Session 3 - Limb development Flashcards
What happens to the notochord in development of the embryo?
-The remnant of the notochord forms nucleus pulposus of vertebrae as it becomes marooned during vertebral column formation from sclerotome
Where abouts on the embryo do limb buds appear?
-Ventero-lateral body
Which limb bud appears first?
-Upper limb bud
NB. lower limb bud is about 2 days behind in development
What happens to the distal end of the limb bud after elongation has occured?
-It flattens producing the primorida of the hand and feet, ie hand and foot plates
What is a limb bud?
-A core of proliferating mesoderm with an ectodermal covering
When do limb buds appear?
-By the end of week 4
What is the first step in the development of the limb buds?
-Activation and proliferation of somatic lateral mesoderm
What is the apical ectodermal ridge?
-A thickened ridge of ectoderm at the apex
What section of mesoderm forms the bones of the upper limb?
-Somatic lateral mesoderm
What section of mesoderm makes the musculature of the limbs?
-The somites (paraxial mesoderm)
What are the main functions of the AER?
- Drive elongation of the limb bud
- Controls differentiation proximally to distally
How does the AER control elongation and proximal to distal differentiation?
- The AER has an inductive influence on the underlying mesoderm in such a way that it instructs the mesoderm to remain undifferentiated and keep dividing and elongate
- The proximal mesoderm is too far away to receive signal and thus begins to differentiate
What happens if the AER does not develop?
-Elongation of the limb bud will not occur
What happens to the limb bud after it has elongated?
-The distal end flattens into the hand and foot plates and the mesenchyme condenses into discrete organisations known as digital rays
What happens to the AER after the hand and foot plates appear?
-Organised regression of the AER except over the digital rays
What happens to the digitial rays of hand and foot plates?
-Become the bones of the hand/foot through endochondral ossification
What happens to the interdigital spaces between the digital rays?
-They become sculped by apoptosis
What happens to the digital rays before ossification?
-Elongation driven by the AER
Give the postnatal axis of these embryonic axis
i) Anterior
ii) Posterior
iii) Ventral
iiii) Dorsal
i) Superior
ii) inferior
iii) Anterior
iv) Posterior
What contributes to the formation of the limb buds?
- Somites
- Lateral plate mesoderm
What controls dorso-ventral patterning?
- Controlled by the ectoderm
- Dorsal ectoderm produces dorsalising influences whilst ventral ectoderm produces ventralising influences
What controls anterioposterior axial development?
-The zone of proliferating activity
Where is the zone of proliferating activity?
-The posterior base of the limb bud
What is the function of the zone of prolifeating activity?
-Control patterning and maintain the AER
Where does ossification begin?
-The proximal limb bud
Describe endochondral ossification of the limb bud
- Mesoderm condenses and proliferates producing a cartilaginous model of the bone
- Collar of periosteal bone appears in shaft
- Central cartilage calcifies and nutrient artery penetrates-> primary ossifcation centre formed
- Medulla becomes cancellous bone and cartilage forms epiphyseal growth plates
- Epiphyses develop secondary ossification centres
- Epiphyses ossify and growth plates continue to move apart, lengthening bone
- Epiphyseal growth plates replaced by bone
Why are epiphyseal growth plates significant in clinical practice?
-They can look like fractures on radiographs
How is the musculature of the limb bud formed?
- Myogenic precursors migrate from somites into limb buds
- Precursors coalesce into 2 common muscle masses around the skeleton
- Split into individual muscles throughout development
How does innervation of the limb bud occur?
- The myogenic precursors bring their innervation with them, ie the spinal roots are pulled into the limb bud
- The nerves grow into the common muscle masses and then are pulled into individual muscles
What happens if innervation of the limb bud does not properly develop?
-Development of the limb bud stops
How are the limbs orientated initially after development?
- Thumb/big toe up
- Elbow/knee out
How is the correct anatomical orientation of each limb bud acheived?
- The limb buds rotate as they elongate
- Upper limb rotate laterally
- Lower limb rotates medially
What is a dermotome?
-Strip of skin innervated by a single spinal berve which can be clinically examines
What is a myotome?
-Muscle or group of muscles innervated by a single spinal nerve which can be clinically examined
Which limb buds are more commonly effected by defects?
-Upper limb bud
How common are limb defects?
6/10,000
What is the main cause of limb defects?
-Usually hereditary but can be teratogen-induced
Give an example of a teratogen-induced limb defect
-Phocomelia -> interfered with development of AER
What is syndactyly?
-Fusion of digits (can be by CT or bone)
What is polydactyly?
-Extra digit
What is amelia?
-Complete absence of a limb
What is meromelia?
-Partial absence of one or more limbs
Which border of the limb bud is preaxial?
-Thumb/big toe
Which border of a limb is postaxial?
-5th digit
