Zebra fish have been chosen to become a genetic model organism
(The same species that have long been sold in pet stores)
Many are now concentrating their research on them,
and thousands of mutant lines have been isolated.

Advantages:
Can be raised in aquaria
Many eggs produced per individual
Eggs (embryos) develop rapidly
(very!) Transparent embryos
Embryos are permeable to chemicals (unlike other fish, and frogs)
They are vertebrates

Disadavantages: (are very different from humans in many ways)
Evolutionarily, teleost fish have diverged greatly from mammals.
Their pattern of gastrulation is very different from ours.
Have evolved two extraembryonic membranes we don't have
(the yolk syncytial layer and the enveloping layer)
(reptiles, birds and mammals have 4 extraembryonic membranes)

Note that Zebra fish are classified as teleost fish.
Are NOT closely related to sharks, rays, lamperys or hagfish
each of which have very different gastrulation, etc. patterns

Even Gar and Bowfins development is somewhat different!
(for example, their cleavage is not quite meroblastic)
Of the ~40,000 species of vertebrates, ~20,000 are teleosts
Previously, most research on teleosts was done using either trout or killifish (Fundulus heteroclitus).
The trout eggs came from hatcheries
and Fundulus is the common minnow used for bait in salt and brackist water.

The leading fish embryologist was J. P. Trinkaus, who died a year ago this month,was my PhD Professor and good friend. look at the textbook's photograph 11.5 B on page 349
(His autobiography "Embryologist" was published in 2003)
He and I watched thousands of fish eggs develop!

Furthermore, H.V. Wilson also began as a fish embryologist, but worked on the Black Sea Bass. (in the Atlantic, not the Black Sea!)

Interesting special features of teleost development:
(all teleost's eggs develop nearly the same, as far as I know)

(Like reptile and bird eggs) Cleavage is meroblastic.

But the cytoplasm flows to the blastodisc from the whole surface (immediately after fertilization)

Prof. Trinkaus' first PhD student (Charles Huver) discovered that you can induce formation of an entire second blastodisc by putting crystals of salt on the surface at the vegetal pole.
This discovery was never published in any journal, so nobody knows it but me and you in this class!

Cleavage is synchronous, but NOT in such a regular pattern. (contrary to what the textbook says on page 348, it isn't always two rows of 4; often it's 3,3,2. etc.)P> Late during cleavage, several hundred nuclei move down toward the yolk, and never separate into cells ="the yolk syncytial layer" (this has no equivalent in other vertebrates, and does not become any part of the fish body; it is an "extraembryonic" membrane.

About this same time, the outermost layer of cells forms a very thin epithelial sheet ="the enveloping layer" This does not become part of the fish, has no equivalent in birds, mammals (or amphibia) and is an "extraembryonic membrane"

All the other cells are called "the deep cells"
The body of the fish is made entirely of and by deep cells.
The deep cells craw around randomly until gastrulation begins.

Gastrulation is by epiboly; the enveloping layer surrounds the yolk
And contrary to what the textbook says on page 349, the edge of the enveloping layer crawls actively down over the YSL. even though the YSL also expands and speads around the yolk.
One of Prof. Trinkaus' early discoveries was that mechanical detachment of the edge of the enveloping layer will allow it to retract back to where it started, but then it will crawl back down.

This is one of many examples in which embryonic cells can arrive at the same geometric arrangement by any of two or more different sequences of movements.

Another interesting fact about teleost fish development is that their neural tube forms by hollowing-out of what starts out as a solid rod of cells, running down the back.

Notice the contrast between this, compared with the way that amphibians, mammals etc. form our neural tubes, which is by a sheet of cells (the neural plate) folding, and the edges sticking.

I mention this here because it is another example of embryonic cells arriving at the same end result by alternative different routes

There are many cases of this, and I think that it is one of the most interesting (and maybe important) aspects of embryology (but there isn't even a good name for it!
Maybe it should be called " Path Independence" Please think of a better name!

Another example:
If you dissect out neural plate cells of mammals or amphibians,
and dissociate them into a suspension of individual cells,
and let them re-aggregate into solid masses, then they will form tubes by hollowing-out!

In other words, neural plate cells can form tubes either by rolling-up or by hollowing-out!

Maybe if there were some practical way to iron-out teleost neural ectoderm into flat sheets, then these might roll up into tubes?

In bird embryos, the anterior ~80% of the neural tube forms by folding but the posterior ~20% forms by hollowing out of a rod!

Should we think that two different mechanisms are used?
How can the same mechanism either hollow-out or fold-up?

Some biologists think such events can be explained by thermodynamics, as the author of our textbook seems to believe. (but I think they are wrong, and have written papers on this)

During teleost gastrulation, deep cells crawl directionally into a ring, one part of which is thicker;
this thicker part is called the " embryonic shield"

During epiboly, the deep cells form the fish body axis, head-first, from anterior to posterior. There is no blastopore and no Hensen's node, but deep cells do turn under along the axis.

Sometimes deep cells aggregate toward two points instead of one & then you get a two-headed fish.

Simply pushing down on the top of the embryo can cause this.

Researchers are now trying to prove which genes and proteins control deep cell behavior.
Wnt protein (similar to fly wingless ) seems to be important for anterior-posterior patterning.

Other genes have been named Dharma and Bozokok ! (reasons explained in the book)

Two interesting mistakes from past fish embryo research:

Everyone assumed enveloping cells must invaginate somewhere, because every other kind of embryo has surface cells move inward. Researchers use "nile blue" and other non-poisonous dyes to track cell movements and make " fate maps" One scientist used heavy concentrations of nile blue to find where the enveloping cells were invaginating!
But it turned out they normally never invaginate, but treatment with large amouts of this chemical causes abnormal invagination.

The drug actinomycin inhibits mRNA transcription, and because Fundulus egg development was blocked by actinomycin, but only when the eggs were exposed to the drug in the first hour
after fertilization, some researchers concluded they must make all their future m-RNA very quickly early in development.
The true explanation was that the eggs become impermeable then!