Orangogenesis- formation of vertebrate limb (L18)

Question 1

What is the pentadactyl limb?

Answer 1

A conserved structure in tetrapods, modified for different functions yet built to same plan.

Question 2

The limb has what 3 developmental axis?

Answer 2

Proximal-distal
Anterior-posterior
Dorsal-ventral

Question 3

What is the limb bud?

Answer 3

Limb bud is formed from mesodermal cells that accumulate under ectoderm. At first formation, half a limb bud can generate a new limb.

Question 4

The AER (apical ectodermal ridge) is necessary for what

Answer 4

outgrowth of the limb.

Implantation of an extra AER duplicated the limb axis.

Question 5

The mesoderm is essential to maintain AER. The mesoderm specifies nature of the….

Answer 5

limb

Question 6

PROGRESS ZONE THEORY:

Answer 6

If AER is removed the linb ceases to grow outwards. Cell positional information depends on time spent in progress zone.
Theory tested by combining old and young limb buds-
If you graft a young limb bud onto the tip of an older limb bud stump there is a duplication of the proximal-distal sequence of bones.

Question 7

THE ZONE OF POLARISING ACTIVITY:

Answer 7

A signalling centre located in posterior region of limb bud. Implantation of cells from the posterior margin of the limb causes a reversal of the posterior to anterior organisation.
EXAMPLE OF GRADIENT THEORY.

Question 8

Fibroblast growth factors are essential for..

Answer 8

induction, maintenance and function of AER.

Question 9

What is Fgf10

Answer 9

Fgf10 is expressed in limb bud mesoderm ad stimluates limb bud growth.

Question 10

Apoptosis in interdigital nerotic zone controls..

Answer 10

amount of webbing between digits

Question 11

Signals for apoptosis are provided by what proteins?

Answer 11

BMP proteins, they are inhibited by gremlin proteins.

Question 12

Example of gremlin proteins working

Answer 12

Duck-like webbing can be produced in chicks by applying gremlin-soaked beads between digits to inhibit BMPS.

Question 13

Abnormal cell death can result in….

Answer 13

Syndactyly

Question 14

EXAMPLES of gene defects that can cause deviation from normal limb formation

Answer 14

Chick limbless mutation- failure of AER development
Polydactyly- several mutations e.g. altered expression of sonic hedgehog
Thalidomoide- a toxin that can disrupt limb development.