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  • Koskinen, Mikko (Helsingin yliopisto, 2014)
    Dendritic spines are small bulbous protrusions extending from dendritic shafts of neurons. These compartments house most of the postsynaptic terminals of excitatory synapses in the mammalian central nervous system. Dendritic spines are formed during early development and their density and morphology undergoes significant changes during maturation. After maturation dendritic spines are not static structures but display constant changes in their morphology and stability. The shape and size of dendritic spines have been linked to synaptic transmission, coupling the form of spines to neuron function. Several neurological diseases and disabilities are characterized by abnormal spine density and morphology. The main structural component of the dendritic spines is the actin filament, F-actin. Actin filaments are dynamic polymers of the monomeric protein actin. The filaments are constantly turning over and reorganizing. Both processes are regulated by actin binding proteins. All structural changes and the maintenance of dendritic spines are dependent on actin dynamics. Current research indicates that the dynamics of actin filaments do not change following spine maturation. Maturation does lead to a decrease in the movement of spines and an increase in stability, indicating changes in F-actin dynamics. In this study I have shown that the dynamics of F-actin do change during maturation. The stable pool of F-actin increases in size and the turnover of the dynamic pool increases. One of the actin binding proteins with a potential to regulate actin stabilization is myosin IIb, a motor protein with capabilities to bind F-actin and to introduce contractility into the filament network. Myosin IIb has been shown to regulate dendritic spine development, size and shape and play a role in memory consolidation. In this study I have shown that myosin IIb regulates dendritic spine F-actin via two distinct mechanisms. Myosin IIb can bind F-actin and stabilize it without affecting the turnover of the dynamic filaments. Myosin IIb-mediated contractility on the other hand can facilitate the turnover of the dynamic filaments. These findings help us to understand the molecular mechanism behind dendritic spine structure regulation and possibly in the future how it is related to synaptic transmission and different pathological states. Due to their small size, dendritic spines pose unique challenges for the study of actin dynamics. Most of the available methods are based on advanced fluorescence microscopy. In this study I have made a critical evaluation of the methods used to measure F-actin turnover in dendritic spines and the analysis of the data. I have also developed a novel approach to use fluorescence anisotropy to measure the level of actin bundling. The method has been previously applied to measure actin polymerization. My findings have led to the conclusion that in actin-dense compartments, such as the dendritic spines, fluorescence anisotropy reflects actin bundling rather than polymerization and that conclusions based on earlier research using similar techniques should be re-evaluated.
  • Linnaluoma, Jessica (Helsingin yliopisto, 2012)
    Despite their small surface area on Earth, freshwater ecosystems have recently been recognized as important components of the global carbon budget. The external loading of terrestrial organic carbon enhances the net heterotrophy in lake ecosystems, leading to CO2 supersaturation in most of the world s lakes, and lacustrine water bodies are therefore clear sources of carbon to the atmosphere. The present study provides information on carbon gas (CO2 and CH4) concentrations and fluxes from three large dimictic lakes in southern Finland with contrasting water quality: Lake Pääjärvi (a humic lake), Lake Ormajärvi (a clear-water lake) and the Enonselkä basin in Lake Vesijärvi (an urban clear-water lake basin). The lakes were intensively sampled throughout the open-water period for general limnology as well as for biology to determine the processes behind the gas fluxes. Greenhouse gases determinations were based on surface water concentrations and gas accumulation in floating closed chambers. Fluxes were analysed at different times of the year, during the stratification period (summer) and mixing periods (spring and autumn). The gas transfer velocities (k600) of CO2 and CH4 were related to wind speed during the mixing periods. The study years contrasted each other, i.e. the summer of 2004 was rainy, whereas the summer of 2005 was warm with precipitation close to the long-term average, allowing a comparison of the lake response to different weather conditions. In this study, the greatest carbon gas evasions from the lake surfaces were measured during the spring and autumn mixing periods. The wind speed had a stronger effect on the gas transfer velocity (k600) of CO2 and CH4 in spring than in the autumn. However, there was distinctive gas exchange variability during the summer after rain events. In the