Dual spindles assemble in bovine zygotes despite the presence of paternal centrosomes

Abstract

The first mitosis of the mammalian embryo must partition the parental genomes contained in two pronuclei. In rodent zygotes, sperm centrosomes are degraded, and instead, acentriolar microtubule organizing centers and microtubule self-organization guide the assembly of two separate spindles around the genomes. In nonrodent mammals, including human or bovine, centrosomes are inherited from the sperm and have been widely assumed to be active. Whether nonrodent zygotes assemble a single centrosomal spindle around both genomes or follow the dual spindle self-assembly pathway is unclear. To address this, we investigated spindle assembly in bovine zygotes by systematic immunofluorescence and real-time light-sheet microscopy. We show that two independent spindles form despite the presence of centrosomes, which had little effect on spindle structure and were only loosely connected to the two spindles. We conclude that the dual spindle assembly pathway is conserved in nonrodent mammals. This could explain whole parental genome loss frequently observed in blastomeres of human IVF embryos.

Volumetric morphometry reveals spindle width as the best predictor of mammalian spindle scaling

“…Our automated analysis of chromosomes and spindles is in many aspects similar to the MATLAB-based algorithms described in Schneider et al (2021) . To make such methodology available to a wider user base, we implemented our image processing in ImgLib2 ( Pietzsch et al, 2012 ) and distributed Spindle3D ( https://github.com/tischi/spindle3d ) as a Fiji plug-in that can be installed by enabling the Spindle3D update site ( https://sites.imagej.net/Spindle3D ).…”

“…While most experimental studies still measure spindle length and cell diameter or collapse them into area information, theoretical arguments regularly use volumetric data ( Good et al, 2013 ; Reber et al, 2013 ; Rieckhoff et al, 2020 ). Previous work has analyzed spindles in 3D ( Dumont and Mitchison, 2009 ; Takagi et al, 2013 ; Reber et al, 2013 ; Takagi et al, 2014 ; Baran et al, 2016 ; Oh et al, 2016 ; Cai et al, 2018 , So et al, 2019 ; Rieckhoff et al, 2020 , Schneider et al, 2021 ), but there is still no standard or automated open-source tool for analyzing spindle morphometrics. Tools available for analyzing spindle size and geometry so far only allow for 2D analysis ( Crowder et al, 2015 ; Grenfell et al, 2016 ).…”

Mechanism of spindle pole organization and instability in human oocytes

“…This sudden reduction in multipolar spindles coincides with a rise in KIFC1 mRNA from the two-cell stage onward. Indeed, bovine oocytes express KIFC1, and multipolar spindles are rare in bovine zygotes (122)(123)(124). Thus, differences in maternal KIFC1 expression may also explain why human zygotes are more prone to assembling multipolar spindles than other mammalian zygotes.…”

The Centriole’s Role in Miscarriages

“…Shortly after fertilization, the two spermatozoan centrioles stay together at the base of the decondensing male pronucleus, recruiting egg PCM proteins, and forming the first zygotic centrosome, which emanates a large aster (aka, the sperm aster). Later, the PC and DC (which is still attached to the axoneme) separate to form two independent centrosomes that are first located at the junction of the male and female pronuclei and, later, at the pole periphery of the male and female parallel spindles ( Fishman et al, 2018 ; Cavazza et al, 2021 ; Kai et al, 2021 ; Schneider et al, 2021 ). The two sperm centrioles appear to be essential post-fertilization, as it is impossible to achieve live birth by fertilizing the egg with only sperm heads in humans and most other mammal species (with the notorious exception of mice and other murine species–see below) ( Moomjy et al, 1999 ; Avidor-Reiss et al, 2019 ).…”