formatting the body Flashcards
why did edward conkilin use sea squirts (Styella patita) to study developmental patterning
because the blastomeres of very early embryos shows a yellow pigment that is easy to follow
what did the yellow pigmented cells of the Styella partita develop into
the posterior muscle elements of the tail
how can we track specific cells in development to create fate maps
Inject fluorescent dye into a few cells, use a laser to activate v small number of cells (5) so only they are labelled with the dye, then have a fate map
differences between development of human vs mouse embryos
Human epiblast turns into a disk whereas mouse develop as a cup
what is the first thing to form in an embryo
the posterior to anterior axis with the formation of the primitive streak
what is Hensons node and where does it form
aka primitive node - a thickening at the cranial/anterior end of the primitive streak - will be the organiser (in a chick embryo)
what occurs when you transplant pieces of the primitive streak from and early and late embryo donor into another organism eg quail to chick
the graft from the early embryo will turn into a more complete structure with a wider combination of donor and recipient cells used (greater ability to induce)
the graft from the later stage embryo will only induce a more specific structure (eg just the trunk) and be mainly made up on donor cells
how would you experimentally induce a human axis in a recipient embryo
Take pluripotent human embryo cells, induce with Wnt + activin (organiser fate) and graft into a donor chick embryo
how does the head process form in a chick embryo
once hensons node has reached the most anterior part some cells will carry on under the epiblast and form the head process
what determines the A-P streak in the chick
via gravity (embryo rotations cause the heaviest elements to go to the bottom to begin the posterior ingression of the primitive streak)
describe the conversion of radial symmetry to bilateral symmetry in chick embryos
As the ovum passes through the hen’s reproductive tract, it rotates. This spinning, shifts the yolk such that its lighter components lie beneath one side of the blastoderm.
This imbalance tips up one end of the blastoderm, and that end becomes the posterior marginal zone (PMZ), adjacent to where primitive streak formation begins.
what ultimately determines left right asymmetry in the chick
Nodal and PitX (only expressed on the left side
describe the signalling cascade that determine left right symmetry in the chick
Shh is expressed from the node
on the left side: Shh activates cerebrus, which activates BMPs, which activates Nodal and PitX
on the right side
describe the signalling cascade that determine left right symmetry in the chick
Shh is expressed from the node
on the left side: Shh activates cerebrus, which activates BMPs, which activates Nodal and PitX
on the right side: Shh expressed causing activin expression, inhibiting Fgf8 which inhibits cerebrus so there is no downstream signalling of nodal or pitx
what determines A-P axis formation in the Xenopus
site of sperm entry
(cortical roation and definintion of the animal and vegetal pole then organiser formation at the site of entry)
what structures form when you transplant a portion of the dorsal lip from young and advanced gastrulas
young dorsal lip = anterior structures
later stage dorsal lip = more trunk peices
what is the next step after axis formation
segmentation
what transcription factors confer identity to the segments
Hox genes
what is Hox gene collinearity
relates the gene order of the Hox cluster in the chromosome (telomeric to centromeric end) with the serial activation of these genes in the ontogenetic units along the Anterior-Posterior embryonic axis
the 3 and 5 prime ends of the hox cluster determine what
3' = head 5' = posterior structures
invertebrates how many clusters of Hox genes are there
4 or more
mouse 4 chick 7
hox genes have a sharp __________ boundary and a more diffuse ____________
sharp anterior boundary and more diffuse posterially
what is homeotic transformation
Homeotic transformation is a transformation of tissues in which the developmental fate of an tissue is changed to that of another caused by misexpression of developmental/ Hox genes
if a hox gene is knocked out what occurs
the more anterior hox gene continues its expression
what are somites
precursor populations of cells that give rise to important structures associated with the vertebrate body plan and will eventually differentiate into dermis, skeletal muscle, cartilage, tendons, and vertebrae.
where are osmite derived from
paraxial mesoderm (presomitic mesoderm)
what is somitogenesis
a highly regulated process that determines “what, when, where, and how many” somites an organism makes. Anterior to posterior direction. – blocks of somites in the mesoderm on either side of (flank) the neural tube in the midline
- regulated periodic budding of somites
where do somites form
on either side of the midline (neural tube)
somitogenesis occurs simultaneously with________
the closing of the neural tube
the mesoderm is partitioned into four distinct zones, what are they
lateral plate mesoderm, intermediate mesoderm, paraxial mesoderm and chordamesoderm
the type of mesoderm is determined by …..
relation to the midline
paraxial mesoderm differentiates into head and somite. somite differentiates into what 5 things and what do they develop into
sclerotome (cartilage) syndetome (tendons) myotome (skeletal muscles) endotome (endothelial cells and dorsal aorta) dermatome (dermis, skeletal muscles)
what provides the signals that induce the somite lineage
the surrounding tissues: Signalling from lateral plate, notochord (Shh) and ectoderm (Wnt)
what are the somite called when they are formed, forming and not yet formed
S1 formed
S0 is being formed
S-1 soon to be formed
clock genes are components of what three signalling pathways
notch, fgf and wnt
how are clock genes expressed
cyclic genes expression in an oscillatory pattern
what direction does the wavefront move
anteriorly - moves from the posterior to anterior
what direction are somites formed
anterior to posterior
what system works in conjunction with the clock genes
a gradient system - the wavefront
what opposing gradients control the wavefront/determination front
: retinoic acid in the anterior decreasing towards posterior, FGF and Wnt higher in the posterior moving anterior – where they intersect is where you get the wave front where the somites will form
the intersection of what gradients is the location of somite formation
retinoic acid and fgf&Wnt
what ways do the clock genes interact
The clock genes oscillate out of phase with the other clock signalling pathways – and also work in conjunction with each other
what types of signalling is the notch pathway
juxtacrine signalling - physical contact between the cells is required
briefly describe the notch singalling pathway
physical contact between notch receptor on one cell and ligand on another cell causes cleavage that allow the notch intracellular domain to induce expression of the target gene
which end of the axis is retinoic acid highest
the anterior
which end of the axis is fgf highest
in the posterior
describe the process of somite boundary formation
Notch expressed in the presumptive somite activates Mesp2 in this area, downregulated in the caudal part just being expressed in the rostral (lower/posterior) part -> induces ephrin (ephrinA4) leading to epithelialisation (morphological boundary develops)
what are the different timescale in somite formation in 4 vertebrates
In zebrafish = every 30 mins
In chick = every 90 mins
In mouse = every 2.5 hours
in human = every 5-6 hours
when is the temporal order/identity of the somite determined
before somitogenesis early in development
what confers identity of the segments
Hox genes
what experimentation showed when somite identity is determined
transplanting presomitic mesoderm and flipping it, still showed the correct ordering of the development of somites
take some presomitic mesoderm transplant into a different position and you get the same structure it would originally be
what causes segmental defects in humans
when somites arent formed in the synchronous fashion at the right times you get a mismatch of somites and vertebrae - caused by mutations in notch and other related signalling pathways
whats structures other than the spine are segmented
skull and brain
