Limb development Flashcards
Each limb bud has an external layer of ______ and a core of ______ Muscle and tendon cells originate in the _____ and later migrate into the limb bud
Each limb bud has an external layer of ectoderm and a core of somatic (lateral plate) mesoderm Muscle and tendon cells originate in the somite and later migrate into the limb bud
Muscles and tendons of the limbs come from _______
Muscles and tendons of the limbs come from myoblasts (somites)
Bones, connective tissue (fascia), dermis of the limbs comes from _______
Bones, connective tissue (fascia), dermis of the limbs comes from somatic lateral plate mesoderm (all connective tissue of limb bud)
What is the Apical Ectodermal Ridge (AER)?
What induces its formation?
What factor is expressed in the AER
- APICAL ECTODERMAL RIDGE (AER)
- A distinct ridge of ectoderm at the distal end of the limb bud
- Signals in the underlying somatic mesoderm induce formation of the AER
- AER express fibroblast growth factor 8 (FGF8)
Limb buds grow from _______ axis
- ____ maintains the cells behind the AER in a proliferative, undifferentiated state
- As the AER extends further _____, some cells escape the _____ signal and start to differentiate
- Thus, the AER drives growth of the limb bud along the _______ axis
Limb buds grow from proximal to distal axis
- FGF8 maintains the cells behind the AER in a proliferative, undifferentiated state
- As the AER extends further distally, some cells escape the FGF8 signal and start to differentiate
- Thus, the AER drives growth of the limb bud along the proximal-distal axis
As limbs grow they are patterned along 3 axes:
- ________ (eg shoulder to fingertips)
- _______ (eg thumb to pinky)
- ______ (eg back of hand vs palm)
As limbs grow they are patterned along 3 axes:
- proximal-distal (eg shoulder to fingertips)
- cranial-caudal (eg thumb to pinky)
- dorsal-ventral (eg back of hand vs palm)
What are the three parts that the limb is separated into in the Proximal-distal axis?
- Stylopod
- Zeugopod
- Autopod
What are the three parts that the limb is separated into in the Proximal-distal axis?
- Stylopod
- Zeugopod
- Autopod
Proximal-distal axis
- What controls the order of differentiation
- What do we pattern first?
Proximal-distal axis
- What controls the order of differentiation
- AER (apical ectodermal ridge)
- Halted growth by cutting AER at various stages
- What do we pattern first?
- Sylopod
Proximal-distal axis:
-How do the differentiating cells know what to become?
- Each region of the limb expresses a different Hox gene code, leading to formation of different structures along the P-D axis
- Hox gene mutations lead to missing segments
What is the timing model of proximal-distal limb patterning?
- Temporal differentiation
- Cells know the order in which they are differentiating (eg count cell divisions and know what to become based on their cell division number)
What is the morphogen model of proximal-distal limb patterning
- FGFs (fibroblast growth factors) in the AER and Retinoic acid in the flank form two opposing morphogen gradients along the proximal-distal axis
Both morphogen gradients and timing mechanism are needed to pattern entire P-D axis:
- _______ pattern stylopod/zeugopod
- _______ patterns autopod
Both morphogen gradients and timing mechanism are needed to pattern entire P-D axis:
- morphogens pattern stylopod/zeugopod
- timing mechanisms patterns autopod
Morphogens in the _______ pattern the dorsal-ventral axis of the limb
Morphogens in the overlying ectoderm (developing skin) pattern the dorsal-ventral axis of the limb
Cranial Caudal axis:
- A ________ in the caudal limb bud patterns the cranial-caudal axis
- _______ is expressed by cells in [this region]
- As ____ is secreted, a gradient forms (____ caudal, ___ cranial) to pattern the axis
Cranial Caudal axis:
- A zone of polarizing activity in the caudal limb bud patterns the cranial-caudal axis
- Sonic hedgehog is expressed by cells in the ZPA
- As SHH is secreted, a gradient forms (high caudal, low cranial) to pattern the axis
- ie thumb forms where SHH is lowest (cranial)
How was the ZPA discovered?
