Body plans and segmentation Flashcards
LO
- Historical thoughts on body plans
- Body plans in chordates and vertebrates
- Organizers and axis formation
- Hox genes
- Segmentation similarities and differences across the animal Kingdom
What does the body plan give?
Body plan gives distinct set of features that makes us the phylum that we are. Helps classify the organisms that we are
What are the historical observations of body plans?
Etienna Geoffroy St. Hillaire
Ernst Haeckel
Tell me the pattern that German Embryologists created and what its known as/ show
The pattern known as von Baerian divergence, as illustrated by the embryos of four vertebrates, fish, hen, cow and human, shown at three different stages – early, middle and late. Note the pattern of early similarity giving way to later differences. (Redrawn from A Theory of the Evolution of Development, John Wiley & Sons, Ltd., 1988.)
What are the two views of recapitulation?
Two views of recapitulation:
- Left: ‘perfect’ recapitulation, in which evolution adds new developmental stages at the end of the ancestral ontogeny. This does not occur.
- Right: ‘imperfect’ recapitulation, in which some features of ancestral ontogenies are repeated (or recapitulated) in the development of descendants, even though they may not lead to functional adult structures in the descendants. This form of recapitulation does occur and is due to the fact that it is often hard for natural selection to alter early stages. The two forms of recapitulation are shown in terms of abstract developmental stages (A to D) in the top panel; and in terms of a particular example (bottom panel) where A is a vertebrate zygote; B is a vertebrate embryo with gill clefts; C is an adult fish; and D is an adult human
But: you can see why the early embryologists would have been confused
Tell me the historical observations of body plans
Tell me about body plans in chordates and vertebrates
What is the interrelationship of the chordates?
Chordate: an animal of the large phylum Chordata, comprising the vertebrates together with the sea squirts and lancelets.
Tell me the summary of the main types of clevage patterns
Tell me about the types of cell movement during gastrulation
Why is the rearranging of cells crucial?
- Rearranging cells is crucial to make the right structures, cells give rise to tissues, give rise to organs
- Early patterns of development are crucial for getting cells, not only into the correct germ layer, but into the right place to set up the body axes
Tell me about organisers and axis formation?
What is A-P and D-V related to and what does gravity give?
Tell me about L-R asymmetry in the chick
Organisers and axis formation
What are the stages through fertilisation to hatching called?
embryo genesis
What is development controlled through?
Development is controlled through a number of different regions of the embryo called organizers
What sets up a gradient during organisation and what regulates the activity of Hox?
The absence of BMPs, Wnts and FGFs from the head region of mammalian embryos sets up a gradient. Posterior expression feeding into Cdx pathways, regulates the activity of Hox
Hox genes
What is a hox gene?
Hox genes, a subset of homeobox genes, are a group of related genes that specify regions of the body plan of an embryo along the head-tail axis of animals. Hox proteins encode and specify the characteristics of ‘position’, ensuring that the correct structures form in the correct places of the body.
Hox genes: arthropods
Hox genes: annelids
Hox genes
What is a segment and what is segmentation?
A segment is a repeated unit along the body axis
Segmentation is the division of the units along the body axis to form a segment
Tell me about the similarities and differences in segmentation across the metazoa
And tell me how the evolutionary ideas about segmentation evolved
- Segmentation in animals typically falls into three types, characteristic of different arthropods, vertebrates, and annelids
The Choanozoa are unicellular aquatic protists that sometimes form colonies. The rest of the animals – including fish, birds, amphibians, reptiles and mammals – are all multicellular and are collectively referred to as the Metazoa
Tell me about one version if the ‘clock-and-wavefront’ model of vertebrate somitogenesis
One version of the ‘clock-and-wavefront’ model of vertebrate somitogenesis.
- Top – time measured by clock cycles
- Bottom – left and right bands of paraxial mesoderm (separated at the anterior end, joined at the posterior) in seven successive embryonic stages.
- The determination front (diagonal line) represents the boundary between RA (green) and FGF/Wnt (purple) signalling, and the threshold for cells becoming competent to form segments.
- Orange (shown in LHS of each stage only) – wave of clock expression.
- A somite becomes determined (black, RHS) when a clock pulse sweeps through cells that have become competent as the wavefront moves posteriorly.
- Somites (green blocks) form in the paraxial mesoderm in antero- posterior order, as shown)
Tell me about somite formation in the chick
Cycle of FGF and Wnt, Pre-somitic mesoderm–> somitic mesoderm and retinoic acid production happens every 90 minutes
Tell me about the notch signalling pathway
(possibly watch videos on this area as the explanations are complex and long)
Tell me about FGF and retinoic acid gradients pattern a-v axis in the mouse
- FGF and Retinoic acid gradients define the A-P axis of the mouse
- Somites form where the FGF is at a low enough threshold. As the node moves posteriorly, the level drops, and a somite formed
- Retinoic acid ‘buffers the developing somite, to give it some symmetry to its development.
When presomitic mesoderm was transplanted and flipped what did it still show?
Transplanting presomitic mesoderm and flipping it, still showed the correct ordering of the development of somites
Tell me about the termporal order of somites and how they are specified early in embryonic development
- Somites form by a cycling of the cascade of genes that are required for them to form in the mesoderm.
- Chick 90 minutes
- Mouse 120 min
- Xenopus 45 min
- Zebrafish 30 min
- Even if you excise and invert presomitic mesoderm, the order of formation is unchanged.
- Before somites form, the pre-somitic mesoderm has already been programmed with its identiy
What does pre-somitic mesoderm acquire before somite formation?
positional ID
NB. The somites (outdated term: primitive segments) are a set of bilaterally paired blocks of paraxial mesoderm that form in the embryonic stage of somitogenesis, along the head-to-tail axis in segmented animals.
What is it that gives the positional identity to these early embryos?
Hox
What can gene activity provide?
Positional values
Segmentation; similarities and differences across the Metazoa
Tell me about development and segmentation of the head
- The evolution of pharyngeal arch structures in the vertebrate head.
- The cells that form the gills supports in fish form the middle ear bones of mammals, and jaw structures in amphibians, birds and ‘reptiles’
Where are hox genes expressed and what is their patterning/ function in this location?
Hox genes are expressed in rhombomeres, and pattern the neural crest that migrates to branchial arches
Summary: Vertebrate axis determination
Summary: patterning of the vertebrate axial body plan
Segmentation, is it homologous across phyla?
Homology across phyla?
Summary
- Historical thoughts on body plans
- Body plans in chordates and vertebrates
- Organisers and axis formation
- Hox genes
- Segmentation similarities and differences across the animal kingdom
