Asymmetric and Unequal Cell Divisions in Ascidian Embryos

The complete cell lineage and MAPK- and Otx-dependent specification of the dopaminergic cells in the Ciona brain

The Ciona larva has served as a unique model for understanding the development of dopaminergic cells at single-cell resolution owing to the exceptionally small number of neurons in its brain and its fixed cell lineage during embryogenesis. A recent study suggested that the transcription factors Fer2 and Meis directly regulate the dopamine synthesis genes in Ciona, but the dopaminergic cell lineage and the gene regulatory networks that control the development of dopaminergic cells have not been fully elucidated. Here, we reveal that the dopaminergic cells in Ciona are derived from a bilateral pair of cells called a9.37 cells at the center of the neural plate. The a9.37 cells divide along the anterior-posterior axis, and all of the descendants of the posterior daughter cells differentiate into the dopaminergic cells. We show that the MAPK pathway and the transcription factor Otx are required for the expression of Fer2 in the dopaminergic cell lineage. Our findings establish the cellular and molecular framework for fully understanding the commitment to dopaminergic cells in the simple chordate brain.

Physically asymmetric division of the C. elegans zygote ensures invariably successful embryogenesis

Asymmetric divisions that yield daughter cells of different sizes are frequent during early embryogenesis, but the importance of such a physical difference for successful development remains poorly understood. Here, we investigated this question using the first division of Caenorhabditis elegans embryos, which yields a large AB cell and a small P1 cell. We equalized AB and P1 sizes using acute genetic inactivation or optogenetic manipulation of the spindle positioning protein LIN-5. We uncovered that only some embryos tolerated equalization, and that there was a size asymmetry threshold for viability. Cell lineage analysis of equalized embryos revealed an array of defects, including faster cell cycle progression in P1 descendants, as well as defects in cell positioning, division orientation, and cell fate. Moreover, equalized embryos were more susceptible to external compression. Overall, we conclude that unequal first cleavage is essential for invariably successful embryonic development of C. elegans.

Positioning of the Centrosome and Golgi Complex

For over a century, the centrosome has been an organelle more easily tracked than understood, and the study of its peregrinations within the cell remains a chief underpinning of its functional investigation. Increasing attention and new approaches have been brought to bear on mechanisms that control centrosome localization in the context of cleavage plane determination, ciliogenesis, directional migration, and immunological synapse formation, among other cellular and developmental processes. The Golgi complex, often linked with the centrosome, presents a contrasting case of a pleiomorphic organelle for which functional studies advanced somewhat more rapidly than positional tracking. However, Golgi orientation and distribution has emerged as an area of considerable interest with respect to polarized cellular function. This chapter will review our current understanding of the mechanism and significance of the positioning of these organelles.

Dynamic interplay of cell fate, polarity and force generation in ascidian embryos

A fundamental challenge in developmental biology is to understand how forces produced by individual cells are patterned in space and time and then integrated to produce stereotyped changes in tissue-level or embryo-level morphology. Ascidians offer a unique opportunity to address this challenge by studying how small groups of cells collectively execute complex, but highly stereotyped morphogenetic movements. Here we highlight recent progress and open questions in the study of ascidian morphogenesis, emphasizing the dynamic interplay of cell fate determination, cellular force generation and tissue-level mechanics.