Question

4. You are given an animal whose development has never been characterized before. You note that at the four-cell stage, one blastomere transiently shows dark pigmentation. However, as embryogenesis proceeds, the dark pigmentation fades and you are unable to determine what becomes of the blastomere in the fully-developed embryo.

a) [4pts] Describe a simple experiment you can do to determine the fate of the dark blastomere. Provide sufficient detail to show that you understand the technique and its interpretation.

b) [5pts] For the animal described above, you find that the dark-pigmented blastomere at the four-cell stage ultimately gives rise to all of the central nervous system of the embryo. Using this information, devise an experiment to determine whether this cell is specified to be CNS at the 4-cell stage. In addition to describing the experimental procedure, describe the possible results and how you would interpret them.

c) [1pt] As this embryo undergoes gastrulation you notice that the ectodermal cells surround the endoderm similar to Xenopus. What is the name to describe the movement of these ectodermal cells?

Answer 1

a) the gray crescent is the area that determines the secondary axis. This is the dark blastomere at the four celled stage. Speman conducted experiments with this gray crescent to determine its fate and role.

the Spemann experiment is given below-

He constricted the embryo in a way that the grey crescent is equally divided between two halves. This division lead to development of two normal embryos.

he then constructed another embryo in a way that only one part received grey crescent and one part was without it. The part that had grey crescent developed in a normal embryo but the part that had no grey crescent formed a mass of cell without any organisation. This mass was called belly piece.

this experiment shows that that dark pigmented blastomere is very important for the development of the normal embryo. Its fate is to form the region which determines the dorsal-ventral axis of the cells.

on removing the grey crescent area with needles, no development of neural tissues takes place indicating that it specifies dorsal-ventral axis by inducting CNS formation at the dorsal most side .

b) Except for the isolation experiment described above, most experiments on frog blastula and early gastrula stage embryos suggested that cell fate was not determined until the mid gastrula stage. At mid gastrulation stage, the grey crescent area has given rise to autonomously determined region that forms CNS. Experiment that proves that this region is not specified at the four celled stage is given below.

hence, at 4 celled stage that come even before early gastrula, there is no chance that the dark pigmented blastomere is specified.

c) this movement of cell is called epiboly. Epibolic movement is the way in which a layer epithelial cells spreads over another layer of cells.