Embryology Biology 441 Spring 2009 Albert Harris
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Lecture notes for Jan 21This lecture will be about descriptions of vertebrate embryos, and also early development of sea urchin and other echinoderm embryos, up to their larval stage (a larval echinoderm is called a pluteus).Echinoderms are not vertebrates, but are more closely related to us than most invertebrates are; and early development of echinoderms is unusually simple, easy to visualize, and has been the subject of very much research. Pluteus larvae are to sea urchins what tadpoles are to frogs, and as caterpillars are to butterflies. After living as plankton for a while, they undergo drastic anatomical changes (metamorphosis) to form sea urchins, star fish, etc.
The original discoverers of pluteus larvae (back before the mid 1800s)
thought they were a new (separate) kind of animal, and "pluteus" was the
genus name they chose. It's the Latin word for, among other things "bookcase". Germ layers and body organs are formed by combinations of the following (10) "morphogenetic cell movements": Invagination, involution, ingression (& egression), (cell) locomotion, (epithelial) epiboly, intercalation (including "convergent extension"), cavitation (including "delamination") Fluid inflation of closed cavities is another important mechanical cause of embryonic shapes and shape changes, and another physical cause is osmotic swelling. For many years everyone assumed that growth of cells should be an important cause of embryonic shapes, both in the sense of cell enlargement or directional mitotic cleavage; but this has mostly turned out not to be true, except maybe in mosaic cleavage patterns. Everyone still talks about, for example, an embryo "growing" an eye or a kidney, etc. but cell rearrangement has much more to do with it than growth. Each of these morphogenetic cell movements have their own molecular genetic causes. For example, acto-myosin polymerization contraction is a major driving force, and gain and loss of special cell-cell adhesion proteins controls many changes.
My research specialty happens to be the mechanical causes of these movements. A KEY FACT: The amount and distribution of yolk influences early development. Yolk means food materials stored in egg cell cytoplasms, often partly as crystals. In bird, platypus, and fish embryos there is so much yolk that the cytoplasm, and the embryo during early stages is just a thin layer on one side of the yolk. Only 3 species of mammals lay bird-like eggs, but development in all mammals is geometrically distorted into a thin layer, as in birds. (despite human and most mammal oocytes not having much yolk, our embryos gastrulate in the pattern of reptile and bird eggs) The yolk of a bird's egg is the developing embryo: the "egg white" and the egg shell are layers of non-living materials secreted onto the surface of the embryo as it moves down the oocyte before the bird lays the egg.
A MINOR FACT: Every embryologist I have ever known referred to early embryos
as "eggs", not just before fertilization but for long after.
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Sample questions that you should be able to answer, based on the
lecture given Wednesday, Jan. 21. a) What does a morula stage embryo look like? In sea urchins? In frogs?
b) Does the morula stage come before or after the blastula stage? c) When does the gastrula stage occur, relative to cleavage, morula stage, blastula stage? d) Which kinds of animals have very large amounts of yolk in their egg cells? e) Sea urchin embryos, at the one cell stage, look most similar to the early embryos of which of the many kinds of vertebrates? f) Contrast the geometrical patterns of cell rearrangements in gastrulas of sea urchins, amphibians, birds and mammals? g) Compare the absolute sizes of early embryos (in inches, millimeters, etc) of sea urchins, humans, birds, frogs, salamanders, and fish. h) Sketch the geometry of cell rearrangements for ten different morphogenetic movements. i) For each morphogenetic movement, give at least two examples where it occurs at particular stages of development of a certain kind of vertebrate or echinoderm embryo. (and make a sketch).
j) Do actin and myosin serve any function in morphogenetic movements?
k) Epiblast and hypoblast layers of cells occur in the embryos of which kinds of animals? l) What are the "primitive streak" and the "primitive node"? What are some kinds of animals whose embryos do NOT form either of these two structures? m) What is the blastopore, and how is it similar to the primitive node? n) Does the blastopore connect to the blastocoel? (hint: No) o) To what cavity does the blastopore connect? p) What is a pluteus? Was "Pluteus" ever used as a genus name for species of animals? q) What are at least three unrelated kinds of animals that undergo metamorphosis? r) The mechanical force causing expansion or enlargement of certain embryonic organs often is produced by either by what, or by what else? (not growth) s) What are two special extra-embryonic tissues that develop only in embryos of teleost fish? t) What tissue is formed by primary mesenchyme cells in sea urchin embryos? u) Draw a pluteus. (How do they swim?) v) Draw a mammal blastocyst. How is it different from a blastula? *w) If two or more separate inner cell masses were to form within a single blastocyst, then what would you expect would be the end result?
*x) In terms of the structure and cell rearrangements of the early embryo, what would you guess must be the cause of "conjoined twins" (often called "Siamese twins")? *y) What cytoplasmic flow occurs just after the fertilization of a teleost egg, and suggest at least two mechanical changes that might cause this flow? *z) Guess the reason why development from fertilization up until the blastula or early gastrula stages is LEAST well-understood in BIRD embryology, as compared with these stages in frogs, fish, sea urchins, or even mammals.
I hope the following might interest you: It won't be on any exams.
A use of the word "pluteus" in 'harrius potter et camera secretorum'
page 28: 'amo te quod id dixisti, carissime, sed labor est iniucundus,'
inquit Domina Vislia. 'iam videmus quid Lockhart de re dicat.'
The word "pluteus" is used to here mean what? If you look in Caesar, you can find "pluteus" at least twice in the "Gallic Wars" and at least once in the "Civil War", but to mean something different.
In this brief translation, can you find another word that has been borrowed by science? What declension and gender does the word pluteus seem to be? In which direction do you need to walk to get from Soho Square to the King's Cross train station, platform 9.75?
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