Embryology

Question 1

What do limb buds consist of?

Answer 1

A mere chyme core with an ectoderm covering

Question 2

When and where do limb buds appear?

Answer 2

At the end of week 4 of development (lower limb ~2days after) on the ventro-lateral body wall and initially extend ventrally.

Question 3

What is the limb bud derived from?

Answer 3

The somatic layer of lateral plate mesoderm

Question 4

What is the AER and what is its function?

Answer 4

Apical ectodermal ridge - a thickened ectoderm at the apex of the limb bud. It orchestrates the proximal to distal limb development. It keeps the mesenchyme that immediately underlies it undifferentiated, allowing it to proliferate. Proximal mesenchyme that is too far away to receive signals begins to differentiate. It also induces development of the digits.

Question 5

What is responsible for the asymmetry of limbs (anterior-posterior axis)?

Answer 5

The zone of polarising activity (ZPA) - located at the posterior base of the limb bud.

Question 6

What is responsible for developing the dorsal-ventral axis?

Answer 6

Ectoderm exerts dorsalising and ventralising influences over the mesenchyme core.

Question 7

What boundary does the AER mark?

Answer 7

Dorsal-ventral

Question 8

How do anatomical terms of location change before and after birth?

Answer 8

Anterior/posterior becomes superior/inferior

Ventral/dorsal becomes anterior/posterior

Question 9

What becomes of the remnant of the notochord in adults?

Answer 9

Becomes the nucleus pulposus of the intervertebral discs

Question 9

What are digital rays?

Answer 9

Mesenchyme condensations of hand and foot plates that form cartilaginous models of digital bones.

Question 10

How are the digits developed?

Answer 10

AER breaks up and is only maintained over the tips of the digital rays. Interdigital spaces are progressively sculpted by apoptosis.

Question 11

Outline the formation of bones within the limb buds

Answer 11

Signals from AER to remain undifferentiated eventually stop
Lateral plate mesoderm condenses and differentiates
Cartilage model forms
Endochondral ossification
Primary and secondary ossification centres

Question 12

Outline the formation of muscaluture within the limb buds

Answer 12

Myotonic precursors (from somites) migrate into the limb and coalesce into two common muscle masses around the newly formed skeletal elements. Ventral=flexors and dorsal=extensors. Individual muscles then split off from common masses.

Question 13

Which direction do the upper and lower limbs rotate during their development?

Answer 13

Upper - laterally

Lower - medially

Question 14

Describe the limb innervation during development

Answer 14

Upper limb bud appears opposite caudal cervical spinal segments.
Lower limb appears opposite lumbar and sacral spinal segments.
Spinal nerves enter the limb bud early in development - without them, development stalls.

Question 15

Explain the development of the brachial plexus

Answer 15

Muscles are compartmentalised and nerves grow into common muscle masses.
The anterior divisions for the ventral compartment regroup to form the medial and lateral cord branches, hence they supply the flexors.
The posterior divisions for the dorsal compartment regroup to form the posterior cord, hence supplying the extensors.

Question 16

Outline the prevalence of limb defects

Answer 16

6/10000 live births. Upper limb more commonly affected

Question 17

Describe some common limb defects & causes

Answer 17

Amelia - complete absence of limb
Meromelia - partial absence of limb (eg phocomelia from thalidomide)
Syndactyly - fusion of digits (CT or bone)
Polydactyly - extra digits, usually recessive trait
Causes can be hereditary or teratogen