Spermatogenesis and distinctive mature sperm in the giant freshwater prawn, Macrobrachium rosenbergii (De Man, 1879)

The structures of differentiating male germ cells in the testis of the giant freshwater prawn, Macrobrachium rosenbergii, were studied by light and electron microscopy. Based on ultrastructural characteristics, the developing male germ cells are classified into 12 stages, including spermatogonia, six phases of primary spermatocytes (leptotene, zygotene, pachytene, diplotene, diakinesis and metaphase), secondary spermatocyte, three stages of spermatids and mature sperm. During spermatogenesis, the differentiating germ cells have characteristics similar to those of other invertebrates, but they exhibit some unique characteristics during spermiogenesis. In particular, an early spermatid has a round nucleus with highly condensed heterochromatin, appearing as thick interconnecting cords throughout the nucleus. In contrast to most invertebrates and vertebrates, the chromatin begins to decondense in one-half of the nucleus at the mid spermatid stage. In the late spermatid, the chromatin becomes almost entirely decondensed with only a small crescent-shaped heterochromatin patch remaining at the anterior pole of the nucleus. Mature sperm possess an everted umbrella-shaped plate with a spike covering the anterior pole of the nucleus, whose chromatin is totally decondensed as only small traces of histones H3 and H2B remain. The acrosome appears at the ruffled border of the spike plate as small sac-like structures. Few mitochondria remain in the cytoplasm at the posterior pole.     

An evaluation of the phylogenetic position of the dinoflagellateCrypthecodinium cohnii based on 5S rRNA characterization

Summary Partial nucleotide sequences for the 5S and 5.8S rRNAs from the dinoflagellateCrypthecodinium cohnii have been determined, using a rapid chemical sequencing method, for the purpose of studying dinoflagellate phylogeny. The 5S RNA sequence shows the most homology (75%) with the 5S sequences of higher animals and the least homology (< 60%) with prokaryotic sequences. In addition, it lacks certain residues which are highly conserved in prokaryotic molecules but are generally missing in eukaryotes. These findings suggest a distant relationship between dinoflagellates and the prokaryotes. Using two different sequence alignments and several different methods for selecting an optimum phylogenetic tree for a collection of 5S sequences including higher plants and animals, fungi, and bacteria in addition to theC. cohnii sequence, the dinoflagellate lineage was joined to the tree at the point of the plant-animal divergence, well above the branching point of the fungi. This result is of interest because it implies that the well-documented absence in dinoflagellates of histones and the typical nucleosomal subunit structure of eukaryotic chromatin is the result of secondary loss. and not anindication of an extremely primitive state, as was previously suggested. Computer simulations of 5S RNA evolution have been carried out in order to demonstrate that the above-mentioned phylogenetic placement is not likely to be the result of random sequence convergence.We have also constructed a phylogeny for 5.8S RNA sequences in which plants, animals, fungi and the dinoflagellates are again represented. While the order of branching on this tree is the same as in the 5S tree for the organisms represented, because it lacks prokaryotes, the 5.8S tree cannot be considered a strong independent confirmation of the 5S result. Moreover, 5.8S RNA appears to have experienced very different rates of evolution in different lineages indicating that it may not be the best indicator of evolutionary relationships.We have also considered the existing biological data regarding dinoflagellate evolution in relation to our molecular phylogenetic evidence.     

Progression into and out of mitosis

Progression through mitosis is controlled by cyclin-dependent kinases, which drive cells into metaphase, and by the anaphase-promoting complex/cyclosome, a ubiquitin ligase that triggers sister chromatid separation and exit from mitosis. Recent work has shown how the mutual regulation between cyclin-dependent kinases and the anaphase-promoting complex/cyclosome ensures that cell-cycle events occur in the right order. The analysis of complexes required for sister chromatid cohesion and chromosome condensation has revealed how cyclin-dependent kinases and the anaphase-promoting complex/cyclosome control the behaviour of chromosomes.     

Functional behaviour of bone around dental implants

Achieving a long-term stable implant interface is a significant clinical issue when there is insufficient cortical bone stabilisation at implant placement. Clinical outcomes studies suggest that the higher risk implants are those placed in compromised cortical bone (thin, porous, etc.) in anatomical sites with minimal existing trabecular bone (characterised as type IV bone). In establishing and maintaining an implant interface in such an environment, one needs to consider the impact of masticatory forces, the response of bone to these forces and the impact of age on the adaptive capacity of bone. These forces, in turn, have the potential to create localised changes in interfacial stiffness through viscoelastic changes at the interface. Changes in bone as a function of age (e.g. localised hypermineralised osteopetrosis and localised areas of osteopenia) will alter the communication between osteocytes and osteoblasts creating the potential for differences in response of osteoblastic cells in the older population. A key to understanding the biomechanical and functional behaviour of implants in the older population is to control the anticipated modelling and remodelling behaviour through implant design that takes into account how tissues respond to the mechanically active environment.     

Positioning the genome within the nucleus

Higher eukaryotic genomes contain both housekeeping genes and genes of which the expression is restricted to a defined time and space. It is well established that a correlation exists between structural organization of the genome and gene expression control. The functional mechanisms underlying this correlation are still poorly understood. Here I describe several observations that are the basis of present concepts of genome organization and nuclear architecture related to functionality. Regarding the relationship between positioning and disturbed cell functionality, I describe observations showing that the proximity of selected gene loci is statistically correlated with their propensity for oncogenic translocations as well as observations of patterns occurring in neurodegenerative disorders where unstable repeats are translated into an expanded polyglutamine tract. Such observations underscore the importance to understand how genetic perturbations lead to the global reorganization of nuclear architecture, chromatin structure and widespread changes in gene expression.