title: Induction of cytoplasmic polarity in heterokaryons of mouse 4-cell-stage blastomeres fused with 8-cell- and 16-cell-stage blastomeres.
abstract: Cell surface and cytoplasmic polarity is exhibited by the blastomeres of mouse preimplantation embryos following compaction at the 8-cell stage of cleavage. It has been hypothesized that cytoplasmic polarity is initiated by plasma membrane functions of polar blastomeres that are absent from apolar blastomeres. To test this hypothesis the plasma membranes of "test" polar and apolar 8-cell- and 16-cell-stage blastomeres were inserted into the plasma membrane of "carrier" 4-cell-stage blastomeres by polyethylene glycol-mediated fusion of carrier-test blastomere pairs. After a 4-hr culture period each heterokaryon was scored for the distribution of two marker organelles--lipid droplets and nuclei--with respect to their proximity to the plasma membrane insert from the test blastomere. Plasma membrane inserts from polar test blastomeres were identified by labeling their apical domains with fluorescently tagged (succinylated) concanavalin A. The incidence of polar heterokaryons (those exhibiting a discrete fluorescently labeled area of plasma membrane corresponding to the apical domain inherited from the test blastomere) was 55/85 (69%) and 48/79 (61%) for 8-cell-stage and 16-cell-stage test blastomeres, respectively. In all polar heterokaryons, both nuclei were subjacent to the fluorescent label (apical domain of a polar plasma membrane insert), while the majority of lipid droplets resided in the hemisphere opposite the fluorescent label. In all 61 apolar heterokaryons examined (those lacking a discrete fluorescently labeled plasma membrane area) both nuclei were centrally located and lipid droplets were randomly distributed. These observations are consistent with the hypothesis that cytoplasmic polarity can be initiated by properties that distinguish the plasma membranes of polar blastomeres from those of apolar blastomeres.
authors.affiliation: Department of Obstetrics and Gynecology, University of California, Davis 95616.
authors.forename: L M
authors.initials: LM
authors.lastname: Wiley
authors.forename: M F
authors.initials: MF
authors.lastname: Obasaju
mesh_terms: D000818:Animals; D001757:Blastomeres / Q000166:cytology*; D002451:Cell Compartmentation; D002459:Cell Fusion; D002462:Cell Membrane / Q000502:physiology; D002467:Cell Nucleus / Q000648:ultrastructure; D002970:Cleavage Stage, Ovum / Q000166:cytology*; D003593:Cytoplasm / Q000648:ultrastructure; D050356:Lipid Metabolism; D051379:Mice; D009028:Morula / Q000166:cytology*