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10/06/12 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 Later Processes • Neurula-on • Organogenesis • Morphogenesis 1 10/06/12 Figure 47.15-1 Ectoderm Figure 47.15-2 Ectoderm Neural plate Microtubules Figure 47.15-3 Ectoderm Neural plate Microtubules Actin filaments 2 10/06/12 Figure 47.15-4 Ectoderm Neural plate Microtubules Actin filaments Figure 47.15-5 Ectoderm 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 • 1st multicellular-organism (animal) that has a completely sequenced genome. • completely sequenced at the end of 1998 • 40% homologous with humans 3 10/06/12 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? © 2011 Pearson Education, Inc. Figure 47.17 Epidermis Central nervous system Notochord Epidermis Mesoderm Endoderm Blastula Neural tube stage (transverse section) (a) Fate map of a frog embryo 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 © 2011 Pearson Education, Inc. 4 10/06/12 • 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 © 2011 Pearson Education, Inc. Figure 47.20 20 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 © 2011 Pearson Education, Inc. 5 10/06/12 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 © 2011 Pearson Education, Inc. 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) © 2011 Pearson Education, Inc. • 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 © 2011 Pearson Education, Inc. 6 10/06/12 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 © 2011 Pearson Education, Inc. Figure 47.24 Anterior Limb bud AER ZPA Posterior Limb buds 2 50 µm Digits Apical ectodermal ridge (AER) Anterior 3 4 Ventral Proximal Distal Dorsal Posterior (a) Organizer regions (b) Wing of chick embryo • 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 © 2011 Pearson Education, Inc. 7 10/06/12 • 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 © 2011 Pearson Education, Inc. 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 © 2011 Pearson Education, Inc. 8 10/06/12 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 © 2011 Pearson Education, Inc. Figure 47.26 Lungs Heart Liver Spleen Stomach Large intestine Normal location of internal organs Location in situs inversus 9
