Rediscovering Boveri's centrosome in Ascaris (1888): its impact on human fertility and development

Nematode chromosomes

The nematode Caenorhabditis elegans has shed light on many aspects of eukaryotic biology, including genetics, development, cell biology, and genomics. A major factor in the success of C. elegans as a model organism has been the availability, since the late 1990s, of an essentially gap-free and well-annotated nuclear genome sequence, divided among 6 chromosomes. In this review, we discuss the structure, function, and biology of C. elegans chromosomes and then provide a general perspective on chromosome biology in other diverse nematode species. We highlight malleable chromosome features including centromeres, telomeres, and repetitive elements, as well as the remarkable process of programmed DNA elimination (historically described as chromatin diminution) that induces loss of portions of the genome in somatic cells of a handful of nematode species. An exciting future prospect is that nematode species may enable experimental approaches to study chromosome features and to test models of chromosome evolution. In the long term, fundamental insights regarding how speciation is integrated with chromosome biology may be revealed.

Distinct mechanisms eliminate mother and daughter centrioles in meiosis of starfish oocytes

Centriole elimination is an essential process that occurs in female meiosis of metazoa to reset centriole number in the zygote at fertilization. How centrioles are eliminated remains poorly understood. Here we visualize the entire elimination process live in starfish oocytes. Using specific fluorescent markers, we demonstrate that the two older, mother centrioles are selectively removed from the oocyte by extrusion into polar bodies. We show that this requires specific positioning of the second meiotic spindle, achieved by dynein-driven transport, and anchorage of the mother centriole to the plasma membrane via mother-specific appendages. In contrast, the single daughter centriole remaining in the egg is eliminated before the first embryonic cleavage. We demonstrate that these distinct elimination mechanisms are necessary because if mother centrioles are artificially retained, they cannot be inactivated, resulting in multipolar zygotic spindles. Thus, our findings reveal a dual mechanism to eliminate centrioles: mothers are physically removed, whereas daughters are eliminated in the cytoplasm, preparing the egg for fertilization.

Impact of insemination technique, semen quality and oocyte cryopreservation on pronuclear morphology of zygotes derived from sibling oocytes

Pronuclear morphology seems to be an important predictive value of zygote development and integrity. In this study we want to evaluate the effect of insemination technique, male factor and oocyte cryopreservation on pronuclear morphology of zygotes derived from sibling oocytes in our Centre of Reproductive Medicine, Department of Obstetrics and Gynecology, Arcispedale S. Maria Nuova, Reggio Emilia, Italy. Subjects (n = 190) were submitted to IVF cycles with non-frozen and frozen sibling oocytes. Morphological evaluations were assessed using zygote pronuclear morphology (pronuclei, nucleoli and axis) in four groups: Group 1: 144 zygotes from 85 conventional IVF cycles with non-frozen oocytes; Group 2: 164 zygotes from 85 intracytoplasmic sperm injection (ICSI) cycles with Group 1 patients' sibling frozen oocytes; Group 3: 221 zygotes from 123 ICSI cycles with non-frozen oocytes; Group 4: 197 zygotes from 123 ICSI cycles with Group 3 patients' sibling frozen oocytes. No differences between Group 1 and Group 2 were seen. Group 3 was statistically different from Group 4 in relation to the nucleolar morphology. Oocyte cryopreservation procedure modified the nucleolar morphology of zygotes only in the presence of poor semen quality.

Increased sperm ubiquitination correlates with abnormal chromatin integrity

Ubiquitin, a 8.5 kDa peptide that marks other proteins for proteasomal degradation, tags defective spermatozoa during epididymal passage and is proposed as a biomarker for sperm quality. The present study was designed to evaluate the relationships between sperm ubiquitination, sperm chromatin integrity and semen parameters. Semen samples from 63 couples were collected and analysed according to World Health Organization criteria. Each sample was evaluated for sperm ubiquitination by the direct immunofluorescence method, using anti-ubiquitin antibodies. Chromatin integrity of the same samples was analysed using acridine orange (AO) and toluidine blue (TB) tests. A positive correlation was found between ubiquitinated spermatozoa and the percentage of spermatozoa with abnormal chromatin (AO: r = 0.58, P < 0.001 and TB: r = 0.48, P < 0.001). Negative correlations were obtained between sperm ubiquitination and: sperm count (r = -0.2, P = 0.048), sperm morphology (r = -0.36, P = 0.003), rapidly progressive motility (r = -0.25, P = 0.044) and slow progressive motility (r = -0.28, P = 0.022). Sperm ubiquitination was positively correlated with the percentage of immotile spermatozoa. These results show that among semen parameters, chromatin abnormality is more closely associated with sperm ubiquitination and further validate sperm ubiquitination as a suitable marker for sperm quality.

On fertilization in Chaetopleura apiculata and selected Chitonida

Early events of fertilization are described in Chaetopleura apiculata and other selected Chitonida. C. apiculata egg hulls are elaborated into multi-branched spines with interlocking polygonal bases. Around the perimeter of each base are a series of open pores, ranging in size from 0.1-0.5 microm, which permit sperm direct access to the vitelline layer. In Callochitonidae (Chitonida) even larger pores occur in egg jelly coats, but this is considered to be the plesiomorphic condition, found also in Lepidopleurida such as Deshayesiella curvata. Other Chitonina, such as Rhyssoplax tulipa and Acanthopleura granulata, have a continuous outer dense layer that lacks pores and must be digested by penetrating sperm. Fertilization in Chitonida is unique and involves injection of chromatin into the egg via a narrow tubular nuclear extension that appears to exclude other sperm organelles, including mitochondria, centrioles, and flagellum. New evidence from studies of fertilization in Mopalia muscosa (Chitonida: Acanthochitonina) supports this hypothesis. This type of fertilization implies maternal inheritance of both mitochondria and centrioles, which is highly unusual, because in most animals one sperm centriole assists movements of pronuclei and regulates organization of the mitotic spindle. This mechanism of fertilization is defined by a series of apomorphic characters that unify the order Chitonida.

Centriole inheritance

Early cell biologists perceived centrosomes to be permanent cellular structures. Centrosomes were observed to reproduce once each cycle and to orchestrate assembly a transient mitotic apparatus that segregated chromosomes and a centrosome to each daughter at the completion of cell division. Centrosomes are composed of a pair of centrioles buried in a complex pericentriolar matrix. The bulk of microtubules in cells lie with one end buried in the pericentriolar matrix and the other extending outward into the cytoplasm. Centrioles recruit and organize pericentriolar material. As a result, centrioles dominate microtubule organization and spindle assembly in cells born with centrosomes. Centrioles duplicate in concert with chromosomes during the cell cycle. At the onset of mitosis, sibling centrosomes separate and establish a bipolar spindle that partitions a set of chromosomes and a centrosome to each daughter cell at the completion of mitosis and cell division. Centriole inheritance has historically been ascribed to a template mechanism in which the parental centriole contributed to, if not directed, assembly of a single new centriole once each cell cycle. It is now clear that neither centrioles nor centrosomes are essential to cell proliferation. This review examines the recent literature on inheritance of centrioles in animal cells.