Cleavage Plane Specification in C. elegans: How to Divide the Spoils

The two-step process of MA alignment is not confined to the germ lineage. The EMS blastomere also undergoes an asymmetric, differentiative division to produce the E blastomere, which is the clonal progenitor of the gut, and the MS blastomere, which gives rise to a variety of tissues including components of the pharynx (see Figure 2Figure 2). Prior to the division of EMS, a 90° rotational alignment of the centrosome–nucleus complex occurs. In contrast with the isolated P cell results discussed above, EMS in isolation does not undergo rotational alignment. However, when EMS is placed in contact with P2, one centrosome in EMS rotates toward the site of cell–cell contact (Goldstein 1995axGoldstein, B. J. Cell Biol. 1995; 129: 1071–1080Crossref | PubMedSee all ReferencesGoldstein 1995a). Thus, P2 acts to establish an axis of polarity in EMS. When EMS is placed in contact with two P2 cells, each EMS centrosome moves close to a point of P2 contact (Goldstein 1995axGoldstein, B. J. Cell Biol. 1995; 129: 1071–1080Crossref | PubMedSee all ReferencesGoldstein 1995a). This suggests that microtubule attachment sites, perhaps similar to those in the P cells, are induced to form at points of EMS–P2 contact. The attachment sites apparently function as they do in the P cells to pull a centrosome toward the site. Interestingly, PAR-3 is uniformly distributed around the cortex of EMS (Etemad-Moghadam et al. 1995xEtemad-Moghadam, B, Guo, S, and Kemphues, K.J. Cell. 1995; 83: 743–752Abstract | Full Text PDF | PubMed | Scopus (288)See all ReferencesEtemad-Moghadam et al. 1995). This finding suggests either that the model for how PAR-3 controls rotational alignment is incorrect or that rotational alignment in EMS is controlled differently than in the P cells.Interaction with P2 polarizes EMS and induces EMS to divide into developmentally distinct cells, with E (the gut precursor) next to P2. 6xGoldstein, B. J. Cell Biol. 1995; 129: 1071–1080Crossref | PubMedSee all References, 7xGoldstein, B. Development. 1995; 121: 1227–1236PubMedSee all References showed that MA alignment and induction of gut are separable events. For rotation to occur, EMS has to contact P2 in a critical time period 9–10 min before EMS cleaves. If P2 and EMS contact is established after this time, the orientation of the MA is random, but gut induction can still occur if the division axis is such that the cleavage furrow does not pass through the point of EMS–P2 contact (i.e., orthogonal to the normal situation). These observations suggest that contact with P2 induces a segregation event, either of gut-specifying factors to the future E side of EMS or of gut-inhibiting factors to the future MS side of EMS. Contact with P2 also controls alignment of the MA, assuring that the segregated factors are passed to the appropriate daughter cell.In conclusion, rotational alignment of the MA is associated with the differentiative divisions of the early embryo of C. elegans. It appears to be a microtubule-based mechanism whereby the MA becomes oriented along a preformed axis of polarity. In the case of the P lineage, this axis appears to be intrinsically defined, whereas in the case of EMS, it is defined by a specific cell contact. In most cases, actin-based mechanisms are needed to establish polarity. Polarized distributions of the cell fate determinants Numb and Prospero have been seen prior to cell division in Drosophila (Kroblich et al. 1995xKroblich, J.A, Jan, L.Y, and Jan, Y.N. Nature. 1995; 377: 624–627Crossref | PubMedSee all ReferencesKroblich et al. 1995), as has Notch1 in mitotic cells in the developing mammalian cerebral cortex (Chenn and McConnell 1995xChenn, A and McConnell, S.K. Cell. 1995; 82: 631–641Abstract | Full Text PDF | PubMed | Scopus (545)See all ReferencesChenn and McConnell 1995). It will be interesting to see whether functionally similar rotational alignment mechanisms operate in such systems to ensure the partitioning of these determinants to the appropriate daughters.