Microtubule configurations during fertilization, mitosis, and early development in the mouse and the requirement for egg microtubule-mediated motility during mammalian fertilization

Paternal contributions to mammalian zygote - Beyond sperm-oocyte fusion

Contrary to a common misconception that the fertilizing spermatozoon acts solely as a vehicle for paternal genome delivery to the zygote, this chapter aims to illustrate how the male gamete makes other essential contributions , including the sperm borne-oocyte activation factors, centrosome components, and components of the sperm proteome and transcriptome that help to lay the foundation for pregnancy establishment and maintenance to term, and the newborn and adult health. Our inquiry starts immediately after sperm plasma membrane fusion with its oocyte counterpart, the oolemma. Parallel to and following sperm incorporation in the egg cytoplasm, some of the sperm structures (perinuclear theca) are dissolved and spent to induce development, others (nucleus, centriole) are transformed into zygotic structures enabling it, and yet others (mitochondrial and fibrous sheath, axonemal microtubules and outer dense fibers) are recycled as to not stand in its way. Noteworthy advances in this research include the identification of several sperm-borne oocyte activating factor candidates, the role of autophagy in the post-fertilization sperm mitochondrion degradation, new insight into zygotic centrosome origins and function, and the contributions of sperm-delivered RNA cargos to early embryo development. In concluding remarks, the unresolved issues, and clinical and biotechnological implications of sperm-vectored paternal inheritance are discussed.

A cytokinetic ring-driven cell rotation achieves Hertwig's rule in early development

Hertwig's rule states that cells divide along their longest axis, usually driven by forces acting on the mitotic spindle. Here, we show that in contrast to this rule, microtubule-based pulling forces in early Caenorhabditis elegans embryos align the spindle with the short axis of the cell. We combine theory with experiments to reveal that in order to correct this misalignment, inward forces generated by the constricting cytokinetic ring rotate the entire cell until the spindle is aligned with the cell's long axis. Experiments with slightly compressed mouse zygotes indicate that this cytokinetic ring-driven mechanism of ensuring Hertwig's rule is general for cells capable of rotating inside a confining shell, a scenario that applies to early cell divisions of many systems.

The evolution of centriole degradation in mouse sperm

Centrioles are subcellular organelles found at the cilia base with an evolutionarily conserved structure and a shock absorber-like function. In sperm, centrioles are found at the flagellum base and are essential for embryo development in basal animals. Yet, sperm centrioles have evolved diverse forms, sometimes acting like a transmission system, as in cattle, and sometimes becoming dispensable, as in house mice. How the essential sperm centriole evolved to become dispensable in some organisms is unclear. Here, we test the hypothesis that this transition occurred through a cascade of evolutionary changes to the proteins, structure, and function of sperm centrioles and was possibly driven by sperm competition. We found that the final steps in this cascade are associated with a change in the primary structure of the centriolar inner scaffold protein FAM161A in rodents. This information provides the first insight into the molecular mechanisms and adaptive evolution underlying a major evolutionary transition within the internal structure of the mammalian sperm neck.

Whole-Mount Immunofluorescence Staining to Visualize Cell Cycle Progression in Mouse Oocyte Meiosis

Whole-mount immunofluorescence allows direct visualization of the cellular architecture within cells. Here, we apply this technique to mouse oocytes to visualize spindle morphology and microtubule attachments to kinetochores, using a technique we call "cold treatment," at various phases of the meiotic cell cycle. This method allows the analysis of spindle structures at different meiosis I stages and at metaphase II. An adaptation of the protocol to the cell cycle stage of interest is described.

Mean-field theory approach to three-dimensional nematic phase transitions in microtubules

Microtubules are dynamic intracellular fibers that have been observed experimentally to undergo spontaneous self-alignment. We formulate a three-dimensional (3D) mean-field theory model to analyze the nematic phase transition of microtubules growing and interacting within a 3D space, then make a comparison with computational simulations. We identify a control parameter G_{eff} and predict a unique critical value G_{eff}=1.56 for which a phase transition can occur. Furthermore, we show both analytically and using simulations that this predicted critical value does not depend on the presence of zippering. The mean-field theory developed here provides an analytical estimate of microtubule patterning characteristics without running time-consuming simulations and is a step towards bridging scales from microtubule behavior to multicellular simulations.

CAPA-IVM improves the cytoplasmic quality of in vitro-matured oocytes from unstimulated mice

Ovarian tissue oocyte (OTO) in vitro maturation (IVM) is a strategy to improve fertility preservation efficiency. Here, the effects of capacitation IVM (CAPA-IVM) on OTO function were investigated. Immature cumulus-oocyte complexes (COCs) from unstimulated 28-day-old mouse ovaries (mimicking OTOs) underwent CAPA-IVM, standard IVM (S-IVM) or in vivo maturation following ovarian stimulation (OS; positive control), and oocyte meiotic maturation and cytoplasmic quality were assessed. CAPA-IVM resulted in improved oocyte meiotic maturation (P < 0.05) and cumulus expansion (P < 0.0001) compared to S-IVM, with expansion comparable to the OS group. MII OTO ROS was lower after CAPA-IVM than S-IVM (P < 0.0001) but not as low as in the OS group (P = 0.036). CAPA-IVM resulted in a better oocyte mitochondrial distribution than S-IVM (P < 0.05) and was similar to the OS group (P > 0.05). Mitochondrial membrane potential in MII OTOs was higher after CAPA-IVM than S-IVM and OS (P < 0.0001). Compared with S-IVM, CAPA-IVM resulted in lower rates of spindle/chromosome configuration and cortical granule distribution abnormalities (P < 0.05), which were similar to OS levels (P > 0.05). MII OTO intracellular Ca2+ levels were similar in the CAPA-IVM and OS groups (P > 0.05), while S-IVM decreased intracellular Ca2+ (P < 0.05). CAPA-IVM and S-IVM decreased mitochondrial Ca2+ levels (P < 0.05). CAPA-IVM increased expression of antioxidant genes (Sod2 and Sirt1) and Egfr (P < 0.05) but not apoptotic genes (Bcl2, Bax and Bcl2/Bax; P > 0.05). CAPA-IVM increased the OTO maturation rate and quality of oocytes from unstimulated mice to the extent that many features of oocyte cytoplasmic quality were comparable to superovulated in vivo matured oocytes.

