March 9, 2005. Biology 104. Albert Harris
Teleosts, Birds and MammalsCleavage, gastrulation and early development.
Most fish are "teleosts", but sharks, rays, lampreys, hag-fish, lung-fish, bowfins, and even gars and sturgeon are NOT teleosts; and the descriptions in the book apply only to those fish that are teleosts. You do not need to memorize all those other kinds of fish that are not teleost fish; just learn that half the species of all vertebrates ARE teleosts, and so are all the kinds of fish usually kept in aquaria. Intense research is now being concentrated on Zebra Fish (Brachidanio rerio) which may become the vertebrate equivalent of Drosophila melanogaster. Thousands of mutant genes are being created and discovered, the whole genome is being sequenced, etc. Why is so much research concentrated on Zebra fish embryology? 1(All) teleost fish embryos are more transparent that embryos of any other group of vertebrates. 2(Many) teleost fish lay their eggs at the time of fertilization, and the embryos develop outside the body of the mother. [Many other species of teleosts are "live bearers" = the embryos develop inside the mother, & have something like a placenta] 3) Zebra Fish can be easily raised in the laboratory, and have a generation time as short as [I haven't found out yet!] ?? 4) Embryos develop most organs within 24 hours 5) Many eggs per female (although they tend to eat their eggs!)
A digression: about "Model organisms and "Genetic Screens" a) All the scientists in a field agree to concentrate all their research on one species of animal or plant, based on criteria like the following:
b) * Short life-cycles. *Can be easily cultured in laboratories. c) If everybody studies the same species, then results from one lab are directly comparable to those from other labs. (In contrast to if each lab used whatever kind of frog or fish were common locally!) HISTORICALLY, this approach started with Drosophila, with T2 bacteriophages, and with house-mice. Now several dozen "model organisms" have been selected, like Drosophila, Chlamydomonas, Arabidopsis, Zebra Fish, Yeast, C. elegans (a nematode) Corn and Rice are also "model organisms". (but not cows) National "Stock Centers" are funded by the NSF, etc. for each one. "Monograph" books are written with everything about that species Annual national and international meetings are held to bring together hundreds or thousands of researchers who specialize in that species. Expose organisms, or sperm, etc. to mutagen chemicals, and then grow thousands, or millions, of individuals. Look for mutant individuals that are abnormal in whatever property you are interested in (e.g. resistance to salt; or ANY property) Breed those mutants, and start a "mutant line" of organisms that are mutated in that one gene. "Stock centers" culture these forever!
Each mutant line is abnormal in one particular protein.
The foundation of knowledge about teleost embryology came from research on Trout eggs (from hatcheries), and "Killifish" = Fundulus heteroclitus (but much longer life-cycles made them impractical for the accumulation of mutant genes, etc.) The leading researchers on teleosts were J.P. Trinkaus, at Yale, and William Ballard at Dartmouth (both recently deceased) Fish embryos develop from a hemisphere of cytoplasm that forms on one side of a large sphere of yolk, that never cleaves. "Meroblastic cleavage": cleave only of a surface layer of cytoplasm
Birds and reptiles also all have meroblastic cleavage. Embryos of all other mammals have holoblastic cleavage.
But our gastrulation is nearly identical to that of birds; HOWEVER: Teleost blastulas, and their process of gastrulation are VERY, very different from bird or reptile embryos. A) Teleosts have two "extraembryonic membranes" that are completely different from any of the four extraembryonic membranes that embryos of mammals, reptiles, and birds have.
(it does NOT become any part of the fish body) 2) The "Yolk Syncytial Layer" is a few hundred nuclei, that never cleave apart, plus the cytoplasm around them.
(The YSL used to be called the "periblast"; a better name!)
C) The YSL has lots of actin and myosin, and (somehow) spreads
actively over the rest of the yolk, gradually surrounding it.
Teleost gastrulation consists of this spreading of the cells over the surface of the yolk. It is an example of epiboly!
D) At first, the plasma membrane over the yolk is not adhesive
Anyone would guess this is caused by addition of adhesive molecules;
But it turned out to be caused by removal of molecules,
which are removed by pinocytosis of vesicles, that sink into the yolk.
SPECIAL ASPECTS OF TELEOST DEVELOPMENT, etc.E) The neural tube forms by hollowing out ("cavitation") of a solid rod of deep cells, instead of by invagination.This cavitation is their version of neurulation. i) The mechanism of cavitation is not known. It isn't cell death. Cavitation is the rearrangement and polarization of cells, so that all their apical (non-adhesive) ends face an internal cavity. ii) In many species of sea urchins, the coelomic cavity is formed by epithelial folding, analogous to neurulation in amphibian embryos But in many other species of sea urchins, their coelomic cavities are formed by cavitation. iii) All vertebrates form their (our!) coelomic cavity by cavitation in the lateral parts of their mesoderm.
iv) The hind-most 1/6th of bird neural tubes are formed by cavitation of a solid rod of cells, although the front 5/6ths is formed by epithelial folding (invagination) followed by fusion of the edges. v) If you dissociate neural plate cells from amphibian, mammal or bird embryos, then part of their "sorting out" will include cavitation! vi) Folding of the neural tube is caused (mostly, & maybe entirely) by concentrated contraction of cytoplasmic actin and myosin in the apical surfaces of the future neural cells, which makes this surface concave, and rolls it into a tube.
THAT CAVITATION AND INVAGINATION OCCUR INTERCHANGEABLY??
In Embryology, it is not unusual for cells to be able to reach the same end result by means of two or more alternative sequences of intermediate processes. Examples: 1) Cell sorting of differentiated cells in sponges and hydra: Even sponges have their own normal pattern of gastrulation; and so do Hydra. But their normal anatomical arrangements of differentiated cells will also form if these cells are dissociated and randomly mixed. 2) Coeloms form in sea urchin embryos either by out-folding of the sides of the archenteron (epithelial folding) OR by cavitation of solid masses of cells (also on the sides of the archenteron). I would have expected this to be a fundamental evolutionary difference. Several hypotheses about phylogeny do regard such differences ("enterocoely" vs "schizocoely") as fundamental.
3) Neural tubes (and thus brain and spinal cord) become hollow
in by folding of flat epithelia in mammals, frogs and most vertebrates; And bird neural tubes fold at the front & cavitate at the rear! And if you dissociate neural tube cells from any kind of vertebrates, they will form hollow balls by cavitation. "PATH INDEPENDENCE" is the best term I can invent for such phenomena. There is no agreed-upon name for it. Developmental Biologists do not agree what it means.
I) Malcolm Steinberg proposed a "thermodynamic theory" for cell sorting, but it is equally applicable to all these phenomena in which cells reach the same end result by different intermediates. (page 74 of the textbook) II) Some people think the mechanism of epithelial folding and the mechanism of cavitation are fundamentally different, but that some cells are capable of doing both mechanisms, one sometimes and the other othertimes, and sometimes a mixture. III) Other people think that the same basic mechanism can cause either folding or cause cavitation. (Contraction toward one end of each cell transports cell-cell adhesions toward that end; thereby creating an apical end, and re-orienting all cells so that their apical ends are toward the center, thereby causing cavitation. When cells are already epithelial, then contraction concentrated at their apical ends causes folding of the cell sheet. Richard Campbell invented this.) IV) Nobody has discovered the true explanation. Maybe you can?
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