humic Lake Pääjärvi, the high precipitation resulted in a large peak in CO2 and CH4 fluxes, which contributed 46% and 48% to the annual fluxes of CO2 and CH4, respectively. In the clear-water Lake Ormajärvi, the contribution of the rainy period to carbon gas fluxes was 39% and 37% for CH4 and CO2, respectively. The response of the clear-water lake to the high precipitation was not as immediate as in the humic lake, but the outcome was more radical, since before the rainy period the lake took up more carbon than was released to the atmosphere, but as a consequence of the rains the situation was reversed. The urban lake basin, with anoxic hypolimnion, was a source of CO2 and CH4 even though the oxidation of CH4 in the water column was intensive during the stratification period. A clear association between biological mineralization processes and carbon fluxes was observed in the humic lake, contrary to the clear-water lake. In the humic lake, CO2 was equally produced and released during the open-water period, except during the summer flux peak after the rainy period, whereas in the clear-water lake there was an excess of CO2 production. Precipitation generated variability in epilimnetic and metalimnetic concentrations of carbon gases and DOC at the time when the lakes showed their strongest stability. Moreover, changes in biological processes were only observed at the surface, which indicates that the excess of CO2 and CH4 were flushed into the lake from the surrounding terrestrial soil or the littoral area. The summer precipitation clearly increased the carbon emissions to the atmosphere, since when omitting the summer flux peaks in the humic and the clear-water lake, the CO2 fluxes were closer to those measured in the urban lake basin during the summer of average precipitation, and close to average fluxes measured in large Finnish lakes. However, CH4 fluxes were always higher in the urban lake basin, indicating the long history of eutrophication and anoxia. The importance of the lakes in recycling terrestrial carbon was expressed by comparing the lake fluxes with the net ecosystem exchange (NEE) of the forested and peatland catchment areas. The carbon gas (CO2 and CH4) flux from humic Lake Pääjärvi was 4%, that from clear-water Lake Ormajärvi 2% and from Enonselkä basin 7% of the terrestrial NEE of the whole catchment area, demonstrating that lakes also release carbon produced in their catchment areas. Estimates of the global warming potential (GWP) of greenhouse gas emissions from the studied boreal lakes indicated that the contribution of CH4 to the carbon fluxes was higher in the urban lake basin (33%), followed by the clear-water lake (13%) and the humic lake (7%). Thus, the lacustrine GWP is influenced by human activity. Human-induced nutrient loading into lakes enhances autochthonous production and the decomposition of organic matter, which may generate large emissions of CH4, as was seen in the urban study lake.
  • Batchu, Krishna chaithanya (Helsingin yliopisto, 2016)
    The aim of this thesis was to study the substrate specificity of A-type phospholipases (PLAs) that belong to different sub-families in order to understand the key factors that regulate their activity. The experimental part of this thesis consists of three studies each one focusing on PLAs that belong to a specific subgroup. In the first study, we developed a mass-spectrometric (MS) assay and implemented it to study in detail the effect of acyl chain length and unsaturation of glycerophospholipids (GPLs) on their hydrolysis by three different secretory PLA2s (sPLA2s) from various sources. The key finding of this study was that efflux of the GPL substrate from the bilayer largely determines the rate of hydrolysis by these PLAs. In micelles, accommodation of the GPL acyl chains in the catalytic active site seems to be more important for substrate specificity. In the next study we used the MS assay developed in study I, to investigate whether substrate efflux propensity regulates the activity of the Ca2+ -independent PLA-Beta (iPLAβ). Our results strongly suggest that the activity of iPLAβ is also determined by the efflux of the GPL substrate from the membrane bilayer. Our last study was on the cytoplasmic PLA-alpha (cPLA2α) that has been implicated in the initiation of the inflammatory lipid-mediator cascade generating eicosanoids and platelet-activating factor. The study was carried out to understand to what extent accommodation in the catalytic site determines specificity for arachidonic acid (AA) and if efflux propensity plays a role in the substrate specificity of cPLA2α. Our results indicate that while accommodation of the substrate in the active site greatly contributes to the preferential hydrolysis of AA-containing GPLs by cPLA2α, efflux of the substrate from a membrane bilayer also plays a significant role. In summary, these studies not only provide information on the factors regulating the substrate specificity of various PLAs but also indicate that lateral arrangement of GPLs could be a key regulator of homeostatic PLAs like iPLAβ.
