Studies on the formation and state of determination of the trunk organizer in the newt,Cynops pyrrhogaster

Summary The exact localization of the presumptive trunk organizer was determined by means of vital staining at the initiation of gastrulation (0 h embryo) and subsequently in 6, 9, 12 and 24 h embryos.The progressive changes in the self-differentiation and inductive capacity of the trunk organizer were studied in isolation cultures (sitting drop) and in sandwich cultures with competent gastrula ectoderm. In the 0 and 6 h embryo cultures the excised trunk organizer predominantly formed atypical ectoderm. A dramatic change in differentiation and inductive capacity occurred in the 9 h embryo. The positive cases — 83% of the isolation and 50% of the sandwich cultures — mainly formed notochord and somites, accompanied by spinal cord and hindbrain in the sandwich cultures. Although no further change in self-differentiation occurred from that time onwards, a gradual increase in inductive capacity was recognized.     

NOR-Chromosome morphology and evidence for rDNA selection in phylactolaemates

Silver-stained chromosome spreads from the phylactolaemate ectoprocts Fredericella indica, Fredericella browni, Plumatella repens, Pectinatella magnifica and Asajirella gelatinosa show that each of these species has a single pair of NORs. In the first four species each NOR is immediately adjacent to the centromere of a large metacentric chromosome. Giemsa-stained preparations from Cristatella mucedo show centromeric gapping in a similar large metacentric, indicating the same type of NOR-chromosome. In Asajirella gelatinosa the two NORs are on a pair of smaller chromosomes, and are not immediately adjacent to a centromere. A close NOR-centromere association would inhibit meiotic crossing over in the NOR region. Since in phylactolaemates reproduction is primarily asexual, often by formation of statoblasts, there is potential intraclonal selection for favorable ribosomal DNA (rDNA) mutations, including from simultaneous germination of otherwise genetically identical statoblasts within a limited area and competition between cell lines within statoblasts. Inhibition of meiotic crossing over within the NOR would maintain its integrity during periods when intraclonal selection would not be occurring. This suggests that a more rapid rate of rDNA evolution is possible in phylactolaemates than in most other organisms, and is consistent with a derivation of this group from the morphologically similar phoronids, despite phylogenetic interpretations of rDNA sequence data that suggest the two groups are not closely related.     

Inductive interactions required for mesodermal differentiation in Bufo arenarum gastrulae

 Dorso-marginal epithelium, a distinct crescent-shaped region in the early gastrula of Bufo arenarum, appears after involution as a narrow dorso-median strip of archenteric endoderm close to the notochord. In explant cultures, this layer showed an extreme dorsalizing activity, promoting the formation of notochordal structures from ventro-mesodermal cells. In the presence of ectoderm, this inductive activity was expanded resulting in a wide range of dorso-lateral components such as notochord, muscle, nephric tubules, mesothelium and mesenchyme. The mesodermal origin of these derivatives was confirmed by the use of FDA (fluorescein-dextran-amine)-labelled explants. Extensive mesodermal development in cultures seems to require cell contacts between the inner aspect of the dorso-marginal epithelium and mesodermal cells. When such contacts were prevented, cultures would only differentiate erythrocytes as a result of a purely ectodermal stimulus. Bisection of the dorso-marginal epithelium in whole embryos resulted in the development of a duplicated set of axial structures, clearly showing the role of the epithelium as a dorsal organizer. Received: 30 July 1996 / Accepted: 20 February 1997     

Nucleolar organizer regions in the fine needle aspirates of lung tumours

Nucleolar organizer region associated proteins (AgNOR proteins) have been identified by means of an argyrophil technique in smears of 60 consecutive fine needle aspirates from lung tumours. AgNOR proteins were visualized as silver-positive granules distributed in loose, intranuclear aggregates. Variations in the number as well as differences in the distribution pattern of AgNOR granules were found among different types of tumours. Except for small cell lung carcinoma, the count of AgNOR granules increased when the differentiation of tumours decreased. In particular well differentiated tumours had relatively few AgNOR granules, distributed in cohesive aggregates. Poorly differentiated tumours had many AgNOR granules organized in loose clusters and small cell lung carcinoma had relatively few granules dispersed throughout the nucleoplasm, showing characteristics unique among all lung neoplasms. The application of the AgNOR technique in cyto-preparations is useful in discriminating between small cell and non-small cell lung carcinomas. Moreover, the pattern of distribution of AgNOR proteins may be of diagnostic value in the assessment of tumours displaying overlap in AgNOR counts.