Embryology Biology 441 Spring 2010 Albert Harris
Lecture notes for Wednesday, January 27, 2010
This experiment was first done with embryos of starfish, by Hans Driesch in the 1890s, and the results astonished him & everybody else. Driesch concluded that embryos must contain super-natural spirits.
Driesch borrowed Aristotle's word "entelechy" as a name for such a mind-like spirit. Even if you separate the first four cells of an echinoderm embryo, each of the 4 will develop into a quarter-sized, but normally proportioned, pluteus (larva). Furthermore, you can push two sea urchin embryos closely together, and sometime they will merge (but not fuse) and will develop into a double-sized pluteus! Any time an embryo manages to develop into a normal animal, despite some such major disturbance, "embryonic regulation" is said to have occurred. Embryonic "regulation" means the ability of embryos (of many kinds of animals) to develop normally despite any such major disturbance. It's analogous to organ regeneration or wound repair. Please do not be mislead by the usual implications that the word regulation now has, of some kind of external supervision, like "government regulations". Embryonic regulation is some kind of internal process, of which the mechanism has not yet been discovered, although there are theories. Much of theoretical and experimental embryology consists of attempts to find out how regulation works, and how regulation (in the sense of overcoming disturbance) is related to normal mechanisms of development. Sea urchin embryos are said to be very regulative. Amphibian embryos are somewhat less regulative, because you can't get normal development after separation of the first four cells, and only sometimes get normal development after separating at the two cell stage, or after fusing two one-cell stages.
Mammals have about the most regulative development of any kind of animal. You can even get early mouse embryos to merge with early rat embryos, and develop into fully functional animals. Sheep-goat fusions also succeed!
Animals that result from fusing embryos of different species are called "chimeras" Chimeric mammals could probably be produced by combining early embryos of any pair of mammal species in which the adults are approximately the same size. (!) No good reason has ever been suggested for trying to make chimeras that combine more than a few species, and it should never be attempted using human embryos (because it would work). Horse-cow chimeras would probably be viable, for example. They may have been made. If you have heard about such chimeras, please tell me. Please do not confuse chimeras with hybrids. Hybrids result from fertilizing oocytes of one species with sperm of another species; thus chromosomes from both species are together in the same cells. Chimeras are mixtures of cells of one species (or one genetic line) with cells of a different species (or of a different genetic line). Nevertheless, some cell types develop by fusion of separate cells; this is definitely true of skeletal muscle cells, and may be true of osteoclasts. Genes from both species will be together in skeletal muscle cells of a chimera. The immune systems of chimerical animals do not attack their tissues. i.e. Sheep lymphocytes don't make anti-goat antibodies, etc. in a sheep-goat chimera, etc. etc. (Although they would if goat skin, etc. were grafted to a sheep; and vice-versa). This lack of immune attack is because the normal "self tolerance" mechanism works by weeding out (Killing? Inactivating? Sequestering? Nobody knows for sure.) all lymphocytes whose binding sites fit any cells or molecules that are in the embryo during embryonic development. It is a good illustration that vertebrate immune systems DO NOT work by recognizing differences between self and non-self molecules and cells.
If you have believed that is how immunity works , please get the idea out of your head. Cells and molecules do NOT all have little identification badges, which lymphocytes compare with their own identification badges, and then attack anything whose badge doesn't match. It doesn't work that way. Such misunderstandings hold back invention of real cures for autoimmune diseases like multiple sclerosis, rheumatoid arthritis, lupus - some of the most cruel, life-ruining and incurable diseases. --------------------------------------------------------------------------------------------------------------
The opposite of "regulative" is "mosaic". Sometimes the word "determinate" is used. Nematode worms are the phylum with the most mosaic development. Neither snow nor sleet nor dark of night, nor separation of cells from each other will deter nematode cells from their normal developmental pathway, with some very interesting exceptions discovered by this department's own Bob Goldstein! One of the deeper questions that embryologists wonder and argue about is what causes the difference between mosaic versus regulative development. * One explanation favored by some very smart people has been that it's how early during development that cells become committed to differentiate, or how early this commitment becomes irreversible. Very early commitment to differentiate into each particular cell type means mosaic development. Late decision (of which cell type to differentiate into) means regulative development. ** Another explanation is that mosaic development results from differentiation being controlled by special cytoplasm being put into different cells at the time they form, while regulative development reflects control of cell differentiation by cell-cell signaling, at times that can be much later than cleavage.
