Mechanisms of Epiboly of ectoderm in the Xenopus Laevis embryo
Introduction
Epiboly is a movement of gastrulation in the amphibian embryo, whereby ectodermal precursors expand to cover the entire embryo. This process occurs in the surface and deep layer cells in the animal and marginal regions. Three rounds of cell division occur in the deep cells, while they also rearrange to form fewer layers. Superficial cells elongate by cell division while flattening, which gives them greater surface area and thinner depth. The ectoderm eventually covers the entire embryo, internalizing the endoderm. This process sets up the correct position for the three germ layers, with the ectoderm on the outside, mesoderm in the middle and endoderm on the inside.
Keller (1980) found that superficial cells spread, divide, and undergo rearrangements and a temporary change in shape, which produces an increase in area. The deep cells become thinner and decrease in the number of layers. They do this by a process called radial interdigitation. Radial interdigitation is when the deep cells elongate, extend protrusions between one another along radii of the embryo and interdigitate to form fewer layers with greater area. Once this process is complete, the deep region consists of one layer of columnar cells, which flatten and spread to further increase area. In the dorsal marginal zone the cells also undergo a shape change, which is not seen in the cells of the animal region. The difference may be due to the uniform spreading in the animal region contrasted with extension and convergence that occurs in the dorsal marginal zone.
In his work on time-lapse films of exogastrulae, Keller (1980) found that the ectoderm becomes corrugated by rapid constrictions of the apices of superficial cells and by the appearance of holes in the epithelium. From this, he suggested that shrinkage, rather than expansion aids in epiboly of the ectoderm. He presents a model (see Figure 1) in which the superficial layer is under tension and the force for expansion must come from the deep cells. The expansion of the deep region is resisted by the tension in the superficial layer resulting in an outward curling of the bilayer (deep and superficial layers).
An alternative model (see Figure 2) is also proposed, in which the superficial epithelium is stretched by tension at the margin of the blastopore, which initiates the superficial cells to spread passively (Keller, 1980). While the deep cells rearrange themselves to occupy the areas now available that were once occupied by the superficial cells.
In the mid 1960's, R. G. Edwards and colleagues at Cambridge University began studying differentiation of rabbit embryonic cells in an artificial environment. They manipulated these embryonic cells into specific types of form such as connective tissue and muscle neurons.
Simple epithelia: Epithelia tissue can have cell shapes these are the Columnar, Cuboidal, and Squamous Cell Shapes. All this cells shapes are part of a type of an epithelia tissue which is the simple Epithelia. There are columnar cells, which means column-like cells and squamous cells, which are flattened and scale-like cells, simple squamous epithelia is found in walls of lung alveoli, blood capillaries and bowman’s
Late one night a woman is driving home on the freeway, she’s hit head on by a drunk driver and killed. The man is charged with two accounts of murder; the woman, and her four-week-old embryo inside her. By law, everyone human being is guaranteed rights of life; born or unborn they are equal. The same law should be enforced concerning human embryonic stem cell research. Dr. James A. Thomson discovered stem cells in 1998 and they’ve intrigued scientist ever since. The stem cells themselves are derived from a three to four day old cluster of cells called a blastocyst and they are so coveted because they are pluripotent, meaning they can differentiate into any type of cell in the human body. Although embryonic stem cells show amazing potential to cure various disease such as cancer, congestive heart failure, Alzheimer’s and Parkinson’s disease, muscular dystrophies, and more. The methods by which they are obtained is controversial. Research on embryonic stem cells is unethical, unnecessary, and purely homicide.
Cinderella is just one example of how Disney had to change parts of the source material to make it appropriate for their audience. The Grimm Brothers’ Cinderella is an example of a piece of writing that teaches their lesson in a violent way. In their version of the story Cinderella’s sisters amplified the trait of jealousy. To emphasize jealousy the stepsisters, “poured peas and lentils into the ashes of the hearth so she had to sit there the entire day and separate them. In the evening, when she was tired, there was no bed for her, and she had to lie next to the hearth in the ashes” (Grimm Brothers page 70). The audience would interpret these actions as wrong and understood the Grimm Brothers more drastic style of writing. The audience learned from what the Grimm Brothers wrote and were taught that there was consequences jealousy because of the harsh punishments that the Grimm Brothers described. Since the audience lived in a harsher time, living was not luxurious. As a
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Stem cell research is on the forefront of regenerative medicine and biological science. It is the study of certain cells in the inner mass of the embryo that are produced a few days after the embryo forms during the blastocyst stage. They are the most primitive of all human cells. They are undifferentiated cells, which mean the cells are not designated to be any special type of cell, such as a nerve, muscle, or skin cell. The cell's specialization is later influenced by the molecules, which are usually proteins that surround the cell (Marshak 220-223). The proteins are typically produced by the mother, but under certain laboratory conditions, distinctive proteins can be introduced and a definite, mature cell type is produced. The cells that are produced could be implanted into a subject to replace worn out cells, or cells that have been destroyed due to disease or injury.
Even though the time periods are very different (by 200 years) the formulas for their fairy tales seems to remain constant. Character development, which is very important in fairy tales is both well done and accurately portrays the living situation for a character in the time period of when it was written. Perrault's version seems to put Cinderella's family in a higher, well-off situation of the Grimm's because she is still abided to obey the rules that her dying mother had set for her. Something that you would see a women do in the late 1600's. Her higher class and the rules of her generation has set her to not have revenge on her step-sisters and helps them marry in the end, making a happy ending to the story for everyone. This also gives off the rules of the time to the young girls who would be listening or reading this story back then. They knew their place in society and tales like Perrault's reinforced it. The Grimm's version, titled Ashenputtle, has key elements in the story line that make it very different from Perrault's Cinderella. The theme becomes very different as the end of the tale results in revenge on the step-sisters from Ashenputtle. This variation in the story line represents the setting in which the Grimm's either lived in themselves, or the living situation of the people who related this tale to the Grimm's.
The cilia found in epithelial cells means that substances can pass easier and quicker. Squamous epithelial tissue is very thin and also very flat. It is usually one cell thick and is therefore in contact with the basement membrane. Because it is so thin, it is brilliant at diffusion and filtration, and it is also found in locations where these functions are essential. Squamous epithelial cells also have an egg-shaped nucleus. Columnar epithelial tissues are elongated and column-shaped. They occur in one or more layers. This type of epithelial tissue enables movement of molecules due to the simple thin structure and therefore allows easy exchange. The nucleus is also elongated and are normally located near the base of the cells. Columnar epithelium makes up the lining of the stomach and also the lining of the intestines. Some of which are specialised for sensory reception, for example the nose, ears and taste buds. Unicellular glands are found between the columnar epithelium and the duodenum. They secrete mucus, lubricating the surface of cells. Cuboidal epithelium tissues are a cuboidal shape, and each have a circular nucleus in the middle of them. This type of epithelium tissue is found in the
Fairytales, the short stories that most children heard as they went to bed, are actually folktales from previous decades. The fairytales today are primarily adaptations of older versions recreated by Disney— the pioneers of this generation. With that said, the modern versions consistently display good triumphing over evil, a prince charming that constantly came to the rescue, and a happily ever after ending. However, the original folktale version didn’t always come with fortunate events, but often were more violent and gruesome. With the fairytale Cinderella, Disney maintains a similar theme as its Grimm version; however, the conflicts, events, and characters that support this idea are rather different.
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