A review of the use of antioxidants in bovine sperm preparation protocols

Reactive oxygen species (ROS) and oxidative stress (OS), the imbalance between the production of free radicals and the cellular antioxidant defenses, are discussed in relation to their role in bovine sperm physiology. Oxidative stress has been associated to male infertility and low fertility rates in Assisted Reproductive Techniques (ART). Antioxidant supplementation is an interesting approach to overcome OS-related infertility and assisted reproduction drawbacks. Several studies have been conducted to identify the potential sources of ROS in a typical ART setting and the impact of antioxidant supplementation on semen quality and pregnancy outcome. Procedures such as freezing and thawing, centrifugation and incubation are thought to produce significant amounts of ROS with a negative impact on sperm quality parameters and reproductive competence. Given the important role of ROS in sperm function, the addition of antioxidants in sperm media to prevent OS and to improve the reproductive outcome requires attention. Currently, there is limited evidence to support the ameliorative effect of antioxidant supplementation on fertilization and embryo development in farm animals. This review summarizes the different types and concentrations of antioxidants used in sperm preparation media of bovine species and their effectiveness in neutralizing excessive ROS production while preserving physiological sperm function.

The interconnection of endoplasmic reticulum and microtubule and its implication in Hereditary Spastic Paraplegia

The endoplasmic reticulum (ER) and microtubule (MT) network form extensive contact with each other and their interconnection plays a pivotal role in ER maintenance and distribution as well as MT stability. The ER participates in a variety of biological processes including protein folding and processing, lipid biosynthesis, and Ca²⁺ storage. MTs specifically regulate cellular architecture, provide routes for transport of molecules or organelles, and mediate signaling events. The ER morphology and dynamics are regulated by a class of ER shaping proteins, which also provide the physical contact structure for linking of ER and MT. In addition to these ER-localized and MT-binding proteins, specific motor proteins and adaptor-linking proteins also mediate bidirectional communication between the two structures. In this review, we summarize the current understanding of the structure and function of ER-MT interconnection. We further highlight the morphologic factors which coordinate the ER-MT network and maintain the normal physiological function of neurons, with their defect causing neurodegenerative diseases such as Hereditary Spastic Paraplegia (HSP). These findings promote our understanding of the pathogenesis of HSP and provide important therapeutic targets for treatment of these diseases.

A Comprehensive Study on the Electrostatic Properties of Tubulin-Tubulin Complexes in Microtubules

Microtubules are key players in several stages of the cell cycle and are also involved in the transportation of cellular organelles. Microtubules are polymerized by α/β tubulin dimers with a highly dynamic feature, especially at the plus ends of the microtubules. Therefore, understanding the interactions among tubulins is crucial for characterizing microtubule dynamics. Studying microtubule dynamics can help researchers make advances in the treatment of neurodegenerative diseases and cancer. In this study, we utilize a series of computational approaches to study the electrostatic interactions at the binding interfaces of tubulin monomers. Our study revealed that among all the four types of tubulin-tubulin binding modes, the electrostatic attractive interactions in the α/β tubulin binding are the strongest while the interactions of α/α tubulin binding in the longitudinal direction are the weakest. Our calculations explained that due to the electrostatic interactions, the tubulins always preferred to form α/β tubulin dimers. The interactions between two protofilaments are the weakest. Thus, the protofilaments are easily separated from each other. Furthermore, the important residues involved in the salt bridges at the binding interfaces of the tubulins are identified, which illustrates the details of the interactions in the microtubule. This study elucidates some mechanistic details of microtubule dynamics and also identifies important residues at the binding interfaces as potential drug targets for the inhibition of cancer cells.

Rescue of oocytes recovered from postmortem mouse ovaries

It is well known that the survivability of gametes of postmortem carcass was decreased as time passes after death. In this study, it was examined whether cytoplasmic replacement rescues the survivability of germinal vesicle stage (GV) oocytes of postmortem carcass in the mouse. Reactive oxygen species (ROS) levels and mitochondria numbers in GV oocytes of the dead mice stored at 4 degrees were significantly impaired after 44 h postmortem compared to the control (0 h). However, when kayoplasts of GV oocytes of postmortem carcass was transferred to recipient ooplasts (GV transfer), proportion of in vitro maturation (IVM), normal spindle morphology, in vitro and in vivo developmental ability after in vitro fertilization (IVF) of reconstituted oocytes was improved. Moreover, secondary follicle oocytes of postmortem carcass were developed, matured and fertilized in vitro and developed to go to term, when GV transfer was conducted at the GV phase. Thus, transfer of GV karyoplast recovered from postmortem carcass, which viability was decreased, into fresh GV recipient ooplasm, rescues survivability of reconstituted oocytes. It suggested the effective use of oocytes of dead animals in the mouse and this achievement must apply to other rare animal species, especially animals under control by human.

Insights into the NAD+ biosynthesis pathways involved during meiotic maturation and spindle formation in porcine oocytes

Treatments that elevate NAD+ levels have been found to improve oocyte quality in mice, cattle, and pigs, suggesting that NAD+ is vital during oocyte maturation. This study aimed to examine the influence of different NAD+ biosynthetic pathways on oocyte quality by inhibiting key enzymes. Porcine oocytes from small antral follicles were matured for 44 h in a defined maturation system supplemented with 2-hydroxynicotinic acid [2-HNA, nicotinic acid phosphoribosyltransferase (NAPRT) inhibitor], FK866 [nicotinamide phosphoribosyltransferase (NAMPT) inhibitor], or gallotannin [nicotinamide mononucleotide adenylyltransferase (NMNAT) inhibitor] and their respective NAD+ pathway modulators (nicotinic acid, nicotinamide, and nicotinamide mononucleotide, respectively). Cumulus expansion was assessed after 22 h of maturation. At 44 h, maturation rates were determined and mature oocytes were fixed and stained to assess spindle formation. Each enzyme inhibitor reduced oocyte maturation rate and adversely affected spindle formation, indicating that NAD+ is required for meiotic spindle assembly. Furthermore, NAMPT and NMNAT inhibition reduced cumulus expansion, whereas NAPRT inhibition affected chromosomal segregation. Treating oocytes with gallotannin and nicotinamide mononucleotide together showed improvements in spindle width, while treating oocytes with 2-HNA and nicotinic acid combined showed an improvement in both spindle length and width. These results indicate that the salvage pathway plays a vital role in promoting oocyte meiotic progression, while the Preiss-Handler pathway is essential for spindle assembly.

