the maternal genome makes a substantial developmental contribution. This may be significant with respect to the importance of primate forebrain expansion in the development of complex behavioural strategies and the way in which these are deployed, especially by the matriline. In many primate societies the maintenance of social cohesion and group continuity over successive generations is dependent on the matriline, with high ranking females producing high ranking daughters that stay within the group. Regions of relative contraction are those to which the paternal genome makes a differential contribution and these are target areas for gonadal hormones, which is congruent with the diminished role for gonadal hormones in the emancipation of primate reproductive behaviour.

Stage-specific and cell type-specific aspects of genomic imprinting effects in mammals

Recent studies have revealed that maternal and paternal alleles of some imprinted genes are differentially expressed from the earliest time of expression, with virtually no expression from one of the two alleles, while for other imprinted genes the normally silent allele can be transcribed during early development. In addition, a number of imprinted genes manifest their imprints only in select tissues. These observations indicate that the marks that denote parental chromosome origin need not directly determine allele expression, but rather bias later epigenetic modifications toward a particular allele. Thus, factors expressed at specific stages or in specific cell types are required to silence one parental allele or another. Stage-dependent and tissue-specific epigenetic modifications include the progressive establishment of the mature adult parental allele-specific DNA methylation patterns. These changes resemble and may share a common mechanistic basis with other epigenetic modifications that occur during development. Understanding the mechanisms by which these post-fertilization epigenetic modifications are mediated and regulated will be essential for understanding how genomic imprinting leads to differences in parental allele expression.

Tissue specific loss of proliferative capacity of parthenogenetic cells in fetal mouse chimeras

Parthenogenetic cells are lost from fetal chimeras. This may be due to decreased proliferative potential. To address this question, we have made use of combined cell lineage and cell proliferation analysis. Thus, the incorporation of bromodeoxyuridine in S-phase was determined for both parthenogenetic and normal cells in several tissues of fetal day 13 and 17 chimeras. A pronounced reduction of bromodesoxyuridine incorporation by parthenogenetic cells at both developmental stages was only observed in cartilage. In brain, skeletal muscle, heart and intestinal epithelium, this reduction was either less pronounced or observed only at one of the developmental stages analysed. No difference between parthenogenetic and normal cells was observed in epidermis and ganglia. Our results show that a loss of proliferative potential of parthenogenetic cells during fetal development contributes to their rapid elimination in some tissues. The analysis of the fate of parthenogenetic cells in skeletal muscle and cartilage development demonstrated different selection mechanisms in these tissues. In skeletal muscle, parthenogenetic cells were largely excluded from the myogenic lineage proper by early post-midgestation. In primary hyaline cartilage, parthenogenetic cells persisted into adulthood but were lost from cartilages that undergo ossification during late fetal development.

Bovine parthenogenetic blastocysts following in vitro maturation and oocyte activation with ethanol

The appropriate in vitro bovine oocyte maturation and ethanol activation conditions for preimplantation bovine embryo parthenogenetic development to the blastocyst stage were investigated. A 7% ethanol concentration significantly enhanced (P<0.05) the proportion of activated, in vitro-matured bovine oocytes (7% ethanol, 83.4 +/- 3.2% versus 0% ethanol, 63.9 +/- 2.0%). The proportion of activated oocytes was significantly higher (P<0.05) by treatment with 7% ethanol for a minimum of 2 minutes (2 minutes, 89.8 +/- 4.0% versus 0.5 minutes 63.4 +/- 4.9%). Oocyte maturation for periods ranging from 30, 34, 38 and 44 hours resulted in a significant increase (P<0.05) in the proportion of activated oocytes, and in oocytes displaying 2 or 3 pronuclei versus oocytes matured for 26 hours. The proportion of cleaved, activated oocytes (2-cell stage), 4 -cell stage and parthenogenetic morula/blastocysts was significantly higher (P<0.05) within the 34-hour oocyte maturation treatment group. Although the 44-hour oocyte maturation treatment group displayed the highest proportion of activated oocytes with 2 pronuclei, it did not display the highest cleavage frequency, possibly due to the effects of postovulatory aging. Several morphologically normal parthenogenetic bovine blastocysts developed from oocytes that were in vitro matured for 34 hours. The ability to produce such parthenogenetic embryos will eventually facilitate investigation into the role(s) of the maternal and paternal genomes during bovine early development.

Site of action of imprinted genes revealed by phenotypic analysis of parthenogenetic embryos

The phenotypes of early post-implantation parthenogenetic embryos were examined. The spectrum of phenotypes suggested that three stages are adversely affected by imprinting--implantation, pregastrulation, and postgastrulation. Survival of parthenogenetic embryos past these developmental blocks can be improved but not completely overcome by experimental asynchrony. These results suggest that imprinting may be "leaky" at early stages.

Parthenogenetic stem cells in postnatal mouse chimeras

The ability of parthenogenetic (pg) cells to contribute to proliferating stem cell populations of postnatal aggregation chimeras was investigated. Using DNA in situ analysis, pg participation was observed in highly regenerative epithelia of various regions of the gastrointestinal tract, e.g., stomach, duodenum and colon, in the epithelia of tongue and uterus and in the epidermis. Pg cells also contributed to the epithelium of the urinary bladder, which is characterized by a relatively slow cellular turnover. Using a sensitive proliferation marker to determine division rate of pg and normal (wt) cells in tissues of a 24-day-old chimera, no significant differences between pg and fertilized cells were observed. However, in colon and uterus of a pg &lt;==&gt; wt chimera aged 101 days, a significant loss of proliferative capacity of pg cells was found. In the colon, this loss of proliferative potential was accompanied by an altered morphology of pg crypts. In general, they were situated at the periphery of the epithelium and lacked access to the lumen, with consequent cystic enlargement and flattened epithelium. No obvious morphological changes were observed in the pg-derived areas of the uterine epithelium of this chimera. Our results provide evidence that pg cells can persist as proliferating stem cells in various tissues of early postnatal chimeras. They suggest that pg-derived stem cells may cease to proliferate in restricted areas of the gastrointestinal tract and in the uterine epithelium of pg &lt;==&gt; wt chimeras of advanced age.(ABSTRACT TRUNCATED AT 250 WORDS)