Flashcards in Embryology Deck (33):
What parts of the embryo during early development go on to produce the musculo-skeletal system?
Somites and lateral plate mesoderm.
In what direction do limb buds initially extend?
What can be said about the time of appearance of the upper and lower limb buds?
The upper limb buds are first to appear, because as with the whole embryo the cephalic end is more developed than the caudal.
Explain why the beginning of the development of the limbs is fairly late.
The limbs do not have a use during fetal life.
What is a limb bud? (What tissue is it made up from?)
It is a core of proliferating mesenchymal tissue, with an ectoderm covering.
What are the three planes of asymmetry in the limbs?
Top to bottom, distal is different to proximal. Front to back, we don't have hair on our palms. AP axis, hands are not symmetrical.
What does the remnant of the notochord become?
The nucleus pulposus. This can rupture out through the annulus fibrosus.
What is the AER?
Apical ectodermal ridge. It is thickened ectoderm at the apex of the limb bud.
What is the AER responsible for?
It is responsible for elongation of the limb bud and drives the cells keep dividing.
What is the role of the ectoderm surrounding the limb bud?
This will go on to form the skin. The AER is specialised and responsible for organisation of the limb bud.
Describe when cells in the limb bud begin to differentiate.
The AER has an inductive influences on cells keeping them undifferentiated. As the limb bud elongates, this moves further from the proximal end of the limb and so if the chemical signals are not transmitted then differentiation begins to occur.
How do the hands and feet form?
There is widening of the end of the limb bud leading to formation of hand and foot plates.
What causes the formation of digits on the limbs?
There are mesenchymal condensations which form in the plates and are cartilaginous models for the digit bones and begin to scallop the limb.
By what process is the tissue between the digits removed in limb development?
What happens to the AER in development of the digits?
It breaks and is only present at the end of the digits. When these reach their required length, the AER regresses.
What boundary on the limb does the AER mark?
The dorsal and ventral ectoderm.
What is the ZPA and what does it do?
This is the zone of polarising activity and it determines anterior posterior axis determination. It also controls patterning and AER maintenance.
What controls proximal distal asymmetry of the limb?
The AER ensures correct proximal/distal development of the limb.
What is syndactyly?
This is fusion of the digits. It can just be CT or it can be bones as well.
What is the name for the condition in which a person has extra digits? What commonly causes this?
Polydactyly. This is caused by a genetic recessive trait.
What is Amelia?
This is the complete absence of a limb.
What is it called when a person has partial absence of one or more limb structures?
What is phocomelia?
This is where there is absence of the proximal part of a limb.
What problem can arise from damaged epiphyseal growth plates?
How is the cartilaginous model formed for long bone development?
Lateral plate mesoderm condenses and differentiates.
What is the initial stage in musculature formation?
Myogenic precursors migrate into the limbs from somites and form two clusters around the newly formed skeletal elements.
In which direction does the upper limb rotate in development?
In which direction does the lower limb rotate during development?
Where do the nerves for the limbs originate from?
The upper limb buds appear opposite the cervical nerve roots and the lower limb buds appear opposite the lumbar nerve roots.
Why do we have nerve plexuses in the limbs?
Muscles are compartmentalised and so nerves grow into these common masses.
What is a myotome?
A group of muscles supplied by a single spinal nerve
What is a dermatome?
It is an area of skin supplied by a single spinal nerve