A monoastral mitotic spindle determines lineage fate and position in the mouse embryo

During mammalian development, the first asymmetric cell divisions segregate cells into inner and outer positions of the embryo to establish the pluripotent and trophectoderm lineages. Typically, polarity components differentially regulate the mitotic spindle via astral microtubule arrays to trigger asymmetric division patterns. However, early mouse embryos lack centrosomes, the microtubule-organizing centres (MTOCs) that usually generate microtubule asters. Thus, it remains unknown whether spindle organization regulates lineage segregation. Here we find that heterogeneities in cell polarity in the early 8-cell-stage mouse embryo trigger the assembly of a highly asymmetric spindle organization. This spindle arises in an unusual modular manner, forming a single microtubule aster from an apically localized, non-centrosomal MTOC, before joining it to the rest of the spindle apparatus. When fully assembled, this 'monoastral' spindle triggers spatially asymmetric division patterns to segregate cells into inner and outer positions. Moreover, the asymmetric inheritance of spindle components causes differential cell polarization to determine pluripotent versus trophectoderm lineage fate.

The second polar body contributes to the fate asymmetry in the mouse embryo

The polar bodies (PBs) are extruded microcells during oocyte meiosis and generally regarded as inessentials for embryonic development. Therefore, PBs have been widely used as important materials for pre-implantation genetic diagnosis in human. Here we report that the second PB (PB2) in the mouse zygote may play roles in cell-fate specification and post-implantation development. A subset of mRNAs encoding pluripotency-related factors are enriched in PB2. Nascent proteins may be synthesized in PB2 after fertilization and transport from PB2 to the zygote before the two-cell stage. The PB2-attached blastomere (pbB) at the two-cell stage, compared to the other blastomere (npbB), likely contributes more descendants to the inner cell mass (ICM) lineage in the blastocyst. Removal of PB2 from the zygote or transient blockage of material exchange between PB2 and the zygote by nocodazole treatment appears to cause a loss of the ICM fate bias of pbB. PB2 removal or nocodazole treatment also results in abnormal post-implantation development. Injection of PB2 lysate into pbB of PB2-removed two-cell-stage embryos may reset the cell-fate preference and rescue post-implantation development. Our data collectively suggest that PB2 would demarcate the earliest cell-fate asymmetry of the mouse zygote and be required for post-implantation development.

Cytoskeletal control of early mammalian development

The cytoskeleton - comprising actin filaments, microtubules and intermediate filaments - serves instructive roles in regulating cell function and behaviour during development. However, a key challenge in cell and developmental biology is to dissect how these different structures function and interact in vivo to build complex tissues, with the ultimate aim to understand these processes in a mammalian organism. The preimplantation mouse embryo has emerged as a primary model system for tackling this challenge. Not only does the mouse embryo share many morphological similarities with the human embryo during its initial stages of life, it also permits the combination of genetic manipulations with live-imaging approaches to study cytoskeletal dynamics directly within an intact embryonic system. These advantages have led to the discovery of novel cytoskeletal structures and mechanisms controlling lineage specification, cell-cell communication and the establishment of the first forms of tissue architecture during development. Here we highlight the diverse organization and functions of each of the three cytoskeletal filaments during the key events that shape the early mammalian embryo, and discuss how they work together to perform key developmental tasks, including cell fate specification and morphogenesis of the blastocyst. Collectively, these findings are unveiling a new picture of how cells in the early embryo dynamically remodel their cytoskeleton with unique spatial and temporal precision to drive developmental processes in the rapidly changing in vivo environment.

Getting to Know Endometriosis-Related Infertility Better: A Review on How Endometriosis Affects Oocyte Quality and Embryo Development

Endometriosis-related infertility describes a case of deteriorated fecundity when endometriosis is diagnosed. Numerous mechanisms have been proposed in an effort to delineate the multifaceted pathophysiology that induces impairment of reproductive dynamics in patients with endometriosis. In this critical analysis, authors present the plethora of molecular events that are entailed and elaborate on how they potentially impair the oocyte's and embryo's competence in patients with endometriosis. Reactive oxygen species, dysregulation of the immune system and cellular architectural disruption constitute the crucial mechanisms that detrimentally affect oocyte and embryo developmental potential. The molecular level impairment of the reproductive tissue is discussed, since differentiation, proliferation and apoptosis constitute focal regulatory cellular functions that appear severely compromised in cases of endometriosis. Mapping the precise molecular mechanisms entailed in endometriosis-related infertility may help delineate the complex nature of the disorder and bring us a step closer to a more personalized approach in understanding, diagnosing and managing endometriosis-related infertility.

Two mechanisms drive pronuclear migration in mouse zygotes

A new life begins with the unification of the maternal and paternal chromosomes upon fertilization. The parental chromosomes first become enclosed in two separate pronuclei near the surface of the fertilized egg. The mechanisms that then move the pronuclei inwards for their unification are only poorly understood in mammals. Here, we report two mechanisms that act in concert to unite the parental genomes in fertilized mouse eggs. The male pronucleus assembles within the fertilization cone and is rapidly moved inwards by the flattening cone. Rab11a recruits the actin nucleation factors Spire and Formin-2 into the fertilization cone, where they locally nucleate actin and further accelerate the pronucleus inwards. In parallel, a dynamic network of microtubules assembles that slowly moves the male and female pronuclei towards the cell centre in a dynein-dependent manner. Both mechanisms are partially redundant and act in concert to unite the parental pronuclei in the zygote's centre.

Glutathione Ethyl Ester Protects In Vitro-Maturing Bovine Oocytes against Oxidative Stress Induced by Subsequent Vitrification/Warming

This study aimed to examine whether the addition of glutathione ethyl ester (GSH-OEt) to the in vitro maturation (IVM) medium would improve the resilience of bovine oocytes to withstand vitrification. The effects of GSH-OEt on spindle morphology, levels of reactive oxygen species (ROS), mitochondrial activity and distribution, and embryo developmental potential were assessed together with the expression of genes with a role in apoptosis (BAX, BCL2), oxidative-stress pathways (GPX1, SOD1), water channels (AQP3), implantation (IFN-τ) and gap junctions (CX43) in oocytes and their derived blastocysts. Vitrification gave rise to abnormal spindle microtubule configurations and elevated ROS levels. Supplementation of IVM medium with GSH-OEt before vitrification preserved mitochondrial distribution pattern and diminished both cytoplasmic and mitochondrial ROS contents and percentages of embryos developing beyond the 8-cell stage were similar to those recorded in fresh non-vitrified oocytes. Although not significantly different from control vitrified oocytes, vitrified oocytes after GSH-OEt treatment gave rise to similar day 8-blastocyst and hatching rates to fresh non-vitrified oocytes. No effects of GSH-OEt supplementation were noted on the targeted gene expression of oocytes and derived blastocysts, with the exception of GPX1, AQP3 and CX43 in derived blastocysts. The addition of GSH-OEt to the IVM medium before vitrification may be beneficial for embryo development presumably as the consequence of additional anti-oxidant protection during IVM.

