title: Bovine Pre-gastrulation Development and the role Of Rauber's Layer

authors: van Leeuwen, Jessica

subjects: embryology

abstract: Although conception rates in cattle are estimated to be 90%, little over half of each insemination results in the birth of offspring. The majority of total combined foetal and embryonic loss is sustained between the bovine blastocyst hatching at Day 8 and the onset of gastrulation after Day 14. Post hatching, the bovine embryo develops a morphological architecture which is very different to the mouse, with a flat epiblast and the loss of the polar trophoblast, known as Rauber’s layer. In mice, the polar trophoblast is essential for the induction of gastrulation and the formation of the placenta. The pre-gastrulation layout of cattle embryos is shared by a diverse phylogeny of placental forming mammals, from rabbits to pigs, and it is therefore considered the more typical mode of mammalian embryonic development. Yet, compared to mouse embryonic development, relatively little is known about the gene expression and signalling networks required for successful gastrulation in these species, particularly cattle which are also economically important.

This thesis studied the morphological and gene expression events during Days 11-15 in cattle which coincides with the period of greatest embryonic loss. A detailed morphological staging system is presented that is supported with spatio-temporal gene expression patterns of genes known to play fundamental roles in mouse pre-gastrulation development. Novel morphological structures and processes were identified such as the formation of the cattle anterior visceral hypoblast (equivalent to the mouse AVE) and cavitation of the epiblast in preparation for epithelialisation of this tissue. Both of these processes are considered pre-requisites for the induction of gastrulation in mammals. The spatio-temporal gene expression patterns prior to gastrulation showed overall high concordance with mouse development however, processes that lead to asymmetry in the mouse involving extra-embryonic tissues have diverged and this can be related back to the different embryonic layouts and polar trophoblast fates between cattle and mice.

Functional studies were carried out in cattle embryos to investigate the relative roles of the three first cell lineages (trophoblast, epiblast and hypoblast) in contributing to embryonic loss and to investigate the possible role of Rauber’s layer in pre-gastrulation development. Over-expression of the weak pro-apoptotic factor BAD resulted in the selective loss of the epiblast between Days 7 and 13/14 of development. The trophoblast was not measurably impaired and although the hypoblast was not lost, the gene expression profile of this tissue was dramatically altered. These results indicate a gradient of sensitivity of epiblast>hypoblast>trophoblast tissue and suggest the epiblast contributes more greatly overall to embryonic loss. This result provides insights into the phenomenom of “phantom pregnancies”, which also are due to loss of the epiblast/embryo proper and are thought to occur in 12-22% of cattle pregnancies. 

The loss of Rauber’s layer is thought to occur by apoptosis and the results shown in this thesis support this idea. Over-expression of anti-apoptotic BCL2 resulted in a delayed loss of Rauber’s layer. BCL2 embryos also had an increased epiblast length and one embryo showed ectopic mesoderm formation. These results are consistent with the hypothesis that a maintained Rauber’s layer could lead to an increase in NODAL signalling. 

The findings in this thesis provide a foundational step in the establishment of the bovine embryo as a model for mammalian development. The results also demonstrate functional perturbations of the cattle embryo are experimentally feasible, and how these have revealed insights into cattle embryonic loss and the role of Rauber’s layer in development.

text: Bovine Pre-gastrulation Development and the role Of Rauber's Layer Although conception rates in cattle are estimated to be 90%, little over half of each insemination results in the birth of offspring. The majority of total combined foetal and embryonic loss is sustained between the bovine blastocyst hatching at Day 8 and the onset of gastrulation after Day 14. Post hatching, the bovine embryo develops a morphological architecture which is very different to the mouse, with a flat epiblast and the loss of the polar trophoblast, known as Rauber’s layer. In mice, the polar trophoblast is essential for the induction of gastrulation and the formation of the placenta. The pre-gastrulation layout of cattle embryos is shared by a diverse phylogeny of placental forming mammals, from rabbits to pigs, and it is therefore considered the more typical mode of mammalian embryonic development. Yet, compared to mouse embryonic development, relatively little is known about the gene expression and signalling networks required for successful gastrulation in these species, particularly cattle which are also economically important.

