I think polar bodies are pretty cute. These little nubbins are products of meiotic division, and a simple testament to how amazing and clever a dividing cell (an oocyte, in this case) can be. Sure, polar bodies aren’t around for long, but I thank my own two long-lost polar bodies that let me have enough nutrients to survive. I hardly knew you, Atticus and Grover. Today’s image is from a paper describing anaphase in mouse oocyte divisions.
Meiosis is a special type of cell division that produces eggs and sperm. In mice, the meiotic spindle in the developing egg, or oocyte, is small and positioned in a very asymmetric location. This helps ensure that the nutrient- and organelle-rich cytoplasm stays with the daughter cell (the egg) that will later be fertilized and support early embryonic development. The other daughter of the division is the polar body, a small round structure that eventually is degraded. A recent paper describes results showing the sequence of anaphase events during mouse meiosis. In most cases of cell division, chromosome separation during anaphase is achieved by the shortening of kinetochore microtubules (termed anaphase A) and the lengthening of the entire meiotic spindle (anaphase B). In most cell divisions, anaphase A precedes anaphase B, yet Greg FitzHarris has shown that the reverse is true in mouse oocytes. Early anaphase B helps to determine the final size of the polar body. In addition, this early anaphase B spindle lengthening is triggered by the loss of tension on kinetochore microtubules, which occurs when cohesion between sister chromatids is lost. The images above are timepoints of anaphase in a mouse oocyte, with microtubules (grey) and chromosomes (green) labeled.
FitzHarris, G. (2012). Anaphase B Precedes Anaphase A in the Mouse Egg Current Biology, 22 (5), 437-444 DOI: 10.1016/j.cub.2012.01.041
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