Monday, April 4, 2011
EGG
On page 98 what does Neil Shubin mean when he states, "Just as with heads and limbs, our history is written within our development from egg to adult?" On the topic of embryology, What is embryology? And list the ways fish, amphibians and chicken embryos are alike? -Eryk Fundakowski: arthur2446@comcast.net
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By saying “just as with heads and limbs, our history is written with our development from egg to adult”, Shubin means that our body plans are explained when compared to other animals in similar development stages. He notes that it is similar with heads and limbs because those were also explained previously in the book when compared to other animals. He continues on to state that “all organs in the chicken can be traced to one of three layers of tissue in the developing embryo” (Shubin 99). This helps explain the human body plan (and the body plans of many other organisms) because humans also have the same 3 germ layers-endoderm, ectoderm, and mesoderm- in the embryo after gastrulation. Differentiations in the embryos start developing during this stage, but similarities between the animals help show that evolution from previous animals actually did occur. Shubin justifies our “history” with these connections to species that appeared earlier than us, and he implies that these similar traits must have been passed down through evolution.
ReplyDeleteEmbryology is the study of the development of an embryo from fertilization to the fetus stage. Fish and amphibians both perform external fertilization and external development because the males of both can have their sperm swim in the water to fertilize. On the other hand, chickens must perform internal fertilization because the sperm of the male cannot survive and reach the eggs in open air; however, the female lays eggs, and the embryo develops externally. Also, the aquatic eggs of many fish and amphibians do not require a large amount of yolk like the eggs of chickens because they can obtain nutrients through the water. Because fish and chicken have their yolk concentrated on one side of the egg, they “utilize a form of meroblastic cleavage called discoidal cleavage” (http://www.sbs.utexas.edu/shankland/BIO349/lc05clvg.htm). In contrast, frogs perform holoblastic cleavage. In holoblastic cleavage, the cleavage furrow passes all the way through the cells to divide the embryo completely. In meroblastic cleavage, the cleavage furrow cannot pass through the yolk, and the yolk-rich egg is divided incompletely.
Sources:
Campbell
Shubin
http://www.sbs.utexas.edu/shankland/BIO349/lc05clvg.htm
Austin Lee (austinklee7@gmail.com)
From Shubin's line on page 98, I agree with Austin's interpretation that development from egg to adult has become more intricate after evolution, but the development is still based on the same basic structures. The similar traits that have been passed down in evolution are the germ layers. While humans are triploblastic with an ectoderm, mesoderm, and endoderm, other species are diploblastic, or even have only one germ layer. Animals didn’t suddenly develop with three germ layers, which meant that over time, species became more advanced, allowing for more germ layers with environmental pressures for survival. The first two layers to develop were the ectoderm and endoderm. The ectoderm mainly turns into outer lining of either the body or other organs such as the skin or the enamel on teeth. The endoderm becomes the lining of inner pathways of the body, such as the lungs or the digestive tract. The development of the mesoderm layer gave rise to muscle in animals for more powerful movement (http://medchrome.com/basic-science/anatomy/structures-derived-from-3-germ-layers-embryology/). This gives an advantage to animals with the mesoderm over diploblastic animals that don’t have the strength to escape their triploblastic predators. The relationship between the developing germ layers of animals shows the human connection to the most primitive species, like sponges that only have one germ layer.
ReplyDeleteAccording to News Medical, “Embryology is a science which is about the development of an embryo from the fertilization of the ovum to the fetus stage” (http://www.news-medical.net/health/Embryology-What-is-Embryology.aspx). In addition to Austin’s statement about internal fertilization in many amphibians and all chickens while most fish externally fertilize, amphibians show the link between water animals and terrestrial animals. When looking at the “history…within our development from egg to adult” (Shubin 98), we see that amphibians’ abilities to live both on land and in water starts the change from external fertilization into internal fertilization. Sperm have not yet evolved to lack flagella, and therefore are dependent on an environment of liquid for mobility to fertilize the egg. It is easy for fish to externally fertilize egg because their surroundings are only water, but land animals must rely on internal body fluids to transport the sperm to the eggs. Species of land amphibians would eventually evolve into reptiles and then chickens, which rely on the internal fertilization brought about by amphibians to reproduce.
