Embryology   Biology 441   Spring 2011   Albert Harris


Lecture notes for Friday, Feb 4, 2011

(Teleost) Fish & Bird Embryology



Zebra fish, just like the ones you can buy in pet shops, are getting to be the species whose embryonic development is most intensively studied of all vertebrates.

Over five thousand professional biologists are now concentrating their research on this one species, which originally came from India & Bangladesh. A former phage geneticist at the Univ. of Oregon first developed the use of this species for embryological research.

They have all the advantages you want in a model organism:

a# Can be cultured in the lab through their whole life cycle.
(fairly rapid life cycle; and breed all year)

b# Small, but not tiny.

c# Each mother produces many eggs (hundreds)

d# Transparent eggs & early embryos.

e# Develop rapidly. (& can live for 2 to 5 years)

f# Not too cute or intelligent to create moral issues.
(Although males and females ordinarily mate for life!)

g# Many mutant strains have already been isolated.
(For any model organism, the more discoveries accumulate, then the more useful that organism becomes for further research)

Also h# reasonably permeable to experimental chemicals.

Fundulus, trout, the Medaka, and a European loach were subjects of much research. (Trout because you could get lots of eggs at hatcheries)

Less research has been done on eggs of various non-Teleost fish, including Gars, Sturgeon, Lungfish, Sharks, Lampreys, etc. which I mostly mention here to make the point that many fish are not teleosts, and each have their own, rather different embryology.

About half of all the 40,000 species of vertebrates are teleosts! Fish rule!


Zebra fish and fundulus eggs are spherical until they are fertilized; then cytoplasm flows into a hemispherical bump. (the one cell stage). (bump = "Blastodisc")


I happen to have studied this process of bump formation, and conclude that it is caused by local weakening of the cortex in this area, so that cytoplasm gets passively squeezed into the bump.
The ratio of the curvature of the bump to the curvature of the oocyte should be a measure of the difference in contractile strengths of their surfaces (I think; but would like to be able to prove!).
There are ways to cause more than one bump to form.

The first 5 or 6 cleavages (mitotic divisions) only cut through the blastodisc down to the yolk; so the cells are still connected.

This sort of thing is called "meroblastic cleavage"
(The textbook calls it discoidal meroblastic cleavage).
Bird eggs, reptile eggs, shark eggs also have meroblastic cleavage.

Gastrulation includes a very dramatic epiboly ( = spreading of a sheet of cells).

Two extraembryonic membranes form:

1) The enveloping layer
(this is very this protective epithelial layer over the top; the embryo hatches out of it later)
(it is analogous, but not homologous to our amnion)

2) The Yolk Syncytial Layer (used to be called the periblast)
A thick multi-nuclear layer under the embryo; it serves for nutrition, and also pulls the embryo around the circumference of the spherical yolk.

The outermost edge of the enveloping layer is attached to the YSL (Yolk Syncytial Layer) and is pulled by it and also crawls on it.

A very interesting increase in adhesiveness of the YSL occurs as it spreads radially .

This change in surface properties by withdrawal of selected molecules from the surface membrane, by means of selected pinocytosis, That was discovered by J. P. Trinkaus by diluting a fluorescent dye ("Lucifer Yellow") in the surrounding fluid, leaving it there long enough for pinocytosis to occur; then washing with clean water, and observing with a fluorescent microscope, and making time-lapse videos of fluorecent fluid-filled vesicles sinking into the cytoplasm.

It is an entirely NEW way for membranes to change adhesion properties; many had hypothesized maybe vacuoles could add materials to the plasma membrane by fusing with it, but nobody even guessed that non-adhesive molecules could be pinocytosed out of the membrane instead.

And notice the elegance of the fuorescent dye method used to detect it.

J.P. Trinkaus continued to produce breakthrough research right to the end of his life, and always had a helpful word for others, especially me.

It was a great honor for me to know him for 40 years; he was always encouraging, never boring, helped the weak, and exasperated the strong.

The fish body is (somehow!?) laid down by convergent migration of cells from both sides around the outer rim of the spreading sheet of cells. (which is called "The Germ Band")

The cells that actually form the body are named "Deep Cells" and are blobby, amoeboid-looking cells that crawl very fast and actively (and crawl entirely randomly at first, but as epiboly occurs, the deep cells move in more regular pathways, to form first the head, then the neck, trunk and tail).

Deep cells converge from both sides of the forming body.



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