Dr. Morris Maduro, UC Riverside
Biology 168 – 10F – Lecture 15, page 1
Development: Classical experiments, Wnt pathway
67-73; 75; 83-85.
Some figures in these notes are redrawn from
Wolpert et al.
1. Division of egg shows role of grey crescent.
The role of the grey crescent in specifying the dorsal
structures is shown in the following experiment. After cortical rotation, if the egg is divided into two using a
fine hair tied across a meridian (longitudinal line going through the animal and vegetal poles), only
divisions that retain part of the grey crescent will give rise to dorsal structures:
This experiment tells us that as long as the embryo piece contains part of the grey crescent, it will be able
to regulate and produce dorsal structures (and hence a relatively normal ½-size embryo) – otherwise it
will result in a ventralized embryo. We have previously looked at a similar experiment performed by Roux
in which one of the cells at the 2-cell stage was killed with a hot needle, and this resulted in the
production of a half-embryo from the living half. Now we see that even though half the embryo had been
ablated, it was
still in contact
with the remaining part. Hence like the sea urchin experiment by Dreisch,
the early frog embryo
2. Fate Maps.
One of the central questions about development in a particular animal is whether or not
cells in particular areas of the early embryo contribute reproducibly to the same structures, even though
the cells in an early embryo are more or less featureless. To do this, we need to establish a coordinate
system for consistently identifying precursor cells in the early embryo, and then we need to mark them in
a way that allows their descendants to be identified but which does not interfere with normal development
(e.g. DiI, pronounced ‘dye-eye’; or Nile blue). These dyes are lipid-soluble, so they
go into cells, and they must also remain only within the descendants of the labeled cells – i.e.
. When the results of many labeling experiments are compiled, we generate a
(Here, ‘fate’ simply means ‘outcome’.)
embryo, we anchor our fate map with the
sperm entry point
(SEP) which creates a small
‘wound’ on the cell surface, coupled with the
(marked by polar bodies), and the
. Cells are marked on the surface of the early gastrula (10 h post-fertilization, about 2x10
and development is allowed to proceed. (As recently as 2006, a revision to the
fate map was
proposed, but there is still some controversy.)