This thesis studied the morphological and gene expression events during Days 11-15 in cattle which coincides with the period of greatest embryonic loss. A detailed morphological staging system is presented that is supported with spatio-temporal gene expression patterns of genes known to play fundamental roles in mouse pre-gastrulation development. Novel morphological structures and processes were identified such as the formation of the cattle anterior visceral hypoblast (equivalent to the mouse AVE) and cavitation of the epiblast in preparation for epithelialisation of this tissue. Both of these processes are considered pre-requisites for the induction of gastrulation in mammals. The spatio-temporal gene expression patterns prior to gastrulation showed overall high concordance with mouse development however, processes that lead to asymmetry in the mouse involving extra-embryonic tissues have diverged and this can be related back to the different embryonic layouts and polar trophoblast fates between cattle and mice.

Functional studies were carried out in cattle embryos to investigate the relative roles of the three first cell lineages (trophoblast, epiblast and hypoblast) in contributing to embryonic loss and to investigate the possible role of Rauber’s layer in pre-gastrulation development. Over-expression of the weak pro-apoptotic factor BAD resulted in the selective loss of the epiblast between Days 7 and 13/14 of development. The trophoblast was not measurably impaired and although the hypoblast was not lost, the gene expression profile of this tissue was dramatically altered. These results indicate a gradient of sensitivity of epiblast>hypoblast>trophoblast tissue and suggest the epiblast contributes more greatly overall to embryonic loss. This result provides insights into the phenomenom of “phantom pregnancies”, which also are due to loss of the epiblast/embryo proper and are thought to occur in 12-22% of cattle pregnancies. 

The loss of Rauber’s layer is thought to occur by apoptosis and the results shown in this thesis support this idea. Over-expression of anti-apoptotic BCL2 resulted in a delayed loss of Rauber’s layer. BCL2 embryos also had an increased epiblast length and one embryo showed ectopic mesoderm formation. These results are consistent with the hypothesis that a maintained Rauber’s layer could lead to an increase in NODAL signalling. 

The findings in this thesis provide a foundational step in the establishment of the bovine embryo as a model for mammalian development. The results also demonstrate functional perturbations of the cattle embryo are experimentally feasible, and how these have revealed insights into cattle embryonic loss and the role of Rauber’s layer in development.

year: 2016

title: Identifying temporal and spatial variations in the binding properties of transition metal-organic ligand complexes in cave waters using organic inclusions in speleothem calcite: Implications for paleoclimate research.

authors: Brodnax, Robert

subjects: speleothems

abstract: Speleothems such as stalagmites, stalactites and flowstones, contain paleoclimate information encoded in their geochemical properties. One of these properties, transition metal abundance in speleothem calcite, is tightly controlled by complexation reactions in cave water with dissolved organic matter (DOM). Organic compounds are also preserved in speleothem deposits. DOM structure varies from cave system to cave system and over time, but whether these structural variations meaningfully alter the way DOM controls trace metal incorporation into calcite is unclear. This is relevant to the development of speleothem transition metal content as a paleoclimate proxy, particularly over large spatial and temporal scales where DOM structure variations are likely. This study identifies variations in the binding properties of DOM preserved in speleothem calcite over time and space by extracting organic compounds of known age and origin from speleothems collected from New Zealand, Niuean, British, and Australian cave systems. The kinetic properties of DOM complexes from these extracts were measured using small volume diffusive gradients in thin films (SV-DGT) and the variation in the binding strength of complexes with Cu, Co, and Ni was compared across time and space. DOM binding of Cu and Co were found to vary significantly depending on the cave system from which the DOM originated. Whether this variation was linked to climate changes represented in the age of the DOM or to changes in system characteristics related to geographical setting was unclear. Despite this, it is apparent that there are factors related to the origin of DOM in cave systems that influence the kinetic restrictions applied to transition metals in cave water. This variability should be considered when linking trace element contents in cave precipitates to paleoclimate change.

