MadSci Network: Development |
Hi Michael! Thanks for your question! You've touched on a very interesting point that may not be "obvious" at all. As you probably already know, the egg, or ovum, is a specialized cell (called a gamete) which comprises the maternal contribution to her offspring. This cell not only contains the necessary genetic information within its nucleus, in the form of DNA (deoxyribonucleic acid), it also houses many vital factors in its cytoplasm. One of these important factors is yolk; the yolk is composed mainly of proteins that the developing embryo will use as a source of nourishment. The main reason for such a requirement is that an embryo is not able to feed itself. And, after all, even the youngest of embryos need food! ;-) So, why are human and chick eggs so different in size? Well, the answer is in the yolk. Mammals, including humans and mice, have very little yolk in their eggs, while reptile and bird eggs have much, much more. Incidentally, amphibians, such as frogs, and fish lie somewhere in the middle-their eggs have moderate amounts of yolk. Just in case you are interested in the nitty-gritty numbers, human and mouse eggs are approximately 0.1mm in diameter and can only be seen under a microscope. Frog eggs, on the other hand, are about ten times that size (1mm in diameter) and can be seen with the naked eye. And finally, chick eggs are approximately 3cm (or 30mm) in diameter-that is 30 times the size of a frog egg, and 300 times the size of a human egg! Okay, so you may be wondering why such differences in the amount of yolk exist. What we need to think about now is how humans, frogs, and chicks develop and what the important differences are. Mammalian embryos develop within the mother in the uterus, using a specialized structure called the placenta. The placenta allows the mother to provide nutrition to the embryo. Since the mother can supply nutrients with the help of the placenta, there is really not much need for vast amounts of yolk in the egg. Chickens have an interesting course of development. The egg is fertilized within the mother and shortly after, is packaged within a shell and then laid. From this point on, the chick embryo is an independent, self-contained system. It must develop on its own without the help of a placenta. And, to make things even more difficult, it takes almost three weeks for the chicken to be mature enough to hatch from the egg and find food on its own. Therefore, chick embryos need a lot of yolk! Frogs also develop outside of the mother and thus, do not have the benefits of a placenta. However, they do have an advantage over chickens. These animals go through an intermediate stage before they become mature frogs, known as the larval (or tadpole) stage. As tadpoles, they can swim around independently and find food on their own. So, amphibians need only moderate amounts of yolk to nourish them during the period between fertilization and the tadpole stage. To summarize, we can think of the amount of yolk in the egg (and by extension, egg size) as a means of providing nutrition for the embryo. However, there are other ways of providing food - mammals are able to nourish embryos using a placenta, and therefore, don't need much yolk in their eggs. Without a placenta, embryos depend on yolk to nourish them until they are able to forage for themselves. The period of time for which embryos rely on yolk as a food source varies for different animals. For this reason, the eggs of different animals will have different amounts of yolk according to this need, and therefore, will vary in size. I hope this answers your question! If you need any clarification, please feel free to email me! (nmdavis@fas.harvard.edu) Here are some references you might find useful: 1. Embryos: A Color Atlas of Development; Bard, J. (editor); Wolfe Publishing 1994 ** This book has some gorgeous photographs of a variety of embryos-sea urchins, fruit flies, chicks, mice and more! It also has information on gametes (egg and sperm), fertilization, and early development for various organisms. 2. Developmental Biology, 5th edition; Gilbert, S.; Sinauer Associates, Inc. 1997 This book is a standard developmental biology text that is relatively up-to-date and comprehensive. Excerpts from this text have been put on the Web, along with some images and other developmental biology links (http://zygote.swarthmore.edu/index.html). -Nikki
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