Design of novel oocyte activation methods: The role of zinc

Oocyte activation occurs at the time of fertilization and is a series of cellular events initiated by intracellular Ca2+ increases. Consequently, oocytes are alleviated from their arrested state in meiotic metaphase II (MII), allowing for the completion of meiosis. Oocyte activation is also an essential step for somatic cell nuclear transfer (SCNT) and an important tool to overcome clinical infertility. Traditional artificial activation methods aim to mimic the intracellular Ca2+ changes which occur during fertilization. Recent studies emphasize the importance of cytoplasmic Zn2+ on oocyte maturation and the completion of meiosis, thus suggesting artificial oocyte activation approaches that are centered around the concentration of available Zn2+in oocytes. Depletion of intracellular Zn2+ in oocytes with heavy metal chelators leads to successful oocyte activation in the absence of cellular Ca2+ changes, indicating that successful oocyte activation does not always depends on intracellular Ca2+ increases. Current findings lead to new approaches to artificially activate mammalian oocytes by reducing available Zn2+ contents, and the approaches improve the outcome of oocyte activation when combined with existing Ca2+ based oocyte activation methods. Here, we review the important role of Ca2+ and Zn2+ in mammalian oocyte activation and development of novel oocyte activation approaches based on Zn2+ availability.

The structure of the ovotestis of the common African land snails found in Abeokuta, South Western Nigeria.

A comparative study of the structure of the ovotestis of African land snails found in Abeokuta was investigated. Of all the five snails species found and dissected, Archachatina marginata, Achatina achatina, A. fulica, Helix pomatia and Limcolaria aurora, only the ovotestis of H. pomatia could not be observed. The positioning, shape and arrangement of the ovotestis was the same for all the other four species. The ovotestis is embedded in the digestive gland at the anterior region of the coiled posterior end of the visceral mass. It is cream coloured and made up of sac-like lobes arranged on a single plane. It is differentiated into an ovariaon and testicular region. The number and size of the ovotestis differ significantly (P<0.05) in all the snails. Each lobe is made of several follicles and the number of which varied in the four species. Statistical analysis showed that live weight, shell length, shell width and shell circumference of the snails had no significant influence on the size on their ovotestis. Meiotic metaphase spreads of the ovotestis tissues revealed chromosome numbers 2n=56, 2n=44, 2n=54 and 2n=28 for A. marginata, A. achatina, A. fulica and L. aurora

Two types of highly ordered micro- and macrochromosome arrangement in metaphase plates of butterflies (Lepidoptera)

In karyotype of many organisms, chromosomes form two distinct size groups: macrochromosomes and microchromosomes. During cell divisions, the position of the macro- and microchromosomes is often ordered within metaphase plate. In many reptiles, amphibians, birds, insects of the orthopteran family Tettigoniidae and in some plants, a so called “reptilian” type organization is found, with microchromosomes situated in the center of metaphase plate and with macrochromosomes situated at the periphery. An opposite, “lepidopteran” type is known in butterflies and moths (i.e. in the order Lepidoptera) and is characterized by macrochromosomes situated in the center and by microchromosomes situated at the periphery. The anomalous arrangement found in Lepidoptera was previously explained by holocentric organization of their chromosomes. Here I analyse the structure of meiotic metaphase I plates in ithomiine butterfly, Forbestraolivencia (H. Bates, 1862) (Nymphalidae, Danainae, Ithomiini) which has a clear “reptilian” organization, contrary to previous observations in Lepidoptera. In this species large bivalents (i.e. macrochromosomes) form a regular peripheral circle, whereas the minute bivalents (i.e. microchromosomes) occupy the center of this circle. The reasons and possible mechanisms resulting in two drastically different spatial chromosome organization in butterflies are discussed.

Separation and Loss of Centrioles From Primordidal Germ Cells to Mature Oocytes in the Mouse

Oocytes, including those 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); concomitantly, a rise in single CETN2 pairs is identified. CETN2 dissolution accelerates following meiosis resumption. Remarkably, a single CETN2 pair is retained in the PCM of most meiotic metaphase-I and -II spindle poles. Partial dissolution of the CETN2 foci 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, much later than previously thought.