To elaborate further on the process of cleavage before gastrulation, holoblastic and meroblastic cleavage depend entirely on the environment of the animal as well (Campbell 1027). Like Austin said, the cleavage furrow will completely pass through the blastula due to the lack of yolk. Fish eggs will have absolutely no yolk, while amphibian eggs contain a small amount. The membrane of these eggs of water creatures are much more permeable than the eggs of terrestrial species so that nutrients can diffuse right into the egg to provide for the developing zygote. Nutrients are not found in the air for land animals, and therefore, animals like chickens will produce eggs with a hard outer shell. Before the egg is exposed outside of the chicken, the blastula will go through meroblastic cleavage, in which the egg partially divides. Too much yolk in chicken eggs prevents complete division, but this amount of yolk is necessary for the survival of the egg. Once the egg lies outside of the chicken body, no more nutrients can be given to the zygote other than the nutrients in the yolk. Fortunately, the yolk contains enough nutrients to ensure the survival of the offspring, and will remain inside the egg as long as the hard outer shell does not break.
Claire Yao (claire.yao521@gmail.com)
Although Austin and Claire make a noteworthy point in regards to Shubin’s statement, "Just as with heads and limbs, our history is written within our development from egg to adult?” it may really be directed at transitioning his topic of discussion from the comparison of our “basic design in really primitive animals” to embryology (Shubin, 97). The entire idea of the statement is simply to relay the idea that our development provides as much historical insight to our body plans as our heads and limbs do. The majority of the book has been dedicated to understanding the evolution of these parts, the head and limbs, to discover the body plan we host today. Finding connectivity or similarity between organisms with similarly structured parts allows for the discovery of this evolution, as Austin had explained. In the Chapter Getting Ahead, Shubin says, “Our head may look incredibly complicated, but it is built from a simple and elegant blueprint. There is a pattern common to every skull on earth, whether it belongs to a shark, a bony fish, a salamander, or a human” (93). Shubin here, as he does throughout the book explains that looking for these commonalities increases the historic and evolutionary knowledge that we have on our body plans. This same process is used in embryology where commonalities are found between organisms to discover the historical background of the organism.
ReplyDeleteEmbryology is the study of the development of the embryo. An embryo is the product of a fertilized egg, from the time of a zygote to the fetal stage (“Embryo”, Harvard Stem Cell Institute). Fish and amphibians, as Austin said, both perform external fertilization because their sperm can swim through the water to fertilize. However, chickens must perform internal fertilization because they do not live in water and therefore, need to fertilize internally. Chickens like fish perform meroblastic cleavage because, like fish, they have lots of yolk. Amphibians, however, perform holoblastic cleavage because they have less yolk. This type of cleavage has an animal pole and a vegetal pole. Gastrulation in amphibians is processed by involution and chickens perform gastrulation by a primitive streak (Campbell, Ch. 47). The most common theme that these embryo share fall under their type of fertilization. Shubin generally describes their physical characteristics as similar, and they all had a head with gill arches and brains (99). In this way, they had some uniting characteristics.
Sonia Doshi (soniadoshi7@gmail.com)
I agree with the three posts above in regards to their interpretation of Subin's quote. To fully understand our history through evolution, we have to view the differences and similarities in the way different embryos develop. There are many different ways embryos develop, some are triploblastic, diploblastic, and monoblastic. The three 'derms' are ectoderm, endoderm, and mesoderm. "Ectoderm differentiates to give rise to many important tissues and structures including the outer layer of the skin and its appendages (the sweat glands, hair, and nails), the teeth, the lens of the eye, parts of the inner ear, the nerves, brain, and spinal cord." (http://www.medterms.com/script/main/art.asp?articlekey=20835) The endoderm differentiates into either the digestive tracts or into the lungs. Mesoderm turns into "a wide variety of tissues including bone, muscle, urogenital organs (oviducts, uterus, epididymis), circulatory system (blood and vessels),
ReplyDeleteconnective tissue, kidney, and heart."(http://www.scribd.com/doc/6005126/Endoderm-and-Mesoderm-Development)
According to http://medical-dictionary.thefreedictionary.com, embryology is defined as "The branch of biology that deals with the formation, early growth, and development of living organisms."
What I found intriguing was that through stem cell research, "some cells within ectodermal structures retain their ability to differentiate into other tissues. For example, some cells in brain (ectoderm) can become bone marrow (mesoderm)." )http://www.medterms.com/script/main/art.asp?articlekey=20835) This is important because we were use to thinking that the three germ layers were independent of each other and they created distinct body parts (as I've listed above). What this research shows is that there is much more to embryos than we think and there must be more research done before we can say we understand the process.
Embryology is “the science dealing with the formation, development, structure, and functional activities of embryos” (dictionary.reference.com). Once an egg is fertilized, the rapid succession of division ensues. An important idea to note is that this division is not mitosis, but simply the cleavage of one cell. The division occurs so rapidly that the cell skips the G1 and G2 phases of mitosis, and therefore there is no significant enlargement of the embryo during this period. As the embryo undergoes further cleavage divisions, the fertilized egg is now called a multi-cellular ball known as a blastula, with a blastocoel (a large cavity) in the center.
