whole_EkCarlJoakim2002_thesis.pdf (17.29 MB)
Transfer mechanisms for low molecular weight lipid insoluble molecules into the developing brain
thesisposted on 2023-05-26, 22:09 authored by Ek, Carl Joakim
The internal environment of the central nervous system (CNS) is protected from its surrounding milieu by barriers, which selectively facilitate and restrict the entry of molecules in and out of the CNS. These barriers form early during development, however, their selectivity in the developing brain appears to be different from those that protect the adult brain. The reasons for the differences in the CNS barriers between young and adult animals are still unclear. The experiments in this thesis were performed in order to obtain morphological and physiological explanations for the changes in the transfer of lipid insoluble molecules into the CNS that occur during development. Quantitative studies with radioactive markers in parallel with experiments using a tracer that could be visualised were conducted in an attempt to correlate physiological uptake with the structural studies. A marsupial species, the grey short-tailed South-American opossum (Monodelphis domestica), was used in all experiments since it is born at an early stage of brain development, and thus is a more accessible model than any eutherian animal during a similar developmental period. The transfer of lipid insoluble molecules has previously not been studied at such early stages of brain development. All experiments were carried out in accordance with NHMRC guidelines and with the approval of the University of Tasmania Ethics Committee. Lipid insoluble radioactive markers were injected into opossum pups and blood, cerebrospinal fluid (CSF) and brain samples were collected to assess initial rate of uptake and steady-state CSF/plasma and brain/plasma ratios. These experiments showed that the steady-state ratios for small lipid insoluble molecules were high in early development compared to the adult and that they decreased during development due to a reduction in CNS barrier permeability with age. Possible routes by which lipid insoluble molecules enter the brain were investigated using a 3000 molecular weight biotin-dextran and visualising it under the light and electron microscope. This tracer allowed localisation of a small inert lipid insoluble molecule at the blood-CNS interfaces. Quantitative measurements of CSF uptake showed that the dextran penetrates across the brain barriers to an extent similar to other small lipid insoluble molecules and thus validates the dextran as a suitable tracer. Under the electron microscope, it appeared that cells of the blood-CSF interface at birth already exhibited tight-junctions, which are the fundamental structures of the brain barriers. These junctions seemed to impede the intercellular movement of the biotin-dextran. Uptake into cells at the blood-CSF interface and lack of extracellular biotin-dextran in the brain suggest that in the immature brain the route of penetration from blood to brain may be predominantly via the CSF rather than directly across the cerebral vessels of the developing brain.
Rights statementCopyright 2001 the Author - The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s). Thesis (Ph.D.)--University of Tasmania, 2002. Includes bibliographical references