Embryology Biology 441 Spring 2009 Albert Harris
Lectures Wednesday, January 28th & Friday, January 30th
Samples of some questions that you should now be prepared to answer:
1) What are three major taxonomic groups of animals whose eggs have holoblastic cleavage? (constrict all the way through in cytokinesis)
Hint: birds, teleost fish, reptiles, sharks
2) Draw and label the parts of gastrulating embryos of fish, mammals, sea urchins, birds, salamanders and frogs.
3) Which of the kinds of animals listed in the previous question are vertebrates?
4) Draw, and discuss the differences between the blastula stages of embryos of each of the kinds of animals listed in question #2 above.
5) In the blastocyst stage of a human or other mammal embryo, what does the "inner cell mass" develop into.
6) It is unusual for a developing mammal embryo to form two separate blastocyts, but it does happen occasionally. What would be the result?
(Hint: about half of human identical twins develop this way.)
7) What would be the result if two inner cell masses were to form inside one blastocyst?
8) What would happen if two separate primitive streaks were to develop in the same inner cell mass of a mammal embryo?
(Hint: It is normal for armadillo inner cell masses to form four primitive streaks .)
9) Do you think it could be possible for two Hensen's nodes (=Primitive nodes) were to develop along the same primitive streak?
What would probably be the end result of that happening?
What must have happened in the development of the embryo shown in the photograph above?
10) What is the closest analog of the blastopore in bird and mammal embryos?
11) Gastrulation occurs by invagination in embryos of which kind of animals?
12) In which kinds of animals does gastrulation occur by...
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* Involution?
* Epiboly?
* Ingression (3 different examples)
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* Which ones can still develop normal bodies even after
the first two cells were mechanically separated from each other?
* Which ones are the least consistent in their patterns of cell division and early geometric arrangements?
* Which ones are the MOST consistent and stereotyped in their patterns of cell division and geometric arrangements?
* Which ones take the longest time between one cell division and the next?
* Which ones cleave synchronously?
15) About how many differentiated cell types do mice have?
(About the same number as humans; probably exactly the same number.)
16) List (as many as you can think of) reasons why Dictyostelium discoideum is such a good model organism for studying embryological phenomena?
17) Suggest at least two arguments that someone could reasonably make that Dictyostelium is not a good model organism for embryologists to study?
(It's a plant? Or is it? No embryo develops by aggregating separate cells. There are many fundamentally different kinds of amoeboid locomotion.
18) Which of the following would be worst as a model organism?
(maybe rank them in order, with explanations)
a) Whales b) Pandas c) Coffee plants d) Leopard frogs e) Leopards
f) The bacteria that cause fatal human pneumonia g) Pine trees
g) Penguins h) Sea urchins j) Crinoids (="Sea lilies")
(Need to go through whole reproductive cycle in the lab or greenhouse, without taking up to much space, & produce lots of young per generation, with short generation times, & you usually have to kill them in large numbers.)
19) What is the distinction between "luxury genes" versus "housekeeping genes" and which ones occur in differentiated cells?
(warning: This is sort of a trick question, phrased this way.)
20) What parts of the body develop from mesoderm? What parts develop from endoderm? And from ectoderm?
21) Neurulation subdivides which germ layer into what three fates?
22) Gastrulation subdivides what into what three fates?
23) * If you were creating a new life form, would you use active cell rearrangements to control the subsequent differentiation of embryonic cells? How would you control differentiation?
24) How can breakage and (incorrect) rejoining of chromosomes cause differentiated cells to transcribe an abnormal combination of luxury genes?
25) * Would you expect all the promoter regions of the luxury genes
transcribed in each particular differentiated cell type to have at least
approximately the same DNA base sequence? In fact, they don't.
But, yes, it would have made a lot of sense if they had been very similar, to bind the same transcription factors.
26) ** When a new differentiated cell type is evolved, what happens
at the molecular level to cause this? Does this has to occur by "splitting"
of a previously-existing differentiated cell type into two?
What other possibilities can we think of?
Nobody has done much research on this.
What kind of evidence (Experiments? Statistical analysis of base sequences?) can help to answer this question, and comparable questions.
Molecular genetics of embryology
(The textbook is strongest on this aspect of development, and also has a lot about historical development of embryology; but I will cover this subject in a somewhat different sequence)
I) All the differentiated cells of the body have exactly the same
combination of genes in their nuclei!
(With very few special exceptions)
That is why it was possible for Dolly, the sheep, to develop from a sheep oocyte from which the original nucleus had been removed and a nucleus from an individual tissue culture cell injected into this enucleated oocyte. If the tissue culture cell had lacked genes for some organs or tissues, thn those wouldn't have developed in Dolly. The early death of Dolly may (or may not) have been partly caused by accumulated mutations in the original transplanted nucleus.
Notice that over 200 malformed sheep embryos were produced and only one or two anatomically normal sheep produced.
Since then, techniques have improved, and cats and several other
kinds of mammals produced the same basic way.
(but with high ratios of failures, in which embryos develop abnormally)
Experiments almost exactly like this were first done with frog nuclei,
by Briggs & King in the early 1950s (in the USA)
Micromanipulator needles were used to suck nuclei out of
recently fertilized Rana frog eggs;
and then nuclei from individual cells from blastula, gastrula, or later
stage embryos were injected into the oocytes.