  • Vepsäläinen, Ville (Helsingin yliopisto, 2007)
    The biodiversity of farmland ecosystems has decreased remarkably during the latter half of the 20th century, and this development is due to intensive farming with its various environmental effects. In the countries of the EU the Common Agricultural Policy (CAP) is the main determinant affecting farmland biodiversity, since the agricultural policy defines guidelines of agricultural practices. In addition to policies promoting intensive farming, CAP also includes national agri-environment schemes (AES), in which a part of subsidies paid to farmers is directed to acts that are presumed to promote environmental protection and biodiversity. In order to shape AES into relevant and powerful tools for biodiversity protection, detailed studies on the effects of agriculture on species and species assemblages are needed. In my thesis I investigated the importance of habitat heterogeneity and effects of different habitat and landscape characteristics on farmland bird abundance and diversity in typical cereal cultivation-dominated southern Finnish agricultural environments. The extensive data used were collected by territory mapping. My two main study species were the drastically declined ortolan bunting (Emberiza hortulana) and the phenomenally increased tree sparrow (Passer montanus); in addition I studied assemblages of 20 species breeding in open arable and edge/bush habitats. In light of my results I discuss whether the Finnish AES take into account the habitat needs of farmland birds, and I provide suggestions for improvement of the future AES. My results show that heterogeneity of both uncultivated and cultivated habitats increases abundance and species richness among farmland birds, but in this respect the amount and diversity of uncultivated habitats are essential. Ditches in particular are a keystone structure for farmland birds in boreal landscapes. Ditches lined by trees or bushes increased ortolan bunting abundance. Loss of that kind of ditches (and clearance of forest and bush patches), reduced breeding ortolan buntings, mainly by decreasing availability of song-posts that are important for the breeding groups of the species. Heterogeneity of uncultivated habitats, most importantly open ditches and the habitat patch richness, increased densities and species richnesses of species assemblages of open arable and edge/bush habitats. Human impact (winter-feeding, nest-boxes) affected favourably the tree sparrow s rapid range expansion in southern Finland, but any habitat types had no significant effects. At the moment the Finnish agri-environmental policy does not conserve farmland ditches as a habitat type. Instead, sub-surface drainage is financially promoted. This is a fatal mistake as far as farmland biodiversity is concerned. In addition to the maintenance of ditches, at least the following aspects should be included more than is done previously in the measures of the future AES: 1) promotion of diverse crop rotation (especially by promoting animal husbandry), 2) maintenance of tree and bush vegetation in islets and along ditches, 3) promotion of organic farming.
  • Piiparinen, Jonna (Helsingin yliopisto, 2011)
    The aim of this thesis was to study ecology of Baltic Sea ice from two perspectives. In the first two studies, sea-ice ecology from riverine-influenced fast ice to drift ice in the Bothnian Bay was investigated, whereas the last two studies focus on the sensitivity of sea-ice bacteria and algae to UVA examined in situ. The seasonal sea ice cover is one of the main characteristics of the Baltic Sea, and despite the brackish parental water, the ice structure is similar to polar ice with saline brine inclusions, the sea ice habitat. The decreasing seawater salinity from the northern Baltic Sea to the Bothnian Bay translates to decreasing brine volumes along the gradient, governing the size and community structure of the food webs in ice. However, the drift and fast ice in the Bothnian Bay may differ greatly in this sense, as drift ice may have been formed at more southern locations. Rafting and the formation of snow ice are common processes in the ice field of the Bothnian Bay. As evidenced in this thesis, rafting altered the vertical distribution of organisms and snow-ice formation provided habitable space in the better-illuminated, nitrogen-rich surface layer. The divergence between fast and drift ice became apparent at the more advanced stages, and chlorophyte biomass decreased from fast to drift ice, while the opposite held true for protozoan and metazoan biomass. The brine volumes affected the communities somewhat, and a higher percentage of flagellate species was generally linked to lower brine volumes, whereas chain-forming diatoms were mostly concentrated in layers with larger brine volumes. These results add to knowledge of the ecological significance of the ice cover lasting up to 7 months per year in this area. Sea-ice food webs are generally light-limited, but while increasing light irradiances typically enhance the primary production and further, the secondary production in sea ice, any increase in solar radiation also includes an increase in harmful UVA radiation. The Baltic Sea ice microbial communities were clearly sensitive to UVA and the responses were strongly linked to the earlier light history, as well as to the solar irradiances they were exposed to. The increased biomass of chlorophytes and pennate diatoms, when UVA was excluded, indicates that their normally minor contribution to the biomass in the upper layers of sea ice might be partly dictated by UVA. The effects of UVA on bacterial production in Baltic Sea ice mostly followed the responses in algal growth, but occasionally the exposure to UVA even enhanced the bacterial production. The dominant bacterial class, Flavobacteria, seemed to be UVA-tolerant, whereas all the Alpha-, Beta- and Gammaproteobacteria present in the surface layer showed UVA sensitivity. These results indicate that changes in the light field of ice may alter the community structure and affect the functioning of ice food webs, and are of importance when the effects of thinning of the ice cover are assessed.