Tracking intracellular forces and mechanical property changes in mouse one-cell embryo development

Cells comprise mechanically active matter that governs their functionality, but intracellular mechanics are difficult to study directly and are poorly understood. However, injected nanodevices open up opportunities to analyse intracellular mechanobiology. Here, we identify a programme of forces and changes to the cytoplasmic mechanical properties required for mouse embryo development from fertilization to the first cell division. Injected, fully internalized nanodevices responded to sperm decondensation and recondensation, and subsequent device behaviour suggested a model for pronuclear convergence based on a gradient of effective cytoplasmic stiffness. The nanodevices reported reduced cytoplasmic mechanical activity during chromosome alignment and indicated that cytoplasmic stiffening occurred during embryo elongation, followed by rapid cytoplasmic softening during cytokinesis (cell division). Forces greater than those inside muscle cells were detected within embryos. These results suggest that intracellular forces are part of a concerted programme that is necessary for development at the origin of a new embryonic life.

Improved early development potence of in vitro fertilization embryos by treatment with tubacin increasing acetylated tubulin of matured porcine oocytes

To improve the developmental potential of in vitro embryos is a long-term concern field for human assisted reproduction and animal in vitro embryo production practice. In the current study, we examined the effects and mechanism of an HDAC6 inhibitor, tubacin, on the maturation of porcine oocytes and in vitro development of porcine IVF embryos. It has been demonstrated the effect of tubacin on the acetylation level of α-tubulin in porcine oocytes. As a result, the maturation rate of porcine oocytes was significantly improved (P < 0.05), and the following development potent of blastocysts forming rate was also significantly increased (P < 0.05). We found that the increased acetylation of α-tubulin significantly reduced the abnormal rate of microtubule, furthermore, the proportion of mitochondria in the vicinity of in vitro fertilization (IVF) nucleus was significantly enhanced in Metaphase I (MI) and Metaphase II (MII) stages. The expression levels of microtubule assembly genes (TUBA1A, αTAT1 and MAP2) significantly up-regulated in MI and MII stages. Together, these results suggest that treatment of porcine oocytes during maturation with tubacin could promote their IVF embryos developmental competence by altering spindle formation, mitochondrial concentration and genes expression patterns of matured porcine oocytes.

Inhibition of survivin induces spindle disorganization, chromosome misalignment, and DNA damage during mouse embryo development

The early embryonic development is important for the subsequent embryo implantation, and any defects in this process can lead to embryonic aneuploidy, which causes miscarriage and birth defects. Survivin is the member of inhibitor of apoptosis protein (IAP) family, and it is also an essential subunit of chromosomal passenger complex (CPC), which regulates both apoptosis and cell cycle control in many models. However, the roles of survivin in mouse early embryos remain unclear. In the present study, we showed that survivin activity was essential for mouse early embryo development. Our results showed that survivin mainly accumulated at chromosomes at metaphase stage and located at the spindle midzone at anaphase and telophase stages during the first cleavage. Loss of survivin activity led to the failure of cleavage in early mouse embryos. Further analysis indicated that survivin involved into spindle organization and chromosome alignment. Moreover, inhibition of survivin induced oxidative stress and DNA damage, showing with the increase of ROS level, the positive γH2A signal, and the increase of Rad51 level. We also observed the occurrence of autophagy and apoptosis in the survivin-inhibited embryos. In summary, our study suggested that survivin was a critical regulator for early embryo development through its regulation on spindle organization, chromosome alignment, and DNA damage.

Integrated microscopy resource for biomedical research at the university of wisconsin at madison

The High Voltage Electron Microscopy Laboratory [HVEM] at the University of Wisconsin-Madison, a National Institutes of Health Biomedical Research Technology Resource, has recently been renamed the Integrated Microscopy Resource for Biomedical Research [IMR]. This change is designed to highlight both our increasing abilities to provide sophisticated microscopes for biomedical investigators, and the expansion of our mission beyond furnishing access to a million-volt transmission electron microscope. This abstract will describe the current status of the IMR, some preliminary results, our upcoming plans, and the current procedures for applying for microscope time.

The IMR has five principal facilities: 1.

The IMR houses an AEI-EM7 one million-volt transmission electron microscope.

Femtosecond laser-induced blastomere fusion results in embryo tetraploidy by common metaphase plate formation

The cell fusion is a widespread process, which takes place in many systems in vivo and in vitro. Fusion of cells is frequently related to tetraploidy, which can be found within natural physiological conditions, e.g., placentation, and in pathophysiological conditions, such as cancer and early pregnancy failure in humans. Here we investigate the mechanism of tetraploidization with help of femtosecond laser-induced mouse blastomere fusion by the means of Hoechst staining, GFP, BODIPY dyes and fluorescent species generated intracellularly by a femtosecond laser. We establish diffusive mixing of cytosol, whereas the large components of a cytoplasm (organelles, cytoskeleton) are poorly diffusible and are not completely mixed after cell fusion and a subsequent division. We show that mechanisms which are responsible for the formation of a common metaphase plate triggered tetraploidization in fused mouse embryos and could be a significant factor in polyploidy formation in vivo. Thus, our results suggest that microtubules play a critical role in tetraploidization.

Reproductive hazards of space travel in women and men

Extended travel in deep space poses potential hazards to the reproductive function of female and male astronauts, including exposure to cosmic radiation, microgravity, increased gravity (hypergravity), psychological stress, physical stress and circadian rhythm disruptions. This Review focuses on the effects of microgravity, hypergravity and cosmic radiation. Cosmic radiation contains protons, helium nuclei and high charge and energy (HZE) particles. Studies performed on Earth in which rodents were exposed to experimentally generated HZE particles have demonstrated a high sensitivity of ovarian follicles and spermatogenic cells to HZE particles. Exposure to microgravity during space flight and to simulated microgravity on Earth disrupts spermatogenesis and testicular testosterone synthesis in rodents, whereas the male reproductive system seems to adapt to exposure to moderate hypergravity. A few studies have investigated the effects of microgravity on female reproduction, with findings of disrupted oestrous cycling and in vitro follicle development being cause for concern. Many remaining data gaps need to be addressed, including the effects of microgravity, hypergravity and space radiation on the male and female reproductive tracts, hypothalamic-pituitary regulation of reproduction and prenatal development of the reproductive system as well as the combined effects of the multiple reproductive hazards encountered in space.

