Abstract
Maternal control of development begins with production of the oocyte during oogenesis. All of the factors necessary to complete oocyte maturation, meiosis, fertilization, and early development are produced in the transcriptionally active early oocyte. Active transcription of the maternal genome is a mechanism to ensure that the oocyte and development of the early embryo begin with all of the factors needed for successful embryonic development. To achieve the maximum maternal store, only one functional cell is produced from the meiotic divisions that produce the oocyte. The oocyte receives the bulk of the maternal cytoplasm and thus is significantly larger than its sister cells, the tiny polar bodies, which receive a copy of the maternal genome but essentially none of the maternal cytoplasm. This asymmetric division is accomplished by an enormous cell that is depleted of centrosomes in early oogenesis; thus, meiotic divisions in oocytes are distinct from those of mitotic cells. Therefore, these cells must partition the chromosomes faithfully to ensure euploidy by using mechanisms that do not rely on a conventional centrosome-based mitotic spindle. Several mechanisms that contribute to assembly and maintenance of the meiotic spindle in oocytes have been identified; however, none is fully understood. In recent years, there have been many exciting and significant advances in oogenesis, contributed by studies using a myriad of systems. Regrettably, I cannot adequately cover all of the important advances here and so I apologize to those whose beautiful work has not been included. This review focuses on a few of the most recent studies, conducted by several groups, using invertebrate and vertebrate systems, that have provided mechanistic insight into how microtubule assembly and meiotic spindle morphogenesis are controlled in the absence of centrosomes.
Highlights
During meiosis, the genome is duplicated without immediate cytokinesis, resulting in a cell with twice the number of chromosomes normally found in somatic cells, a 4N cell
The region occupied by the oocyte nucleus prior to germinal vesicle breakdown (GVBD) (NEBD) is defined as the animal pole; this is the side of the cell where the small polar bodies will be eliminated
Three pathways have been found to operate in cells without centrosomes: the Ras-like nuclear (Ran)/Importin pathway, the chromosome passenger complex (CPC) pathway, and the Augmin pathway; these pathways are not fully understood, and the extent to which each of these pathways contributes to spindle assembly and meiotic division within and across species is not known
Summary
The genome is duplicated without immediate cytokinesis, resulting in a cell with twice the number of chromosomes normally found in somatic cells, a 4N cell. Centrosomes serve as a source of new microtubules and associate with the mitotic spindle by a mechanism that RNAi and inhibitor studies indicate depends on Asp-mediated crosslinking and Dynein activity but not on Kinesin 14/Ncd[83,84,85,86] Taken together, these comparisons suggest that differential use of molecular motors seems to allow the meiotic spindle of oocytes to supply forces or activities that are supplied by the centrosome in mitotic cells. Local regulation of the activity of molecular motors with microtubule bundling functions seems to play a conserved role in generating opposing forces to provide for robust spindle assembly in the absence of centrosomes and possibly to provide insurance for equal chromosome segregation between the large oocyte and tiny polar bodies produced from meiotic division
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