"Ooplasmic segregation" is known to occur in the early embryos of sea squirts (which are chordates, like us, but not vertebrates)
Despite much research by excellent scientists, the molecular nature of the cause is not yet known, A second example of ooplasmic segregation is that future oocytes and sperm develop only from cells that contain a special granular cytoplasm. (in flies & nematodes, & maybe others) A third example is that embryos of many snails and other mollusks form large bulges during early cleavages (these bulges are called "polar lobes" (see photographs on pp. 130 and 134 of the textbook), and are at the vegetal pole). Cytoplasm of polar lobes goes to one of the first 4 cells, and promotes mesodermal development. Stimulation of embryonic cell differentiation by cell-cell signaling is called "embryonic induction". Hans Spemann won a Nobel prize for his graduate student's (Hilde Pröscholdt ) discovery that skin ectoderm can be stimulated by notochord cells to differentiate into a second neural tube. (See photographs on p. 102 and 103 of the textbook) The experiment was to cut a piece of future notochord out of one salamander embryo and insert it into the blastocoel of another gastrula-stage salamander. Different colored species of salamanders were used in order to distinguish which cells formed what organs.
Somatic endoderm of the "host" (the embryo into which the 'graft' is made) forms a second neural tube. Another graduate student of Spemann at the same time (Hans Holtfreter) observed cases of exogastrulation in which the mesoderm bulges outward, instead of invaginating, one of the results of which is that no neural tube forms. The conclusion is that in normal development the notochord sends some signal to the ectoderm closest to it, and this signal causes that ectoderm to form a neural tube (& brain etc.) Much inconclusive research was done trying to discover what molecule is the signal. A major problem was that salami, etc. could also induce neural tubes! Dozens of other examples of induction have been discovered.
2) The lens induces the skin to re-differentiate into cornea. 3) The inner and outer layers of teeth both induce each other to differentiate. 3.5 ) It is reported that bird mouth tissue can be induced to form teeth by mouse mouth cells. 4) Endodermal cells can be induced to form either salivary glands, lungs or pancreas, by contact with mesenchymal cells from those. 5) Grafting Hensen's node to abnormal locations in chicken embryos can induce formation of a second neural tube and entire new embryos. It's what forms the notochord after all.
He smoked a pipe, and at seminars would sit in front and blow smoke rings at the speaker. I was among those subjected to this when I was a graduate student. UNC Biology Department's own Bob Goldstein was the first to prove an example of embryonic induction in the development of nematode worms (C. elegans), which made him world famous. See http://www.bio.unc.edu/faculty/goldstein/lab/Nature.pdf --------------------------------------------------------------------------------------------------------------------------------- a) If you separate the first two cells ofÉ? What kinds of animal? What happens? b) Who was Hans Driesch and about when did he make a revolutionary discovery? c) What is an entelechy? Do they really exist? *Would a negative feedback control be one? d) Does embryonic regulation mean external control of embryos? What does it mean? e) Whose embryos are more regulative that any other group of animals? f) If embryonic development in a certain kind of animals is very non-regulative, then it is ...? g) What is a chimera? Where does this word come from? h) What are some differences between chimeras and genetic hybrids? *i) If you mated two chimeric animals, would the offspring be hybrids? (Sometimes? Depending on?) *j) Can you devise an experiment, using a chimera, to test whether or not osteoclasts become multinuclear by fusion of separate cells versus by mitosis without cell division? *k) Suppose that cells of the each species tended only to fuse with cells of the same species; why could that confuse your experiment (in the sense of giving you a mistaken result)? l) What embryonic process needs to be understood and re-activated in order to cure autoimmune diseases such as multiple sclerosis rheumatoid arthritis and lupus? *m) Can you invent experiments using chimerical animals that might help understand this normal immune process, the failure of which causes autoimmune diseases? n) What are three examples of ooplasmic segregation, and in what kinds of animals does each occur? o) What relation might there be between mosaic development and ooplasmic segregation? p) If ooplasmic segregation controls subsequent differentiation, then will cell fates become irreversible earlier or later in embryonic development? q) Can embryonic induction change the fates of cells that have been grafted to abnormal locations? What about if they are grafted long after being formed by cleavage? *r) If there weren't so many invaginations and folding of embryonic cells from place to place, then would induction be such a good means of pattern formation? s) Do you think that birds still have the genes for making teeth. What about the genes for whatever receptors of inductive signals that induce tooth formation?
t) By what embryological experiment might you prove that snakes still have the genes for making legs? u) What are two things that Hensen's node has in common with the dorsal lip of the blastopore? (What organ it differentiates into, and what it can induce if grafted) *v) If Spemann hadn't yet imagined the possibility of induction at the time of the discovery. then what would you guess they were trying to test? *w) How would you guess that Spemann was able to realize the broad significance of Pröscholdt's discovery.
x) Are there any cases in which two tissues each induce changed differentiation in the other? y) How might you test for cases of induction by inserting thin pieces of plastic of mica into early embryos?
z) What was the importance of Holtfreter's studies of exogastrulas, in relation to induction?
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