Lecture 2 The diversity of development

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Lecture 2The diversity of development
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Multicellular development
Plants Green algae
animals
fungi
Slime molds
Colonial protists
600-1000 Myr ago
protists (single-celled eukaryotes)
A few independent solutions
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Metazoan animals

• Division of labor multiple cell types distinct
  germline and soma
• Tissues, organs that themselves communicate
• Distinct embryonic stages
• Gastrulation to form three early embryonic
  tissues (the germ layers)
• Body plan with radial or bilateral symmetry

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model organisms

• most work done on a small set of animals chosen
  for practical reasons
• The main players
• frog, chick, sea urchin (large, experimentally
  accessible)
• fly and worm (small but genetically tractable)
• mouse (token mammal)
• development may not be typical of group, e.g.
  Drosophila unusual compared to most insects

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describing development

• normal tables of morphological stages
• frog Nieuwkoop Faber stages 1-46
• chick
• before laying stages I- XIII (Kochav
  Eyal-Giladi)
• after laying Hamburger-Hamilton (HH) stages
  1-46
• Carnegie stages use features common to all
  vertebrates (including humans)
• mouse constant developmental rate in uterus, so
  staged by days post coitum
• e.g. E9.5 means embryonic day 9.5 p.c.
• also staging by somite number

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Metazoan topology
Outer epithelial layer ectoderm
Middle mesenchymal layer mesoderm
gut
Inner epithelial layer endoderm
three germ layers, so triploblastic cnidarians
lack mesoderm, so diploblastic (primitive?)
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epithelia and mesenchyme

• epithelia made of polarized cells held together
  by cell-cell and cell-ECM junctions
• mesenchyme tissue made up of scattered
  individual cells in an extracellular matrix (ECM)
• all three germ layers can make both epithelial
  and mesenchymal tissues

Slack Fig 4.7
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Body axes
W 1.13
Shorthand to describe asymmetries of body Most
animals bilaterally symmetric around the
head-tail (anterior-posterior, AP)
axis Back-front differences define the
dorsoventral (DV) axis, usually 90 to AP
axis left-right axis constrained by first two
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Oocyte axis
animal pole
asymmetry of oocyte or early embryo often
referred to as the animal-vegetal axis (most
obvious in sea urchins, amphibians,
ascidians) Cell asymmetry that subsequent
processes build on to form embryonic AP or DV
axis, --but NOT the same as them
female pronucleus
yolk
vegetal pole
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A dozen eggs

• Animals
• Triploblastic
• Platyhelminths-- flatworms (4)
• Coelomates
• Protostomes
• Lophotrochozoa molluscs (3)
• Ecdysozoa nematode (5) and fruit fly (6)
• Deuterostomes
• echinoderms sea urchin (1)
• chordates
• tunicates ascidians (7)
• vertebrates
• frog (8) fish (9,10), bird (11), mammal (12)
• Slime molds (2)

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Some movies online

• Center for Cell Dynamics
• http//raven.zoology.washington.edu/celldynamics/i
  ndex.html
• Timelapse movies of early development, lots of
  unusual organisms
• Society for Developmental Biology Cinema
• http//www.sdbonline.org/dbcinema/
• Rather limited but some good stuff
• Bioclips project
• http//bioclips.com/index.php3
• Fancy animations, mostly cell biology

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Eggs

• Variables Size of egg and amount and
  distribution of yolk
• Yolk complex mixture of proteins (containing
  the 9 essential amino acids) and lipids
  (phosphate-rich)
• Yolk always made by somatic cells, taken up by
  oocytes, stored in membrane-bound vesicles
  (platelets)

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Sea urchins
W Fig 6.12

• many small eggs, not much yolk
• animal-vegetal asymmetry
• Cleavage holoblastic with radial symmetry
• larvae have bilateral symmetry, adults radial
  symmetry

