Mitotic cell rounding and epithelial thinning regulate lumen growth and shape

Coupling changes in cell shape to chromosome segregation

Animal cells undergo dramatic changes in shape, mechanics and polarity as they progress through the different stages of cell division. These changes begin at mitotic entry, with cell-substrate adhesion remodelling, assembly of a cortical actomyosin network and osmotic swelling, which together enable cells to adopt a near spherical form even when growing in a crowded tissue environment. These shape changes, which probably aid spindle assembly and positioning, are then reversed at mitotic exit to restore the interphase cell morphology. Here, we discuss the dynamics, regulation and function of these processes, and how cell shape changes and sister chromatid segregation are coupled to ensure that the daughter cells generated through division receive their fair inheritance.

Heterogeneity, Cell Biology and Tissue Mechanics of Pseudostratified Epithelia: Coordination of Cell Divisions and Growth in Tightly Packed Tissues

Pseudostratified epithelia (PSE) are tightly packed proliferative tissues that are important precursors of the development of diverse organs in a plethora of species, invertebrate and vertebrate. PSE consist of elongated epithelial cells that are attached to the apical and basal side of the tissue. The nuclei of these cells undergo interkinetic nuclear migration (IKNM) which leads to all mitotic events taking place at the apical surface of the epithelium. In this review, we discuss the intricacies of proliferation in PSE, considering cell biological, as well as the physical aspects. First, we summarize the principles governing the invariability of apical nuclear migration and apical cell division as well as the importance of apical mitoses for tissue proliferation. Then, we focus on the mechanical and structural features of these tissues. Here, we discuss how the overall architecture of pseudostratified tissues changes with increased cell packing. Lastly, we consider possible mechanical cues resulting from these changes and their potential influence on cell proliferation.

Methods to study the development, anatomy, and function of the zebrafish inner ear across the life course

The inner ear is a remarkably intricate structure able to detect sound, motion, and gravity. During development of the zebrafish embryo, the ear undergoes dynamic morphogenesis from a simple epithelial vesicle into a complex labyrinth, consisting of three semicircular canals and three otolithic sensory organs, each with an array of differentiated cell types. This microcosm of biology has led to advances in understanding molecular and cellular changes in epithelial patterning and morphogenesis, through to mechanisms of mechanosensory transduction and the origins of reflexive behavior. In this chapter, we describe different methods to study the zebrafish ear, including high-speed imaging of otic cilia, confocal microscopy, and light-sheet fluorescent microscopy. Many dyes, antibodies, and transgenic lines for labeling the ear are available, and we provide a comprehensive review of these resources. The developing ear is amenable to genetic, chemical, and physical manipulations, including injection and transplantation. Chemical modulation of developmental signaling pathways has paved the way for zebrafish to be widely used in drug discovery. We describe two chemical screens with relevance to the ear: a fluorescent-based screen for compounds that protect against ototoxicity, and an in situ-based screen for modulators of a signaling pathway involved in semicircular canal development. We also describe methods for dissection and imaging of the adult otic epithelia. We review both manual and automated methods to test the function of the inner ear and lateral line, defects in which can lead to altered locomotor behavior. Finally, we review a collection of zebrafish models that are generating new insights into human deafness and vestibular disorders.

Patterning, morphogenesis, and neurogenesis of zebrafish cranial sensory placodes

Peripheral sensory organs and ganglia found in the vertebrate head arise during embryonic development from distinct ectodermal thickenings, called cranial sensory placodes (adenohypophyseal, olfactory, lens, trigeminal, epibranchial, and otic). A series of patterning events leads to the establishment of these placodes. Subsequently, these placodes undergo specific morphogenetic movements and cell-type specification in order to shape the final placodal derivatives and to produce differentiated cell types necessary for their function. In this chapter, we will focus on recent studies in the zebrafish that have advanced our understanding of cranial sensory placode development. We will summarize the signaling events and their molecular effectors guiding the formation of the so-called preplacodal region, and the subsequent subdivision of this region along the anteroposterior axis that gives rise to specific placode identities as well as those controlling morphogenesis and neurogenesis. Finally, we will highlight the approaches used in zebrafish that have been established to precisely label cell populations, to follow their development, and/or to characterize cell fates within a specific placode.

