<p><strong>Background:</strong> Postnatal growth of preterm infants does not match recommended intrauterine growth, due to the initial weight loss that accompanies healthy body composition rearrangements after birth. Thus, optimal postnatal growth for preterm infants is currently unknown.</p> <p><strong>Objectives: </strong>(1)<strong> </strong>Collect longitudinal postnatal growth data of 30–36 week GA preterm infants with unimpaired postnatal adaptation; (2) Develop regressions that predict the growth trajectory such an infant will adjust to by days of life 7/14/21; (3) Extrapolate and validate the regressions downwards to 25 weeks.</p> <p><strong>Methods:</strong> Infants of 30–36 week GA, born/admitted to 1/5 participating centres between 2008–2012, who met pre-specified criteria for unimpaired postnatal adaptation and who had at minimum 14 days of data were included. Day-specific anthropometric data from birth to discharge were abstracted retrospectively. Z-score regressions for days 7/14/21 were developed. Regressions were then extrapolated to 25 weeks and validated using an independent study population.</p> <p><strong>Results:</strong> Of 6203 infants, 665 met the screening criteria. By day 14, infants adjusted to stable growth trajectories that were 84±13% of the recommended weight-for-age. Using the following predictors: GA, z-score at birth and hospital-centre, regressions accurately predicted z-scores at days 7, 14 (n=665; R²=0.939, 0.889) and 21 (n=333; R²=0.841). Validation using 25-29 week GA infants (n=173) suggested models were also accurate within this age-range.</p> <p><strong>Conclusion: </strong>These results provide robust estimates of a hypothesis of healthy postnatal growth for preterm infants. Future steps include assessing long-term outcomes in a randomized control trial and assessing the quality of growth using body composition analyses.</p> / Master of Science in Medical Sciences (MSMS)

La capacité d’une plante à réguler sa croissance racinaire est une composante importante de l’acclimatation aux stress environnementaux. A l’échelle cellulaire, cette régulation est effectuée via le contrôle de la division et de l’élongation des cellules mais les rôles respectifs de chaque processus et leurs interactions sont peu connus. Notamment, l’activité de production de cellules du méristème apical racinaire (RAM) est trop souvent négligée. Dans cette thèse, l’analyse spatiale de la croissance le long de l’apex racinaire et l’analyse temporelle des trajectoires de croissance des cellules ont été couplées pour comprendre les liens existants entre division et élongation cellulaire. Pour cela, j’ai développé un système de phénotypage de la croissance à haute résolution spatio-temporelle qui a été appliqué à l’étude de racines d’un peuplier euraméricain (Populus deltoides × Populus nigra) en réponse à différents stress (stress osmotique, impédance mécanique). Une forte variabilité du taux de croissance racinaire entre individus ainsi que des variations individuelles cycliques de la croissance ont été observées malgré des conditions environnementales contrôlées. L’utilisation de cette variabilité couplée à la quantification de l’activité du RAM a mis en évidence l’importance du taux de production de cellules pour soutenir la croissance racinaire. Ces travaux analysent une nouvelle échelle de variations spatiales et temporelles de la croissance peu prise en compte jusqu’à présent. Hautement applicable à d’autres questions scientifiques, l’analyse du devenir des cellules une fois sortie du RAM est également discutée pour des conditions de croissance non stables / Regulation of root growth is a crucial capacity of plants for acclimatization to environmental stresses. At cell scale, this regulation is controlled through cell division and cell elongation but respective importance of these processes and interactions between them are still poorly known. Notably, the cell production activity of the root apical meristem (RAM) is often excluded. During this thesis, spatial analyses of growth along the root apex were coupled with temporal analyses of cell trajectories in order to decipher the links between cell division and cell elongation. This required the setup of a system for phenotyping root growth at a high spatiotemporal resolution which was applied to study the growth of roots from an euramerican poplar (Populus deltoides × Populus nigra) in response to different environmental stresses (osmotic stress or mechanical impedance). An important variability of root growth rate between individuals as well as individual cyclic variations of growth along time were observed despite tightly controlled environmental conditions. Use of this variability coupled with quantification of the RAM activity led us to a better understanding of the importance of the cell production rate for sustaining root growth. This work analyses a new spatiotemporal scale of growth variability poorly considered. Widely applicable to others scientific questioning, temporal analyses of cell fate once produced in the RAM is also discussed for non-steady growth conditions