  • Perkola, Noora (Helsingin yliopisto, 2014)
    The newly detected chemicals, the environmental distribution, fate, and effects in the environment of which are not well known, are called emerging compounds. Artificial sweeteners are one group of emerging compounds. The consumption of artificial sweeteners is high, and because they do not significantly metabolise, all that is consumed finds its way to wastewater treatment plants. Two artificial sweeteners, acesulfame and sucralose, do not degrade in wastewater treatment either, leading to elevated concentrations in the receiving water bodies. Another group of emerging compounds is perfluoroalkyl acids. They have been used both in industry and consumer products since the 1950s. The fluorine-carbon chain of perfluoroalkyl acids makes them extremely resistant to biological, chemical and physical degradation. They are ubiquitous in the environment and are suspected to be carsinogenic, immunotoxic and to interfere with reproduction. The aim of this study was to add to knowledge about the environmental distribution, fate and effects of artificial sweeteners and perfluoroalkyl acids. The occurrence of artificial sweeteners and perfluoroalkyl compounds was surveyed in surface waters. Wastewater effluents and sludge, storm water, and landfill leachate were analysed to evaluate the fluxes of perfluoroalkyl acids into the aquatic environment. Artificial sweeteners and perfluorooctanoic acid were irradiated under artificial sun to investigate their potential to transform via direct and indirect photochemical reactions in surface waters and ultraviolet radiation and during germicidal ultraviolet water treatment. Furthermore, Daphnia magna were exposed to artificial sweeteners to evaluate the ecotoxicological effects. It was discovered that artificial sweeteners and perfluoroalkyl acids are ubiquitous in the Finnish aquatic environment. Of the studied emission sources, wastewater effluents were the most important source of perfluoroalkyl acids in environmental waters. Based on the irradiation of perfluorooctanoic acid with a solar simulator, perfluorocarboxylic acids do not transform via direct or indirect photochemical reactions in the environment. Although a decrease in acesulfame was observed under irradiation with artificial sun, the photolytic half-life in surface water is at least one year. The photolytic half-lives of the other sweeteners were estimated to be 3 to 6 years. The ecotoxicological tests suggest that cyclamic acid might hinder the reproduction of Daphnia magna. Perfluorocarboxylic acids, sucralose and acesulfame are persistent and ubiquitous in surface waters.
  • Sievi, Eeva (Helsingin yliopisto, 2002)
  • Öst, Markus (Helsingin yliopisto, 2000)
  • Jukkola, Tomi (Helsingin yliopisto, 2007)
    The neuroectodermal tissue close to the midbrain hindbrain boundary (MHB) is an important secondary organizer in the developing neural tube. This so-called isthmic organizer (IsO) regulates cellular survival, patterning and proliferation in the midbrain (Mb) and rhombomere 1 (R1) of the hindbrain. Signaling molecules of the IsO, such as fibroblast growth factor 8 (FGF8) and WNT1 are expressed in distinct bands of cells around the MHB. It has been previously shown that FGF-receptor 1 (FGFR1) is required for the normal development of this brain region in the mouse embryo. In the present study, we have compared the gene expression profiles of wild-type and Fgfr1 mutant embryos. We show that the loss of Fgfr1 results in the downregulation of several genes expressed close to the MHB and in the disappearance of gene expression gradients in the midbrain and R1. Our microarray screen identified several previously uncharacterized genes which may participate in the development of midbrain R1 region. Our results also show altered neurogenesis in the midbrain and R1 of the Fgfr1 mutants. Interestingly, the neuronal progenitors in midbrain and R1 show different responses to the loss of signaling through FGFR1. As Wnt1 expression at the MHB region requires the FGF signaling pathway, WNT target genes, including Drapc1, were also identified in our screen. The microarray data analysis also suggested that the cells next to the midbrain hindbrain boundary express distinct cell cycle regulators. We showed that the cells close to the border appeared to have unique features. These cells proliferate less rapidly than the surrounding cells. Unlike the cells further away from the boundary, these cells express Fgfr1 but not the other FGF receptors. The slowly proliferating boundary cells are necessary for development of the characteristic isthmic constriction. They may also contribute to compartmentalization of this brain region.
