On page 102 of Subin's book, he talks about how a fertilized egg develops into a full grown animal. Using the text book, describe the different stages the cells go through until it latches on into the uterus. Include terminology of endoderm, ectoderm, and mesoderm and describe what each layer turns into.
Nikhil Pereira (nikhil.pereira3@gmail.com)
After an egg has been fertilized by the sperm, it becomes a zygote and beings to grow into the fetus. As the zygote is traveling down the fallopian tubes, it forms the blastocyst, a ball of cells with an outer shell. The cells towards the inside (inner cell mass) will eventually form the embryo, while the outer group of cells (trophoblast) later becomes the membranes that will nourish the embryo and shield it. The trophoblast also forms a primitive endoderm layer. On day 5, the blastocyst reaches the uterus and implants itself to the wall by extending fingerlike projections. Differentiation now begins as the cells begin to divide and specialize. The inner cell mass then differentiates into the upper layer, which is known as the epiblast, and the lower layer, which is known as the hypoblast. Cells from the epiblast become to move inward during early gastrulation to form mesoderm and endoderm. While this is occurring, the trophoblast and endometrial tissue around it begin to form the placenta, an organ that aids in gas and nutrient exchange to the embryo.
ReplyDeleteThe ectoderm is the outer most layer, and lines the amniotic cavity. As the cell continues to grow, this region will become the skin, brain, and nervous system.
The endoderm, or inner most layer, lines the yolk sac. This region will eventually form the lining of the gut and other internal organs.
The mesoderm, the middle germ layer, fills the space between the ectoderm and endoderm, eventually becoming the muscles, skeletal system, and circulatory system.
Sexual reproduction showcases the theme of evolution very well. With so many complex states, there is a lot of room for error in sexual reproduction. Asexual reproduction is more reliable than sexual reproduction because the steps are less complex, thus with a smaller margin of error. One benefit to sexual reproduction is that it allows a species to diversify and maintain advantageous elements, through the theory of evolution. Since asexual reproduction does not allow for any sort of variation, any bad qualities will be passed on from generation to generation. Sexual reproduction also allows for more continuity and change to occur. Since half of the embryo’s DNA comes from the mother and the other half comes from the father, there are many combinations of genes that could combine, ensuring that no individual is the same as another. But this also means that there are many mutations that could occur, which is what we see with the many developmental mutations that exist, such as holoprosencephaly, in which the embryo does not successfully develop into two hemispheres. This is the direct result of the Hox genes, also mentioned in Shubin’s book, not turning on genes in the middle of the head, allowing the left and right sides to merge into one.
Sources:
www.columbia.edu/itc/hs/medical/human
http://www.umm.edu/ency/article/002398.htm
http://www.ninds.nih.gov/disorders/holoprosencephaly/holoprosencephaly.htm
http://biology.kenyon.edu/courses/biol114/Chap14/Chapter_14.html
http://library.thinkquest.org/22016/contribute/asex_sex.htm
Campbell pg 1034
Anna Leng (annaissbananas@yahoo.com)
Once the sperm fertilizes the egg, the newly formed zygote goes through cleavage, in which the nucleus undergoes mitosis but the cytoplasm is merely split into two (not always equal) parts. Once this process is repeated 5-7 times, a blastula, which is a hollow, fluid filled ball of cells, is formed. Through al of this, exists the animal and vegetal pole and the gray crescent. The poles are created by an uneven distribution of cytoplasm in the cell. The vegetal pole (more cytoplasm) becomes the yolk while the animal pole creates the actual organism and the gray crescent creates the dorsal/ventral orientation. In humans, the vegetal pole is very small as most nutrition comes from our mothers. The vegetal pole later forms the inner cell mass which creates the extraembryonic membrane. During implantation, the trophoblast, the outer cells of the blastocyst, release enzymes to break down part of the uterine endometrium which allows the blastocyst to embed itself in the uterine wall. After implantation occurs, gastrulation occurs. This is the process in which the cells move under the primitive streak to for the endoderm, mesoderm, and ectoderm. These layers specialize into specific cells based on their location. The ectoderm forms the skin and nervous system; The mesoderm creates the muscular circulatory, excretory, reproductive, and lymphatic system, and the endoderm creates linings of the necessary systems created in the mesoderm as well as the liver, pancreas, thymus, and parathyroid gland. (Campbell 47.1)
