In bird, reptile & mammal embryos, before gastrulation two layers of epithelial cells have been formed.
The epiblast is the upper=outer layer; the hypoblast is a thinner layer under it (next to yolk in birds).
Next, an elongate thickening of epiblast forms called the "primitive streak" : (this will be body axis).
Along midline of primitive streak, ingression occurs along line, "primitive groove" instead of blastopore.
Those epiblast cells that undergo ingression, after they have ingressed, are called "mesoblast cells"
The mesoblast cells crawl laterally to either side: most of them will form mesodermal organs
But some mesoblast cells insert themselves into the hypoblast & become part of it;
The hypoblast forms endodermal organs, (stomach, intestine, lungs, liver etc.)
Those epiblast cells that never undergo ingression and are left behind at the surface become ectoderm.
The anterior end of the line where ingression occurs is called the Hensen's node or Primitive node
This node corresponds to the anterior or head end of the body that is being formed <<----
Ingression pregressively ceases, anterior first, etc. so that Hensen's node seems to move
This posterior movement of Hensen's node is called the regression of the node. (& is a big deal)
Note however, that what actually moves is a process, or rather the edge of a process, NOT material.
The cells that ingress at the node itself are the last to ingress in that area: they form the notochord.
The mesoblast/mesoderm cells to either side of the notochord segment into pairs of somites:
Each somite subdivide 4 parts: dermatome, myotome, anterior sclerotome, post. scleroderm
The mesoderm lateral to the somites becomes the intermediate mesoderm (2 rod like strips)
The mesoderm on either side of that becomes the lateral plate mesoderm: peritoneum, heart, etc.
These two sheets of lateral plate mesoderm split ("cavitation") to form the coelomic cavity.
The remaining epiblast that didn't become mesoblast cells becomes ectoderm, as mentioned above.
The part of this directly over the notochord & somites becomes thickened and folds into a tube.
This thickening & rolling is called neurulation: the edges of the folds fuse to form the neural tube.
Cells ingress from the neural folds (where fusion occurs) to form the neural crest ("ectomesenchyme")
Neural crest forms spinal sensory nerves, Schwann cells, (postganglionic) autonomic nerves, pigment cells,
odontoblasts, and other skeletal cells of the face (that would be mesodermal in other parts of the body)
Neural tube ectoderm forms the brain and spinal cord, including motor nerves & also the eye.
The remainder of the ectoderm (not part of this tube) is called the somatic ectoderm.
Somatic ectoderm forms the outer layer of the skin (epidermis, including hair, feathers, and the kinds of scales in reptiles, birds and some mammals). The sensory part of the nose, and also the inner ear, and a few other structures are also formed later from special parts of the somatic ectoderm, which are called placodes. These will be considered in detail later in the course.
The endoderm forms the lining of the digestive tract - esophagus, stomach, intestine and also the lungs, the liver, the pancreas, thyroid gland, gill slits, and maybe some of the salivary glands, all of which form by invaginations of parts of the endoderm.
In many kinds of embryos, gastrulation also includes spreading of epithelial sheets ="epiboly". E.g. the edges of the bird epiblast crawl outward over the inside surface of their vitelline membrane.
In the gastulation of teleost fish, the edges of its blastodisc crawl down over its yolk in a classic case of epiboly; as this occurs, the fish body is laid down along one part of the advancing cell margin.
A puzzling fact is that in teleost fish embryos the neural tube forms by hollowing out a solid rod ("cavitation") instead of by folding; likewise the posterior sixth of bird neural tubes form this way!!
Similarly, some echinoderm species form their coelom by invagination , others by cavitation.
AAnd if you dissociate mammal or amphibian neural plate into separate cells, masses of these cells will cavitate to form little brain-like and spinal-cord-like : What do your think this means? (no one knows!)
Be sure to understand that sea urchins are not vertebrates & don't have notochords, don't have neural tubes, don't have somites, form their coelomic cavities in different ways.
2) The individual cells of Volvox are very similar to (and evolutionarily VERY closely related to!) what genus of organisms? (hint: it's a plant)
3) What properties are desireable for a "model organism"? (like: should they be good to eat, or what?)
4) What are some of the specific organisms that are now being used as model organisms?
5) Would it be practical to do genetic analyses on Sea Urchins? Why not?
6) If you wanted to identify as many as possible of the proteins that are needed for gastrulation, then how could genetics help you do this?
*7) Do you know what is meant by a "genetic screen"?
8) During what process are animal embryos subdivided into the three primary germ layers?
9) What are the names of these three germ layers?
10) Do germ layers contain any germs? (viruses or pathogenic bacteria? Or what?)
11) Our brain and spinal cord develop from which part of which germ layer?
12) What do sensory nerves develop from? (which germ layer? which part?)
13) What about motor nerves (the ones that carry the signals to muscle cells, to stimulate them to contract), from what do they develop?
14) Embryologically, what do the lungs, liver and the cells that line the digestive tract have in common?
15) Name as many different kinds of organs and tissues as you can that develop from mesoderm.
16) Compare gastrulation in sea urchin embryos, in frog embryos, in bird embryos, and in mammal embryos? (similarities between each pair; differences between each pair)
*17) Compare gastrulation in animal embryos to "fruiting" in Dictyostelium cellular slime molds.
**18) Physarum is one of many common genera of amoeboid organisms in which thousands of different nuclei are present in the same cytoplasm. Can you figure out why Physarum is NOT considered to be one of the "Cellular Slime Molds", but Dictyostelium IS a cellular slime mold?
19) What is meant by chemotaxis? Did you learn what is meant by a chemotactic attractant substance?
20) Which was proved first?
A) Which chemical is used by Dictyostelium discoidium as its chemotactic attractant substance?
or B) That cell aggregation in this organism is guided by chemotaxis? (rather than another mechanism)
21) Many people assume that it isn't possible to prove whether chemotaxis is used by a certain organism without first knowing what chemical they use as their chemotactic attractant. Is that right?
*22) What sort of "bioassay" method could you use to try out different chemicals to find out whether any of them act as chemotactic attractants in a certain stage of development in a given species of animal?
*23) In what sense would you not be able to test which substance is the attractant until you had first proven that chemotaxis is being used? (Whether or not you realized you had proven that!)
**24) In the embryonic development of higher animals (sea urchins, people, birds, frogs) chemotaxis might possibly be a good way to guide cell locomotion in which specific processes?
25) If you wanted to find mutant Dictyostelium whose chemotaxis is abnormal in some way, how would you do it? Start with mutants that are unable to detect or respond to gradients of cyclic AMP?
26) How could you use C-AMP to cause these mutants to isolate themselves "automatically"?
*27) If only one Dictyostelium amoeba in ten million had such a mutation, could you find that amoeba?
**28)Can you invent experiments to test whether chemotaxis is the guidance mechanism that is used to guide some particular kind of embryonic cells to their correct anatomical location in the body? How?