Question

7.a Robert thought Barney’s problem resided in his Nieuwkoop center. What do you think?

b Sarah thought it was a problem with his organizer. Do you agree or disagree?

c.Karen believed it was a problem with neurulation. Is this the reason?

d. What do you think is causing Barney’s problem? Do you think he can be “cured”?

These questions are form the case study: Don’t Lose Your Head A Case Study in Dorsal-Ventral Axis Formation in Amphibians

CASE STUDY BELOW

John and Heather Pipiens were very nervous. Heather had recently given birth to a good

size clutch of eggs. Most of the little larvae were doing fine. The one exception was Barney.

His head development was not like the rest of his siblings. His head seemed smaller than it

should be and he hadn’t formed his cement glands yet.

“What do you think could be wrong?” asked a very worried Heather.

“Don’t get all worked up.” Replied John. “I am sure

its nothing. He may just be a little slower than the rest of

the kids. We have 133 perfectly normal, healthy little larvae

and not all of them developed at the exact same rate.

Remember Gracie? She gastrulated almost a day later than

the rest and she’s just fine. And Jason didn’t have his first

cleavage until almost an hour after the rest and he’s doing

great.”

Just then, the nurse came out and called their name.

They were escorted into an exam room. Barney’s height,

weight and somite count were taken. The nurse then told

them that Dr. Triturus would be in a few minutes.

True to her word, the doctor came into the exam

room five minutes later. He took one look at Barney and his

heart sank. He knew something was very wrong. He was

definitely missing part of his forebrain and his entire cement

gland. Based on his somite count both should be very well developed by now

Dr. Triturus turned and looked at the Pipiens and said, “Mr. and Mrs. Pipiens, at first

glance there seems to be a problem with his developmental progress. He does have some head

development abnormalities that I don’t believe are related to timing. Everything else seems to be

ok; he has the correct number of somites for his age and his trunk seems to be developing just

fine. The problem seems to be the front most part of his head. It’s failed to develop. I don’t

know the exact caused just yet. I would like to admit him for some tests.”

“Is this necessary?” asked Barney’s mom. “How serious is it? Can it be fixed?”

“I really can’t say at this time.” replied Dr. Triturus. “I need to do some tests to get a

better idea of what is causing Barney’s problem before I can make a diagnosis. I would like you

to leave him here over the weekend. You can pick him up on Monday.”

“But we want to stay with him.” said both his mom and dad.

“That really isn’t necessary. He is still in his jelly coat and not really aware of what is

going on. Besides you have 133 other children to take care of. Go home. He’ll be in good

hands with us.”

John and Heather kissed Barney good-bye. The doctor had the nurse admit him and got

him settled in his hospital tank. He then called his students together. Dr. Triturus had a pretty

good idea of the cause of Barney’s problems, but wanted to give his residents a chance at the

diagnosis before he ran tests to confirm his hypothesis.

Dr. Triturus began, “We have a 4 day-old male anuran with abnormal head development.

He is showing an absence of forebrain development and his cement glands have failed to form.

Everything else seems to be normal. What are your thoughts?”

The med students looked at each other. Robert thought it might be a problem with his

Nieuwkoop center. Sarah thought it was probably a problem with his organizer. Karen told

them both were wrong and it had to be a problem that occurred during neurulation. It was

obvious that this group of students had no real idea what was going on.

“You are on track, just not the right one. It’s a problem with dorsal-ventral axis

formation. But it’s obvious that most of you don’t know much about the development of the

central nervous system. Go home and do some research on this and be prepared to answer some

questions during rounds tomorrow afternoon. I am disappointed about how little you seem to

know about this subject. It is one of the first developmental decisions an embryo makes and one

of its most important. Go do some research and be prepared to have better answers tomorrow.”

Dr. Triturus went over to Barney’s tank to check on him one more time. Satisfied that he

would be all right until morning, Dr. Triturus went home hopeful that his students would be

better prepared the next time they met. If not, some of them might find themselves cleaning

tanks for the next two weeks.

You are part of Dr. Triturus’s research group. You really don’t want to clean hospital

tanks for the rest of your residency. To prepare for tomorrow’s rounds answer the following

questions and be ready to provide a diagnosis of the cause of Barney’s head abnormalities. Be

prepared to justify your reasoning.

Answer 1

ANSWER 7.A :-

1. Nieuwkoop apparatus can be considered as the controlling center for the Spemann's organizer which in turn plays an important role in specifying the dorsal structures. This was being observed as the cells originating from the dosal lip also called as the Spemann's organizer which leads to the formation of the dosal parts of the embryo.
2. Although, this Nieuwkoop center deals with the release of regulatory molecules such as beta-catenin which in turn controls the differentiation and development of the dorsal structures. This was in turn not associated with the development of the forebrain part which will be discussed in the next questions.

ANSWER 7.B & 7.C :-

1. First of all, lets consider the egg of an amphibian. The egg is made up of animal hemisphere and the vegetal hemisphere. There is cytoplasm in both the zones apart from the cortex which surrounds the cytoplasm and is composed of the cortical cytoplasm.
2. Before the event of fertilization occurs, at the time of sperm entry, there is a mechanism called as cortical rotation which takes place. The cortical part of animal hemisphere is pigmented and dark and the cortical region undergoes a 30o rotation which in turn leads to the shifting of the animal cortical region into the vegetal region. This in turn leaves an empty zone at the opposite side of the sperm entry zone which then gets covered by the cortical region of the vegetal zone. This region forms the Grey crescent.
3. This is the region which is responsible for the formation of the dorsal part and the site through which the sperm enters forms the ventral part and thus the dorsal ventral axis forms. If this process of cortical rotation is altered, chances are that the anterior and ventral structures may get dislocated as can be seen in case for formation of separated trunk part from the body zone carried out during certain experiments.
4. The Spemann's organizer plays a vital role in the formation of the dorsal structures of the embryo associated with the movements of the cells into the interior forming the mesoderm and the endoderm thereby channelizing gastrulation process. So, there is no association with the formation of dorsal ventral axis as differentiation has occured.
5. As in case of Nerulation which is the formation of the nerve tube which in turn is associated with the formation of the notochord and ultimately the development of the regions of the brain and hence this process seems to be mostly dependent upon the initial processes of cortical rotation in order to mediate the structures of the brain positioned ventrally and dorsally. So, Nerulation cannot be considered to have been involved in the formation of the DV axis as has been stated by the doctor.

ANSWER 7.D :-

1. Up-righting process by which the embryo can be manually controlled and rotated can be used to change the cortical displacement depending upon the gravity possessed by the egg by means of which the angle of optical rotation can be controlled.

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