ReplyDeleteThe time between fertilization and implantation only spans a week. However, it is during this time that an item necessary for one of the most controversial processes in biological studies is created. The blastocyst is fully created around day 5. It is during this time that the cell is filled with undifferentiated cells, or stem cells. After this point, the cells begin to differentiate and their scientific significance is lost. However, if the cells are collected on this day they can be used for research for countless processes in regenerative medicine (healing badly damaged organs.) For example, once a person has a heart attack, the dead muscle does not grow back, but becomes scar tissue and is no longer useful. Stem cell research is helping us find ways to replace that scar tissue with live cells and heal the heart, or grow another heart for the patient itself using these stem cells. This would be important because, although transplants are available, it is never fully guaranteed that the antigen markers on the cells will correctly match the body's antigen markers. If they are not a perfect match, the immune system's cytotoxic T-cells begin to destroy the organ, viewing it as the invader. This is what happens when the organ is rejected, giving no use to the recipient if his body will not accept the new heart. By growing the heart out of stem cells, we can make sure the cells match the recipients DNA, therefore, there is a 0% chance of organ rejection since the antigen markers are the exact same. The only problem with this method, is that research with stem cells requires aborting the embryo. For those who believe life starts at conception, to obtain a small amount of stem cells, you take a life. Thus, although the research would be helpful, many people are against the idea of SCR for this reason. http://stemcells.nih.gov/info/basics/basics1.asp)
(Campbell 42.2) (Campbell 948)
Just as Anna said, and as shown on page 102 of Shubin's Inner Fish, the fertilized egg divides into a blastocyst, where implantation will occur around 6 days after fertilization. The fertilized egg divides in a process called cleavage, where the cytoplasm of the large fertilized egg becomes smaller balstomeres (Campbell 1025). Eventually, the blastomeres will form a single layer of cells that surrounds a blastocoel. Building off of what Aparna said about the vegetal pole containing more yolk than the animal pole, this affects the cleavage of the blastula before gastrulation starts. Animals that live in water environments will contain minimal or no yolk at all because nutrients can diffuse from the water into the developing fetus. However, land animals do not have the benefits of diffusion from air, so more yolk is needed to provide the growing organism with sufficient nutrients. When eggs of water species don't contain much yolk, the cleavage furrow can pass completely through the egg in holoblastic cleavage. However, the eggs of land animals develop with much more yolk in the vegetal pole, so an incomplete division of the egg occurs in meroblastic cleavage (Campbell 1027). If the cleavage furrow were to pass completely through an egg containing a lot of yolk, the contents would spill out, leaving the organism with insufficient nutrients to survive. After cleavage is complete, the blastula attatches to the endometrium lining in the uterus to begin the process of gastrulation.
ReplyDeleteGastrulation is the process where the three germ layers will develop, and eventually become different systems and tissues in the body. Species that come earlier in evolutionary history might have had only two germ layers. These species lacked mesoderms and were called diploblastic. Because the mesoderm later becomes tissues in the skeletal system or muscle tissues, triploblastic animals have the advantage of advanced movement. As species of prey become faster, quicker species of predators will become faster as well in a cycle based off of the development of the mesoderm during gastrulation. The ecotderm and the endoderm are the two main germ layers found in both diploblastic and triploblastic species. Like Anna said, the ectoderm becomes the epidermis or epithelial lining of the mucous membrane. However, in more developed species, the ectoderm will also become the central nervous system. This also leads to cephalization, in which the brain developed to process information about the environment around the organism. A developed central nervous system allows for the advantage of better senses such as sight or smell to detect prey and escape predators. Also previously mentioned, the endoderm that lines the yolk sack will eventually line mainly the digestive system as well as the respiratory system. Having a developed digestive system allows the animal more variety in what can be digested. Also, the respiratory system works with the cardiac system (from the mesoderm) to transport oxygen throughout the body of land animals. These three germ layers are crucial to the developed stage of human bodies in order to survive on land.