Segregating Chromosomes in the Mammalian Oocyte

Chromosome segregation errors in human oocytes lead to aneuploid embryos that cause infertility and birth defects. Here we provide an overview of the chromosome-segregation process in the mammalian oocyte, highlighting mechanistic differences between oocytes and somatic cells that render oocytes so prone to segregation error. These differences include the extremely large size of the oocyte cytoplasm, the unique geometry of meiosis-I chromosomes, idiosyncratic function of the spindle assembly checkpoint, and dramatically altered oocyte cell-cycle control and spindle assembly, as compared to typical somatic cells. We summarise recent work suggesting that aging leads to a further deterioration in fidelity of chromosome segregation by impacting multiple components of the chromosome-segregation machinery. In addition, we compare and contrast recent results from mouse and human oocytes, which exhibit overlapping defects to differing extents. We conclude that the striking propensity of the oocyte to mis-segregate chromosomes reflects the unique challenges faced by the spindle in a highly unusual cellular environment.

The Role of Sperm Centrioles in Human Reproduction - The Known and the Unknown

Each human spermatozoon contains two remodeled centrioles that it contributes to the zygote. There, the centrioles reconstitute a centrosome that assembles the sperm aster and participate in pronuclei migration and cleavage. Thus, centriole abnormalities may be a cause of male factor infertility and failure to carry pregnancy to term. However, the precise mechanisms by which sperm centrioles contribute to embryonic development in humans are still unclear, making the search for a link between centriole abnormalities and impaired male fecundity particularly difficult. Most previous investigations into the role of mammalian centrioles during fertilization have been completed in murine models; however, because mouse sperm and zygotes appear to lack centrioles, these studies provide information that is limited in its applicability to humans. Here, we review studies that examine the role of the sperm centrioles in the early embryo, with particular emphasis on humans. Available literature includes case studies and case-control studies, with a few retrospective studies and no prospective studies reported. This literature has provided some insight into the morphological characteristics of sperm centrioles in the zygote and has allowed identification of some centriole abnormalities in rare cases. Many of these studies suggest centriole involvement in early embryogenesis based on phenotypes of the embryo with only indirect evidence for centriole abnormality. Overall, these studies suggest that centriole abnormalities are present in some cases of sperm with asthenoteratozoospermia and unexplained infertility. Yet, most previously published studies have been restricted by the laborious techniques (like electron microscopy) and the limited availability of centriolar markers, resulting in small-scale studies and the lack of solid causational evidence. With recent progress in sperm centriole biology, such as the identification of the unique composition of sperm centrioles and the discovery of the atypical centriole, it is now possible to begin to fill the gaps in sperm centriole epidemiology and to identify the etiology of sperm centriole dysfunction in humans.

Meiotic spindle formation in mammalian oocytes: implications for human infertility

In the final stage of oogenesis, mammalian oocytes generate a meiotic spindle and undergo chromosome segregation to yield an egg that is ready for fertilization. Herein, we describe the recent advances in understanding the mechanisms controlling formation of the meiotic spindle in metaphase I (MI) and metaphase II (MII) in mammalian oocytes, and focus on the differences between mouse and human oocytes. Unlike mitotic cells, mammalian oocytes lack typical centrosomes that consist of two centrioles and the surrounding pericentriolar matrix proteins, which serve as microtubule-organizing centers (MTOCs) in most somatic cells. Instead, oocytes rely on different mechanisms for the formation of microtubules in MI spindles. Two different mechanisms have been described for MI spindle formation in mammalian oocytes. Chromosome-mediated microtubule formation, including RAN-mediated spindle formation and chromosomal passenger complex-mediated spindle elongation, controls the growth of microtubules from chromatin, while acentriolar MTOC-mediated microtubule formation contributes to spindle formation. Mouse oocytes utilize both chromatin- and MTOC-mediated pathways for microtubule formation. The existence of both pathways may provide a fail-safe mechanism to ensure high fidelity of chromosome segregation during meiosis. Unlike mouse oocytes, human oocytes considered unsuitable for clinical in vitro fertilization procedures, lack MTOCs; this may explain why meiosis in human oocytes is often error-prone. Understanding the mechanisms of MI/MII spindle formation, spindle assembly checkpoint, and chromosome segregation, in mammalian oocytes, will provide valuable insights into the molecular mechanisms of human infertility.

High Temporal-Resolution Transcriptome Landscape of Early Maize Seed Development

The early maize (Zea mays) seed undergoes several developmental stages after double fertilization to become fully differentiated within a short period of time, but the genetic control of this highly dynamic and complex developmental process remains largely unknown. Here, we report a high temporal-resolution investigation of transcriptomes using 31 samples collected at an interval of 4 or 6 h within the first six days of seed development. These time-course transcriptomes were clearly separated into four distinct groups corresponding to the stages of double fertilization, coenocyte formation, cellularization, and differentiation. A total of 22,790 expressed genes including 1415 transcription factors (TFs) were detected in early stages of maize seed development. In particular, 1093 genes including 110 TFs were specifically expressed in the seed and displayed high temporal specificity by expressing only in particular period of early seed development. There were 160, 22, 112, and 569 seed-specific genes predominantly expressed in the first 16 h after pollination, coenocyte formation, cellularization, and differentiation stage, respectively. In addition, network analysis predicted 31,256 interactions among 1317 TFs and 14,540 genes. The high temporal-resolution transcriptome atlas reported here provides an important resource for future functional study to unravel the genetic control of seed development.

It takes two (centrioles) to tango

Cells that divide during embryo development require precisely two centrioles during interphase and four centrioles during mitosis. This precise number is maintained by allowing each centriole to nucleate only one centriole per cell cycle (i.e. centriole duplication). Yet, how the first cell of the embryo, the zygote, obtains two centrioles has remained a mystery in most mammals and insects. The mystery arose because the female gamete (oocyte) is thought to have no functional centrioles and the male gamete (spermatozoon) is thought to have only one functional centriole, resulting in a zygote with a single centriole. However, recent studies in fruit flies, beetles and mammals, including humans, suggest an alternative explanation: spermatozoa have a typical centriole and an atypical centriole. The sperm typical centriole has a normal structure but distinct protein composition, whereas the sperm atypical centriole is distinct in both. During fertilization, the atypical centriole is released into the zygote, nucleates a new centriole and participates in spindle pole formation. Thus, the spermatozoa's atypical centriole acts as a second centriole in the zygote. Here, we review centriole biology in general and especially in reproduction, we describe the discovery of the spermatozoon atypical centriole, and we provide an updated model for centriole inherence during sexual reproduction. While we focus on humans and other non-rodent mammals, we also provide a broader evolutionary perspective.

