Ans1: Growth is defined as the increase in size and mass of a particular organism over a period of time. It is measured in terms of biomass (mass of the organic material). An organism grows by mitosis. Growth stops after certain age of the organism. Growth is quantitative and measured by taking the measurement of height, weight, size and shape of body organs like brain, etc .
Development is defined as the process of cells differentiation which helps in the development of the body. It adds complexities to the body (such as organ development). Development of the organism is a lifelong process. Development includes all traits of an organism (like emotions, cognitive skills). Development is quantitative and qualitative process (Cameron, 2008; Tomlin & Axelrod, 2007).
Ans2:- The yolk is made up of nutrient molecule. Yolk is denser and accumulates in the lower half of the egg due to gravity. This is called the vegetal hemisphere. The outermost layer of this cytoplasm (the cortex) is not pigmented.
The upper hemisphere is known as the animal hemisphere. The nucleus and polar bodies are located in this region. The outermost layer of this cytoplasm (the cortex) is heavily pigmented.
Ans3:- The two important stages in the frog development are as follows:
Gastrulation: The formation of gastrula takes place which forms a two-layered embryo with an opening called as blastopore.
Organogenesis: In this stage, cell differentiation takes place which forms three germ layers into tissues and organs (Busa & Nuccitelli, 1985; Moody & Kline, 1990).
Ans 4:- The sperm appears to be transported to the oviduct by the muscular activity of the uterus in the mammalian female reproductive tract. First of all, the fluidity of the sperm plasma membrane is altered by the removal of cholesterol by the albumin proteins. Some specific proteins and carbohydrates on the sperm surface are lost during capactiation. When sperm reaches to the oviduct then it becomes hyperactive due to the presence of some chemicals. It might be due to the presence of viscious fluid that sperm encounter in the oviduct. Soluble factors also help in the movement of the sperm. It is still a speculation that ovarian follicle secrets some chemostatic substances which attracts sperm (Hawk, 1983; Suarez & Pacey, 2006).
Ans 5: Sea urchins are used as a model in fertilization studies due to following reasons:
the availability of a large amount of homogenous material, which facilitates biochemical and molecular biological approaches,
the ease of gamete handling enables detailed analysis of the mechanism of fertilization, and
the transparency and synchrony of fertilized eggs facilitates investigations on cell division and cell cycle (McClay, 2011).
Ans 6: In chemotaxis, sperm are attracted toward eggs of their species by following a gradient of chemical secreted by the egg. In sea urchin (Arbacia punctulata), when Resact (chemotaxis) is released into the sea water containing sperm, the sperm swim in circle into the source of resact in the sea water. As time passes, more sperm recruited around the source of chemotaxis agent (Cameron, 2008; Gilbert, 2000, 2003; Mauch & Schoenwolf, 2001).
Ans7:- The three ways that polyspermy is prevented are as follows:
a) Fast block polyspermy: - It is achieved by changing the electric potential of the egg plasma membrane. The ionic concentration of egg is lightly different from that of its surroundings. Within 1-3 seconds of first sperm binding, the membrane potential shifts from -70 mV to +20 mV.
b) Slow block polyspermy: After a minute of first sperm fusion with the egg, cortical granule reaction takes place which is slower, mechanical blocking. The cortical granules fuse with the egg plasma membrane and release their contents into the space between the plasma membrane and the fibrous mat of vitelline envelope proteins. The fertilization envelope is formed.
c) Egg’s extracellular wall: A number of proteins are released by the cortical granules that construct the extracellular matrix of the embryo (Gould & Stephano, 2003; JAFFE, 1976; Kline, Jaffe, & Tucker, 1985).
