Lecture 5+6 How do cells acquire fates.pdf

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Cell and Systems Biology
William Navarre

CSB328H1 © Lis| Page 1013 L E C T U R E 5 + 6 : H O W D O C E L L S A C Q U I R E F A T E S ? Reading: Gilbert Part 2 (p 108-111) SPECIFICATION LEVELS OF COMMITMENT  from blastomere to specific cell type, the cell must first be committed to a certain fate, & then differentiate  2 stages of the commitment process: 1. specification – the fate of a cell/tissue is specified when it is capable of differentiating autonomously in a neutral envmt  cell commitment is still reversible 2. determination – a cell/tissue is determined when it is capable of differentiating autonomously even when placed in another region of the embryo (non-neutral envmt)  commitment is irreversible 3. differentiation  3 strategies of specification: (diff combinations are used in diff species) 1. autonomous 2. conditional 3. syncytial AUTONOMOUS SPECIFICATION  autonomous specification: diff regions of the egg cytoplasm contain diff morphogenetic determinants (transcription factors) that direct cell dvpmt (Fig P2.1) w/o signals from other cells  ex. differentiation of trochoblast cells in the snail Patella  trochoblast cells become ciliated (Fig P2.1C), even when isolated from other cells (Fig 2.1D-G)  ex. each blastomere from the tunicate embryo forms its respective cell type even when separated (Fig P2.2)  each contain cytoplasm w mRNA for specific transcription factors o ex. yellow-pigmented cytoplasm – contains Macho  tail muscles (if macho-containing yellow cytoplasm is placed into other cells, they will form tail muscles) o microsurgery forcing some of the yellow cytoplasm from B4.1 cells into b4.2 cells allow b4.2 cells to produce muscle cells as well as their normal ectodermal progeny (Fig P2.3) Page 2CSB328H1 © Lisa Zhao2013 CONDITIONAL SPECIFICATION  conditional specification: the ability of cells to achieve their respective fates by interacting w other cells  August Weismann (1888) – germ plasm theory  each cell of the embryo develops autonomously  the sperm & egg provided equal chromosomal contributions to the new organism  the chromosomes carried the inherited potentials of the new organism  not all the determinants on the chromosomesentered every cell of the embryo – diff determinants entered diff cells  only germ cells contained all determinants, somatic cells contain only a subset (Fig P2.5)  Weismann hypothesis: first cleavage blastomeres determined the L & R side of the future organism  Wilhelm Roux used a hot needle to kill one of the blastomeres (defect experiment specification was autonomous, all the instructions for normal dvpmt were present in each cell (Fig P2.6)  Weismann & Roux  segregation of nuclear determinants  Hans Dreisch (1892) performed an isolation experiment on pluteus larva, separated cells, each cell regulated its dvpmt to produce a complete organism  the
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