Gamete Nuclear Migration in Animals and Plants

The migration of male and female gamete nuclei to each other in the fertilized egg is a prerequisite for the blending of genetic materials and the initiation of the next generation. Interestingly, many differences have been found in the mechanism of gamete nuclear movement among animals and plants. Female to male gamete nuclear movement in animals and brown algae relies on microtubules. By contrast, in flowering plants, the male gamete nucleus is carried to the female gamete nucleus by the filamentous actin cytoskeleton. As techniques have developed from light, electron, fluorescence, immunofluorescence, and confocal microscopy to live-cell time-lapse imaging using fluorescently labeled proteins, details of these differences in gamete nuclear migration have emerged in a wide range of eukaryotes. Especially, gamete nuclear migration in flowering plants such as Arabidopsis thaliana, rice, maize, and tobacco has been further investigated, and showed high conservation of the mechanism, yet, with differences among these species. Here, with an emphasis on recent developments in flowering plants, we survey gamete nuclear migration in different eukaryotic groups and highlight the differences and similarities among species.

Context-dependent spindle pole focusing

The formation of a robust, bi-polar spindle apparatus, capable of accurate chromosome segregation, is a complex process requiring the co-ordinated nucleation, sorting, stabilization and organization of microtubules (MTs). Work over the last 25 years has identified protein complexes that act as functional modules to nucleate spindle MTs at distinct cellular sites such as centrosomes, kinetochores, chromatin and pre-existing MTs themselves. There is clear evidence that the extent to which these different MT nucleating pathways contribute to spindle mass both during mitosis and meiosis differs not only between organisms, but also in different cell types within an organism. This plasticity contributes the robustness of spindle formation; however, whether such plasticity is present in other aspects of spindle formation is less well understood. Here, we review the known roles of the protein complexes responsible for spindle pole focusing, investigating the evidence that these, too, act co-ordinately and differentially, depending on cellular context. We describe relationships between MT minus-end directed motors dynein and HSET/Ncd, depolymerases including katanin and MCAK, and direct minus-end binding proteins such as nuclear-mitotic apparatus protein, ASPM and Patronin/CAMSAP. We further explore the idea that the focused spindle pole acts as a non-membrane bound condensate and suggest that the metaphase spindle pole be treated as a transient organelle with context-dependent requirements for function.

Massive cytoplasmic transport and microtubule organization in fertilized chordate eggs

Eggs have developed their own strategies for early development. Amphibian, teleost fish, and ascidian eggs show cortical rotation and an accompanying structure, a cortical parallel microtubule (MT) array, during the one-cell embryonic stage. Cortical rotation is thought to relocate maternal deposits to a certain compartment of the egg and to polarize the embryo. The common features and differences among chordate eggs as well as localized maternal proteins and mRNAs that are related to the organization of MT structures are described in this review. Furthermore, recent studies report progress in elucidating the molecular nature and functions of the noncentrosomal MT organizing center (ncMTOC). The parallel array of MT bundles is presumably organized by ncMTOCs; therefore, the mechanism of ncMTOC control is likely inevitable for these species. Thus, the molecules related to the ncMTOC provide clues for understanding the mechanisms of early developmental systems, which ultimately determine the embryonic axis.

Differential expression profiles of long non‑coding RNAs during the mouse pronuclear stage under normal gravity and simulated microgravity

Pronuclear migration, which is the initial stage of embryonic development and the marker of zygote formation, is a crucial process during mammalian preimplantation embryonic development. Recent studies have revealed that long non-coding RNAs (lncRNAs) serve an important role in early embryonic development. However, the functional regulation of lncRNAs in this process has yet to be elucidated, largely due to the difficulty of assessing gene expression alterations during the very short time in which pronuclear migration occurs. It has previously been reported that migration of the pronucleus of a zygote can be obstructed by simulated microgravity. To investigate pronuclear migration in mice, a rotary cell culture system was employed, which generates simulated microgravity, in order to interfere with murine pronuclear migration. Subsequently, lncRNA sequencing was performed to investigate the mechanism underlying this process. In the present study, a comprehensive analysis of lncRNA profile during the mouse pronuclear stage was conducted, in which 3,307 lncRNAs were identified based on single-cell RNA sequencing data. Furthermore, 52 lncRNAs were identified that were significantly differentially expressed. Subsequently, 10 lncRNAs were selected for validation by reverse transcription-quantitative polymerase chain reaction, in which the same relative expression pattern was observed. The results revealed that 12 lncRNAs (lnc006745, lnc007956, lnc013100, lnc013782, lnc017097, lnc019869, lnc025838, lnc027046, lnc005454, lnc007956, lnc019410 and lnc019607), with tubulin β 4B class IVb or actinin α 4 as target genes, may be associated with the expression of microtubule and microfilament proteins. Binding association was confirmed using a dual-luciferase reporter assay. Finally, Gene Ontology analysis revealed that the target genes of the differentially expressed lncRNAs participated in cellular processes associated with protein transport, binding, catalytic activity, membrane-bounded organelle, protein complex and the cortical cytoskeleton. These findings suggested that these lncRNAs may be associated with migration of the mouse pronucleus.

Separation and Loss of Centrioles From Primordidal Germ Cells To Mature Oocytes In The Mouse

Oocytes, including from mammals, lack centrioles, but neither the mechanism by which mature eggs lose their centrioles nor the exact stage at which centrioles are destroyed during oogenesis is known. To answer questions raised by centriole disappearance during oogenesis, using a transgenic mouse expressing GFP-centrin-2 (GFP CETN2), we traced their presence from e11.5 primordial germ cells (PGCs) through oogenesis and their ultimate dissolution in mature oocytes. We show tightly coupled CETN2 doublets in PGCs, oogonia, and pre-pubertal oocytes. Beginning with follicular recruitment of incompetent germinal vesicle (GV) oocytes, through full oocyte maturation, the CETN2 doublets separate within the pericentriolar material (PCM) and a rise in single CETN2 pairs is identified, mostly at meiotic metaphase-I and -II spindle poles. Partial CETN2 foci dissolution occurs even as other centriole markers, like Cep135, a protein necessary for centriole duplication, are maintained at the PCM. Furthermore, live imaging demonstrates that the link between the two centrioles breaks as meiosis resumes and that centriole association with the PCM is progressively lost. Microtubule inhibition shows that centriole dissolution is uncoupled from microtubule dynamics. Thus, centriole doublets, present in early G2-arrested meiotic prophase oocytes, begin partial reduction during follicular recruitment and meiotic resumption, later than previously thought.

