Question #63

The primitive node can be seen during gastrulation. Why is it important embryologically?

It is the site of the future cloacal membrane

It controls formation of the amniotic cavity

It gives rise to the placenta

It acts as a signalling centre to set up the cranial/caudal body axis

It marks the caudal end of the embryo

At one end of the primitive streak there are so many cells migrating and moving to form the mesoderm layer, a pit forms. This is known as the primitive node and it marks the cranial end of the embryo.

If a primitive node is transplanted to a second embryo it will produce a second cranial/caudal body axis which will ultimately result in two spinal cords and two heads. This suggests that a signal is produced by the primitive node to induce further gastrulation and migration of surrounding cells. i.e. it is a signalling centre.

Any comments or corrections? Please e-mail help@cambridgequestions.co.uk