Asymmetric Cell Division - Asymmetric Cell Division in C. Elegans

Asymmetric Cell Division in C. Elegans

In C. elegans, a series of asymmetric cell divisions in the early embryo are critical in setting up the anterior/posterior, dorsal ventral, and left/right axes of the body plan. After fertilization, events are already occurring in the one cell stage embryo to allow for the first asymmetric cell division. This first division produces two distinctly different blastomeres, termed AB and P1. When the sperm fertilizes the egg, the sperm nucleus and centrosomes are deposited within the egg, which causes a cytoplasmic flux resulting in the movement of the sperm pronucleus and centrosomes towards one pole. The centrosomes deposited by the sperm seem to be responsible for the establishment of the posterior pole within the one cell embryo. Studies have shown that the pole in which the sperm-derived centrosomes reside always becomes the posterior pole. Furthermore, sperm with mutant or absent centrosomes fail to establish a posterior pole, while enucleated sperm with intact centrosomes successfully fertilize the egg and set up the posterior pole. The establishment of this polarity initiates the polarized distribution of a group of proteins present in the zygote called the PAR proteins (partitioning-defective), which are a conserved group of proteins that function in establishing cell polarity during development. These proteins are initially distributed uniformly throughout the zygote and then become polarized with the creation of the posterior pole. This series of events allows the single celled zygote to obtain polarity through an unequal distribution of multiple factors.

The single cell is now set up to undergo an asymmetric cell division, however the orientation in which the division occurs is also an important factor. The mitotic spindle must be oriented correctly to ensure that the proper cell fate determinants are distributed appropriately to the daughter cells. The alignment of the spindle is mediated by the PAR proteins, which regulate the positioning of the centrosomes along the A/P axis as well as the movement of the mitotic spindle along the A/P axis. Following this first asymmetric division, the AB daughter cell divides symmetrically, giving rise to ABa and ABp, while the P1 daughter cell undergoes another asymmetric cell division to produce P2 and EMS. This division is also dependent on the distribution of the PAR proteins.