A Model for Cleavage Plane Determination in Early Amphibian and Fish Embryos

Abstract

Current models for cleavage plane determination propose that metaphase spindles are positioned and oriented by interactions of their astral microtubules with the cellular cortex, followed by cleavage in the plane of the metaphase plate [1, 2]. We show that in early frog and fish embryos, where cells are unusually large, astral microtubules in metaphase are too short to position and orient the spindle. Rather, the preceding interphase aster centers and orients a pair of centrosomes prior to nuclear envelope breakdown, and the spindle assembles between these prepositioned centrosomes. Interphase asters center and orient centrosomes using dynein-mediated pulling forces. These forces act before astral microtubules contact the cortex; thus, dynein must pull from sites in the cytoplasm, not the cell cortex as is usually proposed for smaller cells. Aster shape is determined by interactions of the expanding periphery with the cell cortex, or with an interaction zone that forms between sister-asters in telophase. We propose a model to explain cleavage plane geometry in which the length of astral microtubules is limited by interaction with these boundaries, causing length asymmetries. Dynein anchored in the cytoplasm then generates length–dependent pulling forces, which move and orient centrosomes.

Organization of early frog embryos by chemical waves emanating from centrosomes

“…When Cdk1 activity decreases following fertilization, or anaphase onset, asters grow out rapidly from centrosomes. The sperm aster grows to fill the whole cell (figure 3a), whereas anaphase asters fill half the cell, since they do not interpenetrate across the mid-plane (figure 3c,d) [3,4,56]. The outer edge of the aster expands at approximately 30 mm min 21 in Xenopus zygotes, and approximately 15 mm min 21 in zebrafish first mitosis [2,3].…”

“…It might be possible to co-image the two waves to gain insights into their coordination. In the early zebrafish embryo, actin-associated vesicles change motility behaviour as the zygote proceeds from mitosis to interphase, and particles closer to the centrosome are affected earlier than more distal ones [3,32], suggesting that they are responding to a mitotic exit wave emanating from centrosomes. Importantly, this motility change wave precedes the aster growth wave.…”

“…The sperm aster grows to fill the whole cell (figure 3a), whereas anaphase asters fill half the cell, since they do not interpenetrate across the mid-plane (figure 3c,d) [3,4,56]. The outer edge of the aster expands at approximately 30 mm min 21 in Xenopus zygotes, and approximately 15 mm min 21 in zebrafish first mitosis [2,3]. This rapid growth is essential for the aster to fill the whole cell in time to position the centrosomes for the following mitosis and also to trigger cleavage furrow ingression following anaphase.…”

“…Though centrosomes are obvious candidates as initiators for microtubule waves, it is unknown whether their capability for microtubule nucleation is locally restricted or reaches longer distances [73,74]. Cytoplasmic dynein is thought to exert outward force on astral microtubules [3], so it is possible that minus ends are released and glide outwards (figure 3g). We have so far assumed the minus ends formed away from centrosomes are stable, presumably capped by g-tubulin ring complex [75,76] or the CAMSAP/Patronin family of proteins [77 -79], but they may well be free to depolymerize.…”

“…How can a cell that would take a protein hours to cross by diffusion alone generate the precisely controlled timing required for cell cycle progression on a scale of minutes? We argue, based on observations by others [1] and our group [2][3][4], that these spatial and temporal organizational challenges may be solved by a common class of biophysical mechanism: chemical reaction waves, also called 'trigger waves' [1]. Furthermore, the centrosome plays a key role in organizing these waves, and can be re-imagined as a wave-initiating site.…”