Body plans and segmentation

Question 1

LO

Answer 1

• 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

Question 2

What does the body plan give?

Answer 2

Body plan gives distinct set of features that makes us the phylum that we are. Helps classify the organisms that we are

Question 3

What are the historical observations of body plans?

Answer 3

Etienna Geoffroy St. Hillaire

Ernst Haeckel

Question 4

Tell me the pattern that German Embryologists created and what its known as/ show

Answer 4

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.)

Question 5

What are the two views of recapitulation?

Answer 5

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

Question 6

Tell me the historical observations of body plans

Answer 6

Question 7

Tell me about body plans in chordates and vertebrates

Answer 7

Question 8

What is the interrelationship of the chordates?

Answer 8

Chordate: an animal of the large phylum Chordata, comprising the vertebrates together with the sea squirts and lancelets.

Question 9

Tell me the summary of the main types of clevage patterns

Answer 9

Question 10

Tell me about the types of cell movement during gastrulation

Answer 10

Question 11

Why is the rearranging of cells crucial?

Answer 11

• 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

Question 12

Tell me about organisers and axis formation?

Answer 12

Question 13

What is A-P and D-V related to and what does gravity give?

Answer 13

Question 14

Tell me about L-R asymmetry in the chick

Answer 14

Question 15

Organisers and axis formation

Answer 15

Question 16

What are the stages through fertilisation to hatching called?

Answer 16

embryo genesis

Question 17

What is development controlled through?

Answer 17

Development is controlled through a number of different regions of the embryo called organizers

Question 18

What sets up a gradient during organisation and what regulates the activity of Hox?

Answer 18

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

Question 19

Hox genes

Answer 19

Question 20

What is a hox gene?

Answer 20

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.

Question 21

Hox genes: arthropods

Answer 21

Question 22

Hox genes: annelids

Answer 22

Question 23

Hox genes

Answer 23

Question 24

What is a segment and what is segmentation?

Answer 24

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

Question 25

Tell me about the similarities and differences in segmentation across the metazoa And tell me how the evolutionary ideas about segmentation evolved

Answer 25

* 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**

Question 26

Tell me about one version if the 'clock-and-wavefront' model of vertebrate somitogenesis

Answer 26

**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)

Question 27

Tell me about somite formation in the chick

Answer 27

Cycle of FGF and Wnt, Pre-somitic mesoderm--\> somitic mesoderm and retinoic acid production happens every 90 minutes

Question 28

Tell me about the notch signalling pathway

Answer 28

(possibly watch videos on this area as the explanations are complex and long)

Question 29

Tell me about FGF and retinoic acid gradients pattern a-v axis in the mouse

Answer 29

* 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.

Question 30

When presomitic mesoderm was transplanted and flipped what did it still show?

Answer 30

Transplanting presomitic mesoderm and flipping it, still showed the correct ordering of the development of somites

Question 31

Tell me about the termporal order of somites and how they are specified early in embryonic development

Answer 31

* 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

Question 32

What does pre-somitic mesoderm acquire before somite formation?

Answer 32

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.

Question 33

What is it that gives the positional identity to these early embryos?

Answer 33

Hox

Question 34

What can gene activity provide?

Answer 34

Positional values

Question 35

Segmentation; similarities and differences across the Metazoa

Answer 35

Question 36

Tell me about development and segmentation of the head

Answer 36

* 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’

Question 37

Where are hox genes expressed and what is their patterning/ function in this location?

Answer 37

Hox genes are expressed in rhombomeres, and pattern the neural crest that migrates to branchial arches

Question 38

**Summary: Vertebrate axis determination**

Answer 38

Question 39

**Summary: patterning of the vertebrate axial body plan**

Answer 39

Question 40

Segmentation, is it homologous across phyla?

Answer 40

Question 41

Homology across phyla?

Answer 41

Question 42

**Summary**

Answer 42

* **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**