ReplyDeleteDuring cleavage, the establishment of the three body axes occurs. The anterior-posterior axis is largely determined by the animal-vegetal pole, and its future dorsal side is determined by the gray crescent, which appears after the cortical rotation. An important distinction between humans, frogs, and chickens, is that they cleave differently. The factor that influences how an embryo cleaves is the yolk content of the egg. Chickens have an abundant volume of yolk (stored nutrients) in its egg. This makes the formation of cleavage furrows difficult in areas with yolk, causing solely areas lacking yolk to undergo cleavage. This incomplete division of the egg, meroblastic cleavage, is largely found in organism with rich yolk content in its eggs. On the other hand, organisms without as much yolk undergo a complete division. Known as holoblastic cleavage, humans, frogs, and other animals with low yolk content can have the cleavage furrow pass through all of its cells. The reason why various animals have different volumes of yolk in its egg is largely dependent on its source of nutrition. Since frogs have a reliable source of nutrients during its embryonic stage, they do not require as much yolk as chickens, which must endure its embryonic stage of development on land. A key idea to note is that humans, although terrestrial, undergo holoblastic cleavage. Because the mother can supply the growing embryo with nutrients through its placenta, it does not require as much yolk.
After cleavage, gastrulation occurs. During gastrulation, the embryo rearranges its cells to start the formation of future tissues and organs. Under gastrulation, three germ layers are produced: ectoderm, endoderm, and the mesoderm. The ectoderm forms the outer layer and the future nervous system, the endoderm lines the digestive tract, and the mesoderm forms the future notochord. In Chapter 6, Shubin describes his first encounter with the three germ layers: “In seeing these embryos, I was seeing a common architecture. The species ended up looking different, but they started from from a generally similar place” (Shubin, 99). What Shubin saw were the three germ layers, which eventually rearranged themselves to provide the basic structure of the organism. Although all organisms with true tissues have germ layers, they gastrulate in various ways. In sea urchins, the cells at the vegetal pole invaginate inwards the cell, forming the archenteron. The opening where the archenteron is formed is known as the blastopore, and will become the future anus of the organism. The invagination contines, until a digestive tube is created. An important concept to remember is that an organism that forms the mouth first is known as a protostome (proto meaning first), while an organism that forms the mouth second is known as a deuterostome (deutero meaning second). In frogs, the process is quite similar, but there are several differences. First, after the invagination of the blastula, a crease along the grey crescent forms. This crease is known as a dorsal lip, and will form the future dorsal side of the organism. As the archenteron forms, the blastocoel slowly shrinks, a major difference between gastrulation of sea urchins and frogs. Another difference that makes frogs unique is that the future mesoderm and ectoderm “roll over the edge of the lip into the interior of the embryo, a process called involution” (1030). In chicks, a primitive streak is formed instead of a blastopore. Although similar, the two structures are oriented differently, and will result in a different rearrangement of cells as compared to fishes and sea urchins. Remember, humans cleave like frogs and gastrulate like chickens. Therefore, we undergo holoblastic cleavage and form a primitive streak.
ReplyDeleteOther factors can alter the way an embryo develops. For example, the Sonic Hedgehog was proven to play a major role in the process of vertebrate organogenesis by controlling cell division in certain areas of the body (http://en.wikipedia.org/wiki/Sonic_hedgehog). In another study, a receptor known as EphrinB/EphB was known to signal adhesion and detachment between germ layers (http://www.plosbiology.org/article/info%3Adoi%2F10.1371%2Fjournal.pbio.1000597). We often see the small ideas, but forget that the entire process of development is made possible by the interaction between various proteins, hormones, and cells. If a single gene were to be missing or mutated, the development of the entire body will be affected. Although they are small, they hold a significant impact on the future of the embryo.
While I agree with the points that Austin and Sonia make, I would also like to mention an additional idea of what Shubin means he writes, “our history is written within our development from egg to adult”. I believe Shubin is trying to emphasize the idea of how the short period of time from the fertilization of an egg to the birth of a child holds a significant impact on the structure of the organism. As I explained earlier, all organisms cleave and gastrulate, but undergo these processes in different ways. Therefore, by examining these various factors and steps that have led to the unique formation of the organism, we can truly understand the history of its origination.
ReplyDeleteWorks Consulted:
Campbell Biology
Your Inner Fish
http://dictionary.reference.com/browse/embryology
http://dictionary.reference.com/browse/proto
http://dictionary.reference.com/browse/deutero
http://en.wikipedia.org/wiki/Sonic_hedgehog
http://www.plosbiology.org/article/info%3Adoi%2F10.1371%2Fjournal.pbio.1000597
(Keigo Tanaka; tanakarus3@hotmail.com)