Functions and dysfunctions of the mammalian centrosome in health, disorders, disease, and aging

Since its discovery well over 100 years ago (Flemming, in Sitzungsber Akad Wissensch Wien 71:81-147, 1875; Van Beneden, in Bull Acad R Belg 42:35-97, 1876) the centrosome is increasingly being recognized as a most impactful organelle for its role not only as primary microtubule organizing center (MTOC) but also as a major communication center for signal transduction pathways and as a center for proteolytic activities. Its significance for cell cycle regulation has been well studied and we now also know that centrosome dysfunctions are implicated in numerous diseases and disorders including cancer, Alstrom syndrome, Bardet-Biedl syndrome, Huntington's disease, reproductive disorders, and several other diseases and disorders. The present review is meant to build on information presented in the previous review (Schatten, in Histochem Cell Biol 129:667-686, 2008) and to highlight functions of the mammalian centrosome in health, and dysfunctions in disorders, disease, and aging with six sections focused on (1) centrosome structure and functions, and new insights into the role of centrosomes in cell cycle progression; (2) the role of centrosomes in tumor initiation and progression; (3) primary cilia, centrosome-primary cilia interactions, and consequences for cell cycle functions in health and disease; (4) transitions from centrosome to non-centrosome functions during cellular polarization; (5) other centrosome dysfunctions associated with the pathogenesis of human disease; and (6) centrosome functions in oocyte germ cells and dysfunctions in reproductive disorders and reproductive aging.

Involvement of LIMK1/2 in actin assembly during mouse embryo development

LIMKs (LIMK1 and LIMK2) are serine/threonine protein kinases that involve in various cellular activities such as cell migration, morphogenesis and cytokinesis. However, its roles during mammalian early embryo development are still unclear. In the present study, we disrupted LIMK1/2 activity to explore the functions of LIMK1/2 during mouse early embryo development. We found that p-LIMK1/2 mainly located at the cortex of each blastomeres from 2-cell to 8-cell stage, and p-LIMK1/2 also expressed at morula and blastocyst stage in mouse embryos. Inhibition of LIMK1/2 activity by LIMKi 3 (BMS-5) at the zygote stage caused the failure of embryo early cleavage, and the disruption of LIMK1/2 activity at 8-cell stage caused the defects of embryo compaction and blastocyst formation. Fluorescence staining and intensity analysis results demonstrated that the inhibition of LIMK1/2 activity caused aberrant cortex actin expression and the decrease of phosphorylated cofilin in mouse embryos. Taken together, we identified LIMK1/2 as an important regulator for cofilin phosphorylation and actin assembly during mouse early embryo development.

Noncanonical Biogenesis of Centrioles and Basal Bodies

Centrioles and basal bodies (CBBs) organize centrosomes and cilia within eukaryotic cells. These organelles are composed of microtubules and hundreds of proteins performing multiple functions such as signaling, cytoskeleton remodeling, and cell motility. The CBB is present in all branches of the eukaryotic tree of life and, despite its ultrastructural and protein conservation, there is diversity in its function, occurrence (i.e., presence/absence), and modes of biogenesis across species. In this review, we provide an overview of the multiple pathways through which CBBs are formed in nature, with a special focus on the less studied, noncanonical ways. Despite the differences among each mechanism herein presented, we highlighted some of their common principles. These principles, governing different steps of biogenesis, ensure that CBBs may perform a multitude of functions in a huge diversity of organisms but yet retained their robustness in structure throughout evolution.

A microtubule-organizing center directing intracellular transport in the early mouse embryo

The centrosome is the primary microtubule-organizing center (MTOC) of most animal cells; however, this organelle is absent during early mammalian development. Therefore, the mechanism by which the mammalian embryo organizes its microtubules (MTs) is unclear. We visualize MT bridges connecting pairs of cells and show that the cytokinetic bridge does not undergo stereotypical abscission after cell division. Instead, it serves as scaffold for the accumulation of the MT minus-end-stabilizing protein CAMSAP3 throughout interphase, thereby transforming this structure into a noncentrosomal MTOC. Transport of the cell adhesion molecule E-cadherin to the membrane is coordinated by this MTOC and is required to form the pluripotent inner mass. Our study reveals a noncentrosomal form of MT organization that directs intracellular transport and is essential for mammalian development.

Anillin controls cleavage furrow formation in the course of asymmetric division during mouse oocyte maturation

Anillin is a scaffold protein that recruits several proteins involved in cleavage furrow formation during cytokinesis. The role of anilllin in symmetric cell divisions in somatic cells has been intensively studied, yet its involvement in cleavage furrow formation is still elusive. In this study, we investigated the role of anillin in mammalian oocyte maturation and cytokinesis. We found that anillin is localized around the nucleus during the oocyte germinal-vesicle stage, and spreads to the cytoplasm after germinal vesicle breakdown. Thereafter, anillin concentrates at the site of the cleavage furrow from anaphase I to metaphase II. Disruption of anillin activity by microinjecting oocytes with specific siRNAs resulted in a failure of polar body extrusion and asymmetric division, and caused abnormal chromosome segregation during anaphase I. Furthermore, pharmacological inhibition of myosin light chain using Y-27632 or ML-7 resulted in decreased anillin expression. Collectively, our data suggest that anillin is an essential intracellular component that maintains the integrity of asymmetric division in mouse oocytes. Mol. Reprod. Dev. 83: 792-801, 2016 © 2016 Wiley Periodicals, Inc.

Optical coherence microscopy as a novel, non-invasive method for the 4D live imaging of early mammalian embryos

Imaging of living cells based on traditional fluorescence and confocal laser scanning microscopy has delivered an enormous amount of information critical for understanding biological processes in single cells. However, the requirement for a high numerical aperture and fluorescent markers still limits researchers’ ability to visualize the cellular architecture without causing short- and long-term photodamage. Optical coherence microscopy (OCM) is a promising alternative that circumvents the technical limitations of fluorescence imaging techniques and provides unique access to fundamental aspects of early embryonic development, without the requirement for sample pre-processing or labeling. In the present paper, we utilized the internal motion of cytoplasm, as well as custom scanning and signal processing protocols, to effectively reduce the speckle noise typical for standard OCM and enable high-resolution intracellular time-lapse imaging. To test our imaging system we used mouse and pig oocytes and embryos and visualized them through fertilization and the first embryonic division, as well as at selected stages of oogenesis and preimplantation development. Because all morphological and morphokinetic properties recorded by OCM are believed to be biomarkers of oocyte/embryo quality, OCM may represent a new chapter in imaging-based preimplantation embryo diagnostics.

