Later Development. Caenorhabditis elegans. Later Processes 10/06/12. Cytoplasmic Determinants Fate Mapping & Cell Fate Limb Development

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1 Later Development Cytoplasmic Determinants Fate Mapping & Cell Fate Limb Development Caenorhabditis elegans Nematoda 10,000 worms/petri dish in cultivation short life cycle (~ 3 days egg to egg) wild-type individuals contain a constant 959 cells life span is around 2 to 3 weeks The position of cells is constant = model organism Neurula-on Organogenesis Morphogenesis Later Processes 1

2 Figure Figure Neural plate Microtubules Figure Neural plate Microtubules Actin filaments 2

3 Figure Neural plate Microtubules Actin filaments Figure Neural plate Microtubules Actin filaments Neural tube Caenorhabditis elegans genome size of C. elegans is about a hundred million base pairs ~ 20X bigger than that of E. coli and about 1/30 of that of human 1 st multicellular-organism (animal) that has a completely sequenced genome. completely sequenced at the end of % homologous with humans 3

4 Fate Mapping Fate maps are diagrams showing organs and other structures that arise from each region of an embryo Classic studies using frogs indicated that cell lineage in germ layers is traceable to blastula cells What germ layers are present in a triploblas-c blastula? Figure Epidermis Central nervous system Notochord Mesoderm Epidermis Blastula Endoderm (a) Fate map of a frog embryo Neural tube stage (transverse section) 64-cell embryos Blastomeres injected with dye Larvae (b) Cell lineage analysis in a tunicate Later studies of C. elegans used the abla-on (destruc-on) of single cells to determine the structures that normally arise from each cell The researchers were able to determine the lineage of each of the 959 soma-c cells in the worm 4

5 Germ cells are the specialized cells that give rise to sperm or eggs Complexes of RNA and protein are involved in the specifica-on of germ cell fate In C. elegans, such complexes are called P granules, persist throughout development, and can be detected in germ cells of the adult worm Figure µm 1 Newly fertilized egg 2 Zygote prior to first division 3 Two-cell embryo 4 Four-cell embryo P granules are distributed throughout the newly fer-lized egg and move to the posterior end before the first cleavage division With each subsequent cleavage, the P granules are par--oned into the posterior- most cells P granules act as cytoplasmic determinants, fixing germ cell fate at the earliest stage of development 5

6 Axis Forma1on A body plan with bilateral symmetry is found across a range of animals This body plan exhibits asymmetry across the dorsal- ventral and anterior- posterior axes The right- ley axis is largely symmetrical Restric1ng Developmental Poten1al Hans Spemann performed experiments to determine a cell s developmental poten-al (range of structures to which it can give rise) Embryonic fates are affected by distribu-on of determinants and the pa^ern of cleavage The first two blastomeres of the frog embryo are to.potent (can develop into all the possible cell types) In mammals, embryonic cells remain to-potent un-l the 8- cell stage, much longer than other organisms Progressive restric-on of developmental poten-al is a general feature of development in all animals In general -ssue- specific fates of cells are fixed by the late gastrula stage 6

7 Forma1on of the Vertebrate Limb Induc-ve signals play a major role in pa0ern forma.on, development of spa-al organiza-on The molecular cues that control pa^ern forma-on are called posi.onal informa.on This informa-on tells a cell where it is with respect to the body axes It determines how the cell and its descendents respond to future molecular signals Figure Anterior Limb bud AER Limb buds 50 µm ZPA Posterior 2 Digits Apical ectodermal ridge (AER) Anterior 3 4 Ventral Proximal Distal Dorsal (a) Organizer regions (b) Wing of chick embryo Posterior The embryonic cells in a limb bud respond to posi-onal informa-on indica-ng loca-on along three axes Proximal- distal axis Anterior- posterior axis Dorsal- ventral axis 7

8 One limb- bud regula-ng region is the apical ectodermal ridge (AER) The AER is thickened ectoderm at the bud s -p The second region is the zone of polarizing ac.vity (ZPA) The ZPA is mesodermal -ssue under the ectoderm where the posterior side of the bud is a^ached to the body Tracing Individual Cells Tissue transplantation experiments support the hypothesis that the ZPA produces an inductive signal that conveys positional information indicating posterior Sonic hedgehog is an induc-ve signal for limb development Hox genes also play roles during limb pa^ern forma-on 8

9 Cilia and Cell Fate Ciliary func-on is essen-al for proper specifica-on of cell fate in the human embryo Mo-le cilia play roles in ley- right specifica-on Monocilia (nonmo-le cilia) play roles in normal kidney development Figure Lungs Heart Liver Spleen Stomach Large intestine Normal location of internal organs Location in situs inversus 9