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Title: An investigation into the regulation of segment number in the leech
Author: Desjeux, Isabelle
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 1996
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The control of the number of segments is a problem posed in all segmented phyla: how can there be generation of a constant number of body parts? Here I have studied the leech embryo as an example of an animal where the number of segments is regulated. The leech develops in a stereotypical manner and gives rise, during the cleavage stage, to 5 pairs of teloblasts. These cells divide in a stem-cell manner, producing blast cells, the founders of the segmental body plan. The blast cells are produced one after the other, and stay ordered in such a way that the most anterior blast cells are the oldest and the most posterior the youngest. As they move anteriorly, the bandlets or columns of blast cells from the different lineages converge to form the germinal band. This results in an anteroposterior gradient of development. In most teloblast lineages, one blast cell populates one segment equivalent. However, more blast cells are born than there are segments. I have been used lineage tracing to study the fate of the cells that are eliminated in the process of segment regulation in the leech. In combination, I have been looking for genes that could be involved in such a process of number counting and boundary formation. My results confirm the presence of two types of blast cells, the segmental cells (that go on to form the segmental body) and the supernumerary cells (that die). They suggest (against what was previously thought), that blast cells do not need to be in contact with neighbours of the other lineages (i.e. the germinal band) to divide, but their presence in the germinal band is required for further divisions, and therefore making of segments. my results also show that the pair-rule gene hairy might not be implicated in setting up the boundary between segmental and supernumerary cells, but that the pair-rule gene patched might be indirectly implicated by being part of a signalling pathway between the segmented tissue and overlaying provisional epithelium.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available