Polar bodies are commonly used for preimplantation genetic diagnosis in human embryos. During egg development, polar bodies are extruded to expel the extra DNA made during Meiosis and to form a haploid oocyte. In contrast to the well-studied mechanisms of polar body extrusion, their fate after extrusion is unknown in any system.
We studied the fate of polar bodies using time-lapse imaging of fluorescent reporter strains and a panel of C. elegans mutants. We found that the first polar body becomes trapped between eggshell layers and persists until hatching. In contrast, the second polar body is internalized in a stereotyped manner by one of the anterior AB daughter cells around the 4-cell stage. Prior to internalization, actin and the phagocytic receptor CED-1 are enriched around the second polar body, consistent with formation of a phagocytic cup. CED-1 persists around the phagosome after internalization and is required for polar body internalization. Other CED-10-dependent engulfment pathway proteins, such as CED-2, are also required, indicating that the polar body is internalized via receptor-mediated phagocytosis.
As part of a candidate screen, we also discovered that the second polar body was rarely internalized in PI3K-deficient vps-34 mutants or in rab-5 RNAi-treated embryos. The phagocytic receptor CED-1 is known to be localized to the plasma membrane via retromer-mediated recycling, which has been shown to require PI3K function. Furthermore, PI3K localization is known to depend on the small GTPase Rab5. Therefore, we examined CED-1 localization and discovered that CED-1 is no longer found around the second polar body in vps-34 mutants and in rab-5-depleted embryos, explaining the observed defects in polar body internalization.
Thus, embryos internalize the second polar body via receptor-mediated phagocytosis, showing that the polar body signals to neighboring cells. This observation raises the possibility that polar bodies have a function after extrusion, which could indicate that polar body removal for genetic testing may not be non-invasive.