Question

1.   Describe the Steps in Grey crescent formation in Xenopus embryos (1pt)

2.   Xenopus Laevis embryos undergo Holoblastic mesolecithal radial cleavage. What does this mean in your own words? Describe some particulars( shape, size, etc) of how Xenopus embryos divide. (1pts)

3.   The Xenopus Gastrula is described as Triploblastic. what does this mean? (1pt)

4.   The _____________(blastocoel/archenteron) forms as the xenopus gastrula begins __________(involution/epiboly). The cells that involute in war become the __________ (mesoderm/endoderm) while the cells on the outside become ________(endoderm/ectoderm). (2pts)

5.   How can a scientist determine the “age” of a chick embryo during early development? (1 pt)

6.   What is different about the gallus gallus blastula when compared to the blastulas of the other organism we have looked at? Why must it develop this way? (1pt)


7.   Describe the cells and processes that are involved in the formation of the Primitive groove. Why is this process important? (2pts)




8.   What must occur for neurulation of chicken embryos? Why are chickens useful to study neurulation and organ development? (1pts)

Answer 1

1.In frog embryology, during embryonic development the egg will be converted into a tadpole containing millions of cells but containing the same amount of organic matter.The upper hemisphere of the egg known as animal pole is dark and the lower hemisphere the vegetal pole is light,when deposited in water and it's ready for fertilization the haploid egg is at metaphase of meiosis II, after the completion of meiosis II the cytoplasm of the egg rotates about 30 degrees relative to the poles and these is revealed by the appearance of a light coloured band the gray crescent.The gray crescent forms opposite the point where the sperm entered.

2.Fertilization in xenopus laevis leads to radical cytoplasmic movements that are essential for ensuring the cytoplasmic determinants are located in the correct positions relative to subsequent cleavage events . Mesolecithal means moderate amount of yolk, often unevenly distributed so many eggs are polarized with a yolk rich pole, termed the vegetal pole and a yolk poor pole termed animal pole so in xenopus laevis the zygotic nucleus is generally displaced towards the animal pole ,zygotes with relatively little yolk cleave holoblastically and this is known as mesolecithal holoblastically radical cleavage.

3. Triploblastic is the condition of the blastula in which there are three primary germ layers the ectoderm, mesoderm and endoderm.

4.Blastocoel, ectoderm.

5.The scientists can determine the age of chick embryo during early development through fertilized egg, because it can be easily maintained in humidified incubators and during early stages of development the embryo floats on to of the egg yolk that it is using for nutrition.

6.Gallus gallus is a major model system in embryology It was one of the first organism used for the developmental research in the nineteenth century because the egg could be opened and the development of the embryo inside could be seen without the use of a powerful microscope, the embryo large size and the ability to survive under surgical manipulation gave the chick an advantage over other model system.

7.The primitive streak is a transient structure whose formation, marks the starts of gastrulation the process in which the inner cell mass is converted into the trilaminar embryonic disc, which is comprised of three germ layers ectoderm, endoderm and mesoderm , the primitive streak originates from from the anterior epiblast and appears as an elongating groove known as primitive groove on the dorsal midsagittal surface of the epiblast along the anterior-posterior axis of the embryo.The rosto caudal and medial lateral axes of the embryo are defined as the primitive streak Formation of primitive streak is one of the key elements in the early development of embryos because the streak is the site where during gastrulation the mesendoderm cells ingress to take up their correct topographical positions in the embryo.Thats why this process is important.

8.Neurulation begins with the formation of a neural plate, a thickening of the ectoderm caused when cuboidal epithelial cells become columnar.Changes in cell shape and cell adhesion cause the edges of the plate fold and rise, meeting in the midline to form a tube.The cells at the tips of the neural folds come to lie between the neural tube and the overlying epidermis.These cells become the neural crest cells.Both epidermis and neural plate are capable of giving rise to neural crest cells The notochord is necessary in order to induce neural plate formation.It is useful in chickens because it creates the neural tube which gives rise to the central nervous system and it creates the neural crest which migrates away from the dorsal surface of the neural tube and gives rise to diverse set of cell type.