Actomyosin contraction during cellularization is regulated in part by Src64 control of Actin 5C protein levels

The mechanics of cephalic furrow formation in the Drosophila embryo

Cephalic furrow formation (CFF) is a major morphogenetic movement during gastrulation in Drosophila melanogaster embryos that gives rise to a deep, transitory epithelial invagination. Recent studies have identified the individual cell shape changes that drive the initiation and progression phases of CFF; however, the underlying mechanics are not yet well understood. During the progression phase, the furrow deepens as columnar cells from both the anterior and posterior directions fold inwards rotating by 90°. To analyze the mechanics of this process, we have developed an advanced two-dimensional lateral vertex model that includes multinode representation of cellular membranes and allows us to capture the membrane curvature associated with pressure variation. Our investigations reveal some key potential mechanical features of CFF, as follows. When cells begin to roll over the cephalic furrow cleft, they become wedge shaped as their apical cortices and overlying membranes expand, lateral cortices and overlying membranes release tension, internal pressures drop, and basal cortices and membranes contract. Then, cells reverse this process by shortening apical cortices and membranes, increasing lateral tension, and causing internal pressures to rise. Since the basal membranes expand, the cells recover their rotated columnar shape once in the furrow. Interestingly, our findings indicate that the basal membranes may be passively reactive throughout the progression phase. We also find that the smooth rolling of cells over the cephalic furrow cleft necessitates that internalized cells provide a solid base through high levels of membrane tension and internal pressure, which allows the transmission of tensile force that pulls new cells into the furrow. These results lead us to suggest that CFF helps to establish a baseline tension across the apical surface of the embryo to facilitate cellular coordination of other morphogenetic movements via mechanical stress feedback mechanisms.

The Ringleaders: Understanding the Apicomplexan Basal Complex Through Comparison to Established Contractile Ring Systems

The actomyosin contractile ring is a key feature of eukaryotic cytokinesis, conserved across many eukaryotic kingdoms. Recent research into the cell biology of the divergent eukaryotic clade Apicomplexa has revealed a contractile ring structure required for asexual division in the medically relevant genera Toxoplasma and Plasmodium; however, the structure of the contractile ring, known as the basal complex in these parasites, remains poorly characterized and in the absence of a myosin II homolog, it is unclear how the force required of a cytokinetic contractile ring is generated. Here, we review the literature on the basal complex in Apicomplexans, summarizing what is known about its formation and function, and attempt to provide possible answers to this question and suggest new avenues of study by comparing the Apicomplexan basal complex to well-studied, established cytokinetic contractile rings and their mechanisms in organisms such as S. cerevisiae and D. melanogaster. We also compare the basal complex to structures formed during mitochondrial and plastid division and cytokinetic mechanisms of organisms beyond the Opisthokonts, considering Apicomplexan diversity and divergence.