Biology subjects

Kitajima, T. S.

Publications and source records attributed to Kitajima, T. S..

4 recordsLinked to original sources

bioRxiv2025

Ex vivo live imaging unveils the dynamics of oocyte formation in mice

Oocyte formation in mammals is a tightly regulated process essential for female fertility, yet the underlying mechanisms remain poorly understood. In this study, we establish an ex vivo culture system that faithfully recapitulates in vivo development and enables long-term live imaging of mouse fetal ovaries. Using high resolution imaging, we capture the dynamic behaviors of germ cells during the development from oogonia to nascent oocytes. We identify pronounced blebbing activity during the mitosis-to-meiosis transition. This behavior is regulated by meiotic initiation signals, underscoring its potential developmental relevance, although its precise role remains unclear. A prevailing model suggests that oocyte formation involves organelle transfer from neighboring germ cells during cyst breakdown. However, through photoconversion-based tracking, we observe no detectable transfer of mitochondria or centrosomes, as organelles remain confined to individual cells. These findings point to alternative mechanisms for cytoplasmic enrichment in oocytes. Our study provides new insights into mammalian oocyte formation and establishes a powerful platform for analyzing germ cell dynamics in real time.

developmental biology↗

bioRxiv2025

Kif11-haploinsufficient oocytes reveal spatially differential requirements for chromosome biorientation in the spindle

Bipolar spindle assembly and chromosome biorientation are prerequisites for chromosome segregation during cell division. The kinesin motor KIF11 drives spindle bipolarization by sliding antiparallel microtubules bidirectionally, elongating a spherical spindle into a bipolar-shaped structure in oocytes. This process stretches chromosomes, establishing chromosome biorientation at the spindle equator. The quantitative requirement for KIF11 in spindle bipolarization and chromosome biorientation remains unclear. Here, using a genetic strategy to modulate KIF11 expression levels, we show that Kif11 haploinsufficiency impairs spindle elongation, leading to the formation of a partially bipolarized spindle during meiosis I in mouse oocytes. While the partially bipolarized spindle allows chromosome stretching in the inner region of its equator, it fails to do so in the outer region. These findings demonstrate the necessity of biallelic functional Kif11 for bipolar spindle assembly in oocytes and reveal a spatially differential requirement for chromosome biorientation within the spindle.

cell biology↗

bioRxiv2024

Transcriptomic Signatures of WNT-Driven Pathways and Granulosa Cell-Oocyte Interactions during Primordial Follicle Activation

Primordial follicle activation (PFA) is a pivotal event in female reproductive biology, coordinating the transition from quiescent to growing follicles. This study employed comprehensive single-cell RNA sequencing to gain insights into the detailed regulatory mechanisms governing the synchronized dormancy and activation between granulosa cells (GCs) and oocytes with the progression of the PFA process. Wntless (Wls) conditional knockout (cKO) mice served as a unique model, suppressing the transition from pre-GCs to GCs, and disrupting somatic cell-derived WNT signaling in the ovary. Our data revealed immediate transcriptomic changes in GCs post-PFA in Wls cKO mice, leading to a divergent trajectory, while oocytes exhibited modest transcriptomic alterations. Subpopulation analysis identified the molecular pathways affected by WNT signaling on GC maturation, along with specific gene signatures linked to dormant and activated oocytes. Despite minimal evidence of continuous up-regulation of dormancy-related genes in oocytes, the loss of WNT signaling in (pre-)GCs impacted gene expression in oocytes even before PFA, subsequently influencing them globally. The infertility observed in Wls cKO mice was attributed to compromised GC-oocyte molecular crosstalk and the microenvironment for oocytes. Our study highlights the pivotal role of the WNT-signaling pathway and its molecular signature, emphasizing the importance of intercellular crosstalk between (pre-)GCs and oocytes in orchestrating folliculogenesis. Author summaryIn mammalian ovaries, primordial follicles protect dormant oocytes from wasteful consumption. We explored the complex molecular dynamics of the critical process of primordial follicle activation (PFA) that underlies female reproductive health, with a focus on the role of WNT signaling. By utilizing a unique mouse model with disrupted WNT signaling, our study reveals novel insights into the regulatory dynamics of granulosa cells (GCs) and oocytes during PFA. Single-cell transcriptome profiling uncovered distinct cell populations and highlighted the impact of WNT signaling on ovarian cell types. This study reveals distinct transcriptomic changes in GCs post-PFA, which affects their differentiation trajectories and highlights the critical role of WNT signaling in establishing GC function. We further examined the consequences of disrupted microenvironments on oocytes and GC-oocyte interactions, which elucidated altered oocyte subclusters and potential links to infertility. These findings provide a foundation for understanding the connections between WNT signaling and ovarian function. Our comprehensive analysis contributes valuable knowledge to reproductive biology, emphasizing the clinical relevance of WNT signaling in female fertility.

bioinformatics↗

bioRxiv2020

Stable kinetochore-microtubule attachments restrict MTOC position and spindle elongation in acentrosomal oocytes

In mouse oocytes, acentriolar MTOCs functionally replace centrosomes and act as microtubule nucleation sites. Microtubules nucleated from MTOCs initially assemble into an unorganized ball-like structure, which then transforms into a bipolar spindle carrying MTOCs at its poles, a process called spindle bipolarization. In mouse oocytes, spindle bipolarization is promoted by kinetochores but the mechanism by which kinetochore-microtubule attachments contribute to spindle bipolarity remains unclear. This study demonstrates that the stability of kinetochore-microtubule attachment is essential for confining MTOC positions at the spindle poles and for limiting spindle elongation. MTOC sorting is gradual and continues even in the metaphase spindle. When stable kinetochore-microtubule attachments are disrupted, the spindle is unable to restrict MTOCs at its poles and fails to terminate its elongation. Stable kinetochore fibers are directly connected to MTOCs and to the spindle poles, and thus may serve as a measure that defines proper spindle length. These findings reinforce the hypothesis that kinetochores act as scaffolds for acentrosomal spindle bipolarity.

cell biology↗