These eggs with the injected nuclei often developed into tadpoles
(with all normal organs and differentiated cell types)
but 99% died before becoming adults. This nevertheless
proved that most cells have all the DNA needed to make a whole animal
In the early 1970s, John Gurdon (in Cambridge) using mutant
Xenopus frogs and UV light to kill oocyte nuclei, injected nuclei
from gastrula or tail-bud stages of tadpoles,
and got more success with development to adult frogs.
(He visited the UNC Biology department and presented a lecture
on this and related research in the early 1980s.)
(He was very interested in Rhododendron plants, & was very nice)
The first experiments proving nuclear equivalence were
done by a German, Spemann, who tied a single baby hair
around a 1-cell-stage fertilized frog egg, and constricted it.
Cell divisions occurred only on the side of the constriction
where the nucleus happened to be.
At the 16 cell stage, he loosened the hair constriction enough
for one nucleus to slip over into the uncleaved cytoplasm
on the other side, and then tightened the hair enough to
pinch the embryo in two.
Both half-embryos developed into whole frogs,
proving that all nuclei contained all the genes up until
at least the 16 cell stage.
Many other experiments proved this type of conclusion, Including early nuclear transplantations in salamanders by Prof. Gene Lehman of the UNC Biology Department, who taught this course from the 1940s - 1988.
So the birth of Dolly the sheep was no surprise to embryologists; but the general public knew nothing about Briggs & King, or Spemann.
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Some exceptions to the rule that all the cells have all the genes:
a) Mature mammal red blood cells do not have nuclei;
One stage of their differentiation is extrusion of the nucleus,
presumably to make room for a little bit more hemoglobin.
Red blood cells in birds, reptiles, amphibians & fish do have nuclei.
Their nuclei are shrunken and inactive; & have been useful
in experiments as a source of inactive nuclei that can be reactivated.
b) Amphibian oocytes make hundreds of extra copies of the DNA that codes for ribosomal RNA.
c) A few insects make extra copies of genes for particular proteins that they make in large amounts.
d) THE MOST IMPORTANT EXCEPTION TO THE RULE:
The genes for antibody binding sites.
Lymphocytes, including the "B-lymphocytes" that make and secrete antibodies, go through special processes of random recombination and hyper-mutation of the genes for the binding sites of antibodies.
The amino-acid sequences of antibodies are not inherited; their genes are randomly generated and selected in embryos, in a process almost the same as Darwinian evolution, by random mutation followed by selective survival, during each person's own embryonic development.
"The Clonal Selection Theory" {which has turned out to be true}
The DNA that codes for the binding sites of antibody proteins during embryonic development undergoes random splicing and very high mutation rates with each one of billions of developing B-lymphocytes randomly making its own DNA sequence for the binding site.
For example, no matter which blood type you have, you randomly generate lymphocyte clones whose binding sites exactly fit the A blood group antigen and other lymphocyte clones that make antibodies that fit the B antigen.
If your blood group is O, then you still have both those clones of "anti-A" and "anti-B" lymphocytes, and if you got a transfusion of types A, B or AB blood, then those antigens would attack the injected blood cells.
If your blood type is A, that means your cells have the "A" substance (which is a certain sugar polymer) on their surfaces (and on the outside surfaces of all the other cells of your body, too)
Those of us with blood type A couldn't survive unless some (still unknown) mechanism had 'weeded out' all those clones of lymphocytes who make antibodies whose binding sites fit the A blood group substance. Without this selective elimination "clonal selection" of those and all other "anti-self" lymphocytes, then our immune systems would attack all your own molecules.
No cells have anything like little flags or badges labeling themselves
"SELF! Please Do Not Attack"
as is implied by the ways people talk about "recognizing" "self".
Sixty or seventy years ago, that was how scientists thought immunity worked,
but that turned out to be very wrong. Many Nobel prizes have been
given for replacing those older theories, but they still survive in
the elementary textbooks, and in the vocabulary of "recognize" & "self".
The reason your antibodies do not attack any of your own molecules is because none of them have binding sites that fit your molecules.
It's not anything like them being able to fit self molecules but then desisting because they have "recognized" those are "self".
If antibody binding sites fit a certain molecule, they bind = attack.
If they don't fit, then they don't attack.
Would you say, my key doesn't open that lock because the lock recognizes that the key is a non-self key?
Misguided vocabulary can trap your intuition into very misguided ways of thinking and talking (and reasoning) about phenomena.
And elementary textbooks are nearly all written by non-scientist "educators", who copy from older textbooks and are 50-100 years out of date, spiced by recent discoveries that aren't really new (like Dolly)
MEDICAL IMPORTANCE: Autoimmune Diseases
If those anti-self lymphocytes don't get all get killed or inactivated, or if some of them get reactivated, then they will attack some of your own molecules:
If they attack the myelin sheath around nerves the result is called Multiple Sclerosis or MS
If they attack molecules in your joints, and also other antibody molecules the result is called rheumatoid arthritis (sometimes RA)
If they attack RNA, single stranded DNA, histones &/or collagen the resulting disease is called Lupus
If they attack insulin-secreting cells in the pancreas then the result is called type I diabetes (or childhood diabetes)
And there are dozens and maybe hundreds of other autoimmune diseases, that differ depending on which normal body molecule gets bound to, "attacked" and damaged.