Biology Reference
In-Depth Information
FIGURE 21.1
Avoiding the problem of restricting mobility across a border by making immigrants go native. (a)
Diagram of the embryonic chick brain and spinal cord, showing the locations of the midbrain, hindbrain and their
common boundary (dotted line). (b) Experiments in which cells on one side of the border are labelled and the
embryo allowed to live on reveal that small numbers of cells cross the boundary naturally. This has been drawn in
only one direction but happens in both. (c) If a group of midbrain cells is transplanted into the hindbrain side of the
boundary, at least some of them cease to express the midbrain marker Otx2 and express instead the hindbrain
marker Gbx2. This suggests that they have altered identity to conform to their new environment.
environment rather than their lineage history, with immigrants quickly going native. The
rapid switch of gene expression probably uses signals mediated by Wnt1, and a network of
mutual repression by Otx2 and Gbx2. Later in development, the Otx2-Gbx2 boundary
becomes an important signalling centre in its own right
d
the isthmic organizer.
4
It is important to note that other vertebrates show significantly more restriction to mixing
even at this same site. In zebrafish, mixing seems not to be allowed at all
5
while mice restrict it
to a small number of cells and a short window of developmental time.
6
These differences may
simply reflect differences in the timing of the Otx2/Gbx2 boundary being converted into
a specialist border zone with its own cell type.
PROT
ECTING A BORDER BY CONTROLLING MIGRA
TION
The second approach to solving the mixing problem is for cells in different states to
become immiscible so that they create and respect a mutual boundary between zones, or
