|From what does the yolk form? Endoderm? Mesoderm? Ectoderm?
|Question Date: 2001-04-06|
Actually, you asked a bit of a trick question, as the yolk arises from none of the above. Yolk is a substance within a single cell: the egg. Think of a chicken egg, like the kind you buy in the store. These are unfertilized, single cells. I do not know if ALL eggs have yolk (e.g. human eggs), but some certainly have more than others! This chicken egg, if fertilized, will then divide and give rise to all the tissues mentioned, in a set of complex and highly regulated steps. Which tissues (endoderm, mesoderm, ectoderm) arise from the yolk is another question you might ask. Certainly, in many organisms, the yolk will survive during the development and serve as a food source for the developing embryo or hatched juvanille organism (as with some fish larvae), and so the yolk never develops into any tissue but remain yolk.
For basic info on the stages of development, see
Fertilization and development has been well studied in frog eggs and sea urchin eggs. Frog eggs are composed of two halves or "poles" (the vegital pole and the animal pole). Are all eggs like this? I don't know. I do know that in frog eggs, the two halves are different colors, and so are easy to distinguish from one another. After fertilization, the yolk-rich vegital pole of frog eggs develops into endoderm, mostly, after blastula development. To compare development in a wide variety of animals (frogs, worms, flies, zebrafish, chickens, mice, rats and rabbits), see the web site below.
To trace the developmental stages in a fruit fly, probably one of the most-studied organisms, check out the web site below. There is a lot of detail and jargon, but it gives the stages of development and the order in which they occur, along with some pictures of these events.
Let's limit our discussion to birds (although reptiles and fish are not all that different in this regard).
The "yolk" is actually formed in the mother and packaged into the egg as it develops. After fertilization, only a small part of the egg -- usually at the relatively "less yolky" end called the animal pole -- actually subdivides into embryonic cells and will give rise to the embryo. As the embryo grows, ectodermal cells (of the embryo) expand down around the yolk to cover it so that it appears as if the yolk is "growing" out of the embryo.
The yolk itself is comprised mostly of a protein called vitellogenin and has quite a bit of phospholipid content as well. Yolk protein is made in the mother and transported into the developing oocyte as it grows.
Interestingly, if we look at how mammals develop, it's not all that different from birds and reptiles in many ways! Even though mammalian embryos obtain nutrients directly from the mother instead of from yolk, the structures used by mammalian embryos to do so are very reminiscent of the structures used by a chick embryo to "attach to" the yolk. And if you look at a mammalian embryo, it has the same general "shape" as a bird or reptilian embryo in early development -- it "appears" as if it is sitting on aball of yolk! And a structure even forms called the "yolk sac."
You can learn more about this as well as view movies and animated diagrams by linking through the website for the Society for Developmental Biology at http://sdb.bio.purdue.edu
Well, the yolk sac is edodermal, in fact it stays connected to the gut via the yolk duct. The yolk itself isn't cellular, though. The yolk is not formed by the chick embryo that uses the yolk. How do I know? The eggs I buy at the store all have yolks and I know that none of those eggs are fertilized. Since the 3 embryonic layers form after fertilization, I know that none of them gives rise to the yolk. The female that produces the egg must have made the yolk. So the yolk is not from endoderm, mesoderm, or ectoderm, it is made by an adult tissue, the ovary. Can you find out the origin of the ovary?
I figured that your next question would be how the female makes the egg. Well, all female birds and mammals make their egg cells before they are born or hatch. Obviously a chicken's thousands of eggs are not fully formed when she hatches! Unlike mammals, birds usually only have only one active ovary. An egg builds up the yolk while still inside the ovary, with the material coming from the mother via the blood vessels. The white and shell are added while the egg makes the trip from the ovary through the oviduct. A chicken can lay about one egg a day.
An unfertilized egg is a haploid cell. A fertilized egg is diploid. What do these terms mean?
While looking for more egg info I found out that a hen's diet determines how yellow the yolk is. That was news to me. According to the CMC food web site (http://www.cmcfood.com/facts.html) marigold petals are sometimes added to chicken feed to make yolks more yellow!
Thanks for asking, I thought about something I hadn't really considered much before and learned something new about chickens!
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