-Michelle Layvant, mlayvan2@students.d125.org
Saturday, April 2, 2011
Germ Layers
In chapter six, Shubin talks about developing embryos and their body plans. When Christian Pander looked at embryos he found that all organs in a chicken can be traced to one of three layers of tissue in the developing embryo -- germ layers. The endoderm, ectoderm, and mesoderm all have a specific role in embryogenesis in each animal. Use this time to review and describe what function each of the three layers plays, specifically in a human being. Also, go through each stage starting from the blastocyst stage where there is no body plan up until the embryo has completely formed. How does each layer know what to do/each cell know where to go/what to form? (Discuss the role/function of the Organizer.)
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Generally, the ectoderm forms the outermost parts of the body, the endoderm forms the structures of the respiratory and digestive systems, and the mesoderm forms everything in between the parts of the endoderm and ectoderm. Specifically, the ectoderm forms the epidermis of the skin, the epithelial lining of the mouth and anus, the parts of the eye, the nervous system, the epidermis’s sensory receptors, the adrenal medulla, tooth enamel, and the epithelium of the pineal and pituitary glands. The mesoderm forms the notochord, the skeletal system, the muscles of the body, the excretory system, the Circulatory/lymphatic systems, the reproductive system, the dermis of skin, the body cavity lining, and the adrenal cortex. The endoderm forms the epithelial lining of the digestive tract and the respiratory system, the lining of the urethra/urinary bladder/reproductive system, the liver, the pancreas, the thymus, and the thyroid and parathyroid glands.
ReplyDeleteAround 5 days after fertilization in a human female, a ball of 16 cells is developed called the blastocyst. The blastocyst is the mammalian version of the blastula and has no appearance of a body or resemblance to an animal. The ball of cells is only called a blastocyst after the blastocoel, a fluid-filled cavity, forms. The blastocyst then slows its rate of cell division and starts gastrulation, when groups of cells move to form a three-layered gastrula. These three layers are the ectoderm, mesoderm, and endoderm, whose functions are described above. These layers grow to form organs and a recognizable human body.
The Organizer region is DNA that directs the cells of each of the layers in building the complete body. Organizers are found in all animals with bodies, and can even be swapped between very different species. The Organizer is a portion of our DNA that we share with all of the bodied animals around us, showing that it has been passed down throughout evolutionary history.
-Austin Lee (austinklee7@gmail.com)
Sources:
ReplyDeleteCampbell
Shubin
http://en.wikipedia.org/wiki/Human_embryogenesis#Blastocyst_differentiation
Like Austin said, the ectoderm will eventually become the outer lining of the body or body systems. The main future of the ectoderm is to become the animal’s skin, but the ectoderm also provides many other functions. In addition to becoming the tooth enamel and epithelial lining of the mouth and anus, the ectoderm also becomes the nervous tissue in the central nervous system (http://www.britannica.com/EBchecked/topic/178675/ectoderm). Differentiation of ectoderm cells becomes the neural plate, which further folds into the neural crest cells and neural tube (http://www.benbest.com/science/anatmind/anatmd4.html). The neural tube is what eventually becomes the central nervous system, but it was developed from the ectoderm.
ReplyDeleteThe mesoderm generally becomes any tissues or organs that make up body systems between the ectoderm and endoderm. Muscle tissue or the skeleton of an organism is formed by the mesoderm, as well as the circulatory system and excretory system (http://www.luc.edu/faculty/wwasser/dev/layer.htm). The mesoderm is the third germ layer that appears in triploblastic organisms only. This germ layer signifies that the species is more developed than its diploblastic counterparts because they have the advantage of faster and stronger movement by their muscles and capacity to quickly circulate oxygen to different parts of the body.
The endoderm, like Austin said, mainly forms the archenteron, which will eventually become the lining of the digestive tract (Campbell 1028). Because the most primitive species only had an ectoderm, the development of the endoderm supports the need for a more developed digestive tract in order for organisms to adapt to having larger bodies. With larger bodies, they have the advantage of eating organisms smaller than themselves, and using the nutrients from digested food to keep up their strength. Because the endoderm also lines the respiratory tract, the endoderm shows how animals with bigger bodies needed a more developed respiratory system to transport oxygen throughout the body. The development of the endoderm and the mesoderm became more detailed as animals needed these layers to support the large bodies that they would develop into.
As Austin had previously posted, a blastula forms after the fertilization of the female egg by the male’s sperm. The blastula either goes through holoblastic cleavage or meroblastic cleavage, depending on how much yolk is present in the egg. If barely any yolk is present, the cleavage furrow will pass completely through the egg in holoblastic cleavage. If more yolk is present, the egg will go through an incomplete division known as meroblastic cleavage. After this process, gastrulation occurs, in which the three germ layers are produced. The Organizer is a patch of tissue that contains the coding to determine the fate of the germ layers (Shubin 106). The Organizer makes sure that there is no error in the transcription of DNA to cause the germ layers to develop in the wrong place of the body. From here, the germ layers begin to develop into the structures of the body.
Claire Yao (claire.yao521@gmail.com)