MSK - Development Of The Limbs Flashcards
What do somites develop into? (3)
Dermatone - skin
Myotome - muscles
Sclerotome - bones
What is a somite?
Organisation of paraxial mesoderm into segments.
Somites are bilaterally paired blocks of paraxial mesoderm that form along the head to tail axis of the developing embryo.
1st appear at day 20 in the occipital region.
31 in total
When do limb buds become visible?
End of fourth week of development.
Where do they become visible?
Ventro-lateral body wall
Which limb bud appears first? Upper or lower limb bud?
Upper limb bud appears first followed by the lower limb bud 1 to 2 days later.
What do the limb buds consist of?
Mescenchymal core covered by layer of cuboidal ectoderm
Where is the anterior and posterior axis in the embryo?
Anterior towards the head
Posterior towards the tail
Remember in the adult
Superior = head
Anterior = Tail
What is the proxio-distal axis?
Base of the limb to tips of the digits
Where does the limb bud come from?
Activation of mesenchyme within lateral mesoderm.
Derived from the somatic later of lateral plate mesoderm
What is the difference between Somatic and Splanchic
Somatic - to do with the body
Splanchic - to do with viscera
What contributions to the limb skeleton?
Lateral plate mesoderm
How does the limb bud elongate?
Proliferation of the mescenchyme core. Lots of tissue created not well differentiated
What is the name of the thickened ectoderm at the apex of the limb bud?
Apical ectodermal ridge (AER)
What is the importance of the apical ectodermal ridge?
Critical for limb bud outgrowth
Orchestrates limb development proximal to distal
Final stage is appearance of paddles
AER then regresses - gradually at first then almost entirely.
What does the apical ectodermal ridge do?
Exert an inductive influence on the adjacent mesenchyme, causing it to remain undifferentiated
As the limb grows, what happens to the cells furthest from the apical ectodermal ridge (AER)?
Proximal mesenchyme begins to differentiate into cartilage and muscle.
Molecular signals from the AER can only communicate to cells that are closest to it so the furthest part differentiates.
What would happen in the absence of the apical ectodermal ridge (AER)?
The limb bud would differentiate
What induces the development of the digits within the hand/foot plates?
Apical Ectodermal Ridge (AER)
How are hand and foot plates formed?
Limb buds become flattened
How is the boundary between the dorsal and ventral limb ectoderm marked?
Apical ectodermal ridge
What regulates the patterning of the anteroposterior axis of the limb?
Zone of polarising activity
What is the zone of polarising activity (ZPA) and role?
A cluster of mesenchyme all cells at the posterior border of the limb near the AER.
The cells have an important signalling role at the posterior base of the limb bud.
Controls patterning.
It ensures the AER is maintained until it is no longer needed
Generates asymmetry in the limbs
What happens to mesenchyme to form the ‘digital rays’ hand and foot plates?
Mesenchyme condensations within plates.
Cartilaginous models of the digital bones
How are fingers and toes formed?
Apoptosis (cell death) in the AER seperates the ridge into five parts.
Interdigital spaces progressively sculpted by programmed cell death
Further formation of the digits depends on their continued out growth under the influence of the five segments of ridge ectoderm, condensation of the mesenchyme to form cartilaginous digital Ray’s and the death of intervening tissue between the rays
Which limb, upper or lower, is effected by limb defects more often?
Upper limb
What causes limb defects?
Hereditary but teratogens induced defects have been described
Name four common limb defects.
What happens in each defect?
Syndactyly - fusion of the digits, don’t get sculpting of digital rays. Degree of fusion varies.
Polydactyly - extra digits on either hand or foot, genetic recessive trait
Amelia - complete absence of a limb
Meromelia - partial absence of a limb
Phocomelia - exposure to thalidomide, lost extension of limb bud
When does endochondral ossification begin?
During the end of the embryonic period
What happens during ossification?
What mesoderm is involved?
Lateral plate mesoderm condenses and differentiates.
Cartilage model forms
What type of ossification occurs in long bones?
Endochondral ossification
Describe primary ossification. When are the centres present?
Primary ossification centres are present by the 12th week of development. From the primary centre in the shaft/diaphysis of the bone, endochondral ossification gradually progresses towards the ends of the cartilaginous model.
Where do secondary ossification centres appear?
Epiphyses
At birth, diaphysis is usually completely ossified but the two ends (epiphyses) are still cartilaginous. Shortly after ossification centres arise.
Why are the epiphyseal growth plates important?
Epiphyseal growth plates are important in the growth in length of bones. Allows bone to grow after birth
How do muscles begin to form?
Myogenic precursors migrate into limb buds from somites, they go either side of the developing limb bud
What are the two common muscle masses around the newly formed skeletal elements?
Ventral = flexor Dorsal = extensor
How are individual muscles formed?
Explain segmental innervation of nerves.
They split from the common muscle masses.
They bring the nerve corresponding to that body segment which explains segmental innervation of nerves.
What week of development do limbs rotate?
7th week
What way do the upper and lower limbs rotate?
Limbs rotate in opposite directions.
Upper limb - laterally.
Lower limb - medially
Anatomical position = thumb lateral, big toe medial
What gives rise to the spinal nerves?
The functional segments of the spinal cord
When do spinal nerves enter the limb bud?
Early during development, without innervation development stalls.
Where does the upper and lower limb bud appear for innervation?
Upper limb bud - Opposite the caudal cervical spinal segment
Lower limb bud - Opposite the lumbar and sacral spinal segment
What cords do anterior divisions compartment?
Medial and lateral cords. Flexors are supplied by medial and lateral cords.
What cords to posterior divisions compartment?
Posterior cords
All extensors are supplied by posterior cord
What is the effect of a spinal nerve innervating a somite?
The somite and all of its derivates will be supplied by that one spinal nerve.
