Lecture notes for Wednesday, January 7, 2015
Grading
There will be three 1-hour exams (January 30th, March 4th, April 8th), and a 3-hour final exam (May 4th, at 8 AM).You can drop the lowest hour exam grade... alternatively, you can count the final less.
exam 1 exam 2 exam 3 final
20% 20% 20% 40%
0% 25% 25% 50%
25% 0% 25% 50%
25% 25% 0% 50%
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Textbook
Langman's Medical Embryology concentrates on mammal and human development, and medical problems.Most of the course content is NOT in this textbook (or any available textbooks) "Developmental Biology" textbooks concentrate on molecular genetic aspects, don't have much specifically about mammal development, and have much more about fly development. Hox genes etc. have crowded out all other aspects of embryology.
Biomechanics of development is available only in much more expensive books, and textbooks written in other languages.
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Today's lecture: (followed by sample exam questions)
Mammal embryology compared with sea urchin embryology.
Introductory and core courses sometimes leave impressions that all kinds of embryos are more similar to sea urchin development than they really are.
For example, in mammal development,
1* cleavage isn't synchronous, not 1-2-4-8-16-32 etc.
2* cleavage isn't rapid (16-20 hours from one division to the next, instead of 30-60 minutes)
3* cell cycle "check points" are not turned off in mammals, unlike urchins, frogs, fish, birds
4* early cleavages don't form any special pattern in mammals. (just a pile of cells)
5* Mammal embryos become triploid when the sperm fuses with the oocyte
(That is because the oocyte is still diploid at the time of fertilization, in most mammals.)
(In dogs and foxes, oocytes are tetraploid at fertilization, after which meiosis occurs)
No species of vertebrate finishes both meiotic divisions in oocytes before fertilization.
Sperm precursor cells do become haploid before they differentiate into sperm.
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QUESTIONS FOR CLASS DISCUSSION:
What function is served, do you guess, by keeping extra sets of chromosomes until so late?
Is there really a certain "moment of conception" at which time each individual becomes genetically unique?
Incidentally, sea urchin oocytes really do become haploid before fertilization. Also jelly-fish.
But all other phyla are like vertebrates. Many are like dogs in this respect.
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Similarities:
1) Size 100 micrometers, one tenth of a millimeter, half the thickness of a cover-slip
2) Transparent jelly coat. "vitelline membrane" "zona pellucida"
3) Yolk, but not nearly so much as in other vertebrates (birds, frogs, fish, platypus, all vertebrates)
4) Inhibit "polyspermy" by secreting 20,000 cortical vesicles enzymes. Inhibit adhesion of sperm
5) Electrical voltage change when any sperm fuses with an oocyte. Like nerve action potential.
6) Cleave all the way through (holoblastic cleavage, versus meroblastic cleavage, fish, birds) (versus what arthropod embryos do)
QUESTION FOR CLASS DISCUSSION: Embryonic cell divisions in flies?
7) Cells rearrange into a hollow ball. "Blastula" in urchins and most vertebrates
"Blastocyst" in mammals (which really are different)
8) Morphogenetic cell movements
* folding of epithelial cell sheets,
* fusion of epithelia with each other
* conversion of epithelia to mesenchymal cells
* locomotion, by mesenchymal and also epithelial cells
9) Subdivision of embryo into...
Ectoderm Skin and nervous system
Mesoderm Skeleton, muscles, heart, kidneys
Endoderm Lining of digestive tract, liver, many glands, lungs in mammals
These same subdivisions occur in all multicellular animals (not sponges).
10) Boundaries between subdivisions are created by epithelial folding, etc.
(Recently discovered that undifferentiated "stem cells" can be steered toward differentiation into alternative cell types by different strengths of forces and by geometric arrangements)
11) The genes and proteins that control cell differentiation are almost exactly the same "homologous" in almost all multicellular animals.
12) Vertebrates, and urchins too, have "hox genes" that were first discovered in flies.
Hox genes bunched together (linked) on chromosomes, and are selectively transcribed in stripes, down the length of the body.
Colinearity occurs in vertebrates, in the same geometric pattern as flies.
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Twenty Six Sample Exam Questions:
A) List 5 important differences between sea urchins versus mammal development.
B) List 12 important similarities
Fertilization occurs when, relative to fertilization?
C) In human oocytes?
D) In dog oocytes
E) In Jelly-fish oocytes
F) Name one other phylogenetic groups of animals that are like jelly-fish in this respect.
G) What are the three primary germ layers?
H) What are two organs that develop from each of these, in mammals?
I) Describe at least three active cell rearrangements.
J) Sketch cell arrangement in sea urchins at the 4 cell stage.
K) Sketch cell arrangements in a mammal embryo at the 4 cell stage.
L) Do mammal embryos sometimes not have a 4 cell stage? Explain why they might not.
M) Sketch cell arrangements at the blastula and blastocyst stages.
N) Blastocysts are a stage of embryonic development in…
just sea urchins
just vertebrates
just mammals
just humans
most multicellular animals
O) Blastulas are a stage of embryonic development in…
just sea urchins
just vertebrates
just mammals
just humans
most multicellular animals
P) Morphogenetic cell movements occur during the development of? just sea urchins? etc.
Q) Ectoderm, mesoderm and endoderm occur in? just sea urchins? just vertebrates, etc.
R) Hox genes? Are they just in sea urchins, just in vertebrates, or what?
S) Colinearity, of locations of gene transcription versus locations of genes on chromosomes?
? Urchins, vertebrates, mammals, humans, most animals?
T) What is the difference between holoblastic versus meroblastic cleavage?
U) Rapid voltage changes occur in nerve cells, muscle cells and what other kind of cell?
V) If a human oocyte were simultaneously fertilized by two sperm, instead of one, then how many complete sets of chromosomes would it then have.
W) What normally prevents more that one sperm from fertilizing an oocyte?
X) Invent TWO different new methods of contraception based on the normal biological methods that prevent more than one sperm from fertilizing an oocyte.
Y) Would it surprise you if hox genes were discovered in paramecia, or other kinds of one celled animals. Would this change current beliefs in any important way?
Z) What effect can mechanical forces sometimes have on undifferentiated stem cells?
Suggest what this might explain in normal development.
Invent some way to take advantage of this effect for medical purposes.