Embryology

Question 1

What parts of the embryo during early development go on to produce the musculo-skeletal system?

Answer 1

Somites and lateral plate mesoderm.

Question 2

In what direction do limb buds initially extend?

Answer 2

Ventrally.

Question 3

What can be said about the time of appearance of the upper and lower limb buds?

Answer 3

The upper limb buds are first to appear, because as with the whole embryo the cephalic end is more developed than the caudal.

Question 4

Explain why the beginning of the development of the limbs is fairly late.

Answer 4

The limbs do not have a use during fetal life.

Question 5

What is a limb bud? (What tissue is it made up from?)

Answer 5

It is a core of proliferating mesenchymal tissue, with an ectoderm covering.

Question 6

What are the three planes of asymmetry in the limbs?

Answer 6

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.

Question 7

What does the remnant of the notochord become?

Answer 7

The nucleus pulposus. This can rupture out through the annulus fibrosus.

Question 8

What is the AER?

Answer 8

Apical ectodermal ridge. It is thickened ectoderm at the apex of the limb bud.

Question 9

What is the AER responsible for?

Answer 9

It is responsible for elongation of the limb bud and drives the cells keep dividing.

Question 10

What is the role of the ectoderm surrounding the limb bud?

Answer 10

This will go on to form the skin. The AER is specialised and responsible for organisation of the limb bud.

Question 11

Describe when cells in the limb bud begin to differentiate.

Answer 11

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.

Question 12

How do the hands and feet form?

Answer 12

There is widening of the end of the limb bud leading to formation of hand and foot plates.

Question 13

What causes the formation of digits on the limbs?

Answer 13

There are mesenchymal condensations which form in the plates and are cartilaginous models for the digit bones and begin to scallop the limb.

Question 14

By what process is the tissue between the digits removed in limb development?

Answer 14

Apoptosis

Question 15

What happens to the AER in development of the digits?

Answer 15

It breaks and is only present at the end of the digits. When these reach their required length, the AER regresses.

Question 16

What boundary on the limb does the AER mark?

Answer 16

The dorsal and ventral ectoderm.

Question 17

What is the ZPA and what does it do?

Answer 17

This is the zone of polarising activity and it determines anterior posterior axis determination. It also controls patterning and AER maintenance.

Question 18

What controls proximal distal asymmetry of the limb?

Answer 18

The AER ensures correct proximal/distal development of the limb.

Question 19

What is syndactyly?

Answer 19

This is fusion of the digits. It can just be CT or it can be bones as well.

Question 20

What is the name for the condition in which a person has extra digits? What commonly causes this?

Answer 20

Polydactyly. This is caused by a genetic recessive trait.

Question 21

What is Amelia?

Answer 21

This is the complete absence of a limb.

Question 22

What is it called when a person has partial absence of one or more limb structures?

Answer 22

Meromelia.

Question 23

What is phocomelia?

Answer 23

This is where there is absence of the proximal part of a limb.

Question 24

What problem can arise from damaged epiphyseal growth plates?

Answer 24

Bone deformity.

Question 25

How is the cartilaginous model formed for long bone development?

Answer 25

Lateral plate mesoderm condenses and differentiates.

Question 26

What is the initial stage in musculature formation?

Answer 26

Myogenic precursors migrate into the limbs from somites and form two clusters around the newly formed skeletal elements.

Question 27

In which direction does the upper limb rotate in development?

Answer 27

Laterally.

Question 28

In which direction does the lower limb rotate during development?

Answer 28

Medially

Question 29

Where do the nerves for the limbs originate from?

Answer 29

The upper limb buds appear opposite the cervical nerve roots and the lower limb buds appear opposite the lumbar nerve roots.

Question 30

Why do we have nerve plexuses in the limbs?

Answer 30

Muscles are compartmentalised and so nerves grow into these common masses.

Question 31

What is a myotome?

Answer 31

A group of muscles supplied by a single spinal nerve

Question 32

What is a dermatome?

Answer 32

It is an area of skin supplied by a single spinal nerve

Question 33

What do the terms pre axial and post axial describe?

Answer 33

Pre axial is the great toe/ thumb side and post axial is the side of the little finger/ little toe.