Ans 8:- The five basic steps in the recognition of sperm by the egg are as following:
a) The egg releases chemotaxis molecule which attracts the sperm,
b) The enzymes are released from the exocytosis of the acrosomal vesicle,
c) The sperm binds to the extracellular envelope (vitelline layer or zona pellucida) of the egg.
d) The sperm passes through the extracellular envelope of egg.
e) Egg and sperm fused in the cell plasma membranes (Bishop & Sommerfeldt, 1996; Bloch Qazi, Heifetz, & Wolfner, 2003; Petit, Serres, Bourgeon, Pineau, & Auer, 2013).
Ans9:- a). Genetic similarity to humans: Zebrafish are vertebrates. It shares a high degree of sequence and functional homology with mammals, including humans.
b). Easier to house and care for than rodents: Zebrafish are usually small in size and shows simple nature in their natural environment. So, it is easier to keep and there is minimal effect on the experiments due to stress in the animals.
c). Impact of any genetic mutation or drug treatment is easy to use:- The embryos and larvae of Zebrafish are completely transparent. This allows seeing the impact of genetic manipulation or pharmacological treatment using non-invasive imaging techniques.
d) Lots of offspring: Zebrafish grows quickly and gives 200-300 offspring which ensures constant supply of animals.
Ans 10:- In amphibian embryos, cells containing mainly yolk divide more slowly and thus are larger than cells contain mainly cytoplasm during most of the cleavage period.
In zebrafish, large amount of yolk is present so that cytoplasm is segregated into a small patch at the egg periphery. This is the only part of the egg that divides during the cleavage period. As a result, the developing embryo lies on top of the yolk mass throughout development.
Ans 11:- The germ cells migrate through Hensen’s node by entering the blastocoel move anteriorly, but they do not move as far ventrally as the presumptive foregut endodermal cells. These cells remain between endoderm and epiblast and form mesoderm in animals, avian, amphibian or fish model.
Ans 12:- It is a streak which is formed by the thickening of the epiblast at the posterior region of the embryo, which is anterior to the Koller’s sickle. The primitive streak defines the axes of the embryo. It extends from posterior to anterior; migrating cells enter through its dorsal side and move to its ventral side; and it separates the left portion of the embryo from the right. The thickening of the region is caused by the ingression of endodermal precursors from the epiblast toward the center. When these cells enter the primitive streak, the streak elongates toward the future head region (Bellairs, 1986; Wei & Mikawa, 2000).
Ans 13:- Compaction is a phenomenon in which formation of inner cell mass and differentiation of the inner cells takes place. At this stage, the tropoblast separates from the inner cell mass. It is important for the development of specific tissues and organs.
Ans 14:- The monozygotic and the dizygotic twin grow in different manner. First of all, mammalian cell division is slowest as compared to other groups in the animal kingdom. Secondly, the rotational cleavage takes place (i.e., two blastomers divides meridionally and rest equatorially). Thirdly, the cells don’t divide in the same number and rate (e.g., 2,4,8,16). The most important one is the compaction of the cells which lead to cell differentiation.
Ans.15:- The study of mammalian gastrulation is the most difficult one for the scientist out of three vertebrates. It is because zygote is barely visible to naked eye (human zygote = 200 micrometer in diameter). Secondly, the number of mammalian zygotes produced is very small in number. Lastly, the development of mammalian zygote takes place inside another organism which makes the study difficult one.
Ans.16:-The three distinctive difference between left and right sides of the mammal are the presence of the heart in the left side of the chest cavity and loops to the right, spleen is also on the left side of abdomen, the major lobe of the liver forms on the right side of the abdomen, the large intestine loops right to left as it traverses the abdominal cavity, and the right lung has one more lobe than the left lung.
Ans 17:- Cleavage is defined as the phenomenon in which cell division occurs at an early stage when fertilized egg starts to develop into an embryo.
Rotational cleavage: In mammals, the second cleavage of the blastomers occurs in different fashion that the first cleavage. One blastomer divides meridionally and another one equatorially. This is called as rotational cleavage (Alford, Ng, & Burgess, 2009).