  • Lahti, Laura (Helsingin yliopisto, 2012)
    Embryonic midbrain and hindbrain are structures which will give rise to brain stem and cerebellum in the adult vertebrates. Brain stem contains several nuclei which are essential for the regulation of movements and behavior. They include serotonin-producing neurons, which develop in the hindbrain, and dopamine-producing neurons in the ventral midbrain. Degeneration and malfunction of these neurons leads to various neurological disorders, including schizophrenia, depression, Alzheimer s, and Parkinson s disease. Thus, understanding their development is of high interest. During embryogenesis, a local signaling center called isthmic organizer regulates the development of midbrain and anterior hindbrain. It secretes peptides belonging to fibroblast growth factor (FGF) and Wingless/Int (Wnt) families. These factors bind to their receptors in the surrounding tissues, and activate various downstream signaling pathways which lead to alterations in gene expression. This in turn affects the various developmental processes in this region, such as proliferation, survival, patterning, and neuronal differentiation. In this study we have analyzed the role of FGFs in the development of midbrain and anterior hindbrain, by using mouse as a model organism. We show that FGF receptors cooperate to receive isthmic signals, and cell-autonomously promote cell survival, proliferation, and maintenance of neuronal progenitors. FGF signaling is required for the maintenance of Sox3 and Hes1 expression in progenitors, and Hes1 in turn suppresses the activity of proneural genes. Loss of Hes1 is correlated with increased cell cycle exit and premature neuronal differentiation. We further demonstrate that FGF8 protein forms an antero-posterior gradient in the basal lamina, and might enter the neuronal progenitors via their basal processes. We also analyze the impact of FGF signaling on the various neuronal nuclei in midbrain and hindbrain. Rostral serotonergic neurons appear to require high levels of FGF signaling in order to develop. In the absence of FGF signaling, these neurons are absent. We also show that embryonic meso-diencephalic dopaminergic domain consists of two populations in the anterior-posterior direction, and that these populations display different molecular profiles. The anterior diencephalic domain appears less dependent on isthmic FGFs, and lack several genes typical of midbrain dopaminergic neurons, such as Pitx3 and DAT. In Fgfr compound mutants, midbrain dopaminergic neurons begin to develop but soon adopt characteristics which highly resemble those of diencephalic dopaminergic precursors. Our results indicate that FGF signaling regulates patterning of these two domains cell-autonomously.
  • Saarimäki-Vire, Jonna (Helsingin yliopisto, 2012)
    The embryonic midbrain and hindbrain give rise to brain stem structures and the cerebellum. The ventral midbrain and anterior hindbrain include highly important brain nuclei such as the dopaminergic substantia nigra and the ventral tegmental area, as well as serotonergic dorsal raphe neurons. These specific brain structures are affected in several disorders such as Parkinson s disease, depression, schizophrenia and drug addiction. Between the developing midbrain and hindbrain is a signalling centre called the Isthmic Organizer. This Isthmic Organizer secretes signalling molecules, such as Wnts and Fibroblast growth factors (Fgfs). Fgf8 is able to induce midbrain and anterior hindbrain characteristics in ectopic locations, and thus Fgf8 can act as an organizer molecule. Fgf signals are mediated by Fgf receptors (Fgfr). Of the four Fgfrs, Fgfr1-Fgfr3 are expressed in the nervous system. Fgfr1 is required to maintain coherence of a slowly dividing midbrain-hindbrain boundary cell population. However, the role of Fgfr2 and Fgfr3 in the development of midbrain and anterior hindbrain is poorly understood as well as cell adhesion molecules related to the maintenance of the coherent isthmic constriction. In this study, we elucidated the role of Fgfr2 and Fgfr3 during the development of the mid-brain and hindbrain. We showed that loss of either Fgfr2 or Fgfr3 alone or even both together did not result in any structural abnormalities. Thus, Fgfr1 is the major Fgf receptor in the midbrain and anterior hindbrain region. However, when Fgfr1 and Fgfr2, or all three Fgfr1, Fgfr2 and Fgfr3 were simultaneously inactivated, the defects in the midbrain-hindbrain development were much more severe than in the Fgfr1 mutants alone. Dorsal midbrain structures and the cerebellum were lost. Although some dopaminergic precursors appeared in the ventral midbrain, all dopaminergic neurons and several other ventral neuronal populations were lost by birth. We showed that Fgfr cooperatively regulate cell survival, antero-posterior patterning, and the maintenance of neural progenitor properties. Loss of Fgf signalling in the ventral midbrain resulted in a thinner ventricular zone and premature neurogenesis. This was not caused by shortened cell cycle length or abnormalities in cellular polarity, cellular architecture or the orientation of mitotic spindles. Instead, loss of Fgf signalling lead to a downregulation of neural stem cell transcription factors, which allowed