Condensins promote chromosome individualization and segregation during mitosis, meiosis, and amitosis in Tetrahymena thermophila

Condensin is a protein complex with diverse functions in chromatin packaging and chromosome condensation and segregation. We studied condensin in the evolutionarily distant protist model Tetrahymena, which features noncanonical nuclear organization and divisions. In Tetrahymena, the germline and soma are partitioned into two different nuclei within a single cell. Consistent with their functional specializations in sexual reproduction and gene expression, condensins of the germline nucleus and the polyploid somatic nucleus are composed of different subunits. Mitosis and meiosis of the germline nucleus and amitotic division of the somatic nucleus are all dependent on condensins. In condensin-depleted cells, a chromosome condensation defect was most striking at meiotic metaphase, when Tetrahymena chromosomes are normally most densely packaged. Live imaging of meiotic divisions in condensin-depleted cells showed repeated nuclear stretching and contraction as the chromosomes failed to separate. Condensin depletion also fundamentally altered chromosome arrangement in the polyploid somatic nucleus: multiple copies of homologous chromosomes tended to cluster, consistent with a previous model of condensin suppressing default somatic pairing. We propose that failure to form discrete chromosome territories is the common cause of the defects observed in the absence of condensins.

Meiotic shugoshins differ from mitotic ones by arginine-reach C-terminal motif in yeast, plant, animals, and human

Background. Shugoshins (SGOs) are proteins that protect cohesins located at the centromeres of sister chromatids from their early cleavage during mitosis and meiosis in plants, fungi, and animals. Their function is to prevent premature sister-chromatid disjunction and segregation. Meiotic SGOs prevent segregation of sister chromatids in meiosis I, thus permitting homologous chromosomes to segregate and reduce chromosome number to haploid set. The study focused on the structural differences among shugoshins acting during mitosis and meiosis that cause differences in chromosome behavior in these two types of cell division in different organisms. Methods. A bioinformatics analysis of protein domains, conserved amino acid motifs, and physicochemical properties of 32 proteins from 25 species of plants, fungi, and animals was performed. Results. We identified a C-terminal arginine-reach amino acid motif that is highly evolutionarily conserved among the shugoshins protecting centromere cohesion of sister chromatids in meiotic anaphase I, but not among mitotic shugoshins. The motif looks like “arginine comb” capable of interaction by hydrogen bonds with guanine bases in the small groove of DNA helix. Shugoshins in different eukaryotic kingdoms differ also in the sets and location of amino acid motifs and the number of α-helical regions in the protein molecule. Discussion. Meiosis-specific arginine-reach motif may be responsible for formation of SGO-DNA nucleoprotein complex, thus protecting meiotic shugoshins from degradation during meiotic metaphase I and anaphase I, while mitotic SGOs have a motif with less number of arginine residues. This structural difference between meiotic and mitotic shugoshins, probably, could be a key molecular element of the prolonged shugoshin resistance to degradation during meiotic metaphase I and anaphase I and be one of the molecular elements causing the difference in chromosome behavior in meiosis and mitosis. The finding of differences in SGO structure in plant, fungi and animals supports idea of independent evolution of meiosis in different lineages of multicellular organisms.

Meiotic shugoshins differ from mitotic ones by arginine-reach C-terminal motif in yeast, plant, animals, and human

Background. Shugoshins (SGOs) are proteins that protect cohesins located at the centromeres of sister chromatids from their early cleavage during mitosis and meiosis in plants, fungi, and animals. Their function is to prevent premature sister-chromatid disjunction and segregation. Meiotic SGOs prevent segregation of sister chromatids in meiosis I, thus permitting homologous chromosomes to segregate and reduce chromosome number to haploid set. The study focused on the structural differences among shugoshins acting during mitosis and meiosis that cause differences in chromosome behavior in these two types of cell division in different organisms. Methods. A bioinformatics analysis of protein domains, conserved amino acid motifs, and physicochemical properties of 32 proteins from 25 species of plants, fungi, and animals was performed. Results. We identified a C-terminal arginine-reach amino acid motif that is highly evolutionarily conserved among the shugoshins protecting centromere cohesion of sister chromatids in meiotic anaphase I, but not among mitotic shugoshins. The motif looks like “arginine comb” capable of interaction by hydrogen bonds with guanine bases in the small groove of DNA helix. Shugoshins in different eukaryotic kingdoms differ also in the sets and location of amino acid motifs and the number of α-helical regions in the protein molecule. Discussion. Meiosis-specific arginine-reach motif may be responsible for formation of SGO-DNA nucleoprotein complex, thus protecting meiotic shugoshins from degradation during meiotic metaphase I and anaphase I, while mitotic SGOs have a motif with less number of arginine residues. This structural difference between meiotic and mitotic shugoshins, probably, could be a key molecular element of the prolonged shugoshin resistance to degradation during meiotic metaphase I and anaphase I and be one of the molecular elements causing the difference in chromosome behavior in meiosis and mitosis. The finding of differences in SGO structure in plant, fungi and animals supports idea of independent evolution of meiosis in different lineages of multicellular organisms.