References
Alford, L. M., Ng, M. M., & Burgess, D. R. (2009). Cell polarity emerges at first cleavage in sea urchin embryos. Developmental Biology, 330(1), 12–20. doi:10.1016/j.ydbio.2009.02.039
Bellairs, R. (1986). The primitive streak. Anatomy and Embryology, 174(1986), 1–14. doi:10.1007/BF00318331
Bishop, J. D. D., & Sommerfeldt, a. D. (1996). Autoradiographic investigation of uptake and storage of exogenous sperm by the ovary of the compound ascidian Diplosoma listerianum. Marine Biology, 125(4), 663–670. doi:10.1007/BF00349248
Bloch Qazi, M. C., Heifetz, Y., & Wolfner, M. F. (2003). The developments between gametogenesis and fertilization: Ovulation and female sperm storage in Drosophila melanogaster. Developmental Biology. doi:10.1016/S0012-1606(02)00125-2
Busa, W. B., & Nuccitelli, R. (1985). An elevated free cytosolic Ca2+ wave follows fertilization in eggs of the frog, Xenopus laevis. Journal of Cell Biology, 100(4), 1325–1329. doi:10.1083/jcb.100.4.1325
Cameron, N. (2008). The biology of growth. In Nestle Nutrition Workshop Series: Pediatric Program (Vol. 61, pp. 1–15). doi:10.1159/000113162
Gilbert, S. F. (2000). Recognition of Egg and Sperm. Sinauer Associates. Retrieved from http://www.ncbi.nlm.nih.gov/books/NBK10010/
Gilbert, S. F. (2003). The morphogenesis of evolutionary developmental biology. The International Journal of Developmental Biology, 47(7-8), 467–477.
Gould, M. C., & Stephano, J. L. (2003). Polyspermy prevention in marine invertebrates. Microscopy Research and Technique. doi:10.1002/jemt.10351
Hawk, H. W. (1983). Sperm survival and transport in the female reproductive tract. Journal of Dairy Science, 66(12), 2645–60. doi:10.3168/jds.S0022-0302(83)82138-9
JAFFE, L. A. (1976). Fast block to polyspermy in sea urchin eggs is electrically mediated. Nature, 261(5555), 68–71. doi:10.1038/261068a0
Kline, D., Jaffe, L. a, & Tucker, R. P. (1985). Fertilization potential and polyspermy prevention in the egg of the nemertean, Cerebratulus lacteus. The Journal of Experimental Zoology, 236, 45–52.
Mauch, T. J., & Schoenwolf, G. C. (2001). Developmental Biology. Sixth Edition. By Scott F. Gilbert. American Journal of Medical Genetics (Vol. 99). doi:9780878932436
McClay, D. R. (2011). Evolutionary crossroads in developmental biology: sea urchins. Development, 138(13), 2639–2648. doi:10.1242/dev.048967
Moody, S. A., & Kline, M. J. (1990). Segregation of fate during cleavage of frog (Xenopus laevis) blastomeres. Anatomy and Embryology, 182(4), 347–362. doi:10.1007/BF02433495
Petit, F. M., Serres, C., Bourgeon, F., Pineau, C., & Auer, J. (2013). Identification of sperm head proteins involved in zona pellucida binding. Human Reproduction, 28(4), 852–865. doi:10.1093/humrep/des452
Suarez, S. S., & Pacey, A. A. (2006). Sperm transport in the female reproductive tract. Human Reproduction Update. doi:10.1093/humupd/dmi047
Tomlin, C. J., & Axelrod, J. D. (2007). Biology by numbers: mathematical modelling in developmental biology. Nature Reviews. Genetics, 8(5), 331–40. doi:10.1038/nrg2098
Wei, Y., & Mikawa, T. (2000). Formation of the avian primitive streak from spatially restricted blastoderm: evidence for polarized cell division in the elongating streak. Development (Cambridge, England), 127(1), 87–96.
