Chapter 14 Flashcards
what are the three sections of a limb? describe them
stylopod = humerus / femur
zeugopod = radius & ulna / tibia & fibula
autopod = carpals / tarsals
in relation to the limb, describe the three axes (A/P, P/D, D/V)
anterior = thumb
posterior = pinky
proximal = inside (shoulder)
distal = outside (fingers)
dorsal = back of hand (knuckles)
ventral = palm of hand (pads)
what is a limb field?
area on the body before it becomes a limb
what are the three areas of the limb field? describe them
peribrachial = posterior side, under the arm
free limb = what sticks out
shoulder girdle = anterior side, above the arm
describe Harrisons experiment (1918)
did find it, move it, lose it experiments with mesodermal and ectodermal cells of the LIMB FIELD
- mesodermal = (L) stops limb growth
-ectodermal = (L) delayed limb growth bc it needs new skin to form before growing outwards
removed half of the limb field
- by removing any half of the limb field, it can still generate a fully functioning limb
- this occurs naturally thru parasites
describe how parasites alter the limb field in frogs
Parasites enter an animal and can break tissue into groups
- The body forms scar tissue around the parasite, preventing the cells in the limb field from talking to eachother
- Causes multiple limbs to form
*Indicator species -> if something is wrong with a frog, it can indicate something is wrong with everything in the environment
what hox gene is responsible for forming the forelimb bud?
HoxC6
- removing hox causes anteriorization
- adding RA causes posteriorization
describe Fgf8 in limb development
in the apical ectodermal ridge (AER)
- ECTODERMAL CELLS
- helps cells of the limb grow outwards (distally)
describe Fgf10 in limb development
in the mesodermal cells
- helps cells of the limb grow outwards (distally)
what do Tbx5 and Tbx4 transcription factors do in limb development?
Tbx5 = anteriorly, forelimb
Tbx4 = posteriorly, hindlimb
what is the progress zone (PZ)?
cells right under AER, mesenchymal cells
what are the two models for the progress zone? describe them
progress cell model
- the PZ is induced by Fgfs & as these PZ cells grow & divide, they induce specific structures to form
- not all cells remain w/in inducing range
early specification model
- cells decide what they will be prior to dividing
describes Sauder’s grafting experiment (1968)
AER removed = no limb
AER moved = additional limb
AER forelimb & hindlimb swapped = wing in place of leg
AER replaced w/ Fgf bead = normal limb
limb mesenchymal cells replaced = no limb
AER removed in middle of developing limb = shortened limb
what area is Shh located in the limb? what does it specify in limbs?
zone of polarizing activity (ZPA)
- specifies posterior fates
describe the Riddle & Tabin experiment
implanted cells containing Shh-producing virus into anterior portion of limb bud
- produces mirror-image duplication (two pinkies)
determined that Shh & digit identity is dependent on time of exposure (digit 3) and amount of Shh expression (digit 2) –> POLARIZING ACTIVITY
- no Shh = thumb
- lots of Shh = pinky
what did Chiang do?
made mice w/o Shh
- led to dead mice with one eye (cyclopia)
- mice also had improper motor neurons, no A/P limb structure, no spinal column, missing ribs
what did Ohuchi do? (1997)
replaced anterior part of AER w/ Fgf8 = full limb
replaced anterior part of AER w/ Fgf10 = missing some ant. structures of limb
- takes time for Fgf10 to induce Fgf8
- Fgf10 can maintain ZPA, but Fgf8 does it better
*same thing happened when replacing posterior portion!
what molecule does Fgf10 and Fgf8 use to signal and induce eachother?
Wnt
what other Fgf helps keep Fgf8 expressed?
Fgf4
- in the ectoderm
what does Shh do to Fgf’s?
induces them!!
- keeps Fgf10 and Fgf8 active!
what drives the proximal / distal axis in the limb?
Fgf8 (AER) drives distal growth
what drives anterior / posterior axis in the limb?
Shh (ZPA) drives posterior fates
what drives dorsal / ventral axis in the limb?
Wnt7a drives dorsal fates
- engrailed drives ventral fates (blocks Wnt7a)
where is Wnt7a made?
dorsal ectoderm
describe how webbing is formed
- Shh activates Gremlin
- Gremlin blocks BMPs from killing webbing cells
- webbing stays
what induces the apoptosis of webbing cells? (interdigital zones)
BMPs
what do BMPs do to Fgfs?
blocks Fgfs
- this occurs once the limb as done enough distal growth and Fgf’s have accumulated (makes the AER inactive)
importance of dinosaur digits in limb development
hox genes show a dino-bird relationship
- both have 3 fingers / toes
importance of dwarfism in limb development
no FgfR3 (receptor)
- chondrocytes differentiate instead of dividing
importance of giants in limb development
no estrogen at the end of puberty to ‘cap off’ cartilage growth at the end of bones
importance of bats in limb development
extra BMP-inhibitors in webbing
what is the Lateral Plate mesoderm?
causes the limb bud in the amphibian embryo to bulge outward
- generate skeletal elements of the limb
what do the different types of mesoderm make?
- intermediate
- chorda
- lateral plate
- paraxial
intermediate mesoderm = kidneys, gonads (PRONEPHRON)
chorda mesoderm = notochord
lateral plate mesoderm = splanchnic/circulatory system, somatic/body cavity, extra embryonic
paraxial mesoderm = head, somites (myotome/muscle, dermatome/dermis, sclerotome/cartilage, syndrome/tendons, endothelial cells/blood vessels)
