What the Heck is an Egg Yolk?

A question from Alexis D'Angelo, who is a 7th-grader in Ms. Arlene Hicks' class at Pierce Middle School, Grosse Pointe, Michigan. Alexis' original question concerned the fact that she had heard that the yolk of an egg was a cell - if so, where was the nucleus - where were the ribosomes?
The egg of a bird is what we are talking about here. But, the female of every living thing that has females and males, makes eggs or something very much like an egg.

In the case of animals, the egg consists of a single ovum (cell) which contains one-half of the normal genetic material - one-half of the chromosomes. For human females after puberty, about every 28 days one egg is released from one of the two ovaries. Usually, alternate ovaries are used every month. As the egg travels down the fallopian tube, if it does not interact with a sperm, it will eventually be shed along with the lining prepared in the uterus each month. This lining has a very good blood supply and would provide the nutrients necessary for the development of the embryo if the egg were fertilized. If the egg is not fertilized, this shedding of the egg and lining causes little blood vessels to open, which results in bleeding until the blood vessels heal (usually very rapidly).

If the egg encounters a sperm within the fallopian tube, the egg may become fertilized by this sperm cell (which also contains one-half of the normal chromosomes). These chromosomes are introduced into the cytoplasm of the egg cell, and the nucleus of the sperm fuses with the nucleus of the egg and now there is a new nucleus which contains the complete number of chromosomes for a normal cell for that organism, and the development of the embryo. The egg continues its travel to the uterus, where it becomes implanted within the uterine wall (the lining I talked about) and begins to receive nourishment from the bloodstream of the mother. This fertilization process and implantation of the fertilized egg within the uterus initiates a complex series of reactions which leads to cell division - from one, to two, to four, to eight, etc., and to a complex series of reactions which leads to differentiation (development) of these cells along a specific time-line (temporal) which ultimately leads to individual organ development and finally the birth of a complete, living organism. Astonishing!

In the case of birds, this ovum (egg) is certainly different-looking. The female chicken produces a _single cell_ (has a nucleus) with one-half the chromosomes inside, enclosed within a membrane "bag" filled with what we call "yolk" (nutrients, food). On the outside of this yolk sac is a bunch of protein called albumin, then a thin membrane completely surrounding the albumin, and finally the hard shell of the egg (mostly calcium). So, this egg is one, single, entire cell - pretty large, huh? If you look inside an egg, you will see two little twisted chord-like things at each end of the yolk sac... these little connections, each called a "chalazas" connect the yolk sac to the membrane which lies just beneath the shell. These are the little white "things" you see when you break an egg into a bowl... the ones which are really hard to get out of there... <grin>

The chicken releases this egg inside its body, similar to how humans release an egg... but - this egg is deposited on the outside of the chicken whether or not it is fertilized. If it is fertilized by a sperm from a rooster, the nucleus of the sperm will fuse with the nucleus of inside the egg to form a new nucleus which now contains all of the information for development of a chicken. The formation of the hard shell will occur - again, whether or not the egg is fertilized - takes time to form the shell, so the sperm has to fertilize the egg before the shell completely forms. After the egg is deposited on the outside of the chicken, with warmth (chicken sitting on it) the fertilized cell within the egg continues to divide and a baby chicken will grow inside, using the nutrients which are coming from the yolk. The nutrients will be used until finally, the baby chicken will start to peck (an automatic differentiation response - kind of like breathing when a human baby is born) and this pecking will break the shell, and the baby chicken will crawl out.... snakes do this too, and fish do this.. although of course, neither of these "peck" anything... but - they do force their way out of the egg. Snake and fish egg coverings are soft - don't need a sharp beak to break out... Actually, when little snakes are born, most of the leathery egg covering is gone. Other reptiles (like turtles) lay complete eggs, and the little turtles have to work their way out.

Book: Don't Touch That Doorknob!

Copyright John C. Brown, 1995

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