Zone of Polarizing Activity
- Transplant of the zone of polarizing activity (ZPA) from a donor onto the cranial margin of the wing bud of host induced a mirror duplication of the cranial-caudal axis
How are individual bones established?
- Turing reaction-diffusion mechanism
- uses 2 or more interacting molecules that sort themselves into a more complex pattern
- Need both an ACTIVATOR that activates BOTH ITSELF and AN INHIBITOR
What is the Turing reaction-diffusion mechanism?
- Turing reaction-diffusion mechanism
- uses 2 or more interacting molecules that sort themselves into a more complex pattern
- Need both an ACTIVATOR that activates BOTH ITSELF and AN INHIBITOR
What 3 signaling molecules interact to create individual digits (via turing reaction-diffusion mechanism)?
Which of these drives cartilage formation?
- Wnt
- Bmp
- Sox9
- Create a repeating stripe of Sox9 which will start to form cartilage
What separates the digits?
Apoptosis
How (and when) are the limb bones formed?
- 5th-6th weeks: _______ condensations form from ________
- 6th-8th weeks: _______ (cartilage models form for each bone)
- 7th-12th weeks: ________ in most limb bones
- A few don’t appear until after birth
- 5th-6th weeks: Mesenchymal condensations form from somatic lateral plate mesoderm
- 6th-8th weeks: Chondrification (cartilage models form for each bone)
- 7th-12th weeks: Ossification centers appear in most limb bones
- A few don’t appear until after birth
What are the stages of chondrocyte development?
When do they occur?
Quiescent → proliferation → hypertrophy → death and replacement by osteoblasts
Occur during initial bone formation and at the growth plate
During each step in development, the chondrocytes express a different set of ______
During each step in development, the chondrocytes express a different set of genes
- Mutation in any of these genes depends on the particular role for that gene in bone development
- eg can cause reduced chondrocyte proliferation, premature hypertrophy, reduced differentiation
During each step in development, the chondrocytes express a different set of ______
During each step in development, the chondrocytes express a different set of genes
- Mutation in any of these genes depends on the particular role for that gene in bone development
- eg can cause reduced chondrocyte proliferation, premature hypertrophy, reduced differentiation
What are bony eminences and how are they formed?
What is expressed in bone eminences?
- Bone eminences = bumps for attachment of ligaments and tendons
- specifically pattered during development but form secondary to the main bone
-
Scleraxis is expressed in tendons and ligaments but also in bone eminences
- Creates a Transition Zone between bone and tendon/ligament
- _____ is expressed in tendons and ligaments but also in bone eminences
- Creates a Transition Zone between bone and tendon/ligament
-
Scleraxis is expressed in tendons and ligaments but also in bone eminences
- Creates a Transition Zone between bone and tendon/ligament
How are joints created?
Mesenchyme of future joint
- (+chondrogenesis)*↓ | ↓(- chondrogenesis)
- Articular Cartilage* | Synovial cavity, ligaments, capsule
- Need to both promote and inhibit chondrogenesis
What transcription factor promotes formation of articular cartilage?
Gdf5
Cavitation to form the synovial cavity is achieved by the secretion of ______
Cavitation to form the synovial cavity is achieved by the secretion of hyaluronic acid
- glycosaminoglycan that readily absorbs water - thereby creating tissue spaces (draws fluid in to create jelly-filled cavity)
What promotes hyaluronic acid synthesis required for cavitation to form the synovial cavity
In utero movements
Limb musculature originates from ______ at the _____ edge of the _______
Limb musculature originates from myoblasts at the ventrolateral edge of the myotome
- As myoblasts migrate into the limb bud they are ______
- Become specified as limb muscles by ______
- Divide into dorsal and ventral masses which from the _____ and _____ respectfully
- As myoblasts migrate into the limb bud they are proliferating
- Become specified as limb muscles by mesenchyme (somatic mesoderm)
- Divide into dorsal and ventral masses which from the extensors and flexors respectfully
Dorsal-ventral muscle patterning comes from _____
Dorsal-ventral muscle patterning comes from overlying ectoderm
Movement and refinement:
- Excess muscle fibers (each fiber is a _______) are made initially and then undergo ______. The formation of new fibers ends around birth.