(http://www.newworldencyclopedia.org/entry/Germ_layer)
Claire Yao (claire.yao521@gmail.com)
Fertilization usually happens in one of the fallopian tubes (in a human), as the sperm cell breaches the ovum and creates a zygote. Following fertilization is embryonic cleavage, when the zygote divides itself multiple times. The zygote eventually becomes a morula, which is essentially a ball of cells. After further cleavage, the cell becomes a hollow ball of cells called the blastula. In mammals, the blastula is often referred to as the blastocyst. The blastula’s inner space is called the blastocoel, and it usually forms around five days after fertilization in humans. Also, in avian, reptilian, and mammalian embryonic development, the primitive streak forms. The primitive streak creates bilateral symmetry and also marks the point of gastrulation. Following the blastula stage is the gastrula stage, which forms by gastrulation. Gastrulation occurs when the single-layered blastula forms the triple-layered gastrula, although less complex animals have only 2 layers. In fact, “Platyhelminthes and Nematodes are the first three phyla we have examined that have all three primary germ layers during development” (http://bioweb.wku.edu/courses/biol225/225lab3.html). These layers, the endoderm, mesoderm, and ectoderm, are the specialized germ layers that develop into the specific organs that run the daily processes of animals. Through evolution, the invertebrates of the Platyhelminthes and Nematoda origin passed down the trait of having three layers during development to successive animals. This explains why a great number of the animals on Earth undergo gastrulation and create three germ layers. Like Claire Yao said, triploblastic animals have a selective advantage in advanced movement due to the development of the skeletal system and muscle tissues.
ReplyDeleteThe ectoderm forms the epidermis of the skin, the epithelial lining of the mouth and anus, the parts of the eyes, the nervous system, the epidermis’s sensory receptors, the adrenal medulla, the epithelium of the pineal and pituitary glands, and tooth enamel. The mesoderm creates the notochord, the skeletal system, the muscular system, the excretory system, the circulatory and lymphatic systems, the reproductive system, the dermis of skin, the lining of body cavity, and the adrenal cortex. The endoderm forms the epithelial lining of the digestive tract and the respiratory system, the lining of the reproductive system, the liver, the pancreas, the thymus, and the thyroid and parathyroid glands. Generally, the ectoderm forms the more exterior parts of the body, the endoderm forms the most inner linings and organs of the body (especially the digestive and respiratory systems), and the mesoderm creates everything in between the two germ layers.
The germ layers (especially the mesoderm) create a number of systems in the human body that are extremely important to our wellbeing. The skeletal system gives us a body shape, the muscular system allows us to move, the digestive system allows us to intake and break down food efficiently, the reproductive system lets us create genetically similar offspring, and etc. It is through evolution that we gained these body functions(selective advantages), and they us get through each hour of our lives. Humans were not the first to have a skeletal system, a digestive system, reproductive system, or any other systems. We are simply the receivers of these evolutionary landmarks. One example is that organisms from phyla Nematoda and Platyhelminthes passed down the characteristic of having three germ layers, as explained earlier. Another example is that phylum Nematoda was also the first to have organisms with complete digestive systems.
Sources:
Your Inner Fish by Neil Shubin
Campbell Textbook
http://bioweb.wku.edu/courses/biol225/225lab3.html
http://www.ucmp.berkeley.edu/phyla/ecdysozoa/nematoda.html
http://faculty.clintoncc.suny.edu/faculty/michael.gregory/files/bio%20102/bio%20102%20lectures/animal%20diversity/lower%20invertebrates/sponges.htm
Austin Lee (austinklee7@gmail.com)