Splitting placodes: effects of bone morphogenetic protein and Activin on the patterning and identity of mouse incisors

SUMMARY The single large rodent incisor in each jaw quadrant is evolutionarily derived from a mammalian ancestor with many small incisors. The embryonic placode giving rise to the mouse incisor is considerably larger than the molar placode, and the question remains whether this large incisor placode is a developmental requisite to make a thick incisor. Here we used in vitro culture system to experiment with the molecular mechanism regulating tooth placode development and how mice have thick incisors. We found that large placodes are prone to disintegration and formation of two to three small incisor placodes. The balance between one large or multiple small placodes was altered through the regulation of bone morphogenetic protein (BMP) and Activin signaling. Exogenous Noggin, which inhibits BMP signaling, or exogenous Activin cause the development of two to three incisors. These incisors were more slender than normal incisors. Additionally, two inhibitor molecules, Sostdc1 and Follistatin, which regulate the effects of BMPs and Activin and have opposite expression patterns, are likely to be involved in the incisor placode regulation in vivo. Furthermore, inhibition of BMPs by recombinant Noggin has been previously suggested to cause a change in the tooth identity from the incisor to the molar. This evidence has been used to support a homeobox code in determining tooth identity. Our work provides an alternative interpretation, where the inhibition of BMP signaling can lead to splitting of the large incisor placode and the formation of partly separate incisors, thereby acquiring molar‐like morphology without a change in tooth identity.     

Mitochondria: in sickness and in health

Mitochondria perform diverse yet interconnected functions, producing ATP and many biosynthetic intermediates while also contributing to cellular stress responses such as autophagy and apoptosis. Mitochondria form a dynamic, interconnected network that is intimately integrated with other cellular compartments. In addition, mitochondrial functions extend beyond the boundaries of the cell and influence an organism's physiology by regulating communication between cells and tissues. It is therefore not surprising that mitochondrial dysfunction has emerged as a key factor in a myriad of diseases, including neurodegenerative and metabolic disorders. We provide a current view of how mitochondrial functions impinge on health and disease.     

Temporal variability in detritus resource maintains diversity of bacterial communities

Competition theory generally predicts that diversity is maintained by temporal environmental fluctuations. One of the many suggested mechanisms for maintaining diversity in fluctuating environments is the gleaner-opportunist trade-off, whereby gleaner species have low threshold resource levels and low maximum growth rates in high resource concentration while opportunist species show opposite characteristics. We measured the growth rates of eight heterotrophic aquatic bacteria under different concentrations of chemically complex plant detritus resource. The growth rates revealed gleaner-opportunist trade-offs. The role of environmental variability in maintaining diversity was tested in a 28-day experiment with three different resource fluctuation regimes imposed on two four-species bacterial communities in microcosms. We recorded population densities with serial dilution plating and total biomass as turbidity. Changes in resource availability were measured from filter-sterilised medium by re-introducing the consumer species and recording short-term growth rates. The type of environmental variation had no effect on resource availability, which declined slowly during the experiment and differed in level between the communities. However, the slowly fluctuating environment had the highest Shannon diversity index, biomass, and coefficient of variation of biomass in both communities. We did not find a clear link between the gleaner-opportunist trade-off and diversity in fluctuating environments. Nevertheless, our results do not exclude this explanation and support the general view that temporal environmental variation maintains species diversity also in communities feeding chemically complex resource.     

Achiasmatische Oogenese bei Trichopteren

No chiasmata are formed in the oogenesis of the caddis flies Limnophilus decipiens and L. borealis. In the Trichoptera, in likeness to what has been found in earlier cytologically analysed cases, the achiasmatic meiosis is confined to the heterogametic sex. The orders Trichoptera and Lepidoptera share the following cytological features: a) The female is heterogametic. b) No chiasmata are formed in oogenesis. c) Chromatin elimination takes place in the first meiotic division in the egg. d) In the Lepidoptera the chromosomes are holokinetic. This seems to be the case in the Trichoptera, too. e) Apyrene sperms are common. f) The most frequent chromosome number is practically the same: 31 in the Lepidoptera and 30 in the Trichoptera. These cytological similarities obviously originate from a period preceding the divergence of Trichoptera and Lepidoptera; this branching took place at the latest before the tertiary period. The preservation of these cytological similarities for at least 60 to 70 million years is an indication of the great stability of such cytogenetic system.

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Herrn Professor Dr. J. Seiler zum 80. Geburtstag gewidmet.

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Diese Untersuchung wurde ausgeführt mit Unterstützung der Finnischen Staatlichen Kommission für die Naturwissenschaften.     

Selection of multiresistant coliforms by long-term treatment of hypercholesterolaemia with neomycin.

Patients with hypercholesterolaemia are often treated with the antimicrobial agent neomycin. Such treatment is potentially dangerous, however, as it may favour the emergence of multiresistant, R-factor-carrying, enteric bacteria among the intestinal flora. In 11 out of 14 patients who had received neomycin for three months to eight years most of the faecal coliforms were resistant to at least four antimicrobial drugs and capable of transferring this resistance to others. In contrast, only one out of nine patients who were treated with other lipid-lowering drugs had resistant bacteria in their faeces. Neomycin may cause multiresistant strains to emerge because, like tetracycline, it forms high concentrations in the gut. Long-term treatment of non-infectious conditions like hypercholesterolaemia with neomycin is potentially dangerous not only to the patient but also to the community because of the creation of a reservoir of multiresistant organisms.