Development of the limbs Flashcards
What do somatic mesoderm and somites become?
somatic mesoderm –> limb skeleton
somites –> limb musculature
When do limb buds appear? Where do they appear?
appear at the end of week 4
lower limbs 2 days behind upper limbs
appear on ventro-lateral body walls, extend ventrally (belly)
How do limb buds elongate?
through proliferation of mesenchyme core (flexibl mesoderm)
What is apical ectodermal ridge (AER)? Where is it located and what is its purpose?
orchestrates limb development: proximal to distal
keeps mesenchyme immediately underlying it undifferentiated
undifferentiated mesenchyme proliferates –> elongation
proximal mesenchyme begins to differentiate as it is far from AER - can’t receive molecular signal to stay undifferentiated
located at apex of mesenchyme core
What does AER induce and what does it mark?
induces development of digits in hand/foot plates
AER marks the boundary between dorsal & ventral limb ectoderm - ‘dorsalventro patterning’
ectoderm exerts dorsalising & ventralising morphological changes over the mesenchyme core
What are the 3 degrees of symmetry?
- top & bottom: proximal –> distal
- front & back: dorsal –> ventral
- side to side: anterior –> posterior
What is the apical ectodermal ridge (AER)?
thickened ectoderm at the apex of limb bud
How does limb bud elongate?
through proliferation of mesenchyme core
What is the zone of polarising activity (ZPA)?
a signalling centre at the posterior base of the limb bud generates asymmetry in the limbs (anterior - posterior axis determination) controls patterning (separate dorsal & ventral) & maintains AER
Which component controls which axial specification?
Anterior - Posterior: ZPA (zone of polarising activity)
Proximal - distal: (AER) apical ectodermal ridge
D-V: ectoderm (dorsal & ventral)
Describe the embryo’s axis
top-bottom: anterior - posterior
belly-back: ventral - dorsal
away-close: distal - proximal
Describe an adult’s axis
top-bottom: superior - inferior
front - back: anterior - posterior
close - away: proximal - distal
Hand & foot plates: what are digital rays? which process is required?
mesenchyme condensation (regress) within plates - cartilaginous models of digital bones formed (digital rays - flaring of limb buds = digits) apoptosis require of tissue between digits (sculpts interdigital space) AER only maintained over tips of digits, other break up to prevent proliferation between digits
What is the purpose of apoptosis in hand & foot plates?
to sculpt interdigital space
How are bones formed?
signals from AER to remain undifferentiated stop
lateral plate mesoderm condenses & differentiates
cartilage model forms
endochondral ossification (long bones) - osteoblasts - primary ossification centre in diaphysis of long bones
Further development of long bones? (after birth)
secondary ossification centres appear in the epiphysis
bone growth is maintained by epiphyseal growth plate (cartilage forming regions)
Compartmentalisation of muscles & its positional changes during development
myogenic precursors from somites migrate into limbs
coalesce into 2 common muscle masses around the newly formed skeletal element (ventral: flexor, dorsal: extensor)
individual muscles split (branch) from common masses
Rotation of limbs
limbs extend ventrally at first, but as they elongate, they rotate
upper limb: rotates LATERALLY (thumb)
lower limb: rotates MEDIALLY (big toe)
Describe the position of the limbs before and after rotation
before rotation: upper: thumbs up and elbows out
after rotation: thumbs out and elbows in
before rotation: lower: soles facing in, knees out
after rotation: soles facing down, knees up
Limb innervation of upper & lower limb buds
upper LB: appears OPPOSITE caudal cercival spinal spinal segments (highest 2)
lower LB: appears OPPOSITE lumbar & sacral spinal segments (lowest 2)
When do spinal nerves enter the limb bud? why?
enter limb bud early - because without innervation, development stops
brachial plexus in relation to the nerves innervating
all muscles are compartmentalised, nerves grow into the common muscle masses
all anterior divisions regroup to form medial & lateral chords - innervating flexors (ventral)
all posterior divisions regroup to form posterior chord - innervating extensors (dorsal)
what are dermatomes and myotomes?
derived from paraxial mesoderm
D: strip of skin supplied by a single spinal nerve
M: muscle / group of muscles supplied by single spinal nerve
describe how nerves innervate dermatomes and myotomes
Nerve originate from somite segment (spinal root - C5-T1 upper)
as the embryo grows, morphological changes take the spinal nerve with it to innervate a strip of skin / group of muscles
there is no overlap between the segment of skin / muscle innervated by the nerves