Redistribution of connexin 43 during cell division

Gap junction channels, once clustered into gap junction plaques, allow communication of essential metabolites between cells. Gap junction plaques have been reported to be lost from the cell surface during cell division. The mechanism involved in this loss of gap junction plaques during mitosis is unclear, but we hypothesize that an endoexocytotic mechanism that results in cytoplasmic double-membraned annular gap junction vesicles is involved. In this study, gap junction plaque changes in dividing cells were examined in SW-13 adrenocortical tumor cells. Endogenous gap junction protein, connexin 43 (Cx43), was detected with immunofluorescence, and live cell imaging was used to monitor green fluorescent protein-tagged Cx43 (Cx43-GFP). Mitotic stages were identified by Hoechst chromosomal staining. During interphase, large gap junction plaques were detected; however, the presence of these plaques decreased, whereas cytoplasmic puncta increased beginning with prophase. The cytoplasmic puncta were demonstrated with immunoelectron microscopy to be Cx43- positive annular gap junction vesicles. As gap junction plaques reformed at cleavage furrows between daughter cells, the number of annular gap junctions decreased during cytokinesis. The data are consistent with the mechanism of gap junction plaque loss during mitosis relying on an endoexocytotic process that results in annular gap junction vesicles formation. The rapid formation of gap junction plaques during cytokinesis points to the intriguing possibility of connexin recycling from annular gap junction vesicles to form gap junction plaques as mitosis is completed.

Centriole Amplification in Zebrafish Affects Proliferation and Survival but Not Differentiation of Neural Progenitor Cells

In animal cells, supernumerary centrosomes, resulting from centriole amplification, cause mitotic aberrations and have been associated with diseases, including microcephaly and cancer. To evaluate how centriole amplification impacts organismal development at the cellular and tissue levels, we used the in vivo imaging potential of the zebrafish. We demonstrate that centriole amplification can induce multipolar anaphase, resulting in binucleated cells. Such binucleation causes substantial apoptosis in the neuroepithelium. Interestingly, not all epithelia are similarly sensitive to binucleation, as skin cells tolerate it without entering apoptosis. In the neuroepithelium, however, binucleation leads to tissue degeneration and subsequent organismal death. Notably, this tissue degeneration can be efficiently counterbalanced by compensatory proliferation of wild-type cells. Because the risk for generating a binucleated daughter recurs at every cell division, centriole amplification in the neuroepithelium is especially deleterious during progenitor proliferation. Once cells reach the differentiation phase, however, centriole amplification does not impair neuronal differentiation.

Mechanical control of mitotic progression in single animal cells

Despite the importance of mitotic cell rounding in tissue development and cell proliferation, there remains a paucity of approaches to investigate the mechanical robustness of cell rounding. Here we introduce ion beam-sculpted microcantilevers that enable precise force-feedback-controlled confinement of single cells while characterizing their progression through mitosis. We identify three force regimes according to the cell response: small forces (∼5 nN) that accelerate mitotic progression, intermediate forces where cells resist confinement (50-100 nN), and yield forces (>100 nN) where a significant decline in cell height impinges on microtubule spindle function, thereby inhibiting mitotic progression. Yield forces are coincident with a nonlinear drop in cell height potentiated by persistent blebbing and loss of cortical F-actin homogeneity. Our results suggest that a buildup of actomyosin-dependent cortical tension and intracellular pressure precedes mechanical failure, or herniation, of the cell cortex at the yield force. Thus, we reveal how the mechanical properties of mitotic cells and their response to external forces are linked to mitotic progression under conditions of mechanical confinement.