This lesson will introduce you to Kimball's Biology Pages.
This is an on-line biology text created by Dr. John W. Kimball of
Harvard University as an alternative to standard biology text books. If
you want to review the material in your assigned reading for this
chapter go to the Frog Embryology page of this site. Once you have reviewed this material, go to the Central Nervous System
page to complete this lesson. In this lesson you learn about a classic
experiment done in the 1920's by Hans Spemann and Hilde Mangold.
Spemann was awarded the Nobel Prize in 1935 for his work.
Organizing the Embryo: The Central Nervous System.This short
section introduces you to one of the most fascinating, difficult to
study, and important questions in developmental biology: how does the
single totipotent fertilized egg cell (zygote) differentiate into an
individual with millions of cells with different structure and
function? What controls this process and how? Kimball points out that
this can be accomplished mechanisms within the cell (cell intrinsic) or
by interactions among cells. Cells can produce and send these
developmental controlling signals in a variety of ways.
The Spemann Organizer.
What follows here is a summary of the remarkable experiments done by
Spemann and Mangold. These experiments launched the field of modern
developmental biology which today is still trying to unravel the
details of embryonic cell differentiation. This discussion refers often
to the "gray crescent" and the "neural crest." Go back to your text to
review this material (page 968-971) if you need to refresh your memory.
As with most great experiments, their's was logically simple and
uncomplicated, but it uncovered one of the most complex processes in
all of biology.
The experiment involved moving a part of the gray crescent to another
location in the gastrula. The two important observations are that (1) a
second neural crest and eventually central nervous system developed in
the region of the transplant and (2) this new nervous system developed
from cells of the gastrula originally located in the transplant site,
not from the transplanted gray crescent cells. The first observation
was not all that surprising. It was known before hand that the gray
crescent region of the gastrula developed into the central nervous
system. What was surprising was the transplanted gray crescent material
did not develop into the new nervous tissue itself; rather it induced
the recipient cells next to it to do so. In other words the gray
crescent cells produce and release a signal that influences the
development of other cells.
The mechanism for this induction is not as straight forward as you
might first assume. Study the rest of this section and understand the
sequence of events and the function of the various molecules involved,
especially BMP-4, chordin, and noggin.
A Tail Organizer
Read this section that describes new work in this field and look for
similarities and differences between the development of the tail and
the head.
Questions: Answer these questions and send them to me as an email attachment in MS Word or RTF format. - Why is it more correct to say that Spemann's organizer does not
induce the development of the central nervous system but rather lets it
be induced?
- What is the important role of the mother of the embryo in the induction process?
- Write a brief paragraph comparing the similarities and differences between the tail and head organizers.
Return to Mr.
Birch's Biology Class Pages.
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