Imaging low-mass planets within the habitable zone of α Centauri.

Giant exoplanets on wide orbits have been directly imaged around young stars. If the thermal background in the mid-infrared can be mitigated, then exoplanets with lower masses can also be imaged. Here we present a ground-based mid-infrared observing approach that enables imaging low-mass temperate exoplanets around nearby stars, and in particular within the closest stellar system, α Centauri. Based on 75-80% of the best quality images from 100 h of cumulative observations, we demonstrate sensitivity to warm sub-Neptune-sized planets throughout much of the habitable zone of α Centauri A. This is an order of magnitude more sensitive than state-of-the-art exoplanet imaging mass detection limits. We also discuss a possible exoplanet or exozodiacal disk detection around α Centauri A. However, an instrumental artifact of unknown origin cannot be ruled out. These results demonstrate the feasibility of imaging rocky habitable-zone exoplanets with current and upcoming telescopes.

Chromosome behaviour during early meiotic prophase of mouse primary spermatocytes.

A correlative light- and electron microscopical study has yielded both a description of chromosome behaviour during early meiotic prophase in mouse spermatocytes, and an accurate timing of the zygotene and early pachytene stage. The light microscopic part of the study consists of an analysis of orcein stained or C-banded, air-dried preparations, and series of separately squashed sections of seminiferous tubules. The mice used for air-dried preparations were treated with hyroxyurea and triaziquone, which reduces the spermatocyte population to a small, well-defined group of cells in meiotic interphase. The development of the restricted spermatocyte population is followed during four days. The electron microscopical study consists of an investigation of synaptonemal complexes in thin sections of seminiferous tubules in various stages of the epithelial cycle. It turned out that synaptic pairing starts about half a day after the end of the DNA replication phase. At most one and a half day later chromosome pairing is completed. In very early pachytene spermatocytes a sex vesicle is not visible. The orientation of the X and Y chromosome into a sex vesicle starts about one day after the beginning of the pachytene stage. The existence of a leptotene stage is discussed. The presence of a pre-leptotene contraction phase in male mice is excluded.

The behaviour of silver-positive structures during meiotic prophase of male mice.

Air dried preparations and sections of mouse seminiferous tubules were stained with an ammoniacal silver solution to study the behaviour of silver positive structures in meiotic prophase I nuclei. The presence of RNA was investigated using specific staining techniques and RNase digestion. Pachytene nuclei showed silver precipitation at the paracentromeric region of two to six autosomal bivalents. In spermatogonia at least the chromosome nos. 12, 15, 16, 17 and 18 proved to have transcriptionally active nucleolus organizer regions. From late pachytene the Ag-positive structures migrate towards the sex vesicle. In prediffuse diplotene, when the silver spots reached the sex vesicle, a vacuole-like body appeared near the sex vesicle. At the same time significant amounts of RNA accumulate near to the sex vesicle. Finally, in diffuse diplotene a tripartite structure could be observed, composed of (a) a horseshoe-shaped structure adjacent to the sex vesicle, which contains a great deal of RNA, (b) a vacuole-like body, being enclosed by the horseshoe and (c) an Ag-positive mass, migrated from the nucleolus organizer regions. It is probable that the tripartite structure, or at least a part of it, is a large nucleolus. The significance of the structure is discussed.