Cytoskeletal alterations associated with donor age and culture interval for equine oocytes and potential zygotes that failed to cleave after intracytoplasmic sperm injection

Intracytoplasmic sperm injection (ICSI) is an established method to fertilise equine oocytes, but not all oocytes cleave after ICSI. The aims of the present study were to examine cytoskeleton patterns in oocytes after aging in vitro for 0, 24 or 48h (Experiment 1) and in potential zygotes that failed to cleave after ICSI of oocytes from donors of different ages (Experiment 2). Cytoplasmic multiasters were observed after oocyte aging for 48h (P<0.01). A similar increase in multiasters was observed with an increased interval after ICSI for young mares (9-13 years) but not old (20-25 years) mares. Actin vesicles were observed more frequently in sperm-injected oocytes from old than young mares. In the present study, multiasters appeared to be associated with cell aging, whereas actin vesicles were associated with aging of the oocyte donor.

Centrosome and microtubule functions and dysfunctions in meiosis: implications for age-related infertility and developmental disorders

The effects of oocyte aging on meiotic spindle dynamics have been well recognised, but the mechanisms underlying the effects are not well understood. In this paper we review the role of centrosomes and the microtubule cytoskeleton in meiotic spindle formation and maintenance, and the impact of oocyte aging on spindle integrity resulting in centrosome and microtubule dysfunctions that are associated with aneuploidy. Loss of spindle integrity includes dispersion of proteins from the centrosome core structure and loss of attachment of microtubules to centrosomes and kinetochores, which will result in abnormal chromosome separation. The inability of centrosomal proteins to accurately associate with the centrosome structure may be the result of destabilisation of the core structure itself or of microtubule destabilisation at the centrosome-facing microtubule areas that are acetylated in fresh oocytes but may not be acetylated in aging oocytes. Microtubule destabilisation prevents accurate motor-driven transport of centrosomal proteins along microtubules to form and maintain a functional centrosome. Other factors to form and maintain the MII spindle include signal transductions that affect microtubule dynamics and stability. Understanding the mechanisms underlying centrosome and microtubule dysfunctions during oocyte aging will allow diagnosis and analysis of oocyte quality and abnormalities as important aspects for targeted treatment of aging oocytes to extend or restore viability and developmental capacity. New therapeutic approaches will allow improvements in reproductive success rates in IVF clinics, as well as improvements in reproductive success rates in farm animals. This review is focused on: (1) centrosome and microtubule dynamics in fresh and aging oocytes; (2) regulation of centrosome and/or microtubule dynamics and function; and (3) possible treatments to extend the oocyte's reproductive capacity and viability span.

Non-destructive monitoring of mouse embryo development and its qualitative evaluation at the molecular level using Raman spectroscopy

Current research focuses on embryonic development and quality not only by considering fundamental biology, but also by aiming to improve assisted reproduction technologies, such as in vitro fertilization. In this study, we explored the development of mouse embryo and its quality based on molecular information, obtained nondestructively using Raman spectroscopy. The detailed analysis of Raman spectra measured in situ during embryonic development revealed a temporary increase in protein content after fertilization. Proteins with a β-sheet structure—present in the early stages of embryonic development—are derived from maternal oocytes, while α-helical proteins are additionally generated by switching on a gene after fertilization. The transition from maternal to embryonic control during development can be non-destructively profiled, thus facilitating the in situ assessment of structural changes and component variation in proteins generated by metabolic activity. Furthermore, it was indicated that embryos with low-grade morphology had high concentrations of lipids and hydroxyapatite. This technique could be used for embryo quality testing in the future.

Asymmetries and Symmetries in the Mouse Oocyte and Zygote

Mammalian oocytes grow periodically after puberty thanks to the dialogue with their niche in the follicle. This communication between somatic and germ cells promotes the accumulation, inside the oocyte, of maternal RNAs, proteins and other molecules that will sustain the two gamete divisions and early embryo development up to its implantation. In order to preserve their stock of maternal products, oocytes from all species divide twice minimizing the volume of their daughter cells to their own benefit. For this, they undergo asymmetric divisions in size where one main objective is to locate the division spindle with its chromosomes off-centred. In this chapter, we will review how this main objective is reached with an emphasis on the role of actin microfilaments in this process in mouse oocytes, the most studied example in mammals. This chapter is subdivided into three parts: I-General features of asymmetric divisions in mouse oocytes, II-Mechanism of chromosome positioning by actin in mouse oocytes and III-Switch from asymmetric to symmetric division at the oocyte-to-embryo transition.

Post-Testicular Sperm Maturation: Centriole Pairs, Found in Upper Epididymis, are Destroyed Prior to Sperm's Release at Ejaculation

The fertilizing sperm’s lengthiest unchartered voyage is through the longest, least-investigated organ in a man’s body – the Epididymis. Over six meters long in men, ~80 meters in stallions and over one-hundred times a mouse’s body length, there are few functions known aside from sperm storage and nutrition. While spermatogenesis is completed in the testes, here we demonstrate sperm centriole reduction occurs within the epididymis. Investigations of GFP-CENTR mice and controls demonstrate both the presence of centriole pairs in the upper caput region of the epididymis and, the destruction, first, of the distal and, then, of the proximal centriole as the sperm transits to the cauda and vas deferens in preparation for its climactic release. These centrioles can neither recruit γ-tubulin nor nucleate microtubules when eggs are inseminated or microinjected, yet numerous maternally-nucleated cytasters are found. These sperm centrioles appear as vestigial basal bodies, destroyed in the mid-to-lower corpus. Post-testicular sperm maturation, in which sperm centrioles found in the caput are destroyed prior to ejaculation, is a newly discovered function for the epididymis.

From Meiosis to Mitosis: The Astonishing Flexibility of Cell Division Mechanisms in Early Mammalian Development

The execution of female meiosis and the establishment of the zygote is arguably the most critical stage of mammalian development. The egg can be arrested in the prophase of meiosis I for decades, and when it is activated, the spindle is assembled de novo. This spindle must function with the highest of fidelity and yet its assembly is unusually achieved in the absence of conventional centrosomes and with minimal influence of chromatin. Moreover, its dramatic asymmetric positioning is achieved through remarkable properties of the actin cytoskeleton to ensure elimination of the polar bodies. The second meiotic arrest marks a uniquely prolonged metaphase eventually interrupted by egg activation at fertilization to complete meiosis and mark a period of preparation of the male and female pronuclear genomes not only for their entry into the mitotic cleavage divisions but also for the imminent prospect of their zygotic expression.