- Postnatal growth is through fusion of ________ with existing ________.
- Embryonic movements are important for development of the musculoskeletal system. For example, the proper _______, alignment of _____, growth of ______, development of ______
- Spontaneous movements start as early as _______
- Excess muscle fibers (each fiber is a single, multi- nucleated cell) are made initially and then undergo apoptosis. Formation of new fibers ends around birth.
- Postnatal growth is through fusion of progenitor cells with existing muscle fibers.
- Embryonic movements are important for development of the musculoskeletal system. For example, the proper attachment of tendons, alignment of bones, growth of muscles, development of joints
- Spontaneous movements start as early as 7 weeks.
Limbs are innervated by motor nerves from the ______ and sensory nerves from _______
- _______ plexus = C5-T1
- _______ plexus = L4-S3
Limbs are innervated by motor nerves from the ventral spinal cord and sensory nerves from adjacent dorsal root ganglia
- branchial plexus = C5-T1
- lumbosacral plexus = L4-S3
Motor neuron sorting:
Axons that travel into the dorsal side of the limb innervate _____ muscles
Axons that travel into the ventral part of the limb innervate ____ muscles
Axons that travel into the dorsal side of the limb innervate extensor muscles
Axons that travel into the ventral part of the limb innervate flexor muscles
- Motor neurons from the _____ project to axial and body wall muscles
- Motor neurons of the _____ project to the limb
- The ______ innervates the dorsal limb
- The _____ innervates the ventral limb
- Motor neurons from the MMC (medial motor column) project to axial and body wall muscles
- Motor neurons of the LMC (lateral motor column) project to the limb
- The Lateral pool of the LMC (LMCl) innervates the dorsal limb
- The Medial pool of the LMC (LMCm) innervates the ventral limb
The final step of limb development is Limb Rotation.
- Limb buds initially point _____ and _____
- After joint is added, limbs ______
- Elbow and knew point ______
- Limb buds initially point laterally and caudally
- After joint is added, limbs flex ventrally
- Elbow and knew point laterally
Limb rotation in the 8th week:
- Arms rotate ______ so that the ventral (____) surface faces _____
- Legs rotate ______ so that the dorsal (_____) surfaces faces _______
- Elbows now point _____ when flexed
- Knees now point ____ when flexed
Limb rotation in the 8th week:
- Arms rotate 90° laterally so that the ventral (flexor) surface faces anteriorly
- Legs rotate 90° medially so that the dorsal (extensor) surfaces faces anteriorly
- Elbows now point caudally when flexed
- Knees now point cranially when flexed
Label the 4 congenital limb anomalies
Different ways in which limb development may go wrong, perhaps related to ______ patterning
Common defects: (4)
Different ways in which limb development may go wrong, perhaps related to Hox gene patterning
Common defects: (4)
- absence or partial absence of a limb
- over or undergrowth of limb
- duplications
- failed separation (eg fused fingers)
What kinds of anomalies arise if there is defect in the genes involved in chondrocyte steps?
Skeletal malformations
What are some environmental causes of limb anomalies?
- Anything that compromises blood supply (eg causes vasoconstriction)
- warfarin
- thalidomide
- phenytoin
- valproic acid
- cocaine
- sugar (ie maternal diabetes)
- infections
What are possible physical restraints on growth?
- Amniotic band constriction
- Oligohydramnios
- low amniotic fluid
- impair movement that is important for limb development
What is Club foot?
- more common anomalies
- Foot twisted out of shape an/or position
- Bone and soft tissue abnormalities (eg short tendons, underdeveloped calf muscles
- difficult to correct