text: Identifying temporal and spatial variations in the binding properties of transition metal-organic ligand complexes in cave waters using organic inclusions in speleothem calcite: Implications for paleoclimate research. Speleothems such as stalagmites, stalactites and flowstones, contain paleoclimate information encoded in their geochemical properties. One of these properties, transition metal abundance in speleothem calcite, is tightly controlled by complexation reactions in cave water with dissolved organic matter (DOM). Organic compounds are also preserved in speleothem deposits. DOM structure varies from cave system to cave system and over time, but whether these structural variations meaningfully alter the way DOM controls trace metal incorporation into calcite is unclear. This is relevant to the development of speleothem transition metal content as a paleoclimate proxy, particularly over large spatial and temporal scales where DOM structure variations are likely. This study identifies variations in the binding properties of DOM preserved in speleothem calcite over time and space by extracting organic compounds of known age and origin from speleothems collected from New Zealand, Niuean, British, and Australian cave systems. The kinetic properties of DOM complexes from these extracts were measured using small volume diffusive gradients in thin films (SV-DGT) and the variation in the binding strength of complexes with Cu, Co, and Ni was compared across time and space. DOM binding of Cu and Co were found to vary significantly depending on the cave system from which the DOM originated. Whether this variation was linked to climate changes represented in the age of the DOM or to changes in system characteristics related to geographical setting was unclear. Despite this, it is apparent that there are factors related to the origin of DOM in cave systems that influence the kinetic restrictions applied to transition metals in cave water. This variability should be considered when linking trace element contents in cave precipitates to paleoclimate change.

year: 2024

title: Are they really at risk? Students' stories of success.

authors: Gray, Sheridan

subjects: Student engagement

abstract: In 2011, as the year level dean for year 13, I was surprised and pleased to see a group of students complete their fifth year of secondary schooling, contrary to teacher expectations, and in spite of earlier signs of disengagement and struggle.  Such students are frequently labelled “at risk” in our secondary schools. Truancy, lack of achievement, conduct issues, learning difficulties and health problems are seen as indicators of an at risk student by teachers. 

Statistically in Aotearoa, Māori and Pasifika students and students from low socio-economic families tend to be overrepresented among disengaged or early leavers from secondary schools, and so being from these cultures, and being from a low socio-economic group becomes a “risk” factor.   This can lead to assumptions among teachers about the students’ families, how parents value education, and even about the capacity of these groups to be able to succeed in education.  A range of strategies and programmes have been implemented in an attempt to address these issues, including Te Kotahitanga, Ka Hikitia and the Pasifika Education Plan, Resource Teachers Learning and Behaviour, Restorative Practice, and more recently Positive Behaviour for Learning

While there has been a shift in such approaches from deficit of students to emphasising more positive attitudes in teachers, the perspectives of the students themselves are missing from the discussion about these issues.  This study asks students what factors helped them to stay, and to achieve at least a level one NCEA qualification

The findings show that retention and engagement of “at risk” students requires careful attention to the quality of relationships across the school community: students, teachers and parents. They discredit some commonly held assumptions that “at risk” students and their parents do not value education and that teachers cannot support these students sufficiently. These quality relationships are respectful, responsive and tenacious showing a high level of respect for and interest in the students that goes beyond a teaching and learning relationship.  It also suggests that the discourse of “risk” invites a disrespectful response to retention and engagement of students.

text: Are they really at risk? Students' stories of success. In 2011, as the year level dean for year 13, I was surprised and pleased to see a group of students complete their fifth year of secondary schooling, contrary to teacher expectations, and in spite of earlier signs of disengagement and struggle.  Such students are frequently labelled “at risk” in our secondary schools. Truancy, lack of achievement, conduct issues, learning difficulties and health problems are seen as indicators of an at risk student by teachers. 

Statistically in Aotearoa, Māori and Pasifika students and students from low socio-economic families tend to be overrepresented among disengaged or early leavers from secondary schools, and so being from these cultures, and being from a low socio-economic group becomes a “risk” factor.   This can lead to assumptions among teachers about the students’ families, how parents value education, and even about the capacity of these groups to be able to succeed in education.  A range of strategies and programmes have been implemented in an attempt to address these issues, including Te Kotahitanga, Ka Hikitia and the Pasifika Education Plan, Resource Teachers Learning and Behaviour, Restorative Practice, and more recently Positive Behaviour for Learning

While there has been a shift in such approaches from deficit of students to emphasising more positive attitudes in teachers, the perspectives of the students themselves are missing from the discussion about these issues.  This study asks students what factors helped them to stay, and to achieve at least a level one NCEA qualification

The findings show that retention and engagement of “at risk” students requires careful attention to the quality of relationships across the school community: students, teachers and parents. They discredit some commonly held assumptions that “at risk” students and their parents do not value education and that teachers cannot support these students sufficiently. These quality relationships are respectful, responsive and tenacious showing a high level of respect for and interest in the students that goes beyond a teaching and learning relationship.  It also suggests that the discourse of “risk” invites a disrespectful response to retention and engagement of students.

year: 2012