upregulation of proneural genes. Thus, these gene expression changes drove neural progenitors to exit the cell cycle. In addition, we showed that Fgf8 is localized in the basal membrane. Thus, Fgf signalling may maintain proliferative identity of the midbrain neural progenitors, and the cells likely receive these guiding Fgf signals through their basal processes. Finally, we showed that an Fgf-regulated adhesion molecule Cadherin22 (Cdh22) is not essential for the maintenance of the coherent compartment boundary between the midbrain and the hindbrain. Possibly, Cdh22 acts redundantly with other type II cadherins. In addition, specific expression patterns in distinct brain nuclei suggest roles for Cdh22 in the segregation of neuronal populations cooperatively with other cadherins. In summary, these results demonstrate that Fgf signalling, and especially cooperation of the Fgf receptors, is required for proliferation, cell survival, and patterning of the neural progenitors in the midbrain and anterior hindbrain. A good understanding of developmental processes such as detailed mechanisms of signalling pathways and their regulation elucidates possibilities for therapeutic use.
  • Kislin, Mikhail (Helsingin yliopisto, 2015)
    Acute brain trauma and ischemia are severe injuries that have no adequate treatment to date. In vivo two-photon microscopy allows studying longitudinally the process of injury development and brain recovery. This thesis summarizes the work on: i) animal model of acute brain injury and role of extracellular matrix in neuronal recovery and plasticity, ii) investigation of mitochondria during physiological/pathological calcium elevations in vitro and further implementation of quantitative microscopic analysis of neuronal mitochondrial morphology in vivo, iii) mitochondrial damage and recovery in animal models of acute neurodegenerative disorders in the neocortex of anesthetized mice, iv) a novel approach for awake head-fixed recordings. The results described in this work provide novel approaches for intravital morphological analysis of neurons and of their mitochondria, increase our understanding of pathogenesis after traumatic and ischemic injury in neocortex of rodents, and enable the development of novel therapies for CNS injuries.
  • Suomalainen, Marika (Helsingin yliopisto, 2010)
    Tooth development is regulated by sequential and reciprocal interactions between epithelium and mesenchyme. The molecular mechanisms underlying this regulation are conserved and most of the participating molecules belong to several signalling families. Research focusing on mouse teeth has uncovered many aspects of tooth development, including molecular and evolutionary specifi cs, and in addition offered a valuable system to analyse the regulation of epithelial stem cells. In mice the spatial and temporal regulation of cell differentiation and the mechanisms of patterning during development can be analysed both in vivo and in vitro. Follistatin (Fst), a negative regulator of TGFβ superfamily signalling, is an important inhibitor during embryonic development. We showed the necessity of modulation of TGFβ signalling by Fst in three different regulatory steps during tooth development. First we showed that tinkering with the level of TGFβ signalling by Fst may cause variation in the molar cusp patterning and crown morphogenesis. Second, our results indicated that in the continuously growing mouse incisors asymmetric expression of Fst is responsible for the labial-lingual patterning of ameloblast differentiation and enamel formation. Two TGFβ superfamily signals, BMP and Activin, are required for proper ameloblast differentiation and Fst modulates their effects. Third, we identifi ed a complex signalling network regulating the maintenance and proliferation of epithelial stem cells in the incisor, and showed that Fst is an essential modulator of this regulation. FGF3 in cooperation with FGF10 stimulates proliferation of epithelial stem cells and transit amplifying cells in the labial cervical loop. BMP4 represses Fgf3 expression whereas Activin inhibits the repressive effect of BMP4 on the labial side. Thus, Fst inhibits Activin rather than BMP4 in the cervical loop area and limits the proliferation of lingual epithelium, thereby causing the asymmetric maintenance and proliferation of epithelial stem cells. In addition, we detected Lgr5, a Wnt target gene and an epithelial stem cell marker in the intestine, in the putative epithelial stem cells of the incisor, suggesting that Lgr5 is a marker of incisor stem cells but is not regulated by Wnt/β-catenin signalling in the incisor. Thus the epithelial stem cells in the incisor may not be directly regulated by Wnt/β-catenin signalling. In conclusion, we showed in the mouse incisors that modulating the balance between inductive and inhibitory signals constitutes a key mechanism regulating the epithelial stem cells and ameloblast differentiation. Furthermore, we found additional support for the location of the putative epithelial stem cells and for the stemness of these cells. In the mouse molar we showed the necessity of fi ne-tuning the signalling in the regulation of the crown morphogenesis, and that altering the levels of an inhibitor can cause variation in the crown patterning.
  • Estlander, Satu (Helsingin yliopisto, 2011)
    Humic lakes are abundant in the temperate and cold regions of the Boreal Zone. High levels of water colour and strong thermal stratification of humic lakes limit the potential fish habitats and give a special role to the intraspecific and interspecific interactions. Water colour has different effects on species depending on species-specific life-history traits and trophic interactions. Fish species whose success in predation is based on visual cues are more susceptible to suffer in competition. The main aim of the thesis was to demonstrate the effects of water colour on European perch (Perca fluviatilis) in humic lakes. The contribution of water colour to diet, feeding, growth and competitive interactions of fish was studied both in laboratory and in small humic lakes with varying levels of water colour. The main findings of the thesis were that water colour has different effects on species, depending on species-specific life-history traits and trophic interactions. Water colour affected visually-oriented perch feeding and growth negatively, and the prolonged benthic feeding phase of perch resulting from the increased water colour could increase intraspecific competition in perch populations and may result in a partial bottleneck in growth for perch. Moreover, water colour may act as a proximate factor behind the population dependency of sexual growth dimorphism in perch.
  • Milardi, Marco (Helsingin yliopisto, 2015)
    Fish introductions, unlike many other clades, are often carried out purposefully. This is a worldwide practice, which has been ongoing since ancient times and has found new powerful ways to increase in magnitude and scope through the development of more effective transport and with increasing population wealth. Even though the literature is building up pace fast with this relatively recent phenomenon, information is still lacking on the mechanisms and results of the impact that introduced fish have on freshwater ecosystems. Remote small lakes at high-latitudes can be used to evaluate some of these impacts, as they are of limited size and often host relatively simple food webs with a single species of introduced fish, which makes them ideal model systems. Many areas are not as easy to investigate, as they present much more complex ecosystems where complex species interactions take place. For example, artificial lotic systems with a high number of species interactions can be particularly challenging to tackle as other anthropogenic/environmental stressors might be challenging to disentangle. This thesis focused on three main themes which addressed the reliance of introduced fish on terrestrial energy, their cascading effects on the food web and their interaction with native species. The first two themes used brown trout introduced in remote small lake ecosystems at high latitudes as a model, whereas the last theme used a complex lotic system at low latitudes where multiple species were introduced at different times. The chapters of this thesis used neo- and paleolimnological techniques to investigate the impacts of introduced fish, sometimes in combination. In particular, the first theme was tackled through the use of stomach content, fatty acids and stable isotopes to unravel the feeding ecology of introduced brown trout. The second theme was instead addressed through multiple paleo-proxies in combination with neolimnological analyses and models. The last theme was investigated through the analysis of long-term environmental and fish-assemblage data. Our findings suggest that terrestrial sources could be highly important in supporting introduced brown trout populations in small lakes at high latitudes. Despite challenges inherent to the turnover rates of fish liver and muscle tissues, which were longer than previously thought, this support did not vary across different years or fish densities, but was affected by seasonal factors over the course of the open-water season. Our results also suggest that introduced brown trout affect the food web of host lakes through trophic cascades, altering the abundance of pelagic and benthic micro- and macroinvertebrates, probably through a modification of the distribution of macro-invertebrate communities. According to a bioenergetic and a mass balance model, introduced brown trout nutrient regeneration should have increased lake productivity; however paleo-proxies indicated no such change but rather a shift from pelagic to benthic productivity. Finally, our results suggest that, in artificial lotic systems at low latitudes, environmental chemistry did not play a role in the decline of the native fish species communities. Rather, this was a result of the interaction between some of the introduced fish species and the native ones, enhanced by the habitat simplification and the peculiar water fluctuation regime.