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Eggs
31Mar,2024
Eggs
Eggs are a symbol of life important to many during spring holidays. But what about the science of eggs?
You may have heard that an egg is a single cell. For instance, Alberts et al. (2002) assert that "The eggs of most animals are giant single cells" in their molecular biology textbook. This is because an egg is an oocyte (a.k.a. ovum, or egg cell). Oocytes are gametes - haploid cells that are activated when fertilized, and can then develop into a new organism. In mammals, our oocytes remain small. They're still huge when compared with somatic cells (0.1 mm in diameter versus 0.01 mm to 0.03 mm, respectively), but of course nowhere near the size of, say, a chicken egg. It is easier to imagine a frog or fish egg (pictured) being a single cell than it is a bird egg - the nucleus seems analogous to a yolk, the cytoplasm to the egg white, and the cell membrane to the outer membrane or shell. However, in truth it is only the yolk that is the oocyte - everything else is extracellular material (Gilbert, 2000). Therefore, the largest vertebrate cell is the yolk of an ostrich egg! The nucleus of that oocyte can be found at the germinal disc, sometimes called a blood spot (only visible in fertilized eggs). To be clear, the occasional speck of blood seen in egg whites is different from the blood spot present in fertilized eggs (store-bought eggs are not fertilized).
Okay, so eggs are big oocytes. But shouldn't a cell be circular? Why are so many bird eggs tapered ovals?
Spheres are indeed the strongest three-dimensional shapes. And many cells, and many eggs (i.e., fish and frog eggs) are indeed spherical. However, bird species with more spherical eggs take a huge risk: rolling. A more elliptical shape allows the egg to retain some strength without rolling away like a ball - instead, most eggs roll in a circle, ultimately staying safely in place (especially when held in place by clutch-mates). The shape also allows eggs to share heat with one another, retaining heat. There are, however, other variables to egg shape, as evidenced by the wide range of sizes, colors, and shapes seen in bird eggs. Some evidence has suggested that the spherical versus elliptical shape of eggs is influenced by how strong a flier the bird species is (Crespi & You, 2017; Stoddard et al., 2017). Additional variables include clutch size, calcium conversation, and nesting preferences (rolling risk).
Happy Easter to those celebrating!
Photograph credit:
Sac fry and unhatched rainbow trout eggs: Sam Stukel, USFWS, Public Domain.
References:
Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K., & Walter, P. (2002). Molecular biology of the cell (4th edition). Garland Science. https://www.ncbi.nlm.nih.gov/books/NBK26842/
Crespi, S. & You, J. (2017, June 22). Cracking the mystery of egg shape. Science. https://vis.sciencemag.org/eggs/
Gilbert, S. F. (2000). Developmental biology (6th edition). Sinauer Associates. https://www.ncbi.nlm.nih.gov/books/NBK10070/
Stoddard, M. C., Yong, E H., Akkaynak, D., Sheard, C., Tobias, J. A., & Mahadevan, L. (2017). Avian egg shape: Form, function, and evolution. Science, 356(6344), 1249-1254. DOI: 10.1126/science.aaj1945
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