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Cellular slime molds (Dictyostelium discoideum)
The SLUG differentiation , pattern formation
Aggregation of amoebae directed cell migration
Wolpert pp 212-216
Morphogenesis of slug into fruiting body
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Roundworms (Nematodes)Caenorhabditis elegans

• small number of cells (959 in adult)
• cells divide in invariant pattern from worm to
  worm
• rotational, holoblastic cleavage rapid
  development to larval stage
• Lecture 13-14

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Insects the fruit-fly, Drosophila melanogaster
Nuclei in early fly embryo, courtesy Bill
Sullivans lab

• early mitoses without cytokinesis--embryo is a
  syncytium
• yolk in center of egg nuclei move to surface
• cellularization to form blastoderm, then
  gastrulation
• Lectures 9-12

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Chordates

• 3 subphyla
• vertebrates
• cephalochordate
• urochordates
• Defined by embryonic structure, the notochord
• Made by dorsal mesoderm
• Cartilaginous rod, becomes intervertebral disks
  (in vertebrates)
• Other features dorsal nerve cord, pharyngeal
  slits, post-anal tail

W Fig 15.2 Amphioxus cephalochordate
W Fig 6.19 ascidian larva expressing GFP in
notochord
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Ascidians (tunicates, sea squirts)
W Fig 6.20

• Small eggs, little yolk
• Myoplasm (yellow crescent)
• Cleavage holoblastic, radial first cleavage
  meridional
• Invariant development

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Amphibian development Xenopus laevis
Animal hemisphere (pigmented in some species)
Vegetal hemisphere (yolky)
Cleavage is holoblastic, radial Vegetal
blastomeres bigger
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Frog gastrulation

• Invagination at blastopore (dorsal)
• cells move into blastocoel to form archenteron
  (gut) by convergence and extension movements
• Anterior end becomes mouth

W Fig 2.6
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Frog neurulation

• Elongation along anteroposterior axis
• dorsal ectoderm (neural plate) folds into neural
  tube

W Fig 2.7
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Fish development

• meroblastic cleavage confined to non-yolky part
  of egg

Cleavage and gastrulation resemble frog but are
more constrained by yolk
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Bird (chick) development

• Gigantic super-yolky eggs
• Cleavage confined to blastoderm (2 mm)
• discoidal cleavage, generates blastodisc of
  60,000 cells at time of laying
• best vertebrate embryo for studying organogenesis

W Fig 2.10
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Chick cleavageformation of epiblast and
hypoblast

• epiblast will give rise to embryo
• hypoblast makes extra-embryonic tissues (needed
  for prolonged development in eggshell)

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Chick gastrulation

• Epiblast cells migrate into subgerminal space to
  become mesoderm, endoderm
• primitive streak thickened region where this
  happens equivalent to blastopore of frog
• Defines future dorsal midline of body

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Chick organogenesis

• Hensens node regresses (moves anterior to
  posterior)
• somites form from anterior to posterior
• analogous to frog except on a disc, not in a
  sphere

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Mammals (mouse)

• small eggs (0.1 mm diameter), little yolk
• Cleavage slow, asynchronous
• Compaction at 8-cell stage to form morula
• makes two populations of cells outer
  trophectoderm (TE) and an inner cell mass (ICM)
• blastocyst implants into uterus (day 4.5 in
  mouse, day 5.5 in human)

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Mouse gastrulation

• trophectoderm gives rise to placenta
• Part of ICM becomes epiblast--the future embryo
• epiblast in rodents is cup-shaped egg cylinder
  but gastrulation topologically like that of chick

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Diversity of early development reflects
reproductive strategy

• Many small eggs
• develop rapidly to feeding larval stage that may
  undergo metamorphosis to adult
• most invertebrates but also e.g. amphibians
• A few large eggs
• usually develop to smaller version of adult
• nourished in egg with large yolk supply (birds)
• nourished in uterus with placenta (mammals)
• amniotes must set aside cells at blastula stage
  to make extra-embryonic tissues

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Next questions

• What sets up body axes?
• How are germ layers specified?
